M9 Manufacturing Composite Laminates Learning Unit-1: M9.1 M9.1 Fabrication/Manufacturing Techniques M9.1.0 Introduction Over the last 30 years composite materials, plastics, and ceramics have been the dominant emerging materials. Modern composite materials constitute a significant proportion of the engineered materials market ranging from everyday products to sophisticated niche applications. Increasingly enabled by the introduction of newer polymer resin matrix materials and high performance reinforcement fibres of glass, carbon and aramid, the penetration of these advanced materials has witnessed a steady expansion in uses and volume. The increased volume has resulted in an expected reduction in costs. High performance FRP can now be found in such diverse applications as composite armouring designed to resist explosive impacts, fuel cylinders for natural gas vehicles, windmill blades, industrial drive shafts, support beams of highway bridges and even paper making rollers. Composites also find extensive applications in shuttering supports, special architectural structures imparting aesthetic appearance, ergonomics, large signages etc. besides longer life, low maintenance, fire retardancy etc. Their benefits of corrosion resistance and lightweight have proven attractive in many low stress applications. For certain applications, the use of composites rather than metals has in fact resulted in savings of both cost and weight. Some examples are cascades for engines, curved fairing and fillets, replacements for welded metallic parts, cylinders, tubes, ducts, blade containment bands etc. The design of a structural component using composites involves both material and structural design. Unlike conventional materials (e.g., steel), the properties of the composite material can be designed considering the structural aspects. Composite properties (e.g., stiffness, thermal expansion, etc.) can be varied continuously over a broad range of values under the control of the designer. Manufacturing Processes: Continuous advances in the manufacturing technologies and performance of FRP have lead to significant growth in its market acceptance. Fabricating a composite part is generally concerned with placing and retaining fibres in the direction and form that is required to provide specified characteristics while the part performs its design function. The fabrication of composites is a complex process and it requires simultaneous consideration of various parameters such as component geometry, production volume, reinforcement & matrix types, tooling requirements, and process and market economics. The availability of myriad choices available makes it imperative that the factors of economics, design and manufacturing be integrated during the development process itself. For composites to

become competitive with metals, cost reduction is a necessity besides durability, maintainability and reliability. A host of processes exist for the fabrication of composite components. The multitude of tasks involved in the manufacturing of composite laminates can be categorized into two phases: (1) fabrication/Manufacturing techniques and (2) processing. In the fabrication phase the fiber reinforcement and accompanying matrix material are placed or shaped into a structural form such as a flat or curved plate, a cylinder or other body of revolution, and the like. The fiber and matrix may be in preimpregnated form, or the fiber and matrix material may be combined for the first time during this step of developing the structural form. During the processing phase, heat and pressure are used to densify and consolidate the structure. For thermoset matrices die chemical cross-linking reaction (i.e., curing) solidifies the structure, whereas thermoplastic matrices become hard after cooling from their melting temperature. Fabrication techniques for composites are not dependent on the type of matrix material. In fact, some metal forming techniques have been adapted to composites fabrication (e.g., matched-metal die molding). However, processing conditions are entirely dependent on the type of matrix material used. For instance, thermosets require long processing times, whereas thermoplastics require relatively high pressures and temperatures. In this appendix we present a brief introduction to the manufacturing of composites by addressing three important areas. First, we discuss fabrication techniques. Next, processing issues are presented. We conclude with a short discussion of manufacturing defects. Overall, we approach die topic of manufacturing from a general perspective. However, to keep the discussion in the context of this book, we begin by focusing on the manufacture of structural components from layers, or plies, of preimpregnated material, called prepreg. Specifically, layers of material, with the fibers in each layer aligned in a specific direction, are used to form a laminate. We will assume that the laminate is fabricated by hand, and we will describe the necessary steps of fabrication. We will further assume that this hand-fabricated laminate will be processed in an autoclave, which is a pressurized oven that provides the proper levels of heat and pressure to solidify and consolidate the structure. In the early years of the development of fiber-reinforced materials, structural components were fabricated by hand. Even today, in prototype development, hand fabrication is common, and this is also the case for specialty manufacturing and in many university laboratories. However, as labor costs and the need for consistency have increased, engineers have been charged with designing low-cost automated manufacturing techniques. Now, automated techniques like robotic tow and tape placement methods, injection molding, and Pultrusion have dramatically reduced the cost of manufacturing some composite structures. Later in the appendix we will consider these various other approaches to fabricating and processing a laminate. Common to all manufacturing methods is the use of a die, mold, or mandrel. They provide the structural shape for the composite material, and in this discussion they will be referred to generally as the tool. Tools are usually an inverse, or female, replica of the desired structural shape. The design of the tool is a critical and expensive process. The cost of the tool often far exceeds the material and labor costs to produce a composite structure. Also common to all manufacturing methods is, as mentioned, the need to apply temperature and pressure to the structural component after the fiber and matrix are brought together into the desired structural

form. The pressure takes two forms: actual pressure, ideally hydrostatic, to consolidate the tows and layers; a vacuum to remove air entrapped between the layers and to reduce the amount of unwanted gases given off by the resin as it cures. The application of pressure can be in the form of closing both halves of the tool or, as with a flat structural component, pressing the laminate in a hot press. More commonly, however, and as will be assumed here, pressure is applied by putting the uncured structural component into an autoclave. Finally, the vacuum requirement is met by enclosing the structural component in a vacuum-tight bag and drawing a vacuum. M9.1.1 Tooling and Specialty Materials M9.1.1.1 Tooling of Composites Introduction for Tooling of Composites: As all fabrication methods require tools to provide the shape of the composite structure during processing, the design and construction of the tool are critical components of the manufacturing process. Because the tool is heated and pressurized, especially critical is the choice of tooling material. Factors which must be considered in tool material selection are dimensional stability and compatibility, cost, surface finish, and durability. Table M9.1.1 lists the coefficients of thermal expansion for several tool materials. Of the metals, tool steel most closely matches that of the composites. Steel tools are also highly durable and have good thermal conductivity; however, they are extremely heavy and they take substantial time to heat and cool. Ceramic tools have the lowest thermal expansion, so their dimensional stability is the best, and they also have a thermal conductivity close to that of tool steels. However, they are brittle and must be protected from chipping and cracking. Sometimes ceramic inserts are used in steel tools to combine the best characteristics of both materials.

Material system Coefficient of thermal expansion (X ) 610 / C− o

Polymer matrix composites (fiber dir., 1α ) • Aramid or graphite-reinforced • Boron fiber-reinforced • Glass fiber-reinforced

-1.5 to 1.1 2.3 to 3.0 6.3 to 8.4

Slip cast and fired ceramic 0.83 Tool steel 11.1 Electroformed iron 11.7 Electroformed nickel 12.8 Plaster 13.9 High-temperature epoxy 19.4 Aluminum 23.3

Table M9.1.1 Thermal expansion of tooling materials Aluminum tools are easily machined and less expensive than steel or ceramic. They are lighter than steel tools and they heat and cool faster than steel; however, they are not as durable as steel tools and their thermal expansion is excessive. Plaster tools are sometimes used when durability

is not required. They can be made easily by pouring the uncured plaster around a model. Once the plaster has been cast, it is cured and then hardened by coating with a varnish. Actually, graphite- or glass-reinforced composite materials can be used to fabricate a tool. If this is the case the thermal expansion of the tool can be exactly matched to that of the composite structure. Composite tools are durable, their surface finish is excellent, and they are less expensive than steel tools. However, they usually require that a plaster casting be made of the structure first. The manufacture of composite detailed parts and assemblies requires that some kind of accurate repeatable tool surface be provided, which is capable of withstanding repeated exposures to the cure cycle environment of high temperature sand pressures. Individual composite parts or details will require a variety of support tooling-beyond the initial cure tool, which are as follows:

• Master model reference patterns, • Trim or router tools, • Precision hole location drill tools, • Assembly fixtures, • Ply locating templates, and • Associated shop aids.

The primary objective of any tool for composite fabrication is to make an accurate repeatable part within the confines of the process parameters defined by the manufacturer/designer and the detail performance characteristics meeting the requirements of the end user. Design of the initial tool becomes the most pressing initial issue of tooling for composites. Factors, which govern the basic tool design, are as follows:

• Coefficient of thermal expansion (CTE). One of the most critical parameters in the design of tooling for composites is the difference between the CTE of the problems. The greater the difference between the CTE of the composite detail and the tool, the more pronounced the effect would be. One of the effects that occurs as a function of these dimensional differences is called spring back Composite details, when cured, hold the specific molded shape, as defined by the tool, as a result of the cured combination of resin and reinforcement. The spring back, or more accurately defined as a warpage condition, occurs when the composite detail is cured into a tool, that at a specific temperature has one definite dimensional tolerance and then upon cooling to ambient temperature, contracts to its original ambient dimensions. Warpage occurs when stresses are induced to the composite as the tool begins to return to the ambient dimensions. This condition will become predominant as the temperature difference between ambient and cure temperature increases and the dimensional size of tool increases. A common method of minimizing the effect of spring back or warpage of a composite detail during and after cure cycle is to determine the CTE of the composite part being fabricated and the CTE of the tooling material selected. During the design of tooling, CTE of the tooling material should be matched to that of the composite detail. Another condition leading to warpage of the laminate includes an unbalanced laminate orientation where the numbers of plies of material are more dominant in one direction than other.

• Using CTE in the design of tooling for composites. Following two methods are commonly used to minimize the effect of CTE when designing tooling for the fabrication of composite details:

Careful selection of the appropriate tooling material. Difference between

CTE of tooling material and composite detail to be fabricated should be as close as possible.

Use of shrink factors in calculation of dimensions prior to tool fabrication.

• Choice of material in the design of tooling. Careful selection of the appropriate material for tool use must include review of the following criteria:

Anticipated tool usage i.e. expected life of tool. The life expectancy of

any tool fabricated for the lay-up and cure of composite details is dependent on a variety of factors. Material selection, shop handling procedures and cure cycle time all affect the ability of tool to withstand long usage.

Cost available for tool fabrication. Cost of tool fabrication is difficult to quantify due to variable factors such as material and manpower cost.

Material available for tool construction. Available methods for tool fabrication. Methods of manufacturing vary

depending on equipment and personnel resources available. Plaster type master models and wet lay-up type molds take minimum facility requirements and basic shop skills. Use of prepreg materials requires additional skill levels and expenditure on account of sophisticated ovens and autoclaves.

Level of dimensional tolerances required from composite details. Based on the type of manufacturing method and the type of material selected, different levels of dimensional tolerances are possible. Initially, the dimensional tolerance for the composite detail to be fabricated is determined. Compliance to this tolerance is critical in meeting structural demands and conformance to any form, fit or functional requirements.

• Designing tools for resistance to failure. Due to the abusive environment experienced by tooling during the fabrication of composite details, life expectancy of the tooling is always short of anticipated value. Repetitive cycling from ambient to over 177°C (350°F),inadequate care and handling procedures, incorrect fabrication techniques have lead to a variety of defects resulting in premature, temporary or permanent failure of the tool. Failure modes common to composite lay-up tools fabricated by both wet lay-up and prepreg methods generally involve fibre separation and delamination. This is due to a variation in CTE between the resin matrix and the fibre. Possible solution to the problem of delamination between layers of prepreg tooling is the use of resin systems with glass transition temperature values at or slightly above the maximum usable temperature. For example, if the tool is intended to be cycled repeatedly at 177°C (350°F), glass transition temperature value of the resin system in the 220°C (425°F) range will allow more cycles.

M9.1.1.1 Master Models

A master model is a master source identified with holes, scribe lines, trim lines or any other feature of the part that requires duplicating to other tools. The master model is a physical representation of the design or a point of reference to which all supporting tooling, both for fabrication and inspection, would be indexed. Since this surface will provide the reference pattern for all subsequent operations beyond initial fabrication, extreme care must always be taken to protect the master model. Master models may be fabricated from a variety of materials such as plaster, machined urethane or epoxy board stock, monolithic graphite or ferrous and non-ferrous metals. Each material offers distinct advantages and disadvantages. To determine which material is the most feasible, the entire tooling philosophy must be reviewed. Master models are generally stored indefinitely so that they may be referred to over the life cycle of the manufactured parts. Due to hygroscopic nature of plaster, care must be taken to protect the master model from the environment to maintain accuracy. Adequate storage conditions should be utilized for complete protection of master model throughout the life expectancy. M9.1.1.1.1 Plaster Masters One of the oldest methods of producing a master is from plaster. Plaster is made from the mineral called gypsum (CaSO4) which is finely ground and dehydrated to produce a fine powder with uniform properties. With the addition of water to form workable slurry, a reaction occur which produces heat and the inert gypsum gets dried. Plaster is manufactured in various textures or grades. Coarse grades are used to buildup the master model surface and then followed by fine grades which allow precise details such as trim lines or other identifications to be scribed into the surface. Depending on grade being used, plaster has a setting expansion of approximately 0.080% and a thermal expansion in the dried state of a maximum of 0.027ºC (0.0156ºF). M9.1.1.1.2Template Method There are several techniques of building a plaster master determined by the shape of the part. If the part is not symmetrical and does not have a constant cross-section or the size is large, the master model is made from a series of templates secured to a flat base to form a 3D full-scale model of the part. Space between the templates is relative to the degree of abruptness of the contour. For normal gentle contours, a space of 15-20mm is common. Templates are usually made of aluminium to prevent corrosion. For temporary masters, steel is some times used, however, because of the amount of moisture present during mixing and application of the plaster, steel templates may rust. A schematic of typical template plaster master is shown in Figure M9.1.1.

Figure M9.1.1

If electronic data is available, the templates can be NC-machined or cut with water or laser jet directly from the flat pattern generated by the data. Except for NC-method, deburring is generally required to remove spurs or sharp edges from the templates prior to use. Holes are drilled to the templates for threaded rod spacer sand screen support rods. For large models, air passages are cut into the bottom of the template to allow for even curing of the plaster. Once sufficient templates have been prepared, bluing is applied to a flat-ridged steel table and scribed with a pointed tool to denote the location of each template. Flatness of the table is critical and should be within0.127mm. Tooling balls, which indicate the x-, y- and z-direction are some times placed on the table corners as reference points for the system. Each template is attached 90° to the base table with angles. Threaded rods are secured with nuts on each side of the template to provide rigidity to the template face. Wire mesh is placed between the templates and secured to the threaded rod. This is used to hold plaster in place. Slurry of plaster is poured between the templates; surface is made even and left to dry to forma smooth and accurate surface. Because of the tendency of plaster to absorb moisture, it should be sealed after the surface has had adequate time to cure. Commercially available lacquers can be used to seal the surface and provide a suitable protection within the shop environment. M9.1.1.1.3 Follow Board Method A method widely used when a constant cross-section is required to be built is the follow board. A flat surface is required with an accurate side surface to act as a guide rail. A template of the contour is prepared from sheet of aluminium or steel and attached to a wooden guide support. Plaster is mixed and built up on the surface to within 3mm of the final contour. Partial drying is recommended before the final plaster mix is applied. This will prevent shrinking and cracking of the plaster surface which otherwise would affect accuracy. Using the template and guide support, the plaster contour is formed by pushing the template evenly over the surface (Figure M9.1.2).

Figure M9.1.2

M9.1.1.1.4 Sweep Method Sweep method is used when a symmetrical surface such as cone or hemispherical shape is involved. As with the follow board method, a flat surface is required from which a frame of desired shape can be constructed. For large shapes, intermediate templates should be placed to provide adequate support. The sweep itself is made from sheet metal of 3 mm minimum thickness and supported by a wooden guide or other mechanical guides. Plaster can be reinforced to form the desired shapes. All master models fabricated from plaster require, in addition to sealing with commercial grade lacquer, suitable storage, if the model is required to be stored for any period of time outside the shop environment. . M9.1.1.1.5 Lay-up Molds Lay-up molds are used to form the shape of the part to be produced and have the part periphery scribed on the surface. Tools can be made directly from a NC machined master model or from a plastic faced plaster splash taken from a master model not capable of elevated temperature and pressures. The choice of glass or carbon/epoxy for the mold is generally governed by the complexity and CTE of the part to be fabricated. Lay-up molds must be capable of maintaining a vacuum tight environment while being subjected to high temperatures and pressures. NC-Machining Due to the widespread use of CAD systems, a great deal of accuracy can be transferred into the master model through the NC machining operations. Composite Materials Composite tools are usually made from epoxy resin matrix and either E-glass or carbon fibres as reinforcements. Depending on the life cycle required, tools could be made from prepreg or by wet lay-up procedures. Prepregs generally require curing within an autoclave because of the

elevated pressures. Because of the increased compaction available while curing in an autoclave, tooling fabricated from prepregs ate capable of a greater number of cure cycles than the wet lay-up method. In addition to greater compaction, autoclave curing offers better control of resin content and uniformity of reinforcement. M9.1.1.1.6 Prepreg Method There is considerable number of prepregs available as epoxy 'B' staged glass or carbon reinforced cloth. The first step prior to prepreg application on the master surface is to ensure that the prepreg and the master surface are absolutely clean and free from debris and that the surface is smooth and without pin holes. A quick vacuum check is always a good idea at a minimum of 6.2 kPa. A loss of 500 Pa within 5 minutes with the pump non-operating is acceptable. Apply masking tape around the tool periphery for later application of the sealant tape. It is absolutely necessary that the, master surface can be released with a suitable hard wax or other release agents. After the cleaning and releasing processes have been completed, release coated tooling pins should be placed into the holes of the master. Series of steps followed after the above processes are:

• Carefully lay each ply onto the surface and work out wrinkles or air bubbles and maintain the warp direction of each ply in the 0ºdirection.

• An overlap between the plies should be preferably 3-6 mills. • Debulking should be done to ensure that no air is entrapped at the interface and the

smooth surface on the tool. It is accomplished by application of a peel ply net to the edge of the laminate and working out wrinkles and air bubbles.

• Sealant tape should be placed around the periphery to prevent resin flow. Adequate precautions should be taken to allow for resin bleed.

• The orientation for each ply should be such that a balanced system is maintained to minimize stress build up in the laminate.

• As a rule of thumb, Debulking should be done after every 4-5 plies. Final build up of the laminate should be at least 9.5 mm.

• Final vacuum bagging is performed with a layer of peel ply, perforated Teflon, polyester breather and vacuum bag.

• Recommended heat up rate and cure temperature should be followed. Most systems can be initially cured at up to 63°C (145°F) and 586-689 kPa of pressure for 14 hours.

• After the autoclave cycle, carefully remove the bag and films .from the laminate to avoid lift up from the master surface. Attachment of support or back up structure (e.g. egg crate) to the laminate is very important to minimize any potential residual stresses built into the laminate. Support structure details are shown in Figure M9.1.3.

• Separation of the tool from the master should be done carefully to avoid damage to the master or the tool itself. Tooling pins should be removed prior to separation. Once the tool is separated, the surface should be inspected for pin holes or -roughness. Pin holes can be filled with resin and the roughness can be smoothened with fine grit sandpaper. Care must be taken that no fibres are lifted by sanding along1he length of the fibres.

Figure M9.1.3

Once, the tool has been cleaned up, the required check for vacuum integrity is accomplished by placing a layer of polyester breather cloth on the surface and a vacuum bag over it. The acceptance criterion is generally that there is no loss greater that 500 Pa in 5 minutes at a minimum of 6.2 kPa at the start of the test. If possible, depending on the complexity of the tool, place the tool back onto the master and check for any warp or out of contour problems. M9.1.1.1.7 Wet Lay-up Method Wet lay-up of composite tools can be for room or elevated temperatures use. The difference is only in the resin selection. Procedurally, the process is the same except for the cure cycles. Initial steps in wet lay-up method are same as are described in prepreg method namely, cleaning of master surface, checking for vacuum integrity, placing of masking tape around the periphery, releasing with a suitable wax or release agent, placing of tooling pins into the holes of master, etc. For wet lay-up two resins are used, one for the gel or face coat and one for laminating. The gel coat is generally the same as the laminating resin but with additives to thicken it to make it adhere to the contour of the master. Excessive buildup should be avoided incomers or at the bottom of contours while applying the gel coat. Too much resin will result in cracking and crazing later in the tool life cycle. It should also be ensured that air bubbles are removed. Air that remains entrapped either on the tool surface or within the layers of cloth could result in possible blisters and delamination later during tool usage. Application of first few plies should be done carefully to avoid pushing through the gel coat surface. It should be ensured that the plies are completely wet. All wrinkles and entrapped air should be worked out before another ply is added. Overlaps of 6.35 mm between plies should be maintained but the seam should never be placed over a seam from a previous ply. After each ply, additional resin should be added to cover the surface. After the fourth ply, or prior to the resin curing, apply a peel ply to the surface for compaction cycle. Debulking should be performed after every 6-ply or before resin begins to cure. Final laminate thickness should be 9.5 mm. After the final ply has been applied, the compaction step is repeated with the peel ply, mold release film, bleeder and the vacuum bag. Fabricate a support or backup structure of similar material to the laminate to avoid stresses caused by the difference in CTE between the support structure and the laminate. Allow the tool to stand at ambient temperature for a minimum of 24 hours prior to post cure.

After the final 177°C (350°F) post cure, inspect the surface for pinholes and repair any blemishes with gel coat resin. A final vacuum check at 635 mm Hg with a loss of no more than 51 mm is acceptable. M9.1.1.2 Plastic Faced Plaster Plastic faced plasters (PFP) are tooling aids that minimize the wear and tear on masters by duplicating the master surface with a suitable unit that can be used for variety of purposes. PFP sallow for tooling to be directly fabricated from the master surface without exposing the master model to adverse environmental conditions, such as autoclave temperatures and pressures. If taken directly from the mater surface, the PFP is the reverse of the master contour. An intermediate plaster splash is required to get back to the master contour with a PFP. If the surface required is directly from the master model, the PFP will be taken directly from the master surface. If the surface is above or below the master surface, appropriate steps must be taken by either taking additional splashes with or without layers of tooling wax to achieve the appropriate dimension. PFPs can be used in an autoclave (with vacuum integrity) up to approximately 105°C (220°F), however, a limit of one or two runs is all that can be expected. PFPs provide tooling aids for a variety of other room temperature shop applications.

Drill Templates Drill templates or fixtures are used primarily to drill and locate precision holes in the production composite parts. Drill fixtures are fabricated using a room temperature fibre glass/epoxy system. Because the tool is used in the shop environment at ambient conditions, no vacuum integrity or elevated temperature requirements are needed. Trim and Router Templates Trim and router templates are used to trim and rout cured composite parts to a specific dimensional tolerance. Accuracy is required for these tools in order for the composite detail to fit precisely with adjacent details. Trim and router templates can be fabricated directly from the master model, composite tool or tooling aids such as PFP. They are generally fabricated using room temperature cured fibre glass/epoxy system. Since 'trim and routing operations are always carried out at room temperature, CTE is not considered in the design of this type of tooling. Ply Locating Templates Ply locating templates are used during the lay-up of the production part and designate locations for the plies and indexing of detail parts. In addition, these templates may also show individual ply orientation.

M9.1.1.2 Specialty materials Many secondary or specialty materials are used in composites manufacturing. Before we discuss the various types of specialty materials it is helpful to examine a typical lay-up of a composite structure prepared for autoclave processing. Figure M9.1.4 shows a cross section of an autoclave

specimen. In addition to the actual composite laminate, there are release coatings, peel plies, release film, bleeder plies, breather plies, vacuum bags, sealant tape, and damming material. Each of these materials serves a specific function. For instance, release agents are used to prevent the composite material from bonding to the tool. The illustration in Figure M9.1.4 is based on processing a flat laminate, and thus the portion of the tool at the bottom of the figure is actually a flat plate and the portion of the tool toward the top is also a flat plate, sometimes called a caul plate when used in this fashion. However, it is easy to imagine the bottom and top tool components having curvature, or only a curved bottom plate being present and the peel plies, release film, and so on, being draped over the curved uncured laminate.

Figure M9.1.4 Typical autoclave lay-up

Type Form

Examples

Fluorocarbons Films or dispersions Teflon (tetrafluoroethylene), Tedlar

(polyvinylfluoride) Polymer films Coated paper, extruded film Polyvinyl alcohol (PVA), polyamines,

polyethylene, cellophane Silicones Liquids, resins, greases Silicone polymers Waxes Paste Parafin, carnuba, microcrystalline waxes Metal salts

Liquids or particles (external and internal release agents)

Stearic acids (calcium, zinc, lead, aluminum, magnesium salts)

Inorganic Powders Talcum, mica Table M9.1.2 Mold release agents

M9.1.1.3 Release agents: Release agents are used to coat the tool so that the composite structure is prevented from bonding to the surface. They are usually a paste or liquid that is coated onto the tool surface and allowed to dry. Films are also used, but they are limited to flat or single-curvature surfaces. Table M9.1.2 lists several types of release agents that are used.

1. Fluorocarbons. Fluorocarbon polymers are used extensively in autoclave molding. Tedlar®, a polyvinyl fluoride, is a fluorocarbon film manufactured by DuPont. FEP (fluoroethylene propylene) forms a continuous film on the mold surface and is used for composites cured up to 177°C. Above this temperature the fluorine disassociates from the polymer. PTFE (polytetrafluoroethylene), a polymeric dispersion of Teflon®, is stable up to 260°C.

2. Polymer films: These polymers are insoluble in most solvents, so they are applied as extruded or blown films. PVA (polyvinyl alcohol), cellophane, polyamines, and polyethylene have all been used as release agents.

3. Silicones: The commercial silicones are cured polymers with high melting points and low volatility. They are applied in liquid form or as grease. Some special silicone release agents are stable up to 480°C; however, most are limited to about 200°C.

4. Waxes: Carnuba paste wax is cheap and easily applied. It is sometimes polished before the composite is laid onto the tool surface. Carnuba waxes are excellent mold release agents for composites cured below 250°F. Above this temperature the wax begins to degrade.

5. Metal salts: Stearic acid, a fatty acid, is used widely for mold release. It has a sharply defined melting point at 71.5°C and has good wetting properties. The main derivatives of stearic acid (such as calcium, zinc, and lead salts) are also used as release agents. The choice of metallic stearate to use for a specific application depends primarily on the type of polymer matrix.

6. Inorganic compounds: These are probably the oldest known release agents. Because they are insoluble, they are applied as powders. Talcum and mica are the most common compounds used. In some cases they are mixed with metal stearates to improve their release ability.

M9.1.1.4 Peel plies and release films and fabrics: Surfaces are protected from contamination by peel plies, and they are normally removed from the composite structure just before bonding or secondary coating operations. The most common peel plies are heat-cleaned-and-scoured nylon, heat-cleaned lightweight fiberglass, or polyester fabrics. User preference of the surface texture after the removal of the peel ply dictates the choice of a specific type. Release films and fabrics serve many different purposes. Sometimes they are used as separators between successive layers of pre-impregnated material, and they are also used to separate the bleeder or breather materials from the composite laminate. They are most commonly a Teflon-coated fiberglass fabric, and in some cases the release film is porous so that resin can flow through the film. For example, porous release films are used to separate the bleeder plies from the composite laminate. This allows the resin to flow from the laminate through the release film and into the bleeder plies. M9.1.1.5 Bleeder and breather plies: Bleeder and breather plies are porous, high-temperature fabrics which are used to absorb excess resin during processing. Most pre-impregnated materials are supplied with excess resin, which is

subsequently removed during processing. The resin is removed to increase the fiber volume fraction and flush voids from the laminate. The excess resin is trapped and absorbed by the porous bleeder plies. Fiberglass, cellulose, and polyester fabrics are all used as bleeder plies. Breather plies are used to provide a vacuum pathway into the composite laminate, and they also act as a conduit for the removal of volatiles during cure. They must remain porous at high temperatures and pressures. Most bleeder materials can also be used as breather plies. In addition, perforated fluorocarbon or nylon films and Teflon-coated fiberglass are sometimes used. M9.1.1.6 Bagging films: Bagging films form a barrier between the composite laminate and the oven or autoclave environment. The bagging film is sealed around the edge of the lay-up by sealant tape, and the film is drawn down onto the composite laminate by pulling a vacuum under the bag. Vacuum bags must be heat resistant, flexible, nonvolatile, and resistant to tearing. Several high-temperature polymer films are used, including Kapton® (up to 316°C), nylon (180°C), and PVA (121°C). Silicon rubber bags are also used up to about 200°C, and they have the added advantage of being reusable. Learning Unit-2:M9.2 M9.2 Hand Lay-up Even though the method has been replaced with automated techniques, the lay-up of preimpregnated material by hand is the oldest and most common fabrication method for advanced composite structures. Furthermore, the basic features of the method remain unchanged. A pictorial essay showing each step in the hand lay-up of a flat composite laminate is shown in Figures M9.2.1-M9.2.15. Each step must follow in successive fashion in order to obtain a high-quality composite laminate after final processing. A description of these steps follows. Step 1, Figure M9.2.1. The surface of the tool is cleaned and a release agent is applied. If the surface is not clean, then the release agent will not function properly. The release agent can be in liquid form, or it may be a solid film. (In the photo-essay, to provide an indication of scale, a hand-held pointer or knife is included in the photographs.) Step 2, not shown. An optional sacrificial layer is laid up on the tool surface. This layer is usually a fiberglass fabric made with the same resin system as the composite laminate. The sacrificial layer protects the laminate from surface abrasion and surface irregularities during manufacturing. Step 3, Figure M9.2.2. A peel ply is placed on top of the sacrificial layer. The peel ply will be removed after processing. Step 4, Figure M9.2.3. The preimpregnated plies are cut according to design specifications. They can be cut by hand using shears or a steel blade knife. However, automated cutting

machines have largely replaced hand cutting. The Gerber knife is a reciprocating-knife system originally developed for the textile industry. It is extremely fast and can cut up to 20 plies at one time. Lasers have been used for cutting, but they are expensive and have limitations on the number of plies that can be cut at one time. Water-jet cutters are also used extensively, and they can cut a large number of plies (> 40) at one time, but some moisture absorption occurs during the cutting operation. Ultrasonic cutters have been used as well. Step 5, Figures M9.2.4, Figure M9.2.5. The first prepreg ply is oriented and placed upon the tool or mold. Subsequent plies are placed one upon another; a roller or other small hand tool is used to compact the plies and remove entrapped air that could later lead to voids or layer separations. It is important that the preimpregnated material have sufficient tack so that it sticks slightly to the peel ply and to the adjacent plies. Tackiness, a characteristic of preimpregnated material, quantifies the relative stickiness of the plies at room temperature. As the preimpregnated material ages, its tackiness is reduced. Eventually, the plies no longer stick together and they may have to be heated slightly to soften them during lay-up. Oils and dirt on the surface of the preimpregnated plies will contribute to reducing composite strength after processing. Technicians should wear gloves during lay-up so that oils and dirt from the hands do not contaminate the prepreg plies during lay-up. In some cases the hand lay-up procedure may be carried out in a clean room to reduce the risk of contamination of the prepreg plies. Step 6, Figure M9.2.6. A flexible resin dam is anchored to the sacrificial layer approximately 3 mm from the edge of the laminate. The dam prevents resin flow out of the laminate, in the plane of the laminate. Flexible dams can be made from silicon rubber, cork, or release coated metal. (As no sacrificial layer is being used in the procedure here, the flexible dam is anchored to the peel ply.) Step 7, Figure M9.2.7. Another peel ply is placed on top of the laminate to protect the laminate surface. Step 8, not shown. A sheet of porous release film is laid over the dam and the laminate. The porous release film will serve as a barrier to prevent bonding of the composite laminate to the secondary materials to follow. Step 9, Figure M9.2.8. Next, bleeder plies are laid up over the release film, in this case the peel ply. The bleeder plies extend to the edge of the laminate. The number of bleeder plies to be used for a given laminate can be determined by using a resin flow process model or through empirical observation. As the number of bleeder plies increases, the final fiber volume fraction of the composite laminate increases. Eventually, a maximum number of bleeder plies is reached and no further increase in fiber volume fraction occurs. Step 10, not shown. Another porous release ply is next laid up over the bleeder plies extending past the flexible dam. This prevents excessive resin flow into the breather material while maintaining a vacuum pathway into the composite laminate. Step 11, not shown. Breather plies are placed over the entire lay-up. The breather plies will conduct the vacuum path into the laminate. It is critically important that sufficient breather

material is used throughout the entire laminate. Creases and areas with shallow curvature are sometimes reinforced with additional layers of breather material to ensure that the breather plies do not collapse in these areas. Usually, two or three breather plies are sufficient. Step 12, not shown. An edge bleeder is used to connect to the vacuum ports. An edge bleeder is nothing more than a strip of breather material folded along its length several times. It is placed so that it overlays the breather material surrounding the laminate and extends out to a convenient location for the placement of the vacuum port. Step 13, Figure M9.2.9. Caul plates are sometimes placed on top of the lay-up. The caul plate is steel or aluminum plate that protects the surface from sharp temperature increases (it acts as a heat sink) and it gives a smooth non-wavy surface texture. Step 14, Figure M9.2.10. If a caul plate is used, then additional breather or bleeder plies are placed over the plate to protect the vacuum bag from puncture. Step 15, Figure M9.2.11. Sealant tape is placed around the entire periphery of the lay-up. Step 16, Figure M9.2.11. The vacuum bag is cut to size and placed over the lay-up. Step 17, Figure M9.2.12. The bag is sealed by pressing the bag over the sealant tape. It is critically important to ensure that the bag is adequately sealed before proceeding to the processing cycle. Many parts are scrapped because the vacuum fails during processing, causing excessive voids, inadequate resin flow, or incomplete consolidation. Step 18, Figure M9.2.13 and M9.2.14. The vacuum port is installed through the bag and the contents are evacuated. The bag is now checked for leaks. If any are detected, they are repaired before processing. Usually a leak test calls for application of a vacuum to some specified level (cm of Hg), followed by a 30-60 minute hold. During the hold the bag is disconnected from the vacuum source and the pressure level within the bag is monitored. If the bag is sealed well and there are no leaks, then the vacuum level should not change for the 30-60 minutes. Some leaking generally occurs, so it is a question of having sufficient vacuum pump capacity to maintain the specified vacuum level. When the vacuum is satisfactory, the composite part is ready for processing. The specific processing steps depend on the particular composite material being used, and the operation of the autoclave depends on the specific make and model. General discussions of processing and autoclave features are presented in the sections to follow. Obviously, there is a significant amount of skilled labor necessary for the hand lay-up of composite parts. Each step has a specific purpose and function. This type of fabrication is the most time-consuming, but it is also the most flexible and when combined with autoclave processing, it results in high-quality parts. Automated equipment can be used to cut and place the preimpregnated material onto the tool surface. The economics of manufacturing dictate that a relatively large volume of parts must be made to make automated equipment cost-effective. Some of these automated methods will be discussed later.

Figure M9.2.1 Step 1 in the Hand Lay-up method: The mold is covered with a release film.

Figure M9.2.2, Step 3 in the hand lay-up method: A peel ply is laid on top of release film.

No sacrificial layer is used in the example lay-up.

Figure M9.2.3, Step 4 in the hand lay-up method: The prepreg plies are cut to design specifications.

Figure M9.2.4, Step 5 in the hand lay-up method: The prepreg plies are oriented and laid on

the tool surface.

Figure M9.2.5, Step 5 (continued) in the hand lay-up method: The prepreg plies are rolled out

to remove wrinkles and air bubbles trapped during lay-up.

Figure M9.2.6, Step 6 in the hand lay-up method: A flexible resin dam is placed around the

edge of the laminate. The dam prevents resin flow in the plane of the laminate.

Figure M9.2.7, Step 7 in the hand lay-up method: Another peel ply is placed on top of the

laminate to protect the laminate surface.

Figure M9.2.8, Step 9 in the hand lay-up method: Bleeder plies are cut and placed on top of the lay-up to absorb excess resin. Note: No porous release film was used in the example lay-up.

The peel ply serves as a release film in this case.

Figure M9.2.9, Step 13 in the hand lay-up method: A caul plate is placed on top of the lay-up.

Bleeder and breather plies can be seen directly underneath the caul plate.

Figure M9.2.10, Step 14 in the hand lay-up method: Additional breather plies are wrapped

around the entire lay-up to protect the vacuum bag from puncture and to provide a vacuum pathway into the laminate.

Figure M9.2.11, Step 15 and 16 in the hand lay-up method: Sealant tape is placed around the

periphery of the lay-up, and a vacuum bag is cut to size to cover the lay-up.

Figure M9.2.12, Step 17 in the hand lay-up method: The bag is sealed by pressing the bag

over the sealant tape.

Figure M9.2.13, Step 18 in the hand lay-up method: A vacuum port is installed through the vacuum bag.

Figure M9.2.14: Finished lay-up after vacuum has been applied ready for autoclave processing

Learning Unit-3: M9.3 M9.3 Processing M9.3.1 Overall Considerations

Once the matrix and fibers are combined, and they have the desired structural shape, it is necessary to apply the proper temperature and pressure for specific periods of time to produce the fiber-reinforced structure. A judicious choice of temperature, pressure, and time produces composites, that are fully cured, compacted, and of high quality. Slight deviations from the recommended processing conditions can result in unacceptable quality. The temperature cycle is usually referred to as the cure cycle, as it is the heating of the resin that initiates the cure reaction. The overall cycle, which includes pressurization and the temperature cycle, is referred to as the process cycle. The typical cure cycle for thermosetting polymer matrix composites is a two-step cycle shown in Figure M9.3.1. In such cycles the temperature of the material is increased from room temperature to some elevated temperature, and this temperature is held constant for the first dwell period. Afterwards, the temperature is increased again to a second temperature and held constant for the second dwell period. After the second dwell, the part is cooled to room temperature at a constant rate. Because there are two dwell periods, this type of cure cycle is referred to as a two-step cure cycle. The purpose of the first dwell is to allow gases (entrapped air, water vapor, or volatiles) to escape from the matrix material and to allow the matrix to flow, facilitating compaction of the part. Thus, the viscosity must be low during the first dwell. Typical viscosity versus temperature profiles of polymer matrices show that as the temperature is increased, the viscosity of the polymer decreases until a minimum viscosity is reached. As the temperature is increased further, the polymer begins to cure rapidly and the viscosity increases dramatically. Thus, the first dwell temperature must be chosen judiciously to allow the viscosity of the resin to be low, while keeping the cure to a minimum. Isothermal viscosity versus time profiles of the resins involved are useful in determining pot life, namely, the maximum length of time at a specific temperature for the resin to remain fluid like. The first dwell time must be less than the pot life of the polymer at the dwell temperature. The purpose of the second dwell is to allow cross-linking of the resin to take place. Here the strength and related mechanical properties of the composite are developed. What is important to realize is that the cross-linking, or curing, process gives off heat (i.e., it is exothermic). Thus, temperatures can increase during cure even with no heat being added. However, since curing is accelerated by supplying heat, care must be taken not to overheat the composite by a combination of the exothermic nature of cure and the heat added to speed up the process. To characterize the exothermic cross-linking reaction of a thermosetting polymer matrix, a thermal cure monitor technique such as isothermal differential scanning Calorimetry (DSC) is commonly used. Figure M9.3.2 shows a typical isothermal DSC trace for a thermosetting polymer. The resin releases energy as the exothermic cross-linking reaction proceeds. Eventually, the DSC trace approaches a flat line as the cross-linking reaction nears completion. If the applied temperature, T, is increased, the reaction rate increases and the time to complete the reaction decrease. If the applied temperature is decreased, the reactions rate decreases and the time to complete the reaction increases.

Figure M9.3.1 Typical two-step cure cycle

Several competing priorities take place in the choice of the second dwell temperature. First, a low temperature is desirable to ease manufacturing and to reduce thermally induced stresses at the micromechanics level that are a result of the mismatch in the coefficients of thermal expansion between the fiber and matrix, and at the layer level that are a result of the mismatch in coefficients of expansion between layers with different fiber orientations. Second, the processing time should be as short as possible for economic considerations. Because low temperatures require longer dwell times, these two concerns must be compromised. Third, however, the temperatures due to the exothermic nature of curing must be kept in check. Often a vacuum is applied to the part during processing, typically during the first dwell to help facilitate removal of entrapped gases. Vacuum is discontinued after the viscosity of the resin increases significantly, and pressure is then applied to consolidate the laminate and to ensure fiber-matrix interaction. Pressure is removed either after significant cross-linking, or after completion of the process cycle. Thus, we see that the second dwell temperature is one of the most critical parameters in the process cycle. Its choice is largely material dependent. A certain minimum temperature must be reached before the cross-linking reaction begins.

Figure M9.3.2 Typical isothermal DSC relations for a polymer resin: Applied temperature T1 >

applied temperature T2. Demands for increased performance have led to the development of several high-temperature resins (e.g., polyimides and bismaleimides). These high-temperature resins retain good mechanical properties at elevated temperatures. Processing these resins requires higher temperatures than conventional epoxy-matrix composites, and the higher temperatures lead to

higher residual stresses. In some cases, processing-induced residual stresses can be high enough to cause cracking within the matrix even before a load is applied. This micro-cracking of the matrix can expose the fibers to degradation by chemical attack, and strength is adversely affected as stresses are initially present. Chemically, the reinforcing fibers are affected very little during the process cycle. The polymer matrix, on the other hand, will contract by as much as 6% due to chemical shrinkage during cross-linking. Stress relaxation during the second dwell period can reduce chemical shrinkage effects, and this relaxation behavior increases with higher temperatures. However, these higher temperatures will increase thermally induced residual stresses. Increased pressure during cooldown reduces thermal contraction, and thus thermally induced residual stresses are reduced. However, too much pressure could lead to damage of the fibers or matrix cracking. Thus, we can see that the optimization of processing parameters is a complicated and interrelated problem. Once a process cycle is chosen, the resulting mechanical properties of the composite must be evaluated. Significant degradation in strength and stiffness or other mechanical properties would not generally be acceptable. For processing thermoplastic matrix composites, as no cross-linking process occurs, there is no need to maintain elevated temperatures for extended periods. However, the processing temperatures are generally much higher than for thermosets, and therefore thermally induced residual stresses are an issue. The area of process modeling is an attempt to quantify the effects of processing on physical parameters such as degree of cure, temperature, fiber volume fraction, and residual stresses. These models can be used to search for optimal processing conditions for specific material systems and structural shapes. Process modeling is an important component of the analysis of composite materials. What really distinguishes the various processing methods that are available is how pressure, vacuum, and temperature are applied. Autoclave curing is discussed in the next section; other methods are discussed in a later section. M9.3.2 Autoclave Curing The best quality parts are cured using an autoclave. Autoclaves have been used extensively for processing high-performance composite materials in the civilian and military aerospace industries. An autoclave consists of a large cylindrical metal pressure vessel with end enclosures that is thermally insulated and heated. Most autoclaves have a forced-hot-gas circulation system as well. An autoclave is pressurized using air or an inert gas such as nitrogen. What distinguishes the autoclave from the curing oven and hot press, to be discussed later, is the ability to cure parts using large, hydrostatic like pressure. A typical autoclave can pressurize up to 20 atm. The large majority of composite structures can be processed using autoclaves with 2-4 m internal diameter, although some extremely large aerospace structures require autoclaves over 20 m in diameter. The capital equipment costs and operating costs for large autoclaves make this type of processing very costly. However, the high quality and high performance of autoclaved parts makes them attractive for certain applications. A typical autoclave is shown in schematic form in Figure M9.3.3. The primary component is the pressure vessel itself, which is cylindrical and contains

embedded heaters and cooling coils. A door at one end allows access to the interior to load parts and perform periodic maintenance. Also, several ports may be installed through the autoclave wall for access to the interior. Some of these ports are dedicated to vacuum lines connected to the parts to be cured. Others are used for control functions such as thermocouples, dielectric sensors, and pressure sensors, all of which help monitor the curing of the material. The interior of the autoclave is heated by radiation from the vessel walls and convection of hot gases as they circulate through the vessel. A circulating fan forces the hot gases through a series of baffles within the autoclave in a circulation loop that runs the length of the autoclave. Typically, this fan is housed at one end of the autoclave and the interior gases are drawn from the central portion of the cylinder, through the baffles, and they return to the other end through a jacket that covers the interior wall. The autoclave applies a pressure to the outer surface of the composite part through pressurization of the interior gases. This pressure is then transferred through the tool plate(s), breather plies, bleeder plies, and other secondary materials to the laminate surface. From there the pressure is shared between the fiber and matrix during curing. The most important aspect is the matrix resin pressure during cure. If it is too low, then voids can grow in the resin or inadequate resin bleeding may occur. In general, composite structures which have been pro-cessed in an autoclave exhibit uniform thicknesses, good consolidation, and very low void content.

Figure M9.3.3 Schematic of an Autoclave

M9.3.3 Other Manufacturing Processes M9.3.3. Introduction Manufacturing of composite materials involves distinct operations that may vary depending upon available technology, existing facilities and personnel skill. The manufacturing process may also vary due to wide variety of composite materials and their application. Each of the fabrication processes has characteristics that define the type of products to be produced. This is advantageous because this expertise allows the manufacturer to provide the best solution for the customer. Factors considered for selection of most efficient manufacturing process are as follows:

• User needs

• Total production volume • Performai1ce requirements • Economic targets • Size of the product • Labour • Surface complexity • Materials • Appearance • Tooling/assembly • Production rate • Equipment

The goals of the composite manufacturing process are to:

• Achieve a consistent product by controlling Fibre thickness Fibre volume Fibre direction

• Minimize voids • Reduce internal residual stresses • Process in the least costly manner

The procedure to achieve these goals involves series of actions to select the three key components, viz.

• Composite material and its configuration • Tooling • Process

Obviously, hand lay-up represents an extreme that cannot be scaled to high volume or large components; to address these issues, and because fibers are not always in the form of impregnated parallel tows in a layer, other fabrication techniques have been developed. In addition, it may be easier in some instances to bring the fibers and matrix together when forming the structural component. We begin this discussion of other fabrication methods by surveying other ways to make fibers available, either by themselves or with matrix material. M9.3.3.1.1 Fiber-Only Preforms If the resin and fiber are to be combined during the fabrication of a composite structure, then they are both supplied as separate materials. Fibers when supplied as a separate material can come in many different forms. Most commonly they are continuous and grouped into bundles, as with tows. The bundles, or strands, are a collection of many hundreds or thousands of fibers twisted or bound together. These strands are wound onto a spool. The fibers usually have a binder that keeps them together and other coating agents to provide better handleability. A yarn is a twisted assemblage of fibers, usually less than 10,000. By contrast, a tow, for the most part,

is untwisted. The simplest yarn is made from a single strand of fibers, and heavier yarns are obtained by twisting and plying several strands together. Typically, this consists of twisting two or more individual strands together, then twisting two or more twisted strands together. Yarns which are simply twisted will kink, corkscrew, and unravel because the twist is only in one direction. This problem is normally eliminated by countering the twist in the twisted yarns with the opposite twist when plying together the twisted yarns. A woven fabric is a material with interlaced yarns, strands, or fibers. Typical fabrics are manufactured by interlacing warp (lengthwise) yarns or strands with fill (crosswise) yarns or strands on a conventional weaving loom. The weave of a fabric determines how the warp and fill yarns are interlaced. Popular weave patterns include plain, twill, crowfoot satin, long-shaft satin, leno, and unidirectional. The plain weave, shown in Figure M9.3.4, is the oldest and most common textile weave. Each fill yarn is repetitively woven over one warp yarn and under the next. It is the most stable of the weave constructions and mechanically it behaves much like a cross-ply laminate. Twill weaves have one or more warp yarns passing over and under two, three, or more fill yarns in a regular pattern. Theses weaves drape better than the plain weave. In the crowfoot or long-shaft satin weaves, one warp yam is woven over several successive fill yarns, then under one fill yarn. A weave pattern in which one warp end passes over four and under one fill yarn is called a five-harness satin weave and is shown in Figure M9.3.5. Satin weaves are less open than other weaves and the strength is high in both the warp and fills directions. The unidirectional weave has a large number of yarns in the warp direction with fewer and generally smaller yarns in the fill direction, and the resulting product has much greater strength in warp direction. Several types of two-dimensional weaves are shown in Figure M9.3.6. More recently, three-dimensional weaves, shown in Figure M9.3.7, have been developed to provide better through-thickness strength in a composite structure Three-dimensional weaves behave orthotropically because there are three principal material directions.

Figure M9.3.4 Plain weave fiber preform A non-woven fabric is a sheet of parallel yarns or tows held together by an occasional transverse yarn or tow, or by a periodic cross-bond with a binder. Mats are blankets of chopped fibers or continuous fibers formed as a continuous flat sheet. The fibers are evenly and randomly distributed and are held together by a binder. The binder used must be able to dissolve in the liquid matrix once it infiltrates the mat. Low-solubility binders are used when the fabrication procedure is such that the matrix may wash out the fibers and create resin-rich regions during infiltration The binder, due to its low solubility, remains intact until the infiltration process is complete, after which it dissolves in the matrix. A mat acts like a single layer of material with nearly the same properties in all in-plane directions (i.e., isotropic behavior). Continuous fibers can be chopped into short lengths, usually between 3 and 50 mm m length. These chopped strands can then be mixed with a liquid resin and injected into a mold, sprayed onto a mold surface, or sprinkled onto a polymer sheet; these sheets then act like a single isotropic layer.

Figure M9.3.5 Five-harness satin weave fiber preform

M9.3.3.1.2 Other Combined Fiber-Matrix Preforms As with tows, yarns, woven forms, non-woven forms, and mats can be preimpregnated with resin before forming them into a structural shape. Combining the fiber and resin in a separate step makes the fabrication process simpler and results in composite structures with better quality. In particular, the proportion of resin to fiber is kept within very close tolerance and fiber orientation is more controlled and reproducible.

Sheet molding compound (SMC) is a type of preform used in the automotive industry. It consists of chopped glass fibers randomly distributed in a polyester sheet; these sheets are stacked one on top of the other and molded to shape with heat and pressure. Some thermoplastic polymers, for example polyphenylene sulfide (PPS), can be drawn into fiber form. Once the polymer fibers are formed, they can be comingled with reinforcing fibers. The resulting strands of reinforcing fibers and polymer are then wound onto a mandrel or pulled through a heated mold. During fabrication and processing the polymer fibers melt and infiltrate the reinforcement.

Figure M9.3.6: Several two-dimensional weaves for fiber preforms

Figure M9.3.7: Examples of three-dimensional weave patterns for fiber preforms.

As reinforcement for composite material, the choice between unidirectional tape and woven fabric is made on the basis of the greater strength and modulus attainable with the tape particularly in applications in which compression strength is important. Salient advantages and disadvantages of tape (Fiber-only preforms) and fabric (Combined Fiber-Matrix preforms) for their selection are given below: Tape (Fiber-only preforms) Advantages:

• Best modulus and strength efficiency • High fibre volume achievable • Low scrap rate • Less tendency to trap volatiles • Automated lay-up possible • No discontinuities • Available in thin plies

Tape (Fiber-only preforms) Disadvantages:

• Poor drape on complex shapes • Cured composite more difficult to machine • Lower impact resistance • Multiple plies required for balance and symmetry • Higher labour cost for-hand lay-up

Fabric (Combined Fiber-Matrix preforms) Advantages:

• Better drape for complex shapes • Single ply is balanced and may be essentially symmetric • Can be laid up without resin • Plies stay in line better during cure • Cured parts easier to machine • Better impact resistance • Many forms available

Fabric (Combined Fiber-Matrix preforms) Disadvantages:

• Fibre discontinuities (splices) • Less strength and modulus • Lower fibre volume than tape • More costly than tape • Greater scrap rates • Warp and fill properties differ • Fabric distortion can cause part warping

Another aspect considered important for composite fabrication is appropriate lay-up techniques along with composite cure control. Some of the considerations for choosing lay-up techniques are given in Table M9.3.1:

Consideration Manual Flat Tape Contoured Tape

Orientation accuracy

Least accurate Automatic somewhat accuracy dependent on tape accuracy and computer programme

Ply count Dependent on operator

Dependent on operator

Programme records

Release film retention

Up to operator Automatic Automatic

Tape lengths Longer tapes more difficult

Longer tape is more economical

Longer tape is more economical

Cutting waste Scrap on cutting Less scrap Least scrap Compaction pressure

No pressure Less voids Least voids

Programming N/A N/A Necessary

Table M9.3.1: Some of the considerations for choosing lay-up techniques M9.3.3.2 Classification of Manufacturing Processes Most widely used manufacturing methods for laminated fibre composites are as follows: M9.3.3.2.1 Open Mold Process

• Spray lay-up - Chopped roving and resin sprayed simultaneously, rolled. • Hand lay-up - Lay-up of fibres or woven cloth, impregnate, no heat or pressure. • Filament winding. • Sheet molding compound. • Expansion tool molding. • Contact molding.

M9.3.3.2.2 Closed Mold Process

• Compression molding – Load with raw material, press into shape. • Vacuum bag, pressure bag, autoclave - Prepreg laid up, bagged, cured. • Injection molding – Mold injected under pressure. • Resin Transfer – Fibres in place, resin injected at low temperature.

M9.3.3.2.3 Continuous Process

• Pultrusion. • Braiding.

All the methods described above are discussed in detail in the following paras. M9.3.3.2.1 Open Mold Processes Open molding offers a number of process and product advantage over other high volume and complex application methods. These include:

• Freedom of design • Easy to change design • Low mold and/or tooling cost • Tailored properties possible • High strength large parts possible • On-site production possible

Disadvantages associated with the open molding process include:

• Low to medium number of parts • Long cycle times per molding • Not the cleanest application process • Only one surface has aesthetic appearance • Operator skill dependent

M9.3.3.2.1.1 Spray Lay-up In a spray lay-up method, the fibre is chopped in a hand held gun and fed into a spray of catalyzed liquid resin directed at the mold (Figure M9.3.8). The sprayed, catalyzed liquid resin will wet the reinforcement fibres, which are simultaneously chopped in the same spray gun. The deposited materials are left to cure under standard atmospheric conditions.

Figure M9.3.8: Spray lay-up method

Advantages

• Widely used form any years. • Low cost way of quickly depositing fibre and resin. • Low cost tooling.

Disadvantages

• Laminates tend to be very resin-rich and, therefore, excessively heavy. • Only short fibres are incorporated, which severely limits the mechanical properties of

the laminate. • Resins need to be low in viscosity to be sprayable. This generally compromises their

mechanical/thermal properties. • The high styrene content of spray lay-up resins generally means that they have the

potential to be more harmful and their lower viscosity means that they have an increased tendency to penetrate clothing etc.

Applications Simple enclosures, lightly loaded structural panels, e.g. caravan bodies, truck fairings, bathtubs, shower trays, some small dinghies. M9.3.3.2.1.2 Wet Lay-up/Hand Lay-up Although we discussed Hand lay-up in M9.2, we will again discussing same briefly here as to get exact picture of process. The hand (wet) lay-up is one of the oldest and most commonly used methods for manufacture of composite parts. Hand lay-up composites are a case of continuous fibre reinforced composites. Layers of unidirectional or woven composites are combined to result in a material exhibiting desirable properties in one or more directions. Each layer is oriented to achieve the maximum utilization of its properties. Layers of different materials (different fibres in different directions) can be combined to further enhance the overall performance of the laminated composite material. Resins are impregnated by hand into fibres, which are in the form of woven, knitted, stitched or bonded fabrics. This is usually accomplished by rollers or brushes, with an increasing use of nip-roller type impregnators for forcing resin into the fabrics by means of rotating rollers and a bath of resin. Laminates are left to cure under standard atmospheric conditions. A typical hand lay-up method is shown in Figure M9.3.9.

Figure M9.3.9: A Typical hand lay-up method

Some of the advantages and disadvantages of hand lay-up of composite structures are as follows: Advantages

• Design flexibility. • Large and complex items can be produced. • Tooling cost is low. • Design changes are easily effected. • Sandwich constructions are possible. • Semi-skilled workers are needed. • Higher fibre content and longer fibres than with spray lay-up.

Disadvantages

• Only one molded surface is obtained. • Quality is related to the skill of the operator. • Low volume process. • Longer cure times required. • Resins need to be low in viscosity to be workable by hand. This generally compromises

their mechanical/thermal properties. • The waste factor can be high.

Applications

• Standard wind-turbine blades, production boats, architectural moldings.

M9.3.3.2.1.3 Filament Winding

Filament winding offers an economic way of producing symmetrical composite components if the production volume and the level of automation is high and the part is well designed. In the recent times, more advanced & highly sophisticated multi-axis Computer Numerical Controlled (CNC) machines have been introduced in the market. From its early use on defence and aerospace products in 1940s and 1950s, filament winding has moved more into the non-aerospace industries for the manufacture of GRP pipes and sporting goods. The products such as piping system, pressure vessels, chemical storage tanks (oxygen, hydrogen .etc), under water breathing apparatus, cylinders, golf club shafts etc. could be fabricated by filament winding technology. Winding of composite filaments over aluminum liners is found to be an ideal replacement for heavy steel cylinders thereby reducing the overall weight without compromising the necessary safety requirements. Filament winding is automated processes for creating parts of simple geometry wherein continuous resin impregnated fibres are wound over a rotating male tool called mandrel. Figure M9.3.10 depicts a filament winding process where a continuous fibre roving passes through a shuttle, which rotates and the roving is wrapped around a revolving or stationary mandrel. Two basic types of filament winding are in use - (i) the polar or planer method, and (ii) the high helical pattern winding.

Figure M9.3.10: Filament winding process

The polar or planer method of winding utilizes a fixed mandrel and a shuttle that revolves around the longitudinal axis of the part to form longitudinal winding patterns. This type of winding is used if the longitudinal fibres are required with angle less than 25° to the mandrel axis. The polar winding schematic is shown in Figure M9.3.10.

Figure M9.3.10: The schematic polar winding process.

In the high helical pattern winding, the mandrel rotates while the shuttle transverses back and forth. Both the mandrel rotation and shuttle movement are in the horizontal plane. By controlling the mandrel rotation and shuttle speed, the fibre angle can be controlled. Angles of 25°-85° to the mandrel rotation axis are possible. The helical winding schematic is shown in Figure M9.3.11.

Figure M9.3.11: The schematic helical winding process.

After completion of the winding, the filament wound structure is cured at room temperature or in an oven. The mandrel is removed after the curing. The mandrel, which determines accurate internal geometry for the component, is generally the only major tool. Low cost mandrel materials such as cardboard or wood can be used for winding low cost routine parts. For critical

parts requiring close tolerances, expensive mandrels designed for long term use may be required. For high temperature cure 315°C (600°F), graphite mandrels with low thermal expansion may be advantageous. However, attention should be paid for potential difficulties for mandrel removal. Mandrels are either removable or non-removable. Removable mandrels are classified according to the removal techniques as:

• Entirely removed. • Collapsible. • Breakable or soluble.

The selection of mandrel involves several considerations. These include part size and complexity, size of openings, resin system and its curing and the number of components to be fabricated. The requirements for a mandrel are:

• It must be stiff and strong enough to support its own weight and the weight of the applied composite while resisting the fibre tension pressure from winding and curing.

• It must be dimensionally stable and should have thermal coefficient of expansion greater than the transverse coefficient of the composite structure.

Different materials used for construction of mandrel are:

• Low melting temperature alloys used for small diameter applications. • Sand, soluble plaster and eutectic salts used for irregular shapes. • Inflatable material used in applications where sometimes the mandrel remains a part of

the structure. • Segmented metal used for high production rates, and where the mandrel can be

withdrawn through a small hole in the part. Of the above, segmented metal and inflatable mandrel are the reusable mandrels. Advantages • . Excellent mechanical properties due to use of continuous fibres. • High degree of design flexibility due to controlled fibre orientation and lower cost of large

number of composites. • This is a very fast and economic method of laying down material. • Resin content can be controlled by metering the resin onto each fibre tow through nips or

dies. Disadvantages

• Difficulty to wind complex shapes, which may require complex equipment. • Poor external finish. • The process is limited to convex shaped components.

• Fibre cannot easily be laid exactly along the length of a component. • Mandrel costs for large components can be high. • Low viscosity resins usually need to be used with lower mechanical properties.

M9.3.3.2.1.4 Sheet Molding Compound Sheet molding compound (SMC) refers to both a material and a process for producing glass fibre reinforced polyester resin items. The material is typically composed of a filled, thermosetting resin and a chopped or continuous strand reinforcement of glass fibre. A SMC processing machine, (Figure M9.3.12) produces molding compound in sheet form. The glass fibre is added to a resin mixture that is carried onto a plastic carrier film. After partial cure, the carrier films are removed. The sheet molding material is cut into lengths and placed onto matched metal dies under heat and pressure. Salient advantages of SMC molding process are as follows:

• High volume production. • Excellent part reproducibility. • Minimum material scrap. • Excellent design flexibility. • Parts consolidation.

Figure M9.3.12: The Sheet molding compound processing machine.

M9.3.3.2.1.5 Expansion Tool Molding Expansion tool molding makes use of rubber inserts in a metal or epoxy tool that expands when heated to provide the molding pressure. The advantage of expansion tool molding is its ability to fabricate parts without an autoclave. The method is dependent upon materials with high coefficients of thermal expansion. It is designed to utilize the difference in thermal expansion between rubber and the tooling material. The female areas of the mold are made of a material with a low coefficient of thermal expansion. The male plug is made of a silicon rubber or other rubber type tool material having comparatively high coefficient of thermal expansion. When the

tool is heated, the rubber male plug expands at a much greater rate than the surrounding female tool. Pressure up to 14 MPa (2000 psi) can be achieved at 175°C (350°F), which acts in all directions. The molding pressure can be controlled by controlling the temperature, rubber composition, rubber thickness and the ratio of rubber volume to the female mold volume. Thermal expansion molding techniques are utilized for special applications of small complex composite structures and composite tubing with critical outside surfaces. Figure M9.3.13 illustrates the methods allowing the expansion of the silicone rubber to provide the required pressure for compacting of the composite materials.

Figure M9.3.13: The methods allowing the expansion of the silicone rubber

The linear thermal coefficient of most silicone rubbers fall in the range of 1-2.1x105. This range is consistent over a 23-246°C (75-480°F) temperature range. The rubbers have a linear expansion of approximately 17 times that of carbon steels due' to which they are used to mold composites by thermal expansion molding techniques. Silicone rubber is very slow to cool down and extra time must be allowed because the rubber is impossible to remove from the composite part until it has shrunk back to its original size. M9.3.3.2.1.6 Contact Molding Contact molding involves the application of molding material to an open mold where it is allowed to cure. The process has been basically developed for the manufacture of large fibre glass components such as boats, automotive parts, etc. The process requires minimal tooling and equipment cost and thus is ideally suited for low volume production. The method has two basic approaches, namely hand lay-up and spray lay-up, which differ only in the manner in which. The material is applied to the mold. The method produces high quality surface finish on only one side of the final product.

Before the lay-up, the mold surface is coated with a thin layer of gel. After the lay-up, the part is allowed to cure at room temperature. The curing can be accelerated by using heat lamps. The major advantages of the method are its simplicity and low cost. The disadvantages of the method are that only one good surface is produced while the other side is very rough. Secondly, the method is relatively slow due to long lay-up and cure time and thus it is not suitable for high volume production. M9.3.3.2.2 Closed Mold Processes M9.3.3.2.2.1 Compression Molding Compression molding is one of the oldest manufacturing techniques in the composites industry. The recent development of high strength, fast cure, sheet molding compounds bulk molding compounds and advancement in press technology is making the compression molding process very popular for mass production of composite parts. Fully formed parts are molded in matched metal compression molds that give the final part shape. In comparison with the injection molding process, better physical and mechanical properties can be obtained in compression molding. This process utilizes large tonnage presses wherein the part is cured between two matched steel dies under pressure and high temperature. The moving platen is heated either by steam or electricity to promote thermal curing. Curing of the part is affected by the following factors:

• Size of platen, which determines the length and width of the part, which can be cured. • Total tonnage of the press, which determines the pressure to be exerted on the projected

surface area of the part. After placing the laminate to be cured called the 'charge' in the core of the mold, the cavity is then closed at a rate of usually 4-12 mm/sec. In most cases the mold is heated to 150°C (302°F), which causes the charge viscosity to be reduced. With increasing mold pressure as the mold is closed, the charge flows towards the cavity extremities, forcing air out of the cavity. The molding pressure based on projected part area ranges from 0.7 to 9 MPa (100 to 1200 psi). Higher molding pressure causes sink marks, while lower pressure cause scumming of the mold and porosity. The curing time is usually between 25 sec to 3 minutes depending on several factors including' resin-initiator-inhibitor reactivity, part thickness, component complexity and mold temperature. The exertion of high pressure eliminates the problem of development of voids. The primary advantage of the compression molding is its ability of producing large number of parts with little dimensional variations, if any, from part to part. A wide variety of shapes, sizes and complexity can be produced by compression molding. An important factor to be considered before deciding to use compression molding is the high tooling cost and the need for large heated presses. Thus, this method is not practical for low volume production. Cure time is very critical. If the resin cure exotherm is not properly controlled, cracking, blistering or warping may occur. Figure M9.3.14 shows a typical compression mold.

Figure M9.3.14: A typical compression mold.

M9.3.3.2.2.2 Vacuum Bag Molding Today bag molded (vacuum and pressure) composites provide higher performance that results from optimization of process controls, design refinements and improved materials. Vacuum bagging techniques have been developed for fabricating a variety of aerospace components and structures. The process is principally suited to prepreg materials. This method utilizes a flexible film or rubber bag that covers the part lay-up. The bag permits evacuation of the air to apply atmospheric pressure. The primary limitation of this method is the limited pressure that can be applied. The bag used in this method has two fold objectives:

• It provides a means for removing volatile products during cure; and • It provides a means for the application of a pressure of one atom which is adequate for

some materials. The essential steps in the process are the lay-up, preparation of bleeder system and the bagging operation. The required number of plies are cut to size and positioned in a mold. When individual plies of a prepreg material are formed to the lay-up tool, certain amount of voids exists between layers. The lay-up is covered with a flexible membrane or vacuum bag, which is sealed around the edges of the mold by a sealant. An edge bleeder is also placed near the edges of the lay-up. Its function is to absorb excess resin, which may flow during curing. Requirement for proper bagging are:

• Bag to be impervious to air pressure, • Bag to uniformly apply the cure pressure,

• Bag not to leak under over-pressure, and • Good and high capacity vacuum path to be provided to evacuate air from between the bag

and the tool. A vacuum is drawn on the lay-up, which helps in eliminating entrapped air. A maximum pressure of about 104 kPa (15 psi) is achieved in this method. The removal of excess resin results in higher fibre content and improved mechanical properties. The lay-up is usually allowed to cool at room temperature. To reduce the cure time, oven is preferred. A typical vacuum bag molding is shown in Figure M9.3.14.

Figure M9.3.14: A typical vacuum bag molding

Functions of vacuum bag components are given in Table M9.3.2 as follows:

No. Component Functions

1 Bag sealant Temporarily bonds vacuum bag to tool

2 Vacuum fitting and hardware

Exhausts air, provides convenient connection to vacuum pump

3 Bagging film Encloses part, allows for vacuum and pressure

4 Open weave breather mat

Allows air or vacuum transfer to all of part

5 Polyester tape (wide) Holds other components of bag in place 6 Polyester tape (narrow) Holds components in place 7 Caul sheet Imparts desired contour and surface finish to

composites 8 Perforated release film Allows flow of resin or air without adhesion 9 Non-perforated release Prevents adhesion of laminate film resin to tool

surface

10 Peel ply Imparts a bondable surface to cured laminate 11 Laminate 12 Glass breather manifold Allows transfer of air or vacuum. 13 Glass bleeder ply Soaks up excess resin 14 Stacked silicon edge

dam Forces excess resin to flow vertically, increasing fluid pressure

Table M9.3.2: Functions of vacuum bag components Two types of vacuum bags commonly used are - (1) expandable bagging, or (2) reusable bagging. M9.3.3.2.2.2.1 Expendable Vacuum Bagging Bleed-out systems are devised to maintain reduced pressures within the bags contents. The bagged lay-up includes the bleed-out system designed for the composite part. Bagged lay-ups can be bled in two ways, namely vertically or edge bled. The basic difference between the two methods is shown in Figure M9.3.15 (a) and (b).

Figure M9.3.15 (a): Vertically bled Bagged lay-up.

Many of today's resin systems are mostly net resin and do not require any resin bleed during cure. This allows for better control of the resin content of the composite structures. If a resin bleed sequence is preferred, the following sequence can be used as a general guide:

• The surface of the mold is prepared with the release agent. • The composite plies are applied and rubbed out to remove the entrapped air. • A perforated release film is applied over the composite laminate and extended

approximately 3.2mm beyond all edges. • A predetermined number of bleeder plies are applied over the release film and extended

to the perimeter of the lay-up.

• A perforated release film is applied over the bleeders and extended 3.2 mm from edge. • One or two layers of a non-woven breather is placed over the lay-up and extended over

the release film. • Sealant tape is applied around the perimeter of the bleeder. • The vacuum bag is positioned and sealed. • The contents are evacuated and the bag is checked and sealed against leaks. • The bagged lay-up is ready for curing.

Figure M9.3.15 (b): Edge bled Bagged lay-up.

Vacuum bag bridging is one of the leading causes of resin rich and excessive voids in corners of composite laminates. Figure M9.3.16 illustrates this common problem.

Figure M9.3.16: The common problem encountered in Vacuum bag bridging

One method of eliminating bridging of the vacuum bag is presented in Figure M9.3.17 by means of 'ears' in the bag. Another method to help reduce resin rich and excessive voids in corners is the placement of an intensifier over the area, usually placed between the separator film and breather. The intensifier can be molded rubber in the radius desired or some sealant tape to fill the corners.

Figure M9.3.17: The method of eliminating bridging of the vacuum bag.

M9.3.3.2.2.2.2 Re-usable (Non-disposable) Vacuum Bagging Due to material and recurring cost associated with the use of expendable vacuum bags, use of silicone rubber reusable bags are the solution. Salient advantages and disadvantages of the vacuum bagging method are as follows:

Advantages

• Higher fibre content laminates can usually be achieved. • Lower void contents are achieved than with wet lay-up. • Better fibre wet-out due to pressure and resin flow throughout structural fibres, with

excess into bagging materials. • The vacuum bag reduces the amount of volatiles emitted during cure.

Disadvantages

• The extra process adds cost both in labour and in disposable bagging materials. • A higher level of skills required by the operators. • Mixing and control of resin contents till largely determined by operator skill.

M9.3.3.2.2.3 Autoclave Molding (also refer M9.3.2) Autoclave molding is similar to vacuum bag process except that the lay-up is subjected to greater pressures and compact parts are produced. Primary disadvantage is high initial and recurring operating cost. The advantage is to produce parts with complex configuration and very large sizes. Autoclave curing of composites is of prime importance for manufacturing high quality aerospace laminates. Curing is achieved through a combination of pressure, temperature and heat under inert conditions in an enclosed vessel. Processing materials must be added to a composite ply lay-up before autoclave curing. These materials control the resin content of the cured part and ensure proper application of autoclave pressure to the lay-up. The materials usually used in preparing a lay-up for autoclave curing are peel ply, separator, bleeder, barrier, breather, dam and vacuum bag. The materials are compatible with the maximum cure temperature and pressures required for the matrix system being cured. The peel ply if used is placed immediately on top of or under the composite laminate. It is ultimately removed just before bonding or painting operations so that a clean, bondable surface is available. A separator (release material) is placed on top of or under the laminate and peel ply. It allows volatile and air to escape from the laminate and excess resin to be bled from the laminate into the bleeder plies during cure. It also gives the cured part a smooth surface. The purpose of the bleeder is to absorb excess resin from the lay-up during cure, thereby producing the desired fibre volume. Fibre glass fabric or other absorbent materials are used for this purpose. The amount of bleeder used is a function of its absorbency, the fibre volume desired in the part, and the resin content of the prepreg material used in the lay-up. In advanced composites essentially all excess resin is bled from the surface of the laminate, with edge bleeding being minimized by properly damming the lay-up edges.

The barrier is commonly placed between the bleeder plies, and breather plies. In the case of epoxy resin, un-perforated film is used so that resin removal from the part can be controlled. For resins that produce volatile by-products during cure, a film with small perforations and large spacing is used to prevent the breather materials from becoming clogged with resin. The breather is a material placed on top of the barrier film to allow uniform application of vacuum pressure over the lay-up and removal of entrapped air or volatile during cure. It may be droppable or loosely woven fabric. The dam is sometimes located peripherally to minimize edge bleeding. It maybe an integral part of the tool or built-in position using materials such as pressure-sensitive tape, silicone rubber or Teflon or metal bars. The vacuum bag is used to contain any vacuum pressure applied to the lay-up before and during cure and to transmit external autoclave pressure to the part. It prevents any gaseous pressurizing medium used in the autoclave (air or inert gas) from permeating the part and causing porosity and poor or unacceptable part quality. Major components of autoclave are - a vessel to contain pressure; sources to heat the gas stream and circulate it uniformly within the vessel; a sub-system to pressurize the gas stream; a sub-system to apply vacuum to parts covered by a vacuum bag; a sub-system to control operating parameters; and a sub-system to load the models into the autoclaves. The materials, which can be processed in the autoclave, are metal bonding adhesives, reinforced epoxy laminates, thermoplastic laminates, ceramics, carbon matrix materials; and many other applications. An autoclave system allows a complex chemical reaction to occur inside a pressure vessel according to a specified schedule in order to process a variety of materials. The pressure and temperature requirements are governed by the type of material to be cured. The evolution of materials and processes has taken autoclave operating conditions from 120°C (248°F) and 276 kPa (40 psi) pressure to well over 760°C (1400°F) and 69 MPa (10000 psi). Epoxy matrix composites, in general, use autoclave cure cycles, which involve 487-690 kPa (85-100 psi) pressure and 175°C (350°F) temperature. The materials processed in autoclaves range from metal bonding adhesives, reinforced epoxy laminates, thermoplastic laminates, metal, ceramic and carbon matrix materials, to many other aerospace and electronic components. Autoclave is generally provided with automatic programmable controllers, which monitor and maintain the required heat up and cool down cycles. The vacuum surrounding the part lay-up is also controlled and is discontinued after initial temperature increase to prevent excess resin flow. Brief functioning of various parts of autoclave is discussed in the following paras. Pressure Vessel It provides the means to retain pressure inside the work space. Typically, the pressure vessel is fabricated from pressure vessel quality carbon steel. It is thermally insulated. All autoclave

vessels and closures are required to be inspected at regular interval. This will reveal any potentially dangerous situation. Gas Stream Heating Several heating methods are available for autoclave systems. Most common method for large autoclaves is indirect gas firing in which products of combustion are passed from external chamber to an internal coil. Gas heating is regularly used in autoclaves with maximum operating temperature of 450°-540°C (8500-1000°F). Earlier, hot oil was used as a heating medium in which oil was circulated from an external heater to an internal coil. This system became obsolete due to contamination problems of bonding area leading to improper processing of parts. Steam heating is often used for autoclaves operating in the 150°-175°C (3000-350°F) range. The superheated steam is passed through a coil in the autoclave to heat the circulating gas. Most small autoclaves are electrically heated. Gas circulation, within the autoclave, is essential to provide temperature uniformity and proper heat transfer to the loaded part. The gas circulation is maintained at 1 to 3 m/s (250to 300 ft/min) in the workspace. Gas Stream Pressurization The three pressurizing gases typically used for autoclaves are (a) air (b) nitrogen and (c) carbon dioxide. Air is used for most of the 120°C (250°F) cures. However, disadvantage of air is that it sustains combustion and maybe hazardous at temperatures above 150°C (300°F). Nitrogen is commonly used in the autoclaves. Liquid Nitrogen is stored in cryogenic form and then vaporized at approximately 1380 to 1552 kPa (200 to 225 psi). Carbon dioxide is the second most commonly used gas. It is stored as refrigerated liquid at approximately 2070 kPa (300 psi). Disadvantages of carbon dioxide are its high density, hazards to personnel, etc. Vacuum Systems Most parts processed in autoclaves are covered with vacuum bags, which are used primarily for compaction of laminates and to provide vacuum for removal of volatile products. The bag allows the part to be subjected to differential pressure in the autoclave without being directly exposed to the autoclave atmosphere. The vacuum bag is also used to apply varying levels of vacuum to the part. The ability to provide pressure on the part under the bag by means of vacuum results in reduced void content. Control System Control system means ability to monitor and control cure cycles. The cure cycle is controlled by feedback from thermocouples, transducers and sensors, which are provided at different locations in the autoclave.

Loading System Carts or trolleys help in placing the parts horizontally and vertically in the autoclave. The parts are loaded s that they are accessible to enable repair of bag leaks. All vacuum sources and vacuum sensor lines are connected to the part when loaded on cart and desired vacuum maintained. The cart must be gently rolled into the autoclave. At present, computer controlled autoclaves with facility for storage of up to 60 different curing cycles programme are available in the world market. In industry, autoclave curing of composites is used to improve cured product quality and reduce fabrication costs by providing:

• Process optimization. • Reduced process inconsistencies and product rejections. Accurate, real-time quality

assurance with rapid error detection and correction. • Verification of process reaction behaviour kinetics. • Non-destructive verification of cured properties. • Accurate, permanent process documentation. • Flexibility in adapting to new or modified processes.

M9.3.3.2.2.4 Injection Molding Injection molding refers to a process that generally involves forcing or injecting a fluid plastic material into a closed mold. It is differentiated from compression molding, in which plastic materials in a soft but not fluid condition are formed by transferring them into an open mold, which is then forcibly closed. This method is not normally used in polymeric matrix compound processes due to fibre damage in the barrel. The molding compound is fed into injection chamber through the feed hopper. The molding compound is heated in the injection chamber wherein it changes into liquid form. It is forced into the injection mold by the plunger. A typical injection mold is shown in Figure M9.3.18. The injection molding process generally has the following advantages over the compression molding:

• More readily automated process, • Permits finer part detail.

Figure M9.3.18: A typical injection mold.

The part and the mold can often be designed in a manner so that no subsequent trimming or machining operations are required. However, all plastic materials cannot be injection molded successfully. There are two basic categories of plastic injection molding (a) thermoplastic and (b) thermoset. In the former, a thermoplastic material is melted and forced through an orifice into the mold, which is kept relatively cool. The material solidifies in the mold from which it can then be removed. In thermoset injection molding, the solidification occurs at high temperature. Therefore, a reaction material is forced into a generally warm mold in which the material further polymerizes into a solid part. This method is normally used for high-volume and low-cost component manufacturing. The disadvantage of the method is that it is limited to materials with very short fibre lengths. Also, since there is large amount of flow during the process, material non-uniformities do exist. M9.3.3.2.2.5 Resin Transfer Molding (RTM) This is a low-pressure, closed mould semi-mechanized process. The fibre reinforcement, which may be pre-shaped, is placed in the required arrangement in the cavity of a closed mould and a liquid resin of low viscosity is injected under pressure into the cavity, which is subsequently cured. The main potential advantages of RTM can be summarized as the capability of rapid manufacture of large, complex, high-performance structures with good surface finish on both sides. It also permits the use of foam and other removable cores to yield three-dimensional parts and hollow components as well. Resin transfer molding is a closed mold low pressure process that allows the fabrication of composites ranging in complexity from simple, low performance to complex, high performance parts and in size from small to large (Figure M9.3.19). The process is differentiated from other molding processes in that the dry reinforcement and the resin are combined within the mold to form the composite component. The fibre reinforcement, which may be pre-shaped, is placed into a tool cavity, which is then closed. A tube connects the closed tool cavity with a supply of liquid resin, which is pumped or transferred into the tool to impregnate the reinforcement for subsequent curing. Injection pressure is normally less than 690 kPa (100 psi). The displaced air is allowed to escape through vents to avoid dry spots. Cure cycle is dependent on part thickness, type of resin system and the temperature of the mold and resin system. The part cures in the mold, normally heated by controllers.

Figure M9.3.19: Resin transfer molding.

The advantages and associated disadvantages of the RTM process are given below: Advantages

• Parts can be made with better reproducibility than with wet lay-up. • Reinforcement and combination of reinforcements can be used to meet specific

properties. • Production cycles are much faster than with wet lay-up. • Using matched tools for the mold, one can improve the finish of all the surfaces. • Mechanical properties of molded parts are comparable to other composite fabrication

processes. • Large and complex shapes can be made efficiently. • Volatile emissions are low because RTM is a closed mold process. • The skill level of operator is less critical. • Mold surfaces can be gel coated to improve surface performance.

Disadvantages

• The mold design is critical and requires good tools or great skill. • Reinforcement movement during resin injection is sometimes a problem. • Control of flow pattern or resin uniformity is difficult. Radii and edges tend to be resin

rich. M9.3.3.2.2.5.1 Vacuum Assisted Resin Transfer Molding (VARTM) In the RTM process, a matched set of molds or closed mold is used. The fibre reinforcements are usually pre-shaped off line to enhance the production cycle time of the molds. Resin is injected through the injection ports and the process is sometimes assisted with vacuum.

However, VARTM is different for many reasons compared to RTM. First, the fabrication of parts can be accomplished on a single open mold. Second, the process uses the injection of resin in combination with a vacuum and captured under a bag to thoroughly impregnate the fibre reinforcement. This process has been used to make both thin and very thick laminates. In addition, complex shapes with unique fibre architectures allow the fabrication of large parts that have a high structural performance. Fabrics are laid up as a dry stack of materials as in RTM and covered with peel ply and a knitted type of non-structural fabric. The whole dry stack is then vacuum bagged, and once bag leaks have been eliminated, resin is allowed to flow into the laminate. The resin distribution over the whole laminate is aided by resin flowing easily through the non structural fabric, and wetting the fabric. Advantages

• Much lower tooling cost due to one half of the tool being a vacuum bag. • Large components can be fabricated. • Standard wet lay-up tools may be modified for this process. • Cored structures can be produce din one operation.

Disadvantages

• Relatively complex process to perform well. • Resins must be very low in viscosity, properties, thus compromising mechanical • Un-impregnated areas can occur resulting in very expensive scrap parts.

M9.3.3.2.2.5.2 Resin Film Infusion (RFI) (Vacuum Infusion Moulding) Selection of a right moulding technique to match the application is critical to successful parts production. Much emphasis has been made especially in the developed countries on adoption of closed moulding technique as an alternative method for fabrication of composite parts with less environmental pollution. Vacuum Infusion (VI) is gaining attention among manufacturers due to its advantages and flexibility. Vacuum infusion produces high and consistent level of laminate quality, low void content and near zero emissions of hazardous air pollutants. The parts such as boat hulls and decks, large structural panels for mass transit vehicles, wind turbine blades etc. could be fabricated by this process. In vacuum infusion process, vacuum is the driving force to impregnate dry reinforcements with liquid resin. The reinforcements (and core material if used) are placed in a mould and then covered with a flexible bag. The peripheral region is sealed and then vacuum is applied which draws resin from a container into the bag along a specified direction (along a symmetrical axis of the component). The process is very much suited to the inclusion of core materials to produce sandwich structures. In resin film infusion process, dry fabrics are laid up interleaved with layers of semi-solid resin film supplied on a release paper. The lay-up is vacuum bagged to remove air through the dry fabrics, and then heated to allow the resin to first melt and flow into the air-free fabrics, and then

after a certain time, to cure. The main attraction of resin film infusion is probably for parts of high surface area. Advantages

• High fibre volumes can be accurately achieved with low void contents. • Good health and safety and a clean lay-up, like prepreg. • High resin mechanical properties due to solid state of initial polymer material and

elevated temperature cure. Disadvantages

• Not widely proven outside the aerospace industry. • Tooling needs to be able to withstand the process temperatures of the resin film. • Core materials need to withstand the process temperatures and pressures.

M9.3.3.2.3 Continuous Processes M9.3.3.2.3.1 Pultrusion Pultrusion is a continuous, automated process that is cost effective for high volume production of parts with uniform cross-section. Due to uniformity in resin dispersion, fibres distribution and alignment, excellent composite structural materials can be fabricated by pultrusion. In recent years composite have started playing an important role in construction of modular bridge construction, towers for telecommunication, post- strengthening of buildings and infrastructure. Pultruded sections such as industrial gratings, walk-ways, cable-trays, hand-rails, ladders etc are being used in corrosion prone areas, chemical plants, off-shore and on-shore operations. On the other hand, the use of composites in bridge construction is taking a new direction towards development of suitable structures, support systems and joints using pultruded profiles. For pultrusion to become acceptable and popular composites manufacturing technology, it must be possible to pultrude complex multi-element cross sections, such as J-stiffened panels and constant airfoil sections. It is used to create shapes by pulling rovings through a shaped and heated die. The use of pultruded parts in aircraft is limited to specialized applications. Practical applications are limited to constant cross-section parts. Pultrusion is used to manufacture constant cross-section shapes, viz., I-beam, box, channels, tubings, etc. The Pultrusion process machine consists of six different parts namely, the creel, the resin bath, the forming die, the heated curing die, the pullers and the cut-off saw. The creel is the beginning of the Pultrusion process and is the material storage system from which the fibres and mat or fabric are drawn in the correct sequence to match the design requirements of the structural shape. Virtually all Pultrusion processes utilize a resin impregnation bath to facilitate the impregnation of the resin into the fibre structure. The use of pre-impregnated fibres eliminates the resin bath.

Two types of dies are used in Pultrusion process, namely, the forming and the heating or curing die. Forming is done immediately after the impregnation process. Forming dies are normally attached to the heating or curing die in order to provide the correct relationship between the forming and the heated curing step. The rovings go through a heated die that represents the cross-section of finished part. Curing is accomplished by heating the die. The product continuously pulled out and as it comes out of the puller mechanism, it is cu the desired length by an automatic saw. The process is continuous and can be used to manufacture extremely long sections. A typical continuous Pultrusion process is shown in Figure M9.3.20.

Figure M9.3.20: A typical continuous Pultrusion process.

There are two categories of Pultrusion products. The first category consist of solid rod and bar stock produced from axial fibre glass reinforcements and polyester resins. These are used to make fishing rods and electrical insulator rods which require high axial tensile strength. The second category consists of structural profiles, which uses a combination of axial fibres and multi-directional fibre mats to create a set of properties that meet the requirements of the application in the transverse and longitudinal directions. More than 90% of all pultruded products are fibre glass reinforced polyester. When better corrosion resistance is required, vinyl ester resins are used. When a combination of superior mechanical and electrical properties is required, epoxy resin is used. Higher temperature resistance and superior mechanical properties generally dictate the use of epoxy resins reinforced with aramid or carbon fibres. The major limitation of this method for aircraft use is the constant cross section requirement. Also, more complex geometries are not feasible by this technique. The chief advantage of the method is the ability to produce consistent parts at very low cost in a short period of time. Pultruded composite parts exhibit all the features produced by other composite processes, such as, high strength to weight ratio, corrosion resistance, dimensional stability, etc. M9.3.3.2.3.1.1 Comparison of Pultruded FRP properties with other structural materials

Physical & Chemical Properties

PultrudedFRP

RigidPVC

MildSteel

StainlessSteel Wood

Specific Gravity 1.8 1.38 7.8 7.92 0.52Thermal Conductivity (Kcal/hr/m /° C)2

24.4 6.4 1220 732.00 0.4

Coeff. of Linear Expansion (cm/cm° C) x 10-6

5.2 37 8 10 1.7

Safe Working Temp. (° C)

130 55 600 600 160

Flame Resistance Good* Poor Excellent Excellent Poor

Corrosion Resistance a. Acidic Excellent Good Poor Excellent Poorb. Alkaline Good Fair Good Excellent Poorc. Solvents Fair Poor Good Excellent Faird. Coastal Environment Excellent Good Poor Excellent Fair

e. Outdoor Exposure Excellent Poor Fair Excellent Fair

f. Effluent Water Excellent Good Poor Excellent Fairg. Steam Good Poor Fair Excellent Fair

* Excellent with special additives Table M9.3.3: Physical & Chemical Properties of Pultruded Profiles Vs. Other Structural

Materials M9.3.3.2.3.1.2 Characteristics of Pultruded Products

Size Forming guide system and equipment pulling capacity influence size limitation

Shape Straight, constant cross sections, some curved sections possible

Length No limitReinforcement Fibre glass, aramid fibre, carbon fibre, thermoplastic

and natural fibresMechanical Strength

Medium to high, primarily unidirectional approaching isotropic

Labor intensity

Low to medium

Mould cost Low to mediumProduction rate

Shape and thickness related

Table M9.3.4: Pultruded Product Characteristics M9.3.3.2.3.1.3 Advantages of Pultrusion Pultrusion is the most cost-effective method for the production of fibre-reinforced composite structural profiles. It brings high performance composites down to commercial products such as light-weight corrosion free structures, electrical non-conductive systems, off-shore platforms and many other innovative new products. The primary advantages of pultruded FRP profiles are listed as below:

• Production is continuous. • Material scrap rate is low. • The requirement for support material is eliminated i.e., breathers, bleeder, cloth, separator

film, bagging film, edge tape, etc. • Labour requirements are low.

Also see the some advantages of pultruded FRP profiles are summarized in the Table M9.3.5:

Features Description Benefits ApplicationsStrong Unit strength in

tension and compression is approx. 20 x that of steel when these properties are combined on the basis of unit density

Optional strength as desired. Exceptionally high impact strength reduces damage potential

Structural process equipment support. Tank supports. Cooling tower ancillaries. Flooring supports. Trusses & joints.

Light Weight

Density of pultruded components is about 20% of steel and 60% of aluminium

Higher performance at less weight. Lower shipping, handling & installation costs. Less operational energy demand.

Automotive leaf springs & bumpers. Prefabricated walkways & platforms. Bus components.

Corrosion Resistant

Unaffected by exposure to a great variety of corrosive environment & chemicals.

Minimum maintenance costs. Long term safety. Longer life.

Chemical plant hand railings, gratings, walkways & bridges. Cable trays. Pipe supports.

Electrical Insulation

Provides strength & rigidity with dielectric properties.

Lesser no. of components. Non-magnetic & safe. Predictable insulation values for wide range of frequencies.

Ladders, Cable trays. Switch gear components. Mounting braces and backboards.

Thermal Insulation

Pultruded components have a low thermal conductivity, 1/250 of

Reduces installation thickness. Eliminates condensation

Bulk head frames. Walk in refrigerator door jams. Window

aluminium & 1/60 of steel.

problems. Reduces energy operation requirements.

frames. Insulated roll up panel doors.

Consolidation

Many individual components can be combined into a large profile.

Reduced assembly cost. Reduced inventory. Fewer parts improve reliability.

Window latch supports. Roll up door

Dimensional Stability

Pultruded components are highly resistant to warping stretch/swelling over a wide range of temperature & stresses.

No permanent deformation under high stress. Close tolerances.

Spring bumpers. Crossing gate arms. Scrubber components.

Safety The pultruded components are very strong & safe to work with. They are microbes and insect proof.

Many gratings suffer from the problem of microbes etc. due to wet or unhygienic working conditions.

This property makes them ideal choice for pharmaceutical & food industries.

Table M9.3.5: Advantages of Pultrusion(Source: Product Information Brochure; DK Fibre Forms, Pune, India)

Disadvantages

• Limited to constant or near constant cross-section components. • Heated die costs can be high.

M9.3.3.2.3.2 Braiding The braiding process involves the weaving of fibres into shape by repeatedly crossing them back and forth over a mandrel. The use of braiding process in the aircraft industry is generally restricted to non-structural applications. The braiding process has been utilized extensively for covering of the electrical wires and fuel lines. The primary advantage is that braiding offers is a rapid, automated method for forming an interwoven structure. The method is a product of textile technology and usually utilizes equipment adapted from the textile industry. The braiding carriers follow a zigzag path in a large circle surrounding the mandrel. The surface of the mandrel is tightly woven with the fibres in a helical pattern. Due to high level of conformability and the damage resistance capability of braided structures, the composite industry had found structural applications of braided composites ranging from rocket launchers to automotive parts to aircraft structures. 2-D braided structures are inter-twined fibrous structures capable of forming structures with 0° and ±θ fibre orientation. Although 2-D braids can be fabricated in tape form, the majority of braided structures are fabricated with tubular geometry. Thickness is built up by over braiding

previously braided layers similar to a ply lay-up process. Braiding can take place vertically or horizontally. A schematic of a horizontal braider is shown in Figure M9.3.21. Although braiding is similar to filament winding, the major difference between braiding and filament winding is that braids are interlaced structures having as many as 144 or more interlacing per braiding cycle.

Figure M9.3.21: A schematic of a horizontal braider

3-D braiding technology is an extension of 2-D braiding in which the fabric is constructed by the inter-twining or orthogonal interlacing of yarns to form an integral structure through position displacement. A unique feature of 3-D braids is their ability to provide through the thickness reinforcement of composites as well as their ready adaptability to the fabrication of a wide range of complex shapes ranging from solid rods to I-beams to thick walled rocket nozzles. A generalized schematic of a 3-D braiding process is shown in Figure M9.3.22.

Figure M9.3.22: A generalized schematic of a 3-D braiding process

*Prepreg Molding

In prepreg molding, fabrics and fibres are pre-impregnated under heat and pressure or with solvent, or a pre-catalyzed resin. The catalyst is largely latent at ambient temperatures giving the material useful life when defrosted. The resin is usually a near solid at ambient temperatures, and so the pre-impregnated materials (prepregs) have a light sticky feel, such as that of adhesive tape. The prepregs are laid up by hand or machine onto a mold surface, vacuum bagged and then heated to typically120-180°C (248-356°F). This allows the resin to flow followed by curing. Additional pressure for the molding is usually provided in an autoclave. Advantages

• Resin/catalyst levels and the resin content in the fibre are accurately set by the materials manufacturer. High fibre contents can be safely achieved.

• The materials have excellent health and safety characteristics and are clean to work with. • Fibre cost is minimized in unidirectional tapes since there is no secondary process to

convert fibre into fabric prior to use. • Resin chemistry can be optimized for mechanical and thermal performance, with the high

viscosity resins being impregnable due to the manufacturing process. • The extended working times (of up to several months at room temperatures) means that

structurally optimized, complex lay-up scan be readily made. • Potential for automation and labour saving. .

Disadvantages

• Materials cost is higher for pre-impregnated fabrics. • Autoclaves are usually required to cure the component. • Tooling needs to be able to withstand the process temperatures involved. • Core materials need to be able to withstand the process temperatures and pressures.

In conclusion, it may be seen that a fibre reinforced composite may be manufactured by one of the many standard fabrication methods as explained above. The selection of the best method for a given application requires the consideration of numerous factors. The selection process must determine the appropriate constituent materials, the actual part design, the required tooling, the fabrication process and the necessary inspection and testing requirements to assure part quality. Learning Unit-4: M9.4 Forming Structural Shapes M9.4.1 Wet Lay-up and Spray-up (Also refer M9.3.3.1.1.2) Two other manual techniques have a long history in the composites industry, wet lay-up and spray lay-up. Both were developed for the fiberglass industry and they are still used very extensively. Wet lay-up is similar to the lay-up of preimpregnated material discussed previously.

The only difference is that the reinforcement and matrix are now combined during fabrication. After the tool is properly prepared and a gel coat (a layer of un-reinforced pigmented polyester that serves as the outer surface of the laminate when completed) is applied on the tool or mold, the reinforcement is put down in the form of chopped strand mat or woven materials. Once the reinforcement is in place, the resin and catalyst are mixed and poured onto the tool surface. To ensure that the resin infiltrates the reinforcement and that no air bubbles remain, it is rolled or brushed into place using hand rollers, brushes, or paddles. The resin mixture can also be sprayed onto the tool surface using a spray gun that automatically meters the appropriate mixture. This type of fabrication is called spray laminating. In some cases, chopped glass fibers are combined with the resin in the metering head and deposited onto the tool surface by spraying. Wet lay-up and spray-up fabrication produces a glass-reinforced structure with 30-50 percent glass content. Higher glass concentrations are not easily fabricated using these techniques; however, the use of prepreg can give higher fiber volume fractions (50-75 percent). M9.4.2 Filament Winding (Also refer M9.3.3.1.1.3) Filament winding is a fabrication technique developed in the late 1940s and early 1950s in which continuous tows of fibers are wound onto a rotating tool, or mandrel. The construction of the mandrel is a key step in the filament-winding process. The choice of material is critical, and of course, the mandrel must be removed from the inside of the finished composite structural form after processing. When complex enclosed shapes are manufactured by filament winding, it is sometimes difficult to remove the mandrel. Mandrels that can be disassembled in sections from inside the shape must be designed, or plaster or sand polyvinyl acetate (PVA) mandrels that can be dissolved with a solvent after processing can be used. The filament-winding machine dispenses, or "pays out," the fiber tows while traversing along the mandrel axis of rotation. Some types of filament-winding machines and control systems allow very complex winding patterns to be generated. Filament winding is an automated fabrication method. Once the mandrel has been installed and the fiber or tow material loaded, an operator can start the machine and the fabrication proceeds automatically. The two types of filament winding are wet winding and dry winding. Wet winding refers to the use of a wet resin during winding. Fibers are passed through a resin bath before being wound onto the mandrel; the reinforcement and matrix are combined during fabrication. In dry winding, preimpregnated fibers, in tape or tow form, are used instead, and the preimpregnated material is wound directly onto the mandrel. Sometimes pre-heaters are used to soften the preimpregnated material before it is placed onto the mandrel surface. If thermoplastic composites are being wound, they are consolidated directly on the mandrel surface by using highly localized heat sources that soften the preimpregnated tape or tow right at the point of contact on the mandrel surface. Surfaces of revolution are appropriate candidates for filament winding. These types of structures include piping, pressure vessels, tubing, rotor blades, or any spherical, conical, or geodesic shape. Other shapes, such as flat panels, can be made by winding onto a rectangular mandrel and then cutting the structure along the winding axis after fabrication.

M9.4.2.1 Winding methods and patterns The two basic types of winding patterns are polar and helical. The polar (or planar) winding pattern results when the mandrel does not rotate, but the head that dispenses the tows, commonly called the payout head, rotates about the longitudinal axis. To produce a given orientation of the tow along the axis, the payout head is inclined at the appropriate winding angle, and the mandrel moves longitudinally as the payout head rotates. This pattern is described as a single-circuit polar wrap and is shown in Figure M9.4.1. As more tows are wound, each tow is placed adjacent to the next. A completed layer consists of many tows integrally woven at plus and minus the orientation angle covering the entire mandrel surface. A helical pattern results when the mandrel rotates continuously while the fiber payout head traverses back and forth over the mandrel, as in Figure M9.4.2. The carriage speed and mandrel rotation are controlled to generate the desired winding angle. A hoop winding can be made by advancing the payout head slowly along the mandrel axis so that the fiber tows are wound transversely to the axis of rotation. As the longitudinal speed of the payout head is increased, the angle that the tows make with the axis of the mandrel rotation decreases. In general, a helical winding pattern does not deposit the next tow adjacent to the previous one. In fact, several circuits may be required before the full mandrel surface is covered. Because of tension in the tow as it is wound on the mandrel, the tows flatten out and the fibers tend to spread into bands. Individual winding patterns can be calculated from the mandrel geometry and the desired lay-up.

Figure M9.4.1: Polar filament winding pattern

M9.4.2.2 Winding machines Filament winders are designed for either polar or helical winding. Polar winders are usually operated with the mandrel in the vertical position to eliminate deflections due to weight. A major advantage of the polar type is that machine control is much simpler. The rotation of the payout head is continuous and at a uniform speed. This eliminates inertial effects that occur in a helical winder when the traverse speed is changed abruptly or when the traverse direction is changed. However, polar winders are generally limited to dispensing only preimpregnated material. Helical winders require at least two degrees of freedom: mandrel rotation and traverse of the payout head along the longitudinal axis. More complicated machines include motion of the payout head perpendicular to the mandrel axis, and rotation of the feed eye; both of these motions permit more accurate placement of the fiber tows around the ends. Figure M9.4.3 shows a typical helical filament winder used in composites manufacturing.

Another component of the filament winder is the fiber tensioner. Fiber tensioning plays an important role in filament winding, as it helps to control the placement of the tow, the resin content of the structure, the width the tow flattens to, and the layer thickness. Improper tensioning of a filament-wound structure can result in unacceptable quality. Tensioning is accomplished through the use of guide eyes, brakes of the drum type, scissor bars, and, for wet winding, the viscous drag through the resin bath. Generally, the fiber tow unwinds from a spool of tow and passes through a series of rollers that are designed to impart a modest amount of tensioning, say, 5 - 50 Newtons/tow. Normally, the tensioning on dry tows is kept to a minimum to reduce abrasion. As the tows pass through the resin bath, the tension level is increased before the tow is placed onto the mandrel.

Figure M9.4.2: Helical filament winding pattern

Figure M9.4.3: En-Tec model 5K240-060-4 four-axis helical filament winder.

M9.4.3 Pultrusion (Refer M9.3.3.1.3.1) M9.4.4 Resin Transfer Molding (RTM) (Refer M9.3.3.1.2.5) M9.4.5 Non-autoclave Curing While the use of an autoclave is the most desirable way to process a composite laminate, other methods of curing can, with care, lead to similar high-quality components. Two of these are described below. M9.4.5.1 Oven Curing Ovens are the least-expensive processing equipment. However, unlike an autoclave, they have no capability to apply pressure during cure. Oven-cured parts are either vacuum bagged so that atmospheric pressure consolidates the part, or they are sealed in a mold with an expansive insert so that thermal expansion forces the composite component against the mold walls. A typical curing oven is a large, thermally insulated, forced-air-circulating metal oven with large access doors at one end. Curing ovens hundreds of cubic meters in volume are common. The most important aspect for curing ovens is temperature uniformity. If the volume of circulating air within the oven is small, then the response time to temperature fluctuations will be long. If there is insufficient baffling of the circulating air, then hot spots or zones will develop in the oven. Often oven-cured parts cured with vacuum-bagging techniques suffer from lack of uniformity in thickness, excessive voids, and lack of consolidation. However, the equipment is relatively inexpensive and for many low-performance applications the quality is sufficient.

M9.4.5.2 Hot Pressing Good quality parts can be obtained by hot pressing. A hot press is a mechanical or hydraulic press with heated platens. Processing using the hot press requires that the composite structure be enclosed between two matched-metal molds. The molds are placed between the platens and the platens are forced together to apply pressure to the mold. The platens are heated so that the mold temperature increases to a sufficient level to promote curing of the matrix. If the composite part is simply a fiat plate, then the "mold" is simply two flat plates, one on the top of the laminate and one on the bottom. Pressurization is by compression of the mold through the thickness; the pressurization forces the laminate to conform to the contour of the mold surface. The mold must be sealed to prevent uncontrolled resin loss and to promote hydrostatic pressurization of the resin. The pressurization of the resin is important to reduce voids created during cure or entrapped during the fabrication procedure. To mimic the autoclave, the composite part can be enclosed in a vacuum bag as part of the curing process. Alternatively, a special diaphragm mold can be used to apply a vacuum to a hot-pressed laminate during curing. Application of the vacuum helps to eliminate voids during cure. The platens in the hot press must be uniformly heated. Typically, this is accomplished by having multiple heating sources embedded in the platens. More elaborate platens have multiple cooling channels bored throughout the platens so that the capability exists to both heat and cool specific regions of the platen. The transfer of thermal energy from the platens to the mold is largely by conduction, although some radiation and convection occurs. Platen and mold materials should be chosen to minimize the response time for thermal conduction. In general, platens are made from tool steel, and molds are either steel or aluminum. Molds are often insulated on the edges to increase temperature uniformity. Hot-pressed composite structures show good consolidation and moderate void content. Some variation in thickness occurs due to lack of uniformity in resin flow and small misalignment of the matched-metal surfaces. M9.4.6 Manufacturing Defects No matter how carefully the manufacturing process is carried out, all composite structures exhibit processing defects. When a material is designated as high-quality, it has relatively few defects. Some processing defects have the potential to be quite detrimental to the mechanical performance of a composite structure. In other cases, they are more of a nuisance than a significant problem. There are many different types of manufacturing defects: voids, delamination, residual stress-induced cracking, resin starvation, resin-rich pockets, damaged fibers, fiber-matrix de-bonding, thermal decomposition, and under-cured and over-cured regions. The processing parameters have a profound impact on when and where these defects occur, and how they can be prevented.

Voids are extremely detrimental to the mechanical performance of composite materials. Their effect on structural performance has been extensively studied in the past. For a number of reasons voids can be created during processing. Air pockets can be trapped during the lay-up procedure, absorbed water in the resin can vaporize during the cure cycle, and gaseous by-products of the cure reaction may be released during curing. In resin transfer molding, voids can occur if the resin flow front does not uniformly infiltrate the reinforcement. The current methodology to control voids in processing is to apply sufficient pressure to prevent their growth during the cure cycle. Delamination or separated layers can arise during processing as a result of transport of gas bubbles, emitted from the resin as it is curing, to the interface between layers. If a sufficient number of voids collect at the interface, then a delamination is produced. In addition, dirt, grease, or other contaminants on the surface of the prepreg layers during lay-up may prevent layer bonding and consolidation. Most aerospace companies nondestructively evaluate each composite laminate for the presence of delamination before releasing the part for service. Residual stresses arise in composite materials due to the mismatch in thermal expansion between the constituents. As the material is cooled down from processing temperatures, this mismatch in thermal expansion leads to the build-up of residual stresses. If the mismatch is too great or if the processing temperature is too high, then residual stresses can lead to matrix cracking during cooldown. If the consolidation has not been properly carried out, then either resin starvation or resin-rich pockets can result. Resin starvation is seen when the applied pressure is too high, causing too much resin to be squeezed out of the material. If too little pressure is applied or if resin flow from the material is not uniform, then resin-rich regions may be created. These regions are weak areas for the composite and they may ultimately be the initiation site for final failure. Similarly, non-uniform curing can lead to weak regions of the material where the local degree of cure is reduced. This type of defect occurs when the temperature distribution during cure is non-uniform. In the extreme case, excessive temperatures may occur when the thickness of the article is large. The energy released by the resin during curing is trapped within the material and the local temperature can rise to very high levels, ultimately leading to thermal decomposition of the matrix. Even when all reasonable precautions are taken, some types of defects are to be expected in composites processing. No process is perfect and a typical component made from a composite material will show some voids, resin-rich regions, de-laminations, and residual stress-induced cracking. Figure M9.4.4 shows such a part with several defect regions labeled. Understanding the effects of these kinds of defects on the mechanical performance of composite structures is the challenge that scientists and engineers must continually address.

Figure M9.4.4: Examples of manufacturing defects in a polymer matrix composite. The material is a graphite-epoxy composite laminate with an embedded shape memory alloy wire.

M9.4.7 Safety Precautions Unlike conventional materials, fabrication and in-service repair of composite parts involve dealing with different types of fibres, matrix materials, solvents, etc. Further, during the curing of composites, solvents and volatile by- products are released. Different types of chemicals are also used for cleaning of the composite parts. Therefore, safety precautions for handling such materials are totally different from those followed while dealing with conventional materials. In addition, all the users of such materials are required to comply with the regulations of Environmental Protection Agency (EPA) and Occupational Safety & Health Administration (OSHA). As per the OSHA standards, all personnel are required to be informed about the known hazards associated with the materials with which they work, regardless of the quantity of the material involved in the operation. In addition, to assist the users in understanding safety and health issues for composite materials, the Safety and Health Sub-committee of the Suppliers of Advanced Composite Materials Association (SACMA) had brought out a publication on Safe Handling of Composite Materials. M9.4.7.1 Terminology in order to assess toxicological data: It is important to understand the terminology in order to assess toxicological data. Some of the basic terms that are applicable to the composite user, especially in reading Material Safety Data Sheets (MSDS) are as follows:

• Toxic- refers to a poison or poisonous substance that may cause a harmful effect in the body. In a broad sense, principle of toxicology is shown in Figure M9.4.7 (a).

• Hazard -takes into account the toxicity of a substance along with exposure to it. Something can be extremely toxic, but if there is no exposure, there is no hazard (Figure

9.4.7 (b)). Care should be taken while handling low toxicity substances, as extreme exposure (high concentration or long exposure time) could result in high hazard.

Figure 9.4.7 (a) Principle of toxicology

Figure 9.4.7 (b) Comparison of exposure and no exposure effect

• Risk -describes the probability or likelihood that a hazard will result in a harmful effect.

Regardless of the toxicity, hazard or risk associated with a substance, care should be taken to minimize the exposure. By and large, toxicity is referred in terms of acute or chronic.

• Acute toxicity occurs when a harmful effect is experienced after a single or short-term exposure to a substance. It usually occurs instantly or within a short time period.

• Chronic toxicity occurs when adverse health effects are manifested after exposure to a substance over a long period of time. These effects can occur following repeated exposures to chemical substances.

In some cases, an allergic reaction to a substance will develop with one exposure or over time with repeated exposures. This is called sensitization. People who are sensitized to one chemical

may react to other similar materials. This is known as ‘cross sensitization’ (Figure M9.4.7 (c)). In such instances, it is important that the person is not exposed further or allowed the opportunity to be exposed.

Figure 9.4.7 (c) Cross sensitization

M9.4.7.2 Basic requirements of a hazard communication programme: The five basic requirements of a hazard communication programme are:

1. Inventory of all hazardous materials to be maintained at the work place. 2. Labeling of all the hazardous materials. 3. Availability of Material Safety Data Sheets (MSDS) for all the materials stored and/or

used in the work place. 4. Training of personnel on proper handling of the hazardous materials. 5. Availability of written programme at the work place complying with the above four

requirements. M9.4.7.3 Scope of MSDS and Classification: MSDS provides information on material hazard, safe handling and disposal procedures. MSDS are available for different cleaning agents, solvents, matrix materials and fibres. MSDS is classified into following nine sections:

1. Product Identification: contains information of manufacturer, contact number in the event of an emergency, chemical name, trade name, etc.

2. Hazardous Ingredients: describes the various hazardous ingredients contained in the product that are more than 1 % of the total, their percentage and exposure limits, if applicable.

3. Physical Data: includes physical properties such as boiling point, vapour pressure, vapour density, solubility in water, specific gravity, percent volatile, evaporation rate, appearance and odour.

4. Fire and Explosive Data: describes the nature of the fire and explosion hazard data. Based on the flash point and other fire and explosive data, appropriate fire extinguishing agents are listed.

5. Reactivity Data: describes the ability of the material to react and release energy or heat under specified conditions.

6. Health Hazard Information: contains possible health hazards in order to assist the user and medical personnel to identify over-exposure, if any.

7. Spill, leak and Disposal Procedure: entails the procedure which is required to be followed in case of an accidental spill or leak.

8. Special Protection: contains information regarding the need for special equipment while using the material such as respiratory equipment, special clothing, type (s) of goggles, eye protection and other special considerations.

9. Special Precautions: contain special handling and storage information. All personnel dealing with composite materials are, therefore, required to have complete knowledge of information contained in the relevant MSDS. M9.4.7.4 Potential exposures posed by composite usage: Periodic exposure assessments should be conducted taking into consideration routes of exposure associated with composite use. Some of the routes of exposure for composite users are given in Table M9.4.1:

Skin and eye Typically hands, lower arms and face are exposed. However, if personal hygiene is not good, other areas of the body may be susceptible to exposure. Exposure should be avoided, especially in cases where dermatitis or sensitization has been confirmed. Contact with chemicals that can be absorbed through the skin should also be avoided.

Inhalation Good ventilation will minimize possible exposure from the release

of solvents or dusts generated.

Ingestion Thorough washing of hands prior to eating or smoking provides significant protection from the effects of accidental ingestion.

Injection Remains from cured composites or brittle fibres, or needles from

weaving process can puncture skin and chemicals could enter the body.

Table M9.4.1: Potential exposures posed by composite usage Two potential exposures posed by composite usage are skin contact with materials that could result in irritation leading to dermatitis or sensitization, and inhalation of particulates from

operations such as cutting, grinding and finishing. Both concerns can be eliminated or minimized with implementation of proper gloves/clothing, good ventilation and process conditions, and effective training: One example of a process hazard in the composites industry is an out-of-control exothermic reaction. This is an unintentional runaway chemical reaction of a resin system, alone or in a prepreg form, typically called an ‘exotherm’. It may occur under any of the following conditions:

• Heating or mixing a resin too long. • Heating a resin too fast; • Allowing a resin to get too hot; and • Contamination or mislabeled chemicals.

Following factors have been known to start or extend an exotherm:

• Deviation from procedures; • Disabling safety equipment; • Casting hot melt resin too deep; • Process equipment malfunction; • Variability in raw material; • Mixing incompatible chemicals and curing agents; • Contaminating chemicals e.g. poor house keeping; • Uneven dispersing or mixing of chemicals; and • Trying to mass cure resin or prepreg in an oven or autoclave.

Exothermic reactions can be avoided and minimized by clearly defining and following the proper procedures, thorough training, making certain that equipment in is good working order and those safety devices or systems are in place and functioning properly. Health and safety hazards associated with various types of epoxies, resin systems, hardeners, curing agents, solvents and fibres are given in Table M9.4.2:

Epoxies • Possible skin sensitizer, low order of acute toxicity, slightly too moderate irritating.

• Irritant to skin and mucous membranes.

Hardeners/Curing • Slightly irritating to skin and mucous membranes. Can Agents cause damage to internal organs (liver) and may decrease ability of blood to transport oxygen to tissues.

• Severe skin and eye irritants, can cause skin and/or respiratory sensitization, certain types can cause temporary visual disturbances with airborne material affecting the cornea.

• Irritating effect on respiratory tract, chest discomfort with reduced lung function.

Fibres • Possible skin sensitization from the fibre sizing.

• Mechanical abrasion and irritation of the skin, possible dermatitis, physico-mechanical properties of the fibres rather than a toxico-chemical reaction.

• Skin, eye or upper respiratory irritation is possible. .Prolonged over exposure to respirable fibrous particles has potential for lasting lung damage.

Resin • Strong/severe irritants to skin and mucous membranes of eye and respiratory tract, skin and respiratory sensitization, systemic toxicity.

• Due to toxicity" and absorption through skin, contact should be avoided.

• Over exposure may cause liver and kidney effects. • Dust or vapour may cause eye, nose or throat irritation. • Vapours generated could cause eye and sinus irritation. • Thermal burns are major effect. • Off-gas materials may cause headaches, dizziness and elevated

blood levels of carboxy-haemoglobin.

Solvents • Mild to moderate skin irritant; moderate to severe eye irritant. Over exposure could result in possible central nervous system depression.

• Irritation of mucous membranes, headache, nausea. Skin contact can cause defatting, dermatitis.

• If ingested, vomiting may cause acute chemical pneumonitis. • Over exposure may cause kidney and liver effects. • Exposure can cause cardiovascular effects (sensitization of the

cardiac muscle), central nervous system depression and drowsiness.

• Irritating to the skin, eyes and mucous membranes, stomach pain, nausea and vomiting, possible link to testicular cancer.

Table M9.4.2: Health and safety hazards associated with various types of epoxies, resin

systems, hardeners, curing agents, solvents and fibres M9.4.7.5 Classification of Safety precautions to take during handling of composites fall: Broadly, safety precautions to be observed during handling of composites fall into the following four sections, namely:

1. General precautions, 2. Precautions during cleaning, 3. Precautions while handling different solvents, and 4. Precautions while handling different resins.

M9.4.7.5.1 General Precautions

• First and foremost safety precaution to be observed while handling composites is that the

work area should be ventilated properly. As explained in the preceding para, different resins, solvents and cleaning agents may cause discomfort or illness when used in confined areas not having proper/adequate ventilation. Adequate ventilation helps to keep vapour concentration of different solvents, resins, etc. below the applicable standards.

• For safety reasons, personnel working on composites should wear mask/safety glasses, impervious gloves and overalls. Gloves used should be chemically/mechanically or thermally resistant depending upon the requirements and their choice must depend on the job and its duration.

• Personnel working with composites should wear dust mask, face shield I and safety glasses during the following operations:

Cutting of the material on the finished composite, Machining, Sanding, Drilling, and Trimming.

• Hand gloves should be used especially while working with fibres. Composite fibres that become embedded in the skin should be removed immediately by pulling them straight out. Bending or flexing the filament will cause breakage of the filaments. If not removed immediately, the filaments tend to work themselves further into the skin.

• Use non-spark producing tools for working on the composites to avoid the risk of any inadvertent fire.

• During in-service repairs, the repair cart and the aircraft should be statically grounded. • Composites decompose during trimming or drilling operation at high speed thereby

creating toxic fumes. Adequate precautions should, therefore, be observed. • Materials should be stored away from heat; sparks, open flame, etc., and also in a well

ventilated area with containers tightly closed. • All hazardous materials must contain identifying labels. Once a container is used for one

type of hazardous material, it should not be used for another type. M9.4.7.5.2 Precautions during Cleaning

Composite surfaces should not be cleaned with scouring powder or harsh cleaning agents. Instead, approved cleaning materials and solutions should be used. Use of non-approved materials may damage the surface protection.

All removed particles after rubbing of composite surface should be removed by blowing oil free compressed air or by means of vacuum cleaner.

Dust, loose fibres and other scrap should be continuously removed by vacuum cleaner. Graphite fibres should be disposed off by landfill burial.

Under no circumstances paper or synthetic wiper material should be used for cleaning plastic surfaces e.g. radomes, other composite parts, etc. as this could create a fire hazard when working with inflammable solvents.

Cleaning liquids like lead-free gasoline are dangerous and highly inflammable. Its fumes should not be inhaled.

M9.4.7.5.3 Precautions While Handling Solvents Safety cans should be used for handling and dispensing inflammable solvents and the cans should be properly labeled. General precautions to be observed while handling solvents are:

Avoid the use of excessive amount of solvents. Use the solvents only in a well ventilated area. Solvents are dangerous for lungs, skin and eyes. Adequate protective clothing should be

worn. By and large solvents like ketone viz. Methyl Ethyl Ketone, Acetone, etc. are toxic

compounds. Therefore, their vapours should not be inhaled. Persons continuously being exposed to solvents must use respiratory protection.

Eye protection, hand gloves and protective clothing should be used to avoid contact of solvents with eyes and skin.

Solvent saturated clothes should be stored or disposed off in metal containers having lids. Smoking or open flame should be prohibited within approximately 7.5 m (25 feet) of any

area where solvents are used. Used solvents should be disposed off as per safety regulations of the State. After cleaning the surface with solvent, it should be left open to allow the solvent to

evaporate before continuing further work. Dry and compressed air should be used to clean and remove all traces of the solvents. It

may be noted that blowing dry air to evaporate solvents can cause propelled droplets, which can cause eye injury.

Solvent is also skin irritant and dries out the natural oils in the skin. Wear protective gloves or use a barrier cream. Wash immediately with clean and running water if skin is splashed with solvent.

Solvents should be used away from the rubber and plastic items as most of them have deleterious effects.

Frequent contact with skin and prolonged inhalation of the vapours should be avoided. Some of these materials have no known antidote.

Due to inflammable characteristics, solvents should not be used in the vicinity of open flame, sparks, sanding area, etc.

Bagging films or peel ply should not be unrolled near the solvents as they may produce static electricity and become a source of fire hazard.

M9.4.7.5.4 Precautions While Handling Resins The following safety precautions are required to be followed while using different resin systems:

• Like the solvents, different resins are also harmful to the skin and eyes. Some people are especially sensitive to these materials.

• Avoid skin contact and wash thoroughly after mixing and applying any resin system. Wear protective gloves to prevent skin contact.

• Use adequate ventilation to avoid vapour inhalation. For example, epoxy resins and hardeners give off potentially -harmful vapours and may be dermatitic.

• Excess resin system should be disposed off in accordance with the safety regulations of the State.

• For overhead work, eye/face protection should be worn. • Catalysts can cause burning of skin and permanent eye damage. Catalyst like Methyl

Ethyl Ketone Peroxide (MEKP), especially used for curing polyester resins and accelerators in direct contact will spontaneously inflame and may lead to explosion. Therefore, proper care should be taken while handling such chemicals.

• For wet lay-up, neoprene/chemical resistant gloves with cotton liners, protective clothing, and eye goggles must be worn to provide adequate protection.