Thermoplastics - An Introduction

What is an engineering thermoplastic? Such a definition is difficult to arrive at and is very subjective, but, for the purposes of this review article, any thermoplastic that can be formed into a load-bearing shape that might otherwise be formed from, for example, steel or wood will be classed as an engineering thermoplastic. This review is restricted to unreinforced and short fibre reinforced thermoplastics where the reinforcing fibre (usually glass or carbon) is typically less than 2-3 mm in length.

Thermoplastics offer many advantages over traditional materials, including: low density; low energy for manufacture; low processing costs; and the ability to make complex shapes relatively easily.

Thermoplastic Characteristics

Thermoplastic materials generally fall within two classes of molecular arrangement, amorphous and semi-crystalline (see figure 1).

Figure 1 Molecular Arrangement of Polymer Chains

Table 1 lists a selection of amorphous and semi-crystalline polymers.

Table 1. Amorphous and semi-crystalline polymers.

Amorphous Semi-crystalline

Polyamideimide Polyetheretherketone

Polyethersulphone Polytetrafluoroethylene

Polyetherimide Polyamide 6,6

Polyarylate Polyamide 11

Polysulphone Polyphenylene sulphide

Polyamide (amorphous) Polyethylene terephthalate

Polymethylmethacrylate Polyoxymethylene

Polyvinylchloride Polypropylene

Acrylonitrile butadiene styrene

High Density Polyethylene

Polystyrene Low Density Polyethylene

Amorphous Polymers

Generally, fully amorphous polymers are stiff, brittle and clear in the virgin state. The temperature and stress state have a profound effect on the molecular arrangement and hence the properties of a polymer. Under the action of sufficient stress, the polymer chains can uncoil and align over a period of time. At elevated temperatures, polymer chains have enough energy to rotate and coil up further.

Figure 2 shows schematically the effect of temperature on the elastic modulus of an amorphous and a semi-crystalline polymer. Below a temperature known as the glass transition temperature, Tg, the structure of amorphous polymers is termed 'glassy', with a random arrangement of the polymer chains, similar to the random molecular arrangement found in glass. As the temperature increases to Tg, the polymer chains have sufficient thermal energy to rotate, resulting in a drop in modulus. One definition of Tg is the temperature at which molecular rotation about single bonds becomes favourable as the temperature increases. At temperatures above Tg but below the melting temperature, Tm, there is a 'rubbery region', where the material can exhibit large elongations under relatively low load.

Amorphous thermoplastics are generally used at temperatures below their Tg, where they can be brittle, just like glass. There are, however, certain exceptions. Polycarbonate (PC) is amorphous yet it is considered tough at temperatures well below its Tg. When it does fracture below its Tg, it does so in a brittle manner, but this requires a large amount of energy and so PC is considered tough, finding use in applications requiring impact resistance, e.g. safety helmets and bullet proof glazing. This behaviour is due to the chemical bonds in polycarbonate rather than the arrangement of the polymer chains.

Figure 2. The effect of temperature on the Elastic Modulus (assuming both types of polymer have the same Tg and Tm)

Semi-Crystalline Polymers

Semi-crystalline materials such as polyamides do not exhibit a clear Tg or 'rubbery' region, although one is often quoted as the amorphous parts of the structure will undergo some transition. For these polymers the main transition occurs at Tm when the crystalline regions break down (see Fig. 2). Some chain rotation in the amorphous regions will occur below Tm, giving some impact resistance at these temperatures. Values of Tg and Tm for a number of polymers are given in Table 2.

Table 2. Glass transition and melting temperatures for a range of thermoplastics.

Polymer Tg (°C) Tm (°C)

High density polyethylene (HDPE) - 135

Polypropylene (PP) -10 175

Polystyrene (PS) 100 -

Polymethylmethacrylate (PMMA) 105 -

Polyvinylchloride (PVC) 65 -

Natural rubber (NR) -75 25

Polydimethyl siloxane (PDMS) -125 -55

Polyoxymethylene (POM) 125 175

Polycarbonate (PC) 150 -

Polyethylene terephthalate (PET) 70 265

Polyetheretherketone (PEEK) 145 335

Nylon 6 (PA6) 50 215

Polyamideimide (PAI) 295 -

Polysulphone (PSul) 195 -

Polyphenylene sulphide (PPS) 90 285

Polyethersulphone (PES) 230 -

Polyetherimide (PEI) 218 -

Polytetrafluoroethylene (PTFE) 20 325

Liquid crystal polymer (LCP) - 420Table 2. Values of Tg and Tm for selected polymers

The selection of a plastic material for a specific application can be a difficult task. After careful consideration, the possibilities may be narrowed to two or three candidates and the final selection is then determined by testing.

The first and most important step in selecting a plastic material from the broad range of available materials (i.e., acrylic, polycarbonate, UHMW, Delrin, nylon, etc.) is to carefully define the requirements of the application, the physical properties required and the environment in which the material will need to perform.

The following list of questions or considerations should be used to define the application as completely as possible. In many cases, the answers to these criteria may be helpful to eliminate a particular plastic or an entire family of plastics. The more completely the application is defined, the better the chance of selecting the best material for the job.

Physical & Mechanical Considerations What are the overall part dimensions (diameter, length, width, thickness)? What load will the part have to carry? Will the design carry high loads? What will the highest load be? What is the maximum stress on the part? What kind of stress is it (tensile, flexural, etc.)? How long will the load be applied? Will the load be continuous or intermittent? Does the part have to retain its dimensional shape? What is the projected life of the part or design?

Thermal Considerations What temperatures will the part see and for how long? What is the maximum temperature the material must sustain? What is the minimum temperature the material will sustain? How long will the material be at these temperatures? Will the material have to withstand impact at the low temperature? What kind of dimensional stability is required (is thermal expansion and

contraction an issue)?

Chemical Considerations Will the material be exposed to chemicals or moisture? Will the material be exposed to normal relative humidity? Will the material be submerged in water? If so, at what temperature? Will the material be exposed to steam? Will the material be painted? If so, what kind of paint? Will the material be glued? If so, what kind of adhesive will be used? Will the material be submerged or wiped with solvents or other

chemicals? If so, which ones? Will the material be exposed to chemical or solvent vapors? If so, which If

so, which ones? Will the material be exposed to other materials that can outgas or leach

detrimental materials, such as plasticizers or petroleum-based chemicals?

Bearing and Wear Considerations

Will the material be used as a bearing? Will it need to resist wear? Will the material be expected to perform as a bearing? If so, what will the

load, shaft diameter, shaft material, shaft finish, and rpm be? What wear or abrasion condition will the material see? Note: Materials

filled with friction reducers (such as PTFE, molybdenum disulfide, or graphite) generally exhibit less wear in rubbing applications.

Other Miscellaneous Considerations Will the part have to meet any regulatory requirements?

o FDA | USDA | Canada AG | 3A-Dairy | NSF | USP Class VI Is UL94 Flame retardant rating required? What level?

o 5VA | 5VB | V-0 | V-1 | V-2 | HB Should the material have a special color and/or appearance?

o Natural | White | Black | Other Colors o Color match to another part or material? o Window-Clear | Transparent | Translucent | Opaque o Smooth | Polished | Textured | One-Side or Both

Will the part be used outdoors? Is UV Resistance needed? Is static dissipation or conductivity important?

o Insulator | Static Dissipative | Conductive

ABS resins are hard, rigid, and tough, even at low temperatures. They consist of particles of a rubberlike toughener suspended in a continuous phase of styrene-acrylonitrile (SAN) copolymer. Various grades of these amorphous, medium-priced thermoplastics are available offering different levels of impact strength, heat resistance, flame retardance, and platability.

Most natural ABS resins are translucent to opaque, and they can be pigmented to almost any color. Grades are available for injection molding, extrusion, blow molding, foam molding, and thermoforming. Molding and extrusion grades provide surface finishes ranging from satin to high gloss. Some ABS grades are designed specifically for electroplating. Their molecular structure is such that the plating process is rapid, easily controlled, and economical.

Compounding of some ABS grades with other resins produces special properties. For example, ABS is alloyed with polycarbonate to provide a better balance of heat resistance and impact properties at an intermediate cost. Deflection temperature is improved by the polycarbonate; molding ease, by the ABS. Other ABS resins are used to modify rigid PVC for use in pipe, sheeting, and molded parts. Reinforced grades containing glass fibers, to 40%, are also available.

Related to ABS is SAN, a copolymer of styrene and acrylonitrile (no butadiene) that is hard, rigid, transparent, and characterized by excellent chemical resistance, dimensional stability, and ease of processing. SAN resins are usually processed by injection molding, but extrusion, injection-blow molding, and compression molding are also used. They can also be thermoformed, provided that no postmold trimming is necessary. (Because the material is not toughened, thermoformed shapes may crack during conventional trimming operations.)

Properties: ABS plastics offer a good balance of tensile strength, impact and abrasion resistance, dimensional stability, surface hardness, rigidity, heat resistance, low-temperature properties, chemical resistance, and electrical characteristics. These materials yield plastically at high stresses, so ultimate elongation is seldom significant in design; a part usually can be bent beyond its elastic limit without breaking, although it does stress-whiten. While not generally considered flexible, ABS parts have enough spring to accommodate snap-fit assembly requirements.

Impact properties of ABS are exceptionally good at room temperature and, with special grades, at temperatures as low as -40°F. Because of its plastic yield at high strain rates, impact failure of ABS is ductile rather than brittle. Also, the skin effect which, in other thermoplastics, accounts for a lower impact resistance in thick sections than in thin ones, is not pronounced in ABS materials. A long-term tensile design stress of 1,000 to 1,500 psi (at 73°F) is recommended for most grades.

General-purpose ABS grades may be adequate for some outdoor applications, but prolonged exposure to sunlight causes color change and reduces surface gloss, impact strength, and ductility. Less affected are tensile strength, flexural strength, hardness, and elastic modulus. Pigmenting the resins black, laminating with opaque acrylic sheet, and applying certain coating systems provide weathering resistance. For maximum color and gloss retention, a compatible coating of opaque, weather-resistant polyurethane can be used on molded parts. For weatherable sheet applications, ABS resins can be coextruded with a compatible weather-resistant polymer on the outside surface.

ABS resins are stable in warm environments and can be decorated with durable coatings that require baking at temperatures to 160°F for 30 to 60 min. Heat-resistant grades can be used for short periods at temperatures to 230°F in light load applications. Low moisture absorption contributes to the dimensional stability of molded ABS parts.

Molded ABS parts are almost completely unaffected by water, salts, most inorganic acids, food acids, and alkalies, but much depends on time, temperature, and especially stress level. FDA acceptance depends to some extent on the pigmentation system used. The resins are soluble in esters and

ketones, and they soften or swell in some chlorinated hydrocarbons, aromatics, and aldehydes.

Properties of SAN resins are controlled primarily through acrylonitrile content and molecular weight of the copolymer. Increasing both improves physical properties, at a slight penalty in processing ease. Properties of the resins can also be enhanced by controlling orientation during molding. Tensile and impact strength, barrier properties, and solvent resistance are improved by this control.

Special grades of SAN are available with improved UV stability, vapor-barrier characteristics, and weatherability. The barrier resins -- designed for the blown-bottle market -- are also tougher and have greater solvent resistance than the standard grades.

Applications: Molded ABS products are used in both protective and decorative applications. Examples include safety helmets, camper tops, automotive instrument panels, and other interior components, pipe fittings, home-security devices and housings for small appliances, communications equipment, and business machines. Chrome-plated ABS has replaced die-cast metals in plumbing hardware and automobile grilles, wheel covers, and mirror housings.

Typical products vacuum-formed from extruded ABS sheet are refrigerator liners, luggage shells, tote trays, mower shrouds, boat hulls, and large components for recreational vehicles. Extruded shapes include weather seals, glass beading, refrigerator breaker strips, conduit, and pipe for drain-waste-vent (DWV) systems. Pipe and fittings comprise one of the largest single application areas for ABS.

Typical applications for molded SAN copolymers include instrument lenses, vacuum-cleaner and humidifier parts, medical syringes, battery cases, refrigerator compartments, food-mixer bowls, computer reels, chair shells, and dishwasher-safe houseware products. Because of their compatibility with many higher-priced resins, SAN resins are also used as color-concentrate carriers for some engineering resins.

Re: ABS cost 01/17/2009 4:29 AM

Just like "oils aint oils", it's also true that "plastics aint plastic".

Abs comes in many different grades (mostly depending on thermal properties) and then the finess of pigment, UV stabilisers used, and filler and stiffening material.

Contact your local industry suppliers and get competitive quotes from all of them, then compare. (Try ICI, DUPONT, GE Plastics, or even do a web search for the material name.)

Indicative costs in $US you could get the following range of prices.

Reprocessed post consumer materials from around $1 per Kg plus freight content.

Basic material with high variation lot to lot, no stabilisers or additives around $4 per Kg.

Modest Engineering materials with some additive and flow modifiers and maybe some level of basic (non boutique) colour around $10 per Kg.

High heat, high additive, low volume specialty materials around $25 per Kg.

Special batch runs for colour matching and testing, around $100 per Kg unless you have long term purchase history with the supplier who will then usually absorb the cost into the overall business plan.

NOTE: Most plastic suppliers also write into sales price option for variation of contract for oil price variation and exchange rates for cross border supplies.

Can you please add your country details and such to your login details so that we can understand what country you are in? I'm in Australia, but we are the only place in the world that relates to $AUS, so that's why I put $US for the prices above as most people have ready reference to their local exchange rate to that currency base.

Just an Engineer is correct in stating that there are many different grades of ABS (Acrylonitrile Butadiene Styrene) and therefore, the prices will changes according to these variations.

Go to:

http://plasticsnews.com/resin-selector.html

http://plasticsnews.com/resin-pricing/all-resins.html

Resin pricing - Commodity Thermoplastics

For January 19, 2009

KEY: Historical Pricing Data

I-Annual volumes greater than 20 million pounds.II-Annual volumes of about 2 million to 5 million pounds.

Volume category

Resin/Grade I II

ABS

Injection Medium Impact 97 - 100 101 - 102

Injection High Impact 102 - 103 105 - 109

Injection Pipe fittings 87 - 88 91 - 95

Injection High heat 123 - 127 132 - 137

Injection Flame retardant 128 - 132 137 - 152

Extrusion Pipe GP 99 - 101 104 - 109

Extrusion Sheet GP 116 - 117 119 - 121

Blends PC/ABS -- 166 - 171

Blends Nylon/ABS -- 171 - 181

Prices are in U.S. cents per pound for prime resin, unfilled, natural color, FOB supplier, unless otherwise indicated.

indicates a market-price increase in our chart in the past week. indicates a market-price decrease in our chart in the past week.

P indicates a price increase for that material is pending. indicates a correction in the published price.

Prices are generated from interviews with North American buyers and suppliers. The information provided is based on sources believed to be reliable but its accuracy or timeliness is not guaranteed and no warranties of any kind are provided.Plastics News does not intend to specify the price of the materials listed. For price quotes on specific materials, contact the supplier.

Polypropylene

The commercial production of polypropylene in the United States began in 1957. That was the

same year that I started working in the plastics industry. Polypropylene and I have now become

senior citizens of the industry. As a result, I have a fondness for the material. It has been

interesting to watch a new material be introduced, develop, and find its place in the plastics

industry.

Polypropylene's (PP) place in the industry is second only to polyethylene. Approximately 15

percent of all of the plastic produced in this country is PP. In large quantities, PP sells for a

published price of $.33 to $.38/lb. With a density of only .903 g/cu cm, PP is the lightest weight of

any of the standard plastics. On a volume basis, PP costs only $.01 to $.013/cu in. This makes

PP the lowest-cost common plastic material suitable for the injection molding process. PP is a

large-volume, lightweight, low-cost plastic material that is well established in the industry.

This material's large volume and low cost caused it to be categorized as a commodity plastic

along with polyethylene, polyvinyl chloride, and polystyrene. This classification is, in my opinion, a

mistake. There is no scientific definition of a commodity plastic. To most plasticians the words

commodity plastic connote a low-cost, large-volume, low-performance material. But PP can be a

high-performance engineering material. Glass-fiber-reinforced PP has a tensile strength in the

same range as nylon.

Many applications that were molded using engineering plastics are now being converted to PP.

For example, nearly all automotive interior trim parts have now been converted from ABS to PP.

Many design engineers mistakenly classify PP as a low-performing commodity material and

overlook its ability to perform as an engineering plastic.

Over the years, many different types of PP have been developed for special applications such as

coatings, fiber, filaments, film, thermoforming, extrusion, and injection molding. All of the PPs can

be divided into two types: homopolymers and copolymers. The homopolymers are favored for

their average lower cost. The copolymers are various combinations of PP and ethylene. The

copolymers are chosen for their improved melt strength, clarity, and impact strength.

As the percent of comonomer increases, tensile strength, stiffness, heat deflection temperature,

and hardness decrease while impact strength increases. Within the various types of

homopolymers and copolymers, the primary distinguishing characteristics are molecular weight

and molecular weight distribution. In general, the higher-molecular-weight PPs are more resistant

to flow but exhibit improved physical properties.

These are easy-flow materials with melt index ranges of less than 1 to more than 35 g/10 min.

The production of fibers and film are the two largest markets for PP. Injection molding ranks third

in the amount of PP processed. PPs are easy materials to injection mold at low temperatures and

pressures. Molders must, however, take into account that PP is a semicrystalline material. A

properly injection molded part is 50 to 60 percent crystalline. The degree of crystallinity has an

effect on the physical properties of a molded part.

As the percent of crystallinity increases, there is a corresponding increase in the material's

density, tensile and flexural strength, mold shrinkage, and heat and chemical resistance. Impact

strength and transparency decrease. Mold shrinkage becomes less uniform. The formation of

PP's crystalline structure is relatively slow. Rapidly cooling a part in the mold results in a

reduction in crystallinity. Minor changes in mold cooling conditions can have a drastic effect on a

part's size and physical properties. Also, rapidly cooled PP parts continue to shrink long after they

are removed from the mold.

Polyethylene (PE) and PP compete for many of the same applications. There is overlap, but PP is

chosen when the application requires a little bit more stiffness and temperature resistance than

that provided by PE.

PP components range in size from micromolded medical and electronic parts weighing less than

a gram to a 9.8-lb minivan interior trim part that measures 87 by 26 inches.

Designing with PP

Part design requirements for the different types of PP are the same with the exception of wall

thickness. PP's wide range of melt indices (MI) must be taken into account. Not all PP can be

molded in the same size and thickness part. A 2-MI PP may not fill a part designed to be molded

in a 10-MI PP. See below for other design considerations:

Wall thicknesses can be as thin as .004 inch for small parts. This is pushing the limits, and a

better minimum wall thickness is .030 to .040 inch. Large PP filter plates have been successfully

injection molded with 3.5-inch-thick walls, but this is an exception. Considering PP's crystallinity

and high mold shrinkage factor, the maximum wall thickness should be limited to .250 inch.

Variation in wall thickness greater than 10 to 15 percent of the part's nominal wall thickness must

be smoothly blended from thick to thin.

Radiusing the corners of PP parts improves melt flow while producing a stronger part with

less molded-in stress. The minimum inside corner radius on a PP part should be at least 25

percent of the part's wall thickness. The stiffness and especially the impact strength of a PP part

can be improved by increasing the size of the radiuses up to 75 percent of the part's wall

thickness.

Draft angles and a good polish are important on PP parts due to their stiffness and high mold

shrinkage factors. A molding draft angle of 1º/side is recommended on inside surfaces that shrink

onto cores in the mold. A minimum draft of .5º/side is normally adequate on outside surfaces that

shrink away from the cavity. Larger draft angles may be required on deep-draw parts, or those

with a lot of geometry. Larger draft angles are always desirable as they result in parts that are

easier and, therefore, less costly to mold.

Projections of all types can be incorporated into PP parts. Their thickness at the junction with

the part's nominal wall should be limited to 50 percent of the part's wall thickness. In cases where

appearance and the absence of sink marks is critical, the thickness of projections can be reduced

to 40 percent of the part's wall thickness.

Depressions, or holes, of any size and shape can be easily molded with PP. The inside

corners of holes should be radiused to minimize molded-in stress. The easy-flow properties of

high-MI PP allow the molding of very small holes without the core-pin bending problems

associated with low-MI PP or other harder-flow plastics. With good venting and proper molding

conditions, good-looking, strong weldlines can be produced.

Tolerances are the same for all types of nonfilled or reinforced PPs. A 1.000-inch-long PP part with a .125-inch thickness can be molded to a commercial tolerance of +/-.007 inch. Longer dimensions require an addition of +/-.005 in/in. A fine tolerance would be +/-.0043 inch for the first inch plus +/-.003 in/in for each additional inch. The commercial tolerance can normally be achieved by any competent injection molder with no cost penalty. The fine tolerances usually result in longer molding cycles and increased cost. In some instances, even smaller tolerances can be achieved but only by mutual agreement between the molder and customer. The ideal tolerance is always the largest tolerance that produces a functional part.

Polyurethane

Extremely wide variations in forms and in physical and mechanical properties are available in polyurethanes. Grades can range in density from 0.5 lb/ft3 in cellular form, to over 70 lb/ft3 in solid form, and in hardness from rigid solids at 85 Shore D to soft, elastomeric compounds.

Polyurethane polymers, produced by the reaction of polyisocyanates with polyester or polyether-based resins can be either thermoplastic or thermosetting.

They have outstanding flex life, cut resistance, and abrasion resistance. Some formulations are as much as 20 times more resistant to abrasion than metals.

The noncellular grades - millable gums and viscous, castable, liquid urethanes are elastomeric thermoset types, processed by conventional rubber methods. These are discussed in the chapter, Thermoset rubber. Grades processed by thermoplastic methods are covered in the chapter, Thermoplastic elastomers. This chapter discusses the cellular polyurethane materials.

Polyurethane foams are thermoset materials that can be made soft and flexible or firm and rigid at equivalent densities. These foams, made from either polyester or polyether-type compounds, are strong, even at low density, and have good chemical resistance. Polyether-based foams havi greater hydrolysis resistance, are easier to process, and cost less. Polyester-based foams have higher mechanical propertie.% better oil resistance, and more uniform Cill structure. Both types can be rayed, sP' molded, foamed in place, or furnished'ir sheets cut from slab stock - buns, 30 to-A in. high, to 80 in. wide, and to 2013 ft 10 ft

A low-pressure molding process reaction-injection molding (RIM) is us most exclusively to produce ureth~ane some weighing as much as 100 lb. In the process, two or more highly reactive liquid systems are injected with high-pressure impingement mixing into a closed mold at low pressure, where they react to form a fin~ ished polymer. Depending on formulation, the polymer can be a rigid, integral-skin, microcellular urethane foam with a flexural tnodulus of over 100,000 psi, a soft, flexible elastomer with a flexural modulus as low as 7,000 psi, or a rigid structural foam having a density of 30 lb/ft3. Cycle time is short; parts can be demolded in less than a minute.

Reinforcement in the form of milled glass fiber, glass flake, or mineral filler increases the stiffness, thermal properties, and dimensional stability of RIM parts. Maximum glass content in reinforced reaction-injection molding (RRIM) is about 25% - a limit determined by the increased viscosity with increasing glass. Natural color of unpigmented RIM urethane parts is tan.

Flexible foams: Glass-transition temperatures (the temperature at which an elastomeric material becomes stiff and brittle) of flexible foams is well below room temperature. The foams can be pigmented to any color but, regardless of pigmentation, they yellow when exposed to air and light. Some types of flexible foams are excellent liquid-absorbing inedia, and can hold up to 40 times their weight of water.

Polyether-type foams are not affected by h igb -temperature aging, either wet or dry, but UV exposure produces brittleness and reduces properties. In use, these foams are always covered with a fabric or other material.

Most solvents and corrosive solutions decrease tear resistance and tensile strength and cause swelling of flexible foams. Swelling is not permanent, however, if the solvent is removed and the foam dried. However, the foams can be destroyed by strong oxidizing agents and hydrolyzed in strong acids or bases. Generally, the polyether foams are more resistant to hydrolytic degradation: the polyester foams are more resistant to oxidative attack.

Applications for polyester flexible urethane foam include gasketing, air filters, sound-absorbing elements, and clothing interliners (laminated to a textile material). The polyether types are used in automobile and recreational -vehicle seats, carpet underlay, furniture upholstering, bedding, and packaging.

Rigid foams: Bases for rigid foams are polymers having glass-transition temperatures higher than room temperature. The cells of rigid foam are about the same size and uniformity as those of flexible foam, but rigid foams usually consist of 90% closed cells. For this reason, water absorption is low. Compressing the foam beyond its elastic limit damages the cellular structure.

Rigid foams are blown with either carbon dioxide or fluorocarbons. Gas generated by vaporizatioq_of fluorocarbons, entrapped in the closed cells, gives the foam a very low thermal conductivity of 0.11 to 0.14 Btuin./hr-ft°- °F. Conductivity increases with age, however, to a constant value of aboul 0. 16. Conductivity Of C02-blown foam starts at about 0.22 Btu-in./hr-ft'-'F.

Properties of rigid foams vary with density and formulation. Compressive strength of a 2 lb/ft' foam is 30 to 40 psi for a polyether type and 25 to 40 for a polyester type (parallel to the direction of foam rise). The values increase in a 12 lb/ft° foam to 560 and 420 psi. Strengths are lower, by about 50%, in the direction perpendicular to foam rise.

Rigid urethane foams are used for thermal insulation of refrigerators, refrigerated trucks and railroad cars, cold-storage warehouses, and process tanks because of their low conductivity and high strength-toweight ratio. Other applications include flotation devices, encapsulation, structural and decorative furniture components, and sheathing and roof insulation for buildings.

Integral-skin foam: Urethane foams that are formed with integral skins range from soft and flexible types to impact-absorbing grades and rigid foams used in structural parts. Color can be added, but since the foams yellow on aging, black is most practical for the surface color. If other colors are required, coatings are recommended. The tough, high-density, integral skin is formed against the mold surface and the low-density core is produced by a blowing agent - usually a fluorocarbon.

Elastomeric foams of this type are used in automotive bumper and fascia systems and, most recently (reinforced with milledglass fibers), in fenders and

other exterior body panels. The semirigid types are used in athletic protective gear, in automotive crash-protection areas, horn buttons, sun visors, and arm rests. Applications for the rigid structural foams include housings for computer systems, chair shells, furniture drawers, and sports equipment.

PVC

Among the vinyl polymers and copolymers, the polyvinyl-chloride (PVC) thermoplastics are the most commercially significant. With various plasticizers, fillers, stabilizers, lubricants, and impact modifiers, PVC is compounded to be flexible or rigid, opaque or transparent, to have high or low modulus, or to have any of a wide spectrum of properties or processing characteristics.

PVC resin can also be chlorinated (CPVC) and it can be alloyed with other polymers such as ABS, acrylic, polyurethane, and nitrile rubber to improve impact resistance, tear strength, resilience, heat-deflection temperature, or processibility.

PVC compounds are processed by extrusion, injection molding, calendering, compression molding, and blow molding. PVC coatings are applied by fluidized-bed and electrostatic powder-coating methods. The resins are also used for dip molding and coating, in the form of plastisols and organosol dispersions or water dispersions (latexes). Cellular PVC products are made by introducing gas into the resin during molding or extrusion. Foams can be open or closed cell, and can be elastomeric or rigid, depending on plasticizer content.

PVC compounds can be made waterwhite in flexible compounds, very clear in rigid compounds, and they can be pigmented to almost any color.

Properties: With so many property variations attainable by compounding methods, no single compound can be considered typical of polyvinyl chloride. For example, creep rate of rigid compounds is so low and predictable that they can be used to make pressure pipe for water distribution; flexible compounds can be soft enough, yet impermeable, so that they are used for baby pants and for an excellent imitation suede, or they can be transparent, nontoxic, and tough enough to be used for mineral-water bottles.

Rigid PVC, sometimes called the "poor man's engineering plastic" is a hard, tough material that can be compounded to a wide range of properties. Noteworthy among its properties is low combustibility; it has high resistance to ignition and is self-extinguishing. It also provides good corrosion and stain resistance, thermal and electrical insulation, and weatherability. However, PVC is attacked by aromatic solvents, ketones, aldehydes, naphthalenes, and some chloride, acetate, and acrylate esters. Some impact modifiers used in rigid PVC reduce chemical resistance. In general, normal-impact grades have better chemical resistance than the high-impact grades.

Most PVC compounds are not recommended for continuous use above 140°F. Chlorination increases heat-deflection temperature, flame retardancy, and density and extends the continuous-use temperature to 176 to 212°F, depending on the amount of chlorination.

By Design: Polyvinyl chloride part design

By Glenn Beall

Published: December 4th, 2002

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his

special perspective on issues important to design engineers and the molding industry.

During World War II, Germany could miraculously restore fresh water in a matter of hours to cities

that had been demolished by bombs. Their secret was easy-to-assemble plastic pipe. Plumbing,

along with film and electrical insulation, were the first uses for polyvinyl chloride (PVC) with

commercial production starting in Germany in 1931.

Throughout the war, PVC was treated as a military secret. These early PVCs were rigid, brittle,

heat-sensitive materials that could be compression molded, calendered, and extruded. They

were, however, extremely difficult to mold using the rudimentary ram injection molding machines

available at the time.

Commercial applications for PVC in the United States were limited until 1926 when B.F.

Goodrich’s Waldo Semon discovered that the material could be softened with a plasticizer. These

softened PVCs found large markets in the plumbing, construction, and electrical industries. Film,

blowmolded, and thermoformed packaging applications were developed and the B.F. Goodrich

Co. prospered. (Waldo Semon also invented bubble gum, which enjoyed faster customer

acceptance than PVC.)

Today PVC is the third-largest-volume material after polyethylene and polypropylene.

Approximately 15 percent of all plastic used in the U.S. is PVC. Large quantities of general

purpose PVC now sell for $.32/lb.

The material’s large volume and low cost classify it as a commodity plastic. This is a

misconception, however, as PVC has mechanical properties that rival ABS. There are three

different types of PVC with many special grades within each type.

Plastisols are liquid suspensions of PVC powder in a plasticizer. The plastisols are not suitable

for IM, but they are widely used for casting, coatings, slush, and rotational molding.

Flexible PVCs find their widest usage in pliable film, electrical insulation, flexible garden hose,

and medical tubing. Large quantities of these materials are injection molded into medical,

electrical, and special commercial products. Flexible PVC is a low-cost alternative for

thermoplastic elastomers. These materials can be tricky to injection mold. However, some

processors have prospered by specializing in this area.

Rigid PVCs are the most interesting injection moldable members of this family of materials. The

first PVCs were rigid but their brittleness limited their use. Their high viscosity and heat sensitivity

made them difficult to injection mold.

Bad Reputation

Many uninformed processors treated PVC the same as the polystyrene, acrylic, and nylon

materials with which they were already familiar. In some cases they thermally degraded the PVC,

producing a corrosive gas that attacked molds and other bare metal surfaces. As a result, PVC

developed an unjustified reputation as a difficult and dangerous material to injection mold.

The remnant of that bad reputation lingers on even today. I spent 10 of my early years in the

medical device industry, where injection molding of PVC was a routine occurrence. I have never

considered PVC difficult or dangerous. It was just a material that had to be processed differently.

In the ensuing years polymer chemists have made impressive improvements in PVC plasticizers

and stabilizers. The advent of the inline reciprocating screw injection molding machines in the

1950s and improvements in screw design and temperature controls have rendered today’s PVCs

much easier to mold than in the early years.

Defining Characteristics

Rigid PVC’s good resistance to water and weathering, plus its self-extinguishing characteristics,

account for its wide use in the electrical and construction industries. Most rigid PVCs contain

fillers that make them opaque; however, a few transparent grades are now available.

PVC is the lowest-cost plastic with a tensile strength of up to 7400 and a flexural modulus of

540,000 psi, with a minimum notched Izod impact strength of -1.0 and a maximum of 17.0 ft-lb/in.

Fiber-reinforced grades have a tensile strength in the range of 15,000 psi.

Rigid PVC is specified for many load-bearing applications, but its maximum heat deflection

temperature of only 180F at a 264-psi loading limits its use. There are always exceptions and

some stress-free injection molded parts have withstood steam autoclaving at 285F.

Designing with PVC

The rigid PVCs are low-mold-shrinkage, amorphous materials with a high molecular weight.

Wall thickness on small injection molded parts can be less, but .050 inch is a good minimum

for PVC. Some PVCs have flow lengths of 40 inches with thicknesses in the range of .150 to .200

inch. The maximum thickness can be whatever is required, but .250 inch is a good upper limit.

Radiusing inside corners less than 40 percent of the wall thickness produces high levels of

molded-in stress. Larger radiuses are easier to mold into stronger parts with improved impact

strength and reduced warp.

Draft angles of ½° can be specified on draws up to 1 inch. Larger draws require a minimum of

1° per side. These are rigid materials and a smooth polish is desirable.

Projections of all types can be specified. Sink marks can be eliminated and molded-in stress

minimized if the thickness of projections is 50 percent of the adjoining wall. The maximum

thickness should be limited to 75 percent of the thickness of the wall to which they are attached.

Depressions, or holes, can have good-appearing, strong weldlines with proper molding

conditions. These are rigid materials and holes require draft angles and smoothly contoured

surfaces. Since rigid PVC is a hard-flow material, the depth of holes must be limited to two to

three times the thickness of the core to avoid the bending of small, unsupported core pins.

Tolerances on a 1-inch-long, .125-inch-thick PVC part can be ±.008 inch. Rigid PVCs have

mold shrinkage of only .001 to .005 in/in. With some grades a fine tolerance can be ±.004 inch.

Rigid PVC is an underused material. Engineers searching for a less-costly alternative for a low-

temperature ABS, PPO, or polycarbonate application should consider PVC.

By Design: Polyethylene part design

By Glenn Beall

Published: March 31st, 2002

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his

special perspective on issues important to design engineers and the molding industry.

In 1933, scientists at Great Britain's Imperial Chemical Industries noted traces of a waxy, white

substance on the inside of a pressure vessel. This was the precursor to the polyethylene (PE)

material that would one day dominate the plastics industry. During the second World War, this

new material was rushed into service as electrical insulation, which became scarce as enemy

forces overran the rubber plantations in the Pacific.

During the war, all PE was allocated for military purposes. Following the war, PE became

available for commercial use at a price of $5.00/lb. Large quantities of PE now sell for only $.32 to

$.56/lb. Today PE is the largest volume material accounting for approximately one-third of the

plastic material used in the U.S.

Polyethylene is actually a whole family of similar materials. The first materials were low- and

medium-density polyethylene (LDPE and MDPE). High-density polyethylene (HDPE) was

discovered in 1957 and linear low-density

polyethylene (LLDPE) was introduced in Canada in 1965. Crosslinked polyethylene (XLPE) and

ultrahigh-molecular-weight polyethylene (UHMWPE) have now joined the family, but these two

materials are difficult to injection mold.

Each PE is unique, but like any family, all of its members have similar characteristics. On the

positive side, all PEs are low in cost, light in weight, and have good impact and chemical

resistance. Many meet FDA and NSF requirements. Of special interest to injection molders is

PE's thermal stability and ease of processing at low temperatures and pressures.

On the negative side, PEs are limited by their relatively high mold shrinkage factor, lack of

stiffness, and low temperature resistance. In spite of these limitations, PE has captured a giant

share of the market.

Molecular Structure Dictates Properties

Any PE with a density of .191 to .925 g/cu cm is classified as an LDPE. A material with a density

of .941 to .959 would be HDPE. The MDPE and LLDPE materials have densities between these

two. In general, as the density of a PE increases, tensile strength, stiffness, heat deflection

temperature, hardness, surface gloss, mold shrinkage, permeation, and chemical resistance also

increase; elongation, impact strength, and environmental stress crack resistance decrease.

All PEs are composed of long molecular chains of carbon and hydrogen. The HDPE molecules

are basically linear with few side branches. The LDPE molecules have approximately 10 times

more side branches than HDPE. These side branches become entangled with the branches on

other molecules. This intermolecular entanglement accounts for LDPE's increased elongation and

impact strength. The LLDPE's molecules also have a lot of side branches, but these branches are

shorter and arranged in an orderly manner along the length of the molecules. The physical

properties of LLDPE are, in general, between HDPE and LDPE.

The lack of side branches on HDPE allows these molecules to align close to each other. This

tight packing of the molecules encourages the formation of crystalline structures in the material.

Properly molded HDPE is 70 to 90 percent crystalline. The side branches on LDPE do not allow

the molecules to pack tightly together, and this discourages crystallinity. A properly molded LDPE

has crystallinity in the range of 45 to 65 percent. Generally speaking, as crystallinity increases

there is a corresponding increase in the material's density, tensile and flexural strength, mold

shrinkage factor, and heat and chemical resistance. Impact strength and transparency decrease;

mold shrinkage is nonuniform.

The degree of crystallinity of an injection molded part can be changed by the way the material is

molded. Molten PEs do not have crystalline structures. If the PE is allowed to cool slowly in the

mold, the crystals reform. If the material is cooled quickly, the crystals do not have time enough to

reform. Minor changes in the molding process can have a significant effect on the degree of

crystallinity and the physical properties of a molded part. These changes in the degree of

crystallinity account for some of the batch-to-batch variation in injection molded PE parts.

All of the PEs are known for their good melt flow properties, but some members of the family flow

better than others. The HDPE molecules are approximately 50 times longer than LDPE

molecules. The shorter the molecule, the easier it is to inject the melt through tiny gates and

restricted cavities. Large, thin-walled parts can be molded in LDPE; however, this material's

relatively low physical properties may favor the use of a higher-density PE.

Designing with PE

Most design engineers proportion all PE parts the same, independent of which member of the

family will be used to mold the part. This is a mistake, as each member of the family responds to

the injection molding process in a different way.

Following are a few factors to consider when when designing with the different PEs:

Wall thickness determinations override all other considerations. Thickness is determined

by the functional requirements of the product and molding considerations. Functional

requirements must, of necessity, take priority over ease of molding. If the product does

not function properly no one will buy it and there will not be any molding problems.

Always remember that nothing happens until somebody buys something.

It is difficult to be definitive about the maximum and minimum allowable wall thickness

for PEs. Micromolding and thin-wall molding have redefined the minimal allowable wall

thickness. Setting aside these two special molding techniques, LDPE can be molded

as small parts with thicknesses of only .010 inch. HDPE parts are difficult to mold with

thicknesses of less than .020 inch. These are the minimum thicknesses for these two

materials; however, a better thickness would be in the range of .030 to .040 inch.

Wall thicknesses of up to 3 inches have been compression and flow molded with PE.

These materials are, however, at their best with thicknesses of .250 inch or less. Their

high mold-shrinkage factor and low thermal conductivity make thicker walls costly and

more difficult to mold. The lower mold shrinkage factor and reduced crystallinity of

LDPE make it the better of the two for thick-walled parts.

Once a wall thickness has been chosen, every effort must be made to maintain that

same wall thickness throughout the part. Thicker walls stay hot longer and shrink more

than thinner walls. The high mold shrinkage of PE is

troublesome in

this regard. An acceptable wall thickness variation in a

PE part would be 10 percent. A 15 percent change in

wall thickness begins to cause nonuniform melt flow

and variations in mold shrinkage. In instances where

greater thickness variations cannot be avoided, the thick to thin walls must have a

gradual transition in thickness.

Radiusing the corners on PE parts improves melt flow and minimizes molded-in stress.

Sharp corners are stress concentrators. Rounded corners are stronger and more

resistant to impact type forces. The minimum inside radius for a PE part should be

equal to 25 percent of the part's wall thickness. Smaller radiuses result in increased

levels of molded-in stress. Maximum part strength is achieved with an inside radius of

75 percent of the part's thickness.

The highly branched LDPE molecules have excellent impact strength, and they can be

designed with radiuses near the low end of the radius size range. The lower impact

strength of HDPE requires radiuses near the maximum side of the range.

Draft angles reduce ejection forces and minimize the cooling part of the molding cycle.

Draft angles are desirable on all molded parts. However, the slippery surfaces on

HDPE allow many parts to be molded without draft. With HDPE the best results are

achieved with polished cores and cavities and molding draft angles of 1/2 to 1º per

side.

The softness of LDPE requires that these parts be thoroughly cooled to develop

strength enough to resist the force of ejection. A molding draft angle of 1º per side is

beneficial. Highly polished surfaces encourage LDPE to stick to cores and cavities. In

many cases a light mat finish or a liquid honed surface improves release from the mold.

This, in turn, allows the parts to be molded on shorter cycles.

Projections of all types can be incorporated into PE parts. The high mold shrinkage of all

PEs dictates that the thickness of stiffening ribs, bosses, gussets, and other projections

be limited to 50 percent of the part's nominal wall thickness. Thicker projections

produce an unacceptable wall thickness at the junction of the projection and the part's

nominal wall. This increase in thickness encourages sink marks, molded-in stress,

warpage, and longer cycle times. The higher mold shrinkage factor of HDPE is more

troublesome in this regard than LDPE.

Depressions, or holes, are easy to mold in all PE materials. The excellent flow

properties of PE produce good-looking, strong weldlines. The increased level of

crystallinity of HDPE creates more weldline problems than with the same part molded

in LDPE.

The low injection pressures that are possible with PE allow the molding of very small

holes without the core pin bending problems associated with the molding of the harder-

flow plastic materials.

Tolerances are difficult to quantify as they are dependent on many interrelated factors.

As a general guide, a 1-inch-long part with a .125-inch thickness can be held to

+/-.0080 inch with LDPE and +/-.0085 inch with HDPE. A fine tolerance would be

+/-.0045 and +/-.0070 inch, respectively. The larger tolerance of HDPE is due to the

material's higher crystallinity and mold shrinkage factors.

The general tolerance can normally be achieved by any competent injection molder

with no cost penalty. The fine tolerances normally result in longer molding cycles and

increased cost. In some instances, even smaller tolerances can be achieved, but only

by mutual agreement between the molder and customer. The best tolerance is always

the broadest tolerance that produces a functional part.

Designing parts to accommodate the subtle differences in the various members of the

PE family results in stronger parts that are more economical to produce. This improved

molding efficiency may be just what you need in order to survive next year's 5 percent

mandatory cost reduction.

By Design: Designing with polycarbonate

By Glenn Beall

Published: December 31st, 2004

If you need a transparent engineering plastic with high heat and impact resistance, polycarbonate

is a good choice.

If you handle a compact disk today you will be touching polycarbonate (PC). To date PC is the

only material capable of filling the demanding requirements of that application.

In 1859 the Russian chemist Butlerov described a PC type of material. This discovery was

repeated by Einhorn in the late 1800s. Fifty years passed before these discoveries were seriously

pursued by General Electric in the U.S. and Farbenfabriken Bayer in Germany. Both announced

pilot plant quantities in 1956. One hundred years after Butlerov’s discovery, Bayer was producing

commercial quantities under the trade name Makrolon. GE followed with Lexan in 1960 and Dow

introduced Calibre in 1984.

There was a waiting market for this unique plastic material. Worldwide consumption reached 40

million lb/year in 1970, and 218 million lb in 1980. This rapid growth is testimonial to the material’s

usefulness. Today PC is second only to nylon in volume and is now the fastest-growing

engineering polymer.

Designing Characteristics

Polycarbonate is an amorphous thermoplastic that combines transparency with high temperature

and impact resistance. There is no other engineering plastic with this combination of properties.

There are different types of PC, but a high-viscosity grade can be defined as follows.

Physical properties are around a tensile strength of 9000 psi; flexural modulus is 340,000 psi,

with a heat deflection temperature of 270°F at a 264-psi loading. Strength and temperature

resistance can be increased with the use of fillers and reinforcements.

Most polycarbonates have a notched Izod impact strength in the range of 12 to 17 ft-lb/in, which

is retained at low temperatures. Polycarbonates are highly notch-sensitive materials. They

actually have higher impact strength than indicated by the standard notched Izod impact test. For

example, a .125-inch-thick, right-angled-shaped part with an inside corner radius of .010 inch had

an impact strength of 2.5 ft-lb. Increasing that radius to .020 inch resulted in an impact strength of

20.2 ft-lb. In other words, doubling the size of the radius increased the impact strength by a factor

of eight. This is the reason why designers make such a fetish of radiusing the corners on PC

parts.

Light transmission is 86% to 89%. This is just below acrylic at 91% to 92% and general purpose

polystyrene at 88% to 91%.

Flame-retardant grades are available with a UL 94 V-0 and 5V ratings.

Polycarbonate has an excellent balance of physical properties, but it lacks the chemical

resistance of semicrystalline polymers. Specifying PC requires careful attention to the chemical

environment of the application.

Polycarbonate is also available alloyed with ABS, acrylic, polyetherimide, polyurethane, PBT, and

PET polyesters. The base polymer and all of these alloys can be tailored for specific applications

with the addition of fillers and/or fiber reinforcement.

Polycarbonate fills the gap between ABS and PPO and the higher-temperature-resistant and

more costly materials such as polysulfone, polyetherimide, polyphenylene sulfide, and liquid

crystal polymers. The list price for injection molding grades of PC was $1.96/lb in 1988. Today the

material costs $1.38 to $1.65/lb, or an average of $.065/cu in. These prices are, however,

increasing.

Applications

GE is a major user of electrical insulating materials. The good electrical properties of PC are one

of the reasons why GE originally pursued the development of this material. A favorable UL rating,

plus low smoke and corrosive gas emissions, accounts for PC’s use in telephones, computers,

printers, copiers, other business machines, and laboratory and diagnostic equipment.

The combination of transparency, coupled with weatherability and impact strength, allows PC to

be used for bulletproof windows, machine guards, lighting applications, and as window panes,

especially in those instances where vandalism is a problem. Other transparent applications

include greenhouse glazing, optical safety lenses, solar collectors, and automobile head and

taillight lenses. A large new application just now being commercialized is side and rear car

windows.

This FDA-sanctioned material has a long history in the food handling industry as processing

bowls, mugs and glasses, tableware, pitchers, storage containers, baby and water cooler bottles,

and in some instances as microwave cookware. Many of these applications rely on PC’s

transparency and temperature resistance.

The amount of PC used in the construction industry is second only to

PVC. Other markets include medical products, toys, portable tools, and

photographic and sporting equipment, especially in applications where

low-temperature impact strength is important.

PC part design tips

 

Wall thicknesses of .012 to more than 1 inch have been molded with PC. However, a

minimum of .050 inch and a maximum of .375 inch are recommended, with .125 inch

being ideal. Flow lengths of 2 inches with a .030-inch thickness and a 16-inch flow can

be achieved with a .125-inch wall. With proper blending, wall thickness variations can

be as much as 25%. At thicknesses somewhere between .140 and .160 inch, PC’s

room temperature notched Izod impact strength declines with a change from a ductile

to a brittle type of failure.

Radiusing corners improves melt flow and allows notch-sensitive PC to develop its

impressive impact strength. An inside corner radius of 60% of the part’s wall thickness

is ideal. The absolute minimum is .020 inch.

Molding draft angles of ½° to 1° per side will suffice for most PC parts, but there are

exceptions.

Projections of all types are molded with PC. In order to avoid sink marks and molded-in

stress, the thickness of projections should be limited to a maximum of 60% of the part’s

wall thickness.

Depressions, or holes, create weldlines. Acceptable appearance and tensile strength

retention of 99% can be achieved with proper molding conditions. Glass-fiber-

reinforced PC can suffer a loss of 35% to 45% of its tensile strength at weldlines. Holes

require standard molding draft and corner radius considerations. Limiting the depth of

holes to two to three times the thickness of the core pin eliminates pin deflection.

Tolerances on PC parts can be as small as ±.0025 inch on a .125-inch-thick, 1-inch-long

part. A standard, less-costly tolerance on the same part would be ±.004 inch. This

amorphous material, which shrinks uniformly parallel and perpendicular to the direction

of flow, is frequently specified for precision parts requiring a minimum allowable

warpage.

By Design: Designing with nylon – Part 1

By Glenn Beall

Published: January 31st, 2006

The oldest engineering plastic is still the largest-volume high-performance polymer.

The first thermoplastic material with physical properties good enough to be considered an

engineering material was nylon. This wonderful, versatile material traces its roots to a low-

molecular-weight caprolactam polymer discovered in Germany by J. Von Braun in 1907. No

commercial use was found for that material.

In 1935, DuPont’s Wallace Carothers successfully polymerized higher-molecular-weight

caprolactam polymers, leading to the development of nylon 6/6. In 1938, I.G. Farben introduced

nylon 6. Nylons 11 and 12 followed in 1949 and 1966.

DuPont’s original research was aimed at developing a synthetic polymer suitable for textile fiber.

The first commercial application was in the bristles of Dr. West’s miracle toothbrush in 1938. This

was the beginning of the end of animal hair brushes that started in China with the 1498

description of a toothbrush “with hog bristles perpendicular to a handle of bone.” Nylon fiber

proved to be stronger than silk. This led to the 1939 introduction of nylon stockings. Sixty-four

million pairs were sold in the first year of production. The Chinese learned to harvest and weave

silk around 3000 BC. Nylon would take the large hosiery market away from silk, but not until after

it had helped win World War II. Glider tow ropes, parachutes, tire cording, and life rafts were

important military applications.

Molding grades of nylon appeared in 1941 and were immediately classified as strategic war effort

materials. The first application was for coil bobbins, gears, and bearings. Following the war, some

processors made a specialty of molding nylon, which quickly dominated the high-performance

part of the thermoplastics market. By 1960, 25 million lb of nylon were sold in the U.S., and this

quantity increased dramatically with each decade (see chart). Today molded nylon is the most

frequently specified engineering plastic.

Defining characteristics

Nylons are semicrystalline, thermoplastic materials known for their tensile strength, temperature,

and chemical resistance. Abrasion resistance coupled with a low coefficient of friction are

important characteristics of nylon. Flame-retardant grades are available with UL 94 V-2 ratings.

A few of the less crystalline grades of nylon have a slight yellow haze but good see-through

ability. Adding transparency to nylon’s other properties increases its usefulness. None of the

other transparent plastic materials can provide nylon’s combination of properties. Light

transmission can be 85-90%. These are not the best materials for optical lenses, but their clarity

is good enough for many products that require transparency.

No two nylons are the same. The most frequently injection molded nylon is 6/6, which has an as-

molded tensile strength of around 13,000 psi, a flexural modulus of 420,000 psi, a notched Izod

impact of only .7 ft-lb/in, and a heat deflection temperature of 220°F at a loading of 264 psi. All of

these properties can be enhanced by reinforcing the material with different types of fibers and

fillers. Nylon is the largest-volume fiber-reinforced thermoplastic material. That important topic will

be the subject of a future By Design article.

At the end of 2005, the market selling price for truckload quantities of nylon 6/6 was $1.61/lb and

$.066/in3. It is interesting to note that the physical properties of nylon 6 are approaching nylon 6/6

values. The major difference between the two is that nylon 6 is $.14/lb lower in cost. Nylon 6 has

a slightly improved impact strength and a heat deflection temperature that is as much as 70 deg F

lower than nylon 6/6. As the best-known injection molding grade, nylon 6/6 is frequently specified

for low-temperature applications that require the material’s other properties. Nylon 6 can

sometimes be substituted in applications that do not require nylon 6/6’s higher heat deflection

temperatures. That material change can result in a cost reduction of $.14/lb.

Understanding nylon

The chemical name for this family of materials is polyamides, but all of the suppliers refer to them

as nylon. The different materials are identified as nylon followed by a number such as 6 or 12.

This number indicates the number of carbon atoms in the original polymerization monomer. A

material such as nylon 6/6 or 6/12 would result from polymerizing two monomers with different

molecular structures.

All of the different nylons have varying physical properties. In general, as the number of carbons

between amide linkages increases, tensile strength and stiffness decrease as impact strength

increases.

All nylons absorb varying amounts of moisture from the atmosphere. Moisture absorption

decreases with an increase in the number of carbon atoms between amide linkages. A fully

saturated nylon 6/6 can absorb 2.8% of water. The absorbed moisture acts as a plasticizer that

softens the material. For example, the tensile strength and stiffness of nylon 6/6 are reduced to

9400 and 320,000 psi respectively, while impact strength increases to 1.2 ft-lb/in.

One disadvantage of nylon is that the absorption of moisture also results in an increase in linear

dimensions. This is unsettling to the uninitiated. Experienced molders have learned to size

cavities to allow for this moisture-related increase in dimensions. Another negative is that nylon

must be molded in its dry as-received condition or be dried before molding. A just-molded nylon

part is dry and relatively brittle. As the nylon absorbs moisture, it regains its characteristic

toughness. The length of time required to reach equilibrium depends on humidity, temperature,

and the part’s thickness. Material suppliers provide literature defining these relationships.

Nylon is not an ideal material for just-in-time delivery. Just-molded nylon parts are undersized and

may fail due to impact loads applied during assembly, or the part’s inability to flex into position. In

the majority of cases, nylon parts must be moisture conditioned before they are used.

The properties of nylon are also affected by the amount of crystallinity in the material. The degree

of crystallinity is dependent on part design and processing conditions. A thick-walled part that

cools slowly can be 50-60% crystalline. The crystallinity of a rapidly cooled thin-wall part may be

only 10%. Generally speaking, as crystallinity increases, tensile strength, stiffness, mold

shrinkage, and heat and chemical resistance also increase. Impact strength and moisture

absorption decline. It is important to recognize that minor variations in molding conditions can

have a major effect on the size and physical properties of a nylon part.

Part two of the nylon story will review its many uses and the guidelines for designing injection

molded nylon parts.

By Design: Designing with nylon – Part 2

By Glenn Beall

Published: March 31st, 2006

Applications and guidelines for nylon use.

Nylon, the oldest of the engineering thermoplastics, was introduced by DuPont at the 1939

World’s Fair in New York City. Since that time nylon has grown to become the largest-volume

engineering material. The attributes that account for this material’s success were discussed in the

February 2006 “By Design” article (imm net.com/articles/2006/February/2785).

This article is concerned with injection molded nylon applications and design details. Nylon

covers a wide range of products, but its largest market is for fiber. Products include hosiery,

undergarments, brush bristles, rope, heavy-duty industrial fabric, and, believe it or not, Americans

annually consume 3 million miles of nylon dental floss.

Part design tips

Wall thickness. Nylon is an easy-flow material. This allows it to be molded in thinner sections

than other materials with similar physical properties. For example, a 48-inch-long part can be

molded with a .190-inch-thick wall. Walls as thin as .010 inch can be molded, but demonstrate an

increase in shear and molecular orientation, which can result in nonuniform physical properties. A

better minimum wall thickness is .040 inch, which can provide a flow length of around 10 inches.

Nylon is a high-mold-shrinkage-factor crystalline plastic. It is not ideal for molding parts with

thicknesses greater than .250 inch. Thicker walls are susceptible to uncontrollable mold

shrinkage, internal voids, and molded-in residual stress.

Variations in wall thickness should be limited to 10-15% of the part’s

nominal wall thickness and be smoothly blended from thick to thin.

Corner radiuses. Nylon is a notch-sensitive material. Sharp inside corners prevent it from

achieving its characteristic toughness. Maximum strength, impact resistance, and ease of flow

are achieved with an inside corner radius equal to 75% of the part’s wall thickness. The minimum

allowable inside corner radius is .020 inch.

Molding draft angles. Nylon is self-lubricating and frequently can be molded without draft angles.

Many bearings and gears must be molded straight. However, nylon parts are easier to mold on

shorter cycles if they are provided with 1?2-1° draft angles per side.

Projections. Many functional features, such as stiffening ribs, solid bosses, and snapfit latches,

can be molded as projections off of a part’s nominal wall. Their thickness at the junction with the

part should be limited to 50% of the part’s wall thickness. In cases where appearance and the

absence of sink marks is critical, projections can be reduced to 40% of the part’s wall thickness.

Depressions and holes. The main problem associated with depressions is that they create

weldlines. The core pins that form small holes are difficult to cool and are susceptible to bending.

Weldlines can weaken a part while creating cosmetic problems. A properly molded nylon 66 part

can have visible but commercially acceptable weldlines while retaining 95% or more of its original

tensile strength.

Nylon’s ease of flow allows the use of low injection pressures. These lower pressures reduce the

bending forces on the core pins that form small holes. All inside corners on holes should have

standard radiuses. As the material shrinks, it grips the core pins. Providing molding draft on these

core pins reduces ejection force, which allows shorter molding cycles.

Tolerances. As a general rule, the dimensional reproducibility of a plastic material is dependent

on the polymer’s mold shrinkage factor. The higher the shrinkage, the broader the tolerances.

Nylon is an exception. In spite of its high shrinkage factor, nylon is specified for precision parts

such as gears, bearings, and aerosol valves. The tolerances that can be maintained are

dependent on the part’s wall thickness. In one series of experiments, a .031-inch-thick nylon 66

part shrank an average of .010 in/in; a .125-inch part shrank .015 in/in; and a .250-inch part

shrank .022 in/in. In other words, smaller tolerances can be held on thin parts than on thick-

walled parts.

A “commercial” tolerance on a 1-inch-long, .125-inch-thick nylon 66 injection molded part is

±.0046 inch. A more costly “fine” tolerance on the same part is ±.0023 inch.

Nylon is the most often specified fiber-reinforced thermoplastic material. Considerations for

designing with reinforced plastic will be the last part of this series.

By Design: Designing with nylon - Part 3

By Glenn Beall

Published: May 31st, 2006

Nylon with reinforcement differs from straight resin in simple ways, such as volume per pound,

and in more complex ways, such as performance properties and gate positioning.

Reinforcing a material to improve its performance is at least as old as adding straw to mud to

produce adobe building blocks. These bricks were more durable than those made of mud alone.

Injection molders reinforce plastic material for the same reason. It was the reinforcing of

thermoplastics that allowed the replacement of metal in heavy load-bearing applications. The

modern reinforced plastics industry as we know it today started in 1935 when the Owens Corning

Fiberglass Co. learned how to economically produce continuous glass filament. The first

applications relied on thermosetting material as the matrix to bind together and shape the glass

fibers.

Injection moldable, reinforced thermoplastics were introduced in the early 1950s by the Fiberfil

Corp. (now DSM Engineering Plastics). The first application was a military land mine housing.

The transportation industry, which became the largest market for reinforced plastics, started with

a Lincoln Continental instrument panel.

Large quantities of reinforced thermoplastics are now being injection molded, but these articles

have never discussed how to design with those materials. Nylon is the largest-volume reinforced

thermoplastic material. It, therefore, seems appropriate to discuss reinforced thermoplastics in

this third, and last, part of the nylon series.

Some definitions

Today people erroneously use the words fillers and reinforcements interchangeably.

Reinforcements improve the physical properties of a plastic material. Fillers, on the other hand,

give the plastic material bulk. On a volume basis, fillers reduce a material’s cost and must,

therefore, be lower in cost than the plastic material. A filler may or may not improve a material’s

physical properties.

A reinforcement is any fibrous material that is stronger than the plastic material. All kinds of

natural and synthetic fibers have been used. Glass fiber accounts for approximately 95% of the

market. The use of carbon and wood fibers is small, but increasing.

Reinforcing fibers have an aspect ratio. Their length is greater than their width. Fillers have little

or no aspect ratio. The typical glass fiber is .0004 inch in diameter. The minimum fiber length that

will provide significant reinforcing is .008 inch. This length produces an aspect ratio of 20:1.

Fibers increase a material’s strength by distributing a load over many molecules. The longer the

fiber, the more molecules there are to absorb the load. The typical fiber length used in injection

molding is limited to .125-.187 inch, as that is the length of the pellets. In 1985 a British company

learned how to coat longer glass fibers. This technology allowed the production of .500-inch-long

pellets and fibers. These longer fibers provide more reinforcement than shorter fibers.

In a glass-fiber-reinforced thermoplastic, the fibers provide greater strength, heat resistance, and

reduced mold shrinkage. The plastic material contributes chemical and electrical properties, color,

surface finish, and processing characteristics. Common glass fibers used in injection molding

have a tensile strength of around 500,000 psi. Adding 30% by weight of these glass fibers to

nylon 6/6 results in the impressive improvement in properties shown in the table below. There are

many different nylon 6/6 plastics and this table lists average values.

Glass is dense. Adding it to nylon 6/6 increases its specific gravity and reduces the volume of a

pound of that mixture. This reduction and the increase in cost per pound must be considered in

specifying a glass-fiber-reinforced material. The large-volume market price for these materials is

included in the table.

Reinforced materials are different

In these times of thin-walling, design engineers are frequently disappointed in the lack of stiffness

in the resulting part. A common reaction is to add glass fibers and try it again. Glass fibers

increase a part’s stiffness, but they also change a lot of the part’s other characteristics.

For example, a nylon 6/6 part is isotropic; it will for all practical purposes have uniform physical

properties in all directions. Adding glass fibers will make the same part anisotropic, i.e., with

nonuniform physical properties. The primary reason for this lack of uniformity is that the fibers

orient in the direction of melt flow.

In one set of experiments on a .050-inch-thick end-gated part, 90% of the fibers were oriented in

the direction of flow. A similar part with a .250-inch thickness had only 5% of the fibers oriented in

the direction of flow. The thinner the part, the more anisotropic the properties will be.

A part with a high degree of fiber orientation has a high tensile strength and stiffness in the

direction of orientation and parallel to the length of the fibers. The tensile strength and stiffness

transverse to the length of the fibers is much weaker.

Mold shrinkage with nonreinforced crystalline materials, such as nylon 6/6, is slightly greater in

the direction of flow than transverse to the flow direction. Fiber-reinforced parts are the exact

opposite. Mold shrinkage is approximately 50% greater in the direction transverse to flow. These

differing shrinkages are of little concern with nonreinforced parts. The large differences in mold

shrinkage of fiber-reinforced parts result in warpage, which is a constant source of problems for

molders.

Conventional wisdom teaches that gating a part in the center produces the shortest flow path,

while allowing the melt to reach all extremities of the cavity at the same time. This is ideal for

nonreinforced materials. There is no universal agreement, but many molders believe that fiber-

reinforced parts should be gated at one end. The logic for this gate location is that you will at least

know in which direction the fibers are oriented.

Part design tips

With a few exceptions, the design criterion is the same for reinforced or nonreinforced nylon

parts. A good minimum wall thickness for a fiber-reinforced part is .125 inch. Thinner walls will

have more problems resulting from fiber orientation.

• Glass-fiber-reinforced nylon parts are stiffer and more abrasive. Cavities, and especially cores,

must be polished and free of undercuts with a minimum draft of 1° per side.

• Highly polished, blemish-free surfaces are difficult to guarantee. The best results are achieved

with a warm cavity that produces a resin-rich surface.

• Sharp corners are to be avoided, as they impede melt flow and break fibers.

• Holes can be molded, but the resulting weldlines may have only 60% of the material’s tensile

strength. When the two flow fronts that form weldlines meet, the glass fibers from one flow front

do not cross the weldline to mingle with the second flow front. While using fiber-reinforced

materials, every effort must be made to avoid locating weldlines in heavily loaded, high-

appearance areas. The reduced concentration of glass fibers in the area of the weldlines may

also result in a nonuniform surface appearance.

By Design: Polystyrene part design

By Glenn Beall

Published: September 25th, 2002

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his

special perspective on issues important to design engineers and the molding industry.

The first polystyrene (PS) was a natural material distilled from tree resin by the French chemist,

Bonastre, in 1831. Another French chemist, Berthelot, is credited with producing the first man-

made PS based on the synthesis of ethylbenzene in 1869. No commercial uses were found for

this material until production started in England in the early 1930s. In 1937, Dow Chemical Co.

introduced PS to the North American market, and today PS is a major injection molding material.

Approximately 8 percent of the plastic produced in the U.S. is PS—the fourth largest in volume.

Polystyrene is a senior citizen member of the plastics industry. It is intellectually stimulating to

work with the recently introduced materials such as Ultem, liquid crystal polymers, or the new

alloys and blends. But well-established materials have the advantage that only comes with

experience. These older materials are fully developed, well understood, and provide fewer

surprises than the new materials.

The first polystyrenes were crystal clear, rigid, brittle, amorphous thermoplastics that became

known as general purpose polystyrene (GPPS), or simply GPS. In volume, their published selling

price is $.42/lb. With an average density of only 1.045 g/sq cm, the cost is $.0150/cu in. This is a

low cost for a transparent material with a flexural modulus of 500,000 psi and a tensile strength of

up to 8200 psi. However, this material’s Achilles’ heel is its low notched Izod impact strength

of .25 to .45 ft-lb/in.

The brittleness of GPS limited its early use. This deficiency was overcome in the late 1940s by

the addition of rubber to produce high-impact polystyrene, or HIPS. Notched Izod impact strength

increased to .9 to 4.1 ft-lb/in, but there was a price to be paid. HIPS costs $.46/lb and $.0165/cu

in. Flexural modulus and tensile strength declined to 260,000 to 370,000 psi and 2325 to 6000

psi, respectively. Worst of all, HIPS lost GPS’s transparency and lustrous surface finish.

Today, packaging accounts for approximately 30 percent of the PS used in the U.S. Consumer

electronics and appliances, notably televisions, radios, air-conditioner housings, and cassettes,

use 9 percent. Housewares and furniture use 8 percent and toys consume another 7 percent.

Designing With PS

In commercial use for 65 years, polystyrene’s stiffness, transparency, low cost, and especially its

ease of processing make it a favorite for injection molding. It would be reasonable then, but a

mistake, to assume that everyone knows how to design parts for good old PS. The impressive

growth of the plastics industry results in new designers entering the field each year, and many of

these newcomers have yet to have their first experience with PS. In this regard the following

design guidelines will be helpful.

Wall thickness minimums can be as little as .010 inch for small parts and disposable

packaging items. One supplier indicates an impressive flow-length-to-thickness ratio of 150:1.

PS is a low-mold-shrinkage-factor, amorphous material. Theoretically there is no limit to the

maximum thickness that can be produced. Thick-walled parts are, however, only practical when

function is more important than cost, and when the mold is hot enough and the gate is large

enough to allow continuous cavity packing during an extended cooling cycle.

The preferred wall thickness for GPS and HIPS is a minimum of .030 inch and a maximum

of .250 inch. Although not desirable, PS parts can tolerate wall thickness variation of up to 25

percent. As always, changes in thickness should be gradual.

Radiusing inside corners on GPS parts is critical, as this is a hard, brittle material. The

minimum corner radius should be 25 percent of the part wall thickness. The higher elongation and

impact strength of HIPS make it more tolerant of small inside radiuses. Maximum strength is

achieved in both materials with a radius of 75 percent of the wall thickness.

Outside corner radiuses should be equal to the inside radius plus the wall thickness. These

proportions produce a uniform wall thickness around the corner. This, in turn, produces uniform

cooling and mold shrinkage with a reduction in molded-in stress and warpage.

Molding draft angles and a smooth polish are mandatory with GPS due to this material’s

hard, brittle surface as demolded. The inside surfaces of parts should have a draft angle of at

least 1/2 degree  and preferably 1 degree per side. The low mold shrinkage of PS does not allow

it to shrink very far away from the cavity during cooling. A 1 degree per side draft angle is

recommended on outside surfaces. The softer nature of HIPS makes it more tolerant of minimum

draft angles.

Projections such as stiffening ribs, bosses, gussets, and standing walls can have a thickness

equal to 75 percent of the wall to which they are attached. Projections with greater thicknesses

can result in sink marks and molded-in stress. In those instances where the avoidance of sink

marks is mandatory, it is desirable to reduce the thickness of a projection to only 65 percent of

the part’s wall thickness.

Depressions, or holes, are easy to produce in PS. This easy flowing, amorphous material is

capable of producing good-looking, strong weldlines. GPS is more challenging in this regard, as

the best of weldlines will deflect light as it passes through a transparent part.

Draft angles are required along the depth of holes in order to facilitate easy part ejection.

Irregularly shaped holes must avoid sharp inside corners that can produce molded-in stress.

Tolerances for GPS and HIPS are the same. A .125-inch-thick part that is 1 inch long can be

held to a commercial length tolerance of ±.003 in/in. A fine tolerance would be ±.002 in/in.

The low and uniform mold shrinkage of PS renders it a dimensionally stable material. While using

materials of this type there is a natural tendency to take advantage of the situation and specify a

fine instead of a commercial tolerance. This understandable urge must be resisted, as a fine

tolerance can result in higher mold and molding costs. There are always exceptions to this rule,

but the ideal tolerance is the largest tolerance that produces an acceptable part.

By Design: Designing with PPO

By Glenn Beall

Published: September 30th, 2004

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his

special perspective on issues important to design engineers and the molding industry.

You can plate it, leave it in water, and use it in electrical applications with no burning. If you need

something better than ABS, go with PPO.

The initials PPO are the easy way of saying polyphenylene oxide. This is a misnomer for

polyphenylene ether, or PPE, which is the European designation. The original PPEs found limited

applications due to their relatively high cost. The material was also susceptible to thermal

degradation at processing temperatures.

In response to these limitations, General Electric’s (GE’s) John Hay discovered that PPE was

completely miscible with polystyrene (PS). Alloying PS with PPE produced a lower-cost material

with a broader processing window. In 1966, GE Plastics reintroduced this new material as a PPO

under the now well-known trade name of Noryl. This polymer found many applications, but it was

a single-source material. Some large potential users, such as the car and computer companies,

were apprehensive about specifying a material that was only available from one supplier.

This marketplace opportunity was recognized by Borg Warner (BW), a highly respected supplier

of Cycolac ABS materials. In 1982, BW introduced Prevex, a PS-modified PPE with similar

properties. This material found a waiting market. Both Prevex and Noryl continued to expand their

market share. Sales were 29 million lb in 1970 and expanded to an impressive 227 million lb in

1998. Sales in 2003 are estimated at 300 million lb. GE purchased BW’s plastics business in

1988.

Defining Characteristics

The styrene-modified PPOs are amorphous, opaque, engineering materials known for their

stiffness and impact strength coupled with good temperature resistance. Varying the percentages

of PPE and PS allows this material to be adapted to many different molding processes and

market applications. Filled and fiber-reinforced grades are available, some with Underwriters

Laboratories’ 94 flame retardancy rating of V0 and 5V. General purpose, injection molding grades

are priced at $1.80/lb or $.068/cu in in truckload quantities. There are too many grades of PPO to

be summarized here. One popular flame-retardant injection molding grade of Noryl (N 190) has a

tensile strength of 7000 psi and a flexural modulus of 3.25 x 105 psi, with a notched Izod of 7 ft-

lb/in and a heat deflection temperature of 205°F at 66 psi. Mineral-filled and glass-fiber-reinforced

grades have much greater tensile and flexural strength with higher temperature resistance.

Modified PPO is a bridge material that successfully fills the gap between polycarbonate and ABS.

Typical Applications

This versatile material finds uses as communication, medical, appliance, and business equipment

and housings. Transportation applications include wheel covers, instrument panels, grilles, and

electrical components. This material’s low hydrophilic characteristics allow its use in long-term

contact with water for pumps, sprinklers, fittings, tanks, pipe, and water meters. Electroplated

parts are used for EMI/RFI shielding, appearance-type plumbing applications, and as automotive

and appliance trim. PPO’s excellent electrical properties and nonburning characteristics combine

to allow its use as junction boxes and covers, coil bobbins, wiring terminal boards, fuse blocks,

fire alarms, exit sign housings, and current-carrying switch gears.

In those instances when ABS or impact styrene is just not quite good

enough, PPO is the next best choice.

Tips for designing with PPO

 

Wall thicknesses of .060 to .180 inch are ideal for PPO. Thicknesses of .030 to .375 and

even .500 inch have been molded. Flow lengths of 10 inches can be molded with

a .060-inch wall. Walls .125 inch thick can be up to 25 inches long. A tolerable wall

thickness variation can be 25% with proper blending from thick to thin.  

Radiuses improve melt flow and reduce stress. A good inside corner radius for PPO is

one-half of the part’s wall thickness and never less than .015 inch.  

Molding draft angles of ½° to 2° per side are recommended for PPO. Textured surfaces

require a draft angle of 1½° plus 1°/side for each .001 inch of texture depth.  

Projections such as ribs, bosses, and gussets on parts with walls less than .125 inch

can be 50% to 60% of that wall thickness. These values should be reduced to 40% to

50% on parts with thicker walls.  

Depressions, or holes, create weldlines. With proper molding procedures, these

weldlines will not cause appearance or abnormal strength problems. Weldlines are

rarely as strong as the surrounding material. They should be located in low-stress

areas. Holes require standard molding draft and corner radius considerations. Limiting

the depth of holes to two to three times the thickness of the core pin eliminates pin

deflection.  

Tolerances on PPO parts can be as fine as ±.001 inch on a .125-inch-thick, 1.000-inch-

long part. A less costly commercial tolerance on the same part would be ±.002 inch.

There are always exceptions and not all parts can maintain these tolerances all of the

time. This is a low-mold-shrinkage material that shrinks uniformly in all directions. PPO

is frequently chosen for large, precision parts that require a minimum of warpage.

Test Methods

ELECTRICAL PROPERTIES OF PLASTICS

Certain thermoplastics are good electrical insulators and offer freedom of design in electrical applications.

Electrical properties may also be changed by environmental conditions such as moisture and / or

temperature.

A BASIC CONCEPT TO REMEMBER is that electrons must be exchanged between molecules for electric

current to flow through a material. Plastic molecules hold on to their electrons and do not permit the

electrons to flow easily — therefore we can refer to certain plastics as insulators or having insulating

properties.

The molecules in plastics are also "polar" which means that they will tend to act like little magnets and align

themselves in the presence of a voltage or field, the same as the needle in a compass trying to point North.

The electrical properties of thermoplastics can be described by the following:

1. VOLUME RESISTIVITY

2. SURFACE RESISTIVITY

3. DIELECTRIC CONSTANT

4. DIELECTRIC STRENGTH

5. DISSIPATION FACTOR

6. ARC RESISTANCE

MECHANICAL PROPERTIES OF PLASTICS

In this we will cover the technical terms and concepts used to describe the properties or performance of

thermoplastic materials. These STANDARDIZED terms are used by suppliers and users to communicate

how a material behaves under specific conditions. This allows comparisons of different materials.

DESIGN

 

A designer or engineer will often use design equations that work with metals while a part is being designed.

Metals behave like a spring; that is, the force generated by the spring is proportional to its length. A plot

(FIGURE 2) of the force as a function of length is a "straight line."

 

When a material actually works this way it is called "LINEAR" behavior. This allows the performance of

metals and other materials that work like a spring to be quite accurately calculated. A problem occurs when

the designer tries to apply these same equations directly to plastics. Plastics DO NOT BEHAVE LIKE A

SPRING (not a straight line), that is they are "non-linear." Temperature changes the behavior even more. 

How much load or force will the part be required to carry? How will the part be loaded? What are the

direction and size of the forces in the part? These are but a few of the questions that a designer tries to

answer before a material is selected.

1. STRESS

2. STIFFNESS

3. STRAIN

4. YIELD POINT

5. TENSILE STRENGTH

6. ELONGATION

7. COMPRESSIVE STRENGTH

8. SHEAR STRENGTH

9. FLEXURAL STRENGTH

10. TORSIONAL STRENGTH

11. CREEP

12. FATIGUE STRENGTH

13. IMPACT STRENGTH

14. NOTCH SENSITIVITY

THERMAL PROPERTIES OF PLASTICS

With a change in temperature, plastics materials tend to change size considerably more than other

materials, such as steel, ceramics, and even aluminum. A designer must consider these differences in the

sizes. In fact, the shipping environment may expose the part to a much greater temperature variation than

the part will ever see in use. The measure of how much a part changes size as the temperature changes is

called the "THERMAL COEFFICIENT OF EXPANSION".

1. COEFFICIENT OF EXPANSION

2. DEFLECTION TEMPERATURE UNDER LOAD

ELECTRICAL PROPERTIES OF PLASTICS

1. VOLUME RESISTIVITY

The Volume Resistivity is defined as the ratio between the voltage (Direct Current or DC), which is like the

voltage supplied by a battery, and that portion of current which flows through a specific volume of the

specimen. Units are generally ohm per cubic centimeter.

Visualize putting DC electrodes on opposite faces of a one centimeter (.394 inch) cube of a plastic material.

When a voltage is applied, some current will flow in time as the molecules align themselves.

Ohm's Law tells us that a voltage (volts) divided by the current (amps) is equal to a resistance (ohms) or V/I

= R. When the voltage applied to the cube is divided by the current, the resistance for 1 cm of the plastic is

determined or ohm per cm.

Generally plastics are naturally good insulators and have very high resistance. The Volume Resistivity can

change with temperature and the presence of moisture or humidity.

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2. SURFACE RESISTIVITY

The Surface Resistivity is the ratio between the direct voltage (DC) and current along the surface per unit

width. Units are generally ohms. Again refering to Ohm's Law, The Surface Resistivity is a measure of how

much the surface of the material resists the flow of current.

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3. DIELECTRIC CONSTANT

The Dielectric Constant is the ratio of the capacitance (AC voltage) of electrodes with the insulating material

between them to the capacitance of the same electrodes with a vacuum or dry air in between.

The dielectric constant is a measure of how good a material works to separate the plates in a capacitor.

Remember that the molecules are like little magnets and are trying to realign themselves every time the

voltage (current) changes direction. Some materials do it better than others.

The dielectric constant for a vacuum has a value of 1. Dry air is very nearly 1. All other materials have

"dielectric constants" that are greater than 1. The "dielectric constant" for a plastic material can vary with the

presence of moisture, temperature, and the frequency of the alternating current (and voltage) across the

plates.

The units for frequency are usually "HERTZ (Hz)" which means cycles per second. 3 kilohertz is the same

as 3,000 hz and 3 megahertz is the same as 3,000,000 hz.

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4. DIELECTRIC STRENGTH

Dielectric Strength is the voltage difference (DC) between two electrodes at which electrical breakdown

occurs and is measured as volts per mil of thickness. This is an indication of how effective an "insulator" the

material is.

The test is similar to that used for "Volume Resistivity" except the voltage is increased until there is an arc

across the plates. This means that the voltage was strong enough to break down the material and allow a

large current to flow through it. Again this property can be affected by the presence of moisture and

temperature. Frequency may also affect this property when the material is subjected to an Alternating

Current.

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5. DISSIPATION FACTOR

The Dissipation Factor (AC) is the tangent of the loss angle of the insulating material. It can also be

described as the ratio of the true in-phase power to the reactive power, measured with voltage and current

90 degrees out of phase.

This is an indication of the energy lost within the material trying to realign the molecules every time the

current (voltage) changes direction in alternating current. The property varies with moisture, temperature,

and frequency.

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6. ARC RESISTANCE

 

The Arc Resistance is the elapsed time in which the surface of the material will resist the formation of a

continuous conductive path when subjected to a high-voltage (DC), low-current arc under rigidly controlled

conditions.

 

 

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MECHANICAL PROPERTIES OF PLASTICS

1. STRESS

How does one know if a material will be strong enough for a part? If the loads can be predicted and the part

shape is known then the designer can estimate the worst load per unit of cross-sectional area within the

part. Load per unit area is called "STRESS".

 

 

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2. STIFFNESS (Modulus)

Sometimes a designer knows a part can only bend or deflect a certain amount. If the maximum amount of

bending and the shape of the part are known, then the designer can often predict how STIFF a material

must be. The measurement of the STIFFNESS of a material is called the "MODULUS" or "MODULUS OF

ELASTICITY." The higher the modulus number, the stiffer the material; and conversely, the lower the

number, the more flexible the material. The Modulus also changes as the temperature changes

 

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3. STRAIN

The measurement of how much the part bends or changes size under load compared to the original

dimension or shape is called "STRAIN." Strain applies to small changes in size.

STRAIN = (Final Length - Original Length)/Original Length

= Change in Length or Deformation/Original Length

STRESS, STRAIN, and MODULUS are related to each other by the following equation. The modulus or

stiffness of a material can be determined when the material is loaded in different ways, such as tension,

compression, shear, flexural (bending) or torsion (twisting). They will be called TENSILE MODULUS, also

known as plain MODULUS, FLEXURAL MODULUS, TORSIONAL MODULUS, etc.

MODULUS = STRESS/STRAIN or, in other words

MODULUS = Load /change in shape when loaded. (STIFFNESS)

Choose the type of modulus in the property sheet that most nearly duplicates what the customer expects the

major load to be, tension, bending (flexural). If the load is unknown, use the lowest module value of the two.

These numbers can be used for short-term loading if the load is to be applied for only a few days at the

most.

The stress/strain equation is the equation used by designers to predict how a part will distort or change size

and shape when loaded. Predicting the stress and strain within an actual part can become very complex.

Fortunately, the material suppliers use tests that are easy to understand.

THE PERFORMANCE OF A PLASTIC PART IS AFFECTED BY:

WHAT KIND OF LOAD THE PART WILL SEE (Tensile, Impact, Fatigue, etc.)

HOW BIG THE LOAD IS

HOW LONG OR OFTEN THAT LOAD WILL BE APPLIED

HOW HIGH AND/OR LOW A TEMPERATURE THE PART WILL SEE

HOW LONG IT WILL SEE THOSE TEMPERATURES

THE KIND OF ENVIRONMENT THE PART WILL BE USED IN. WILL MOISTURE OR OTHER

CHEMICALS BE PRESENT?

THIS IS WHERE PLASTICS DIFFER IN THEIR BEHAVIOR WHEN COMPARED TO OTHER MATERIALS,

SUCH AS METALS AND CERAMICS. CHOOSING STRESS AND/OR MODULI VALUES THAT ARE TOO

HIGH AND DO NOT ACCOUNT FOR TIME AND TEMPERATURE EFFECTS CAN LEAD TO FAILURE OF

THE PART.

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4. YIELD POINT

The yield point is that point when a material subjected to a load, tensile, compressive, etc. gives (yields) and

will no longer return to its original length or shape when the load is removed. Some materials break before

reaching a yield point, for example, some glass-filled nylons or die cast aluminum.

To try to further visualize this property, take a piece of wire and slightly bend it. It will return to its original

shape when released. Continue to bend and release the wire further and further. Finally the wire will bend

and not return to its original shape. The point at which it stays bent is the "YIELD POINT." The "yield point"

is a very important concept because a part is usually useless after the material has reached that point.

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5. TENSILE STRENGTH

The maximum strength of a material without breaking when the load is trying to pull it apart is shown in

Figure 4. This is the system used by the suppliers to report tensile properties in their literature, such as

strength and elongation.

A good way to visualize this property is to think of pulling a fresh marshmallow apart and then pulling a piece

of taffy apart. The force or pounds required to pull the taffy apart would be much greater than required to pull

the marshmallow apart. If that force is measured and the taffy and marshmallow each had a cross-sectional

area of one square inch, then the taffy has the higher "tensile strength" in terms of pounds per square inch.

Plastics may demonstrate tensile strengths from 1000 psi (pounds per square inch) to 50,000 psi.

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6. ELONGATION

ELONGATION IS ALWAYS ASSOCIATED WITH TENSILE STRENGTH because it is the increase in the

original length at fracture and expressed as a percentage. An example would be to pull on a 1 " wide piece

of paper that is 4" long. It tears with no visible elongation or nearly 0% elongation. Now do the same thing to

a 1" x 4" piece of taffy. It will stretch several times its original 4" length before it fractures. Assume that it is

stretched to a 12" length then (12"/4") (100) = 300% elongation.

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7. COMPRESSIVE STRENGTH

The maximum strength of a material without breaking when the material is loaded as shown in Figure 6.

Check if the material supplier has the information on compressive strength, since it is not always

determined.

This term becomes less meaningful with some of the softer materials. PTFE, for example, does not fracture.

Consequently, the compressive strength continues to increase as the sample is deforming more and more.

A meaningful "compressive strength" would be the maximum force required to deform a material prior to

reaching the yield point. The compressive term similar to "elongation" is "compressive deformation," though

it is not a commonly reported term. It is easy to visualize two identical weights (FIGURE 7), one sitting on a

1" cube of fresh marshmallow and the other on a 1" cube of taffy. The marshmallow would be flattened and

deformed more.

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8. SHEAR STRENGTH

The strength of a material when the material is loaded as shown in Figure 8. The surfaces of the material

are being pulled in opposite directions. Some examples of items that see shear loading are the nail holding a

picture on the wall, the cleats of athletic shoes, and tire tread as a car speeds up or slows down.

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9. FLEXURAL STRENGTH

The strength of a material when a beam of the material is subjected to bending as shown in Figure 9. The

material in the top of the beam is in compression (squeezed together), while the bottom of the beam is in

tension (stretched). Somewhere in between the stretching and squeezing there is a place with no stress and

it is called the neutral plane. A simple beam supported at each end and loaded in the middle is used to

determine the flexural modulus given in properties tables. Skis, a fishing pole, a pole vault pole, and a diving

board are examples of parts needing high flexural strength.

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10. TORSIONAL STRENGTH

The strength of a material when a shape is subjected to a twisting load as shown in Figure 10. An example

of a part with a torsion load is a screw as it is being screwed in. The drive shaft on a car also requires high

torsional strength.

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11. CREEP

Visualize large weights being hung on bars of different materials. All materials will experience some initial

and immediate deformation or stretching when the load is first applied. As long as the yield point has not

been exceeded, a metal sample which acts like a spring will not stretch any more regardless of how long the

weight is left on. When the weight is removed, the metal bar will return to its original shape. The length of a

"thermoplastic" bar will continue to slowly increase as long as the load is applied. This is called CREEP. The

amount of creep increases as the load and/or temperature are increased. Some thermoplastics like nylons

will creep more when they have softened because of the presence of moisture. The "crosslinked" or "3D net"

structure in "thermosets" resists creep better than thermoplastics. Reinforcements like glass and carbon,

which do not creep, greatly reduce the creep of the composite material when mixed with a plastic.

Remember the relationship between stress/strain/modulus is:

Modulus = Stress/Strain

The initial strain or change in length with the weight will give a value for the modulus (this is usually the short

term value reported in the property tables for the tensile modulus or flexural modulus). If the weight (stress)

is left on over a period of time, the amount of bending or elongation continues to increase and the value for

the modulus will decrease with time as shown in Figure 16. This decreasing modulus that is a function of

time (and even temperature) is called the "CREEP MODULUS" or "APPARENT MODULUS."

THIS IS THE MODULUS THAT THE DESIGNER SHOULD BE USING TO MORE ACCURATELY PREDICT

THE BEHAVIOR OF THE PLASTIC MATERIALS. THE VALUE CHOSEN FROM THE SUPPLIER'S

LITERATURE WILL BE BASED ON THE ESTIMATED TIME THE LOAD WILL BE APPLIED, THE

AMOUNT OF THE LOAD, AND THE TEMPERATURE CONDITIONS PRESENT WHEN THE LOAD IS TO

BE APPLIED.

REMEMBER THAT CREEP IS AFFECTED BY:

LOAD (STRESS)

TEMPERATURE

LENGTH OF TIME THE LOAD IS APPLIED

OTHER ENVIRONMENTALS, SUCH AS MOISTURE OR CHEMICALS

Since the STRESS is kept constant, i.e., the weight or load is not changed or removed, the equation

becomes:

Apparent Modulus x Total Strain = Constant (Stress)

or in other words, if the strain goes up, then the Apparent Modulus must come down. Since the strain

increases with time and temperature, the Apparent Modulus decreases with time and temperature.

The data is sometimes presented in supplier literature in terms of Stress Relaxation. This means that the

STRAIN is held constant and the decrease in the load (stress) is measured over time. This is called

"STRESS RELAXATION''. This information is important for applications, such as gaskets, snap fits, press

fits, and parts joined with screws or bolts. The equation becomes:

Apparent Modulus / Stress = Constant (Strain)

or in other words, as the stress goes down because the material moves, then the apparent modulus also

goes down.

Sometimes a supplier will recommend a maximum design stress. This has a similar effect to using the

apparent modulus. The recommended design stress for some acrylic injection molded parts is 500 psi and

yet its tensile strength could be reported to be as much as 10,000 psi in the property chart. Designers will

often look at the 10,000 psi value and cut it in half to be safe; however, it is not really enough and could lead

to failure of the part.

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12. FATIGUE STRENGTH

Plastics, as well as other materials, subjected to cyclic loading will fail at stress levels well below their tensile

or compressive strengths. The combination of tension and compression is the most severe condition. This

information will be presented in S-N Curves or tables. The S-N stand for Stress-Number of cycles. A PART

WILL SURVIVE MORE CYCLES IF THE STRESS IS REDUCED. The stress can be reduced by reducing

the deflection and/or decreasing the thickness of a part.

Some examples of cyclic loading are a motor valve spring or a washing machine agitator. With time, parts

under cyclic loading will fail; however, properly designed and tested they will not fail before several million

loadings have been completed. Figure 24 shows a typical S-N curve.

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13. IMPACT STRENGTH

Many plastics demonstrate excellent impact strength. Impact strength is the ability to withstand a suddenly

applied load. Toughness is usually used to describe the material's ability to withstand an impact or sudden

deformation without breaking. No single test has yet been devised that can predict the impact behavior of a

plastic material under the variety of conditions to which a part can be subjected. Many materials display

reduced impact strength as the temperature is lowered. Thermosets and reinforced thermoplastics may

change less with changes in temperature. Check the supplier literature for any unusual factors that may

affect the impact performance of a part.

Some of the impact tests commonly used in supplier literature are:

Izod Test: designed to measure the effect of a sharp notch on toughness when the test

specimen is suddenly impacted.

Tensile Impact Test: designed to measure the toughness of a small specimen without a notch

when subjected to a sudden tensile stress or load.

Gardner Impact Test: drops a shaped weight and determines the energy required to break the

test sample.

Brittleness Temperature Test: determines ability of the material to continue to absorb impacts

as the temperature is decreased.

Special tests may need to be devised to more nearly duplicate the actual application.

INFORMATION PROVIDED BY THESE TESTS WILL AID IN CHOOSING MATERIAL CANDIDATES;

HOWEVER, THE DESIGNER MUST STILL TEST THE ACTUAL PART UNDER CONDITIONS AS NEAR

AS POSSIBLE TO ACTUAL USE CONDITIONS BEFORE BEING CONFIDENT THAT THE MATERIAL

SELECTION IS ADEQUATE.

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14. NOTCH SENSITIVITY

Some plastic materials have exceptional impact performance and very good load carrying capability;

however, the performance of a material can be greatly reduced by having sharp corners on the part. The

sharp corners can be part of the design or from machining operations. A SHARP CORNER IS A GREAT

PLACE FOR A CRACK TO START. The Izod impact strength of a tough material like polycarbonate is

reduced from 20 to 2 as the radius of the notch is reduced from 0.020"R to 0.005"R respectively.

The sharp corners not only reduce the impact resistance of a part, but also allow for a stress concentration

to occur and encourage the premature failure of a load carrying part.

MINIMIZING SHARP CORNERS MAY MAKE THE MACHINING OPERATION MORE DIFFICULT;

HOWEVER, IT MAY BE CRUCIAL TO THE PART'S SUCCESS.

Edges of sheet being used in impact applications like glazing must also be finished to be free of sharp

notches. This is a concern with acrylics and even tough materials like polycarbonate.

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THERMAL PROPERTIES

1. COEFFICIENT OF EXPANSION

The units are usually given in inches per degree Fahrenheit. It is the change in length (inches) of one inch of

a part caused by changing the temperature one degree.

The change in length = Original length x the coefficient of expansion x the change in temperature = 10

x .00006 x 40 = .024 inches.

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2. DEFLECTION TEMPERATURE UNDER LOAD

In addition to changing size, the strength and modulus of elasticity of plastic materials tend to decrease as

the ambient temperature increases. The standard test for determining the DEFLECTION TEMPERATURE

UNDER LOAD (DTUL) at 66 and 264 psi provides information on the ability of a material to carry a load at

higher temperatures. The 66 psi means a light load and the 264 psi means a heavy load on a beam. The

temperature of the loaded beam is raised until a certain amount of deflection is observed. The temperature

when that deflection is reached is called the DTUL. Plastics usually have a higher DTUL at 66 psi than 264

psi because of the lower load. Note: The DTUL is sometimes referred to as the Heat Distortion Temperature

or HDT.