pultrusion process - composite manufacturing

SEMINAR REPORT ON PULTRUSION PROCESS

MANIPAL INSTITUTE OF TECHNOLOGY

DEPARTMENT OF MECHANICAL AND MANUFATURING

SUBMITTED BY

CHETAN P BHAT

080922016

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Pultrusion Process

Manipal Institute of Technology

Department of Mechanical and Manufacturing 1

Introduction

Pultrusion is a continuous molding process using fiber reinforcement in polyester or other

thermosetting resin matrices. The process is similar to the metal extrusion process, with the

difference being that instead of material being pushed through the die in the extrusion

process, it is pulled through the die in a pultrusion process. Pultrusion creates parts of

constant cross-section and continuous length. Pultrusion is a simple, low-cost, continuous,

and automatic process.

Pultrusion is a continuous, automated closed-moulding process that is cost effective for high

volume production of constant cross section parts. Due to uniformity of cross-section, resin

dispersion, fibre distribution & alignment, excellent composite structural materials can be

fabricated by pultrusion. The basic process usually involves pulling of continuous fibres

through a bath of resin, blended with a catalyst and then into pre-forming fixtures where

the section is partially pre-shaped & excess resin is removed. It is then passed through a

heated die, which determines the sectional geometry and finish of the final product. The

profiles produced with this process can compete with traditional metal profiles made of

steel & aluminium for strength & weight.

The pultrusion process has developed slowly compared to other composite fabrication

processes. The initial pultrusion patent in the United States was issued in 1951. In the early

1950s pultrusion machines for the production of simple solid rod stock were in operation at

several plants. Most of these machines were

the intermittent pull type. In the mid-1950s,

continuous pull machines were available. The

late 1950s were producing pultruded structural

shapes and by 1970, there has been a dramatic

increase in market acceptance, technology

development, and pultrusion industry

sophistication.

The process provides maximum flexibility in

the design of pultruded FRP profiles. Currently,

profiles up to 72 inches wide and 21 inches

high are possible. Since the process is continuous, length variations are limited to shipping

capabilities. Specific strength characteristics can be designed into the composite, optimizing

laminate performance for a particular application by strategic placement of high

performance reinforcements. Color is uniform throughout the cross section of the profile,

eliminating the need for many painting requirements. Processing capabilities include the

production of both simple and complex profiles, eliminating the need for much post-

production assembly of components.

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The tooling required to start producing pultruded parts is fairly inexpensive and

straightforward compared to the complex and sometimes very costly molds that are

necessary for other plastics molding processes. It is worth noting though that open profiles

are generally less expensive to produce than hollow ones.

As a rule of thumb, parts with a small cross section can be manufactured at a speed of

roughly one meter per minute, whereas larger profiles will require up to ten times longer.

Pultrusion is a high-volume manufacturing process and most manufacturers will ask for a

minimum order of 500 meters to start production.

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Basic Raw Materials

One can use a wide variety of fibrous reinforcement and resin system to get a composite

material with a broad spectrum of properties by pultrusion process. Since each fibre and

resin material brings its own contribution to the composite, knowledge of raw material

properties is the first step in designing a satisfactory composite product. The reinforcement

provides mechanical properties such as stiffness, tension and impact strength and the resin

system (matrix) provides physical properties including resistance to fire, weather, ultraviolet

light and corrosive chemicals.

Reinforcement Types

Reinforcements serve as the primary load bearing entity in the part; reinforcements can

enhance functional performances such as electrical conductivity, radar cross section, and

thermal performance. In the process, the reinforcement allows the part to be pulled

through the die acting as both a load transfer media as well as the source of bulk, which

allows the die to be continuously, uniformly filled. Three characteristics must be considered

when choosing reinforcements:

first the fibre type (glass fibre, aramid and carbon); second the form (roving strands, mat &

fabrics) and third the orientation.

Based on Fiber Type

The glass fibre continues to be the most widely used reinforcement, because they are

readily available and comparatively cheaper. Electrical grade E-glass fibres, the most

common, exhibits a tensile strength of approximately 3450 MPa and a tensile modulus of 70

GPa, but they have relatively low elongation of 3 to 4%. A variety of fibre diameters and

yields are available for specific applications. Surface sizing of glass fibres provides optimum

impregnation and chemical bonding between the fibres and matrix resins, thus ensuring

maximum strength development and retention.

S-glass fibre exhibits high tensile strength (4600 Mpa) & tensile modulus (85 Gpa) and is

used for high-performance applications. The Carbon fibre exhibits tensile strength from

2050 to 5500 MPa and tensile modulus from 210 to 830 GPa with elongation of 0.5 to 1.5%.

Carbon fibre has various unique properties like electrical conductivity, high lubricity and low

specific gravity (1.8 versus 2.60 for E-glass).

Very tough composites having good flexural and impact strength can be fabricated by using

Organic fibres such as aramids, having high tensile strength (2750 MPa) and modulus (130

GPa) along with elongations of up to 4%. Polyester fibres with appropriate binders have

been used as a replacement for glass in applications that would benefit from increased

toughness and impact resistance but where tensile and flexural strengths can be sacrificed.

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Based on Form

Rovings

Rovings are continuous fibers, which are one of the primary reinforcements used in

pultrusion. Rovings come in three main forms; Conventional rovings, Single-End Continuous

Rovings or direct rovings, and bulky or texturized rovings.

Conventional Rovings

Conventional or multi-end roving is assembled from a number of forming packages into the

desired final yield or Tex. Conventional rovings are most commonly used in applications

containing large thickness of unidirectional reinforcement. Conventional rovings tend to fill

space at lower glass levels, giving a more resin rich cross-section.

Single-End Continuous Rovings or direct rovings:

Single-End Continuous Rovings or direct rovings are the

most commonly used reinforcements in the pultrusion

process. Single- End Continuous Rovings combine ease

of handling due to low catenary and fuzz, with highly

reproducible mechanical properties in both its standard

unidirectional usage and when used in stitched and

woven fabrics. Single-End Continuous Rovings are widely

used due their excellent processing, and laminate performance. Considerably higher shear

strengths are achieved with single-end rovings compared to conventional rovings

Bulky or Texturized Roving

Bulky, texturized, or fluffy rovings are specialty rovings

designed to fill corners in complex shapes, "clean" the

die, preventing formation of resin rich areas, which

could cause local spaulling. Bulky rovings are intended

to act as local filler, though they do provide some

reinforcement.

Mats, Complexes, and Veils

Mats, fabrics, and veils are used in pultrusion processes to give properties to the part not

achievable using roving reinforcement. Mats give the ability to develop off axis structural

performance, create a higher resin content part, and develop unique surface qualities for

both visual and non-visual attributes, such as corrosion resistance.

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Continuous Filament Mat

Continuous filament mat or swirl mat is the most common mat used in pultrusion.

Continuous filament mats (CFM) provide strength and stiffness in the transverse or non-

pulling direction of the pultruded part. They provide

a degree of bulk, which improves processing and

limits resin rich sections of the part.

This bulk also reduces the glass fraction required for

the processing of a specific cross section. CFM mat

also improves the shear strength of the laminate

produced. CFM used in pultrusion contains a fast-

wetting non-soluble binder that maintains mat

integrity through the preforming operations. It is

less prone to skewing (misalignment) common in

fabrics.

Mats are generally composed of coarse glass

strands, which are highly porous thus ensuring

complete wet out of each individual filament. The

roll is slit to the appropriate width of the part. The variation in slitting widths will cause

some variation in the localized reinforcement contents within the part; hence there will be

an impact on the mechanical strength. Weight variation (as well as resin variation) and

shrinkage will also induce variation in product characteristics.

Continuous strand mat provides the most economical method of obtaining a high degree of

transverse physical properties. The mats are layered with roving; this process forms the

basic composition found in most pultruded products. The ratio of mat to roving determines

the relationship of transverse to longitudinal physical properties.

Fabrics and Stitched Complexes

Fabrics and stitched complexes are the newest generation reinforcements for the pultrusion

process. The construction of fabrics can be tailored to give specific reinforcing properties to

the part in order to achieve the needed strength in parts with demanding design

requirements. When the mix of required physical properties is not satisfied by conventional

mat roving construction, selected fabrics can be used to meet the end use requirements.

Varieties of these products can be used by themselves or in conjunction with the standard

mat roving construction to obtain the necessary results. The fiberglass fabrics are available

in balanced, high longitudinal, high transverse or ± 45° multi-ply construction. Since these

materials are more costly, the composites using these reinforcements are more expensive

than standard construction pultrusion.

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Chopped Strand Mats

The use of chopped strand mats in pultrusion is normally

related to specific needs for improved surface or corrosion

resistance in flat or gently curved laminates. Care must be

taken in selecting a chopped strand mat for pultrusion, as

most existing products are not designed to handle the tension

associated with the pulling of the product through the

process.

Veils

Veils in the pultrusion process are designed to establish a high

quality surface layer on the pultruded part and protect the die

from scarring by the other reinforcements during the pulling

process. Veils can be materials such as spun polyester, glass

veil, and for special requirements carbon veils have been

used. In many cases the veils can have pre-printed designs and

logos, which become the part surface finish aiding the part

appearance. Since pultrusion is a low-pressure process,

fiberglass reinforcements normally appear close to the surface

of the product. These can affect the appearance, corrosion

resistance or handling of the products. The two most

commonly used veils are A-glass and polyester .

Matrix Choice

The resin matrix has several functions in a pultruded composite. The resin's basic functions

are to fill the space between filaments, to fix the strand alignment, and to distribute the

bonding and shearing stresses. Due to the much higher modulus of the glass, and its

normally high percentage of volume in the composite, the strength effect of the resin is

usually quite small. As in the case with all FRP/GRP material systems, the resin plays an

important role in determining the chemical and environmental durability of the total

system. It also controls the thermal, electrical, and visual

The composite properties such as high-temperature performance, corrosion resistance,

dielectric properties, flammability and thermal conductivity are determined exclusively by

the properties of resin matrix.

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Polyester Resins

Unsaturated Polyester resins are most commonly used in pultrusion. Orthophthalic,

isophthalic acids or anhydrides, in combination with maleic anhydride and various glycols,

are the basic elements. Pultrusion polyester has the ability to gel and cure rapidly to form

the strong gel structure required for release at the die wall. Generally resins with the

viscosities of 500 cP are used for pultrusion. Higher viscosity low-reactive monomer versions

can be blended with additional styrene to suit the processing need. The styrene level must

be properly maintained to achieve good cross-link structure without having residual

(unreacted) styrene in the finished composite.

Polyester resins exhibit good corrosion resistance to aliphatic hydrocarbons, water, dilute

acidic & alkaline environments. They do not perform well when exposed to aromatic

hydrocarbons, ketons, and concentrated acids. A high degree of unsaturation in polyester

chain exibits shrinkage up to 7% on curing. This level can be reduced using fillers and low-

profile additives. Composite based on polyesters retains high percentage of their electrical

insulation properties even if used continuously at temperature up to 200oC. Though

polyester supports combustion without modification, hence backbone bromination or the

use of additives greatly improves its flammability and smoke generation properties.

The electrical properties of polyesters make them suitable for use as primary insulators in

many high-voltage applications. Retention of electrical properties even at elevated

temperatures has made polyester insulators the materials of choice in many applications.

The weatherability of polyester is fair to good. Additional protection is usually through a

variety of ultraviolet absorption additives or using polyester surface veils and even painting

(done after pultrusion)

Vinyl Ester Resins

These resins are used when additional performance is sought. Vinyl esters (VE) offer better

corrosion resistance, higher mechanical properties at elevated temperatures, and improved

toughness properties such as impact and shear. They provide very efficient wet out and they

have higher temperature capability with improved flexibility compared to polyester resins.

The chemical structure of vinyl ester resins is such that the reaction sites are at the end of

each polymer chain rather than along the chain resulting in rigid segments along the

polymer backbone. This leads to lower-link density and high-temperature capability of these

materials. VE resins are superior to polyesters, but this advantage comes at a cost in two

ways:

1. VE resins can be as high as double the cost of polyester resins

2. VE resins usually run at speeds about 2/3 the speed of polyester due to their lower cross-

link density. Many VE have a narrow temperature window. A 10°C temperature change can

cause blistering in pultruded profiles. Operators should be aware of the small processing

window which VE resins have.

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Epoxy Resins

Epoxy resins typically offer the highest physical property performance as well as the best

strength retention at high temperatures of all the resins used in pultrusion. Epoxies are

frequently used for primary electrical insulation, aircraft, aerospace, and defence

applications. Epoxies can be used in continuous use applications at 300°F (150°C) and

epoxies provide increased flexural strengths and shear strengths when compared to

polyesters and VE. Epoxies have excellent corrosion resistance and electrical properties.

The disadvantages of epoxy resins can be:

• Poor toughness as a result of their rigid structure

• Can be more expensive to purchase

• Slower processing speeds vs Polyester and VE resins

• Lower pot life

• Mold sticking considerations

• More difficult to clean up

They require a higher reinforcement content than either polyester or vinyl ester

Other Resins

A variety of resin alternatives is also available for specific applications. The resins based on

Methacrylate Vinyl Ester Resins although expensive than polyesters but could be used for

their special properties viz. improved physical properties, very low viscosity which allows

them to be highly filled, rapid processing speeds, smooth profile surfaces and improved

flame retardancy and weathering. One concern with MVE is odor which plant personnel may

find objectionable.

Phynolic resins are also used in pultrusion owing to their high heat resistance and flame-

retardancy/low-smoke characteristic. Phenolic resins are suitable typically for pultruding

natural fibres such as jute.

A desire to improve toughness and post processing formability has lead to the use of

thermoplastic resins. The engineering thermoplastic resins provide excellent heat distortion

properties. The technology for impregnating fibres with thermoplastic resins includes hot-

melt application and solvent solution impregnation.

Filler and additives

Filler and additives are used to enhance specific performance, reduce cost, influence

viscosity, or improve processability of resin systems. Fillers can be incorporated into the

resins in quantities up to 50% of the total resin formulation by weight. ). The usual volume

limitation is based on the development of usable viscosity, which depends on the particle

size and the characteristics of the resin. There are three fillers frequently chosen for use in

pultrusion.

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These fillers are:

Calcium carbonate is the most popular and is used as a volume extender. Calcium carbonate

is generally used where performance is not critical.

Clay (alumina silicate) fillers are used for their corrosion resistance and in profiles requiring

electrical insulation. They can provide very good surface finish.

Alumina trihydrate is used when flame or smoke suppression is desired.

Calcium carbonate is primarily used as a volume extender to provide the lowest-cost-resin

formulation in areas in which performance is not critical.

Special purpose additives include ultraviolet radiation screens for improved weatherability,

antimony oxide for flame retardance, pigments for coloration, and low-profile agents for

surface smoothness and crack suppression characteristics. Mould release agents (metallic

sterates or organic phosphate esters) are important for adequate release from the die wall

to provide smooth surfaces and low processing friction. Pigments may be used to impart

color, weatherability, or flame retardency to the finished part.

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Components of Pultrusion Process

There are four components required in any pultrusion processing operation:

1. Creels

2. Forming or preforming guides

3. Resin impregnation systems

4. Primary die

5. Puller/clamping pads

6. Cut off saws

Creels

The creel should provide a position from which the

roving can be fed to the pultrusion process under

controlled and uniform tension. It also provides a

location for the transfer of the roving strand from

the running package to a second back-up package for

continuous uninterrupted production. There may

even be room for extra roving packages for

replacement or maintenance as required.

The size, shape and type of creel will normally be

determined by space considerations such as roving

package dimensions, the distance the strand must

be conveyed and the number of packages to be

handled. The amount of glass being used on a

continuous basis must also be considered.

The two common types of creels used are shown.

Each creel arrangement has a range of possible

number of packages for the best process efficiency:

Table creel - up to 50 packages

Bookshelf creel - 20 to several hundred packages

The bookshelf-type creel is the most common and

usually provides the best balance of accessibility

and maximum utilization of floor space. The size

will vary widely, but the creels shown provide a

standard module concept for creeling. Shown are

creels for handling 32 packages (16 active-16 transfer) in both a side pull and end pull

configuration. To handle different numbers of packages, the creels can be increased or

decreased in length or multiple creels can be used.

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In the end pull creel, commonly used in pultrusion, it may be necessary to guide each strand

within the creel using steel rods or ceramic guide eyes. This can prevent sagging and

whipping could tangle two or more strands on the same shelf.

Most pultrusion processes use stationary roving packages on bookshelf or table creels,

pulling the roving strand from the inside of the roving package. For some applications

requiring minimal strand twist for better wet-out and a flatter and wider strand profile,

pulling the roving strand from the outside of the roving package may be an option.

Forming guides

It is important to determine how the reinforcement is organized, aligned, and fed into the

primary die. Besides formulation and heating control, preforming system is also critical for

successful and constancy pultrusion. Before entering die, impregnated fibreglass rovings

and mats must be properly arranged and placed. Un-proper preforming system causes

failure of pultrusion, bad quality, and other problems.

Please note, preforming system is far from easy as most people think of. Firstly, it must be

designed based on profile design to meet the

physical requirements. Secondly, it must let

all reinforcements running freely and

smoothly, to avoid any breaking of rovings,

mats or cloth. Thirdly, for dies with

mandrels, the preforming system is the only

device to keep uniform thickness of

pultruded profiles. In-proper preforming

may cause eccentric and even breaking of

pultrusion. Fourthly, for complex profiles,

preforming system is a large challenge. A

preforming die gently shapes the material and removes all but about 10% of the excess resin

prior to entry into the pultrusion die.

There are two primary materials used in the

forming guide tooling: steel and ultra high

molecular weight polyethylene (UHMWPE).

The advantage of steel is that it is less

expensive (if carbon steel is used vs.

stainless) and standard plate, sheet, bar, and

rod can all be used.

A disadvantage is corrosion of non-stainless

steel and the difficulty to machine. The

advantage of UHMWPE is that it is lighter,

resistant to chemical attack, less damaging to the glass reinforcement, and therefore, easier

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to clean. A major advantage is that it is easy to fabricate, on-line, when modifications are

required. UHMWPE is easy to drill and machine slots for mat. A disadvantage is that

UHMWPE wears faster than steel. Except for the primary die the forming guides and the

guiding of the reinforcement into the die is the most important aspect in pultrusion

technology

Resin Impregnation System

The dimensions of resin baths are restricted to minimize the volume of catalyzed resin and

can be heated to control the resin viscosity to promote fiber wetting, although this will

reduce the working life of the bath. To facilitate lacing up, the roller assembly in the resin

bath is manufactured in two parts—a lower fixed set of rollers submerged in the resin bath

and a moveable upper set, under which the fiber is positioned. The assembly is then pressed

down to push the fiber into the bath to contact with the lower set of rollers. This system

facilitates an easy lace-up procedure and ensures good compaction to expel all air and

promote fiber wetting. Alternatively, the fiber can be passed over a drum upon which the

correct amount of resin has been metered and adjusted by a doctor blade.It is extremely

important to allow the resin and reinforcement enough time to fully wet-out. The

impregnation or bath system directly impacts wetout.

There are three resin impregnation systems available today.

They are:

1. Dip bath

2. Straight through bath

3. Resin Injection systems

Dip Bath

The dip bath, or open bath, has the

reinforcements travelling from the creels

down into the bath where the rovings go

through an "S" bar guide, which breaks apart

the roving bundle, allowing better coating of

the filaments by the resin. The bath system is typically used for all roving reinforcements, as

well as for simple mat and roving profiles where the mat can be handled horizontally, or

where taking the mat out of the horizontal plane will not induce a bow into the finished

profile. This system exposes a large amount of resin to the air and permits styrene

evaporation into the plant environment. Styrene emission is a environmental and health

consideration,which needs to be assessed with this impregnation system.

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Straight Through Bath

A straight through resin bath is a trough with

forming cards on each end. These cards can

also be used to begin the forming process. The

cards allow resin to leak from the open areas.

The excess resin is then collected and pumped

back into the bath trough. The advantage of

this design is the reinforcements are taken

from the horizontal plane, allowing the

profiles to be made with fewer tendencies to

warp or bow, and this design also reduces the amount of styrene released into the plant

environment.

Resin Injection System

The resin injection system is the newest

process for impregnation. With resin

injection, a steel chamber is attached to

the front of die. The chamber contains

port(s), which allow resin to be injected

into the cavity. The combination of cavity

design, resin pressure, and movement of

the reinforcement being pulled into the

die generate hydraulic pressure forcing

resin to penetrate the reinforcement

bundle, resulting in impregnation. With resin injection the resin is not open to the plant

environment, reducing the amount of styrene released into the workplace. Resin injection

systems require reduced clean up time, due to resin contact with fewer components. The

disadvantage of resin injection is the potential for incomplete impregnation of profiles with

thick walls, or incomplete impregnation in resin systems with high filler loading

incorporating a high number of mat or veil reinforcements.

Primary Die

The die is the heart of the pultrusion system and is the limiting step in production rate since

the part is both shaped and, usually, cured in the die. The processes of shaping and curing

along with the correspondent line speed are dependent upon the shape of the part, the

type of resin, the internal friction in the die, the heat expansion of the resin, the contraction

of the resin, and mechanical warpage which may occur in the part because of non-

symmetries in the fiber orientations.

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The opening of the die is usually somewhat larger than the final shape, permitting easy

collection of the fibers bundle, and then the die interior dimensions gradually reduce in size

until the final shape is achieved. During this shaping process, the part is cured.

Cure is accomplished either by

thermally heating the die (usually with

common electrical heaters) or by

subjecting the material to rf

frequencies. Both of these systems

have their advantages. The thermal

heating is simple and can be used with

metal dies, thus limiting die wear.

However, the poor heat transfer of the

resin means that as the thickness of

parts increases, the speed of the

pultrusion line must slow. Studies have

shown that thicknesses of about .5 inches can be thermally cured at 2 feet per minute but

that parts thicker than 3 inches cannot be cured at all using just thermal energies, regardless

of the line speed.

If rf curing is used, the thickness of the parts which can be cured and the line speeds

possible are both improved — about 3 times faster with parts that are .5 inches thick.

However, rf curing does not work well for metallic dies nor for conductive parts. Hence with

rf curing, non-metallic dies are generally used and these are prone to rapid erosion and poor

dimensional control. Parts with conductive components (such as carbon fibers) cannot be

effectively cured using rf radiation. These materials are thermally cured. Recent machines

which combine both thermally heated metal dies and rf heating after exiting the die have

proven to give much better performance than either of the methods alone.

Parts must be quite hard

(essentially cured) when they

exit the die so that they will

not be deformed by the

pulling mechanism, although

some curing after exiting the

die is possible if done before

the pullers. Post-die curing

can be done with a tunnel

heater, although this adds

considerable length to the line and is notoriously inefficient in heat use. Another method of

post die curing is to use heated, moving C-shaped dies (also called split dies) that have

cavities in the shape of the finished part and close on the part as it exists the die. These dies

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are mounted on a moving belt or chain and stay in contact with the part long enough to

insure that sufficient cure is achieved to withstand the forces of the puller. Off-line curing

could then be used, as required.

The following are some considerations, which need to be taken into account when

purchasing primary dies:

Material selection

The die material should be capable of withstanding continual heating and cooling without

deformation. Usually this means high-grade tool steel. The material should be capable of

resisting wear from abrasion, and be damage tolerant for repeated assembly and

disassembly. The material should be capable of receiving chrome plating of 0.001-0.002

inches (0.025-0.0508 mm) of thickness for wear resistance. Two widely used steels for die

manufacture are A-2 hardened to 55-60 Rockwell hardness and P-20 prehardened to 28-30

Rockwell hardness.

Shrinkage Factors

It is the nature of most resins to shrink after reaching peak exotherm, and during the cooling

process. Because of this a shrinkage factor must be calculated into the die design. This will

enable the die to form the part to the proper dimensions after the part is completely

cooled. A shrinkage factor cannot be unilaterally determined, as each resin system and

reinforcement lay-up is different, however recommended shrinkage factors are:

Thickness dimensions: 1% shrinkage

All other dimensions 0.3% shrinkage

Die Opening Design

The die opening design must accommodate the smooth entry of reinforcements into the

proper position. Generally a symmetrically shaped die is made to utilize either end of the die

as entry or exit, if possible.This enables longer die life between re-chroming. At the opening

of a die a minimum radius of 0.250 inches should be used. The die inlet is tapered at 7–100,

with well rounded edges to prevent fiber fracture.

One Piece or Split Cavity Dies

The advantage of a one-piece die is that the finished part will not have a parting line.A one-

piece gun barrel drilled die is usually less expensive to manufacture, however it may be

more expensive in the long run. If a part seizes up inside a one-piece die during processing,

the die may be impossible to repair.

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Multiple piece dies have the advantage of being easily opened to allow cleaning and

maintenance, however care needs to be given in designing the die so the parting line

between the mold halves does not cause a problem with the part being molded.

Die Surface Treatments

Due to the abrasive nature of fiberglass reinforcements, a protective surface treatment is

required on the die cavity. The most commonly used treatment is hard chrome plating at

0.001-.002 inches (25-50 microns) thick. For dies expected to perform in long term service,

nitriding may be considered.

Die Maintenance

Die maintenance is one of the most important factors in extending die life. Opening the die

after each production run and recording the number of feet of production and the wear

conditions is important.Testing the die surface with copper sulfate for wear spots is

critical.The sooner die wear can be detected and treated the longer the die will last before

major rework.The best time to perform die inspection and maintenance is just after the die

has been pulled from production, prior to being stored. Inspection just prior to installation is

not recommended, as production pressures may make repairs, if needed, difficult, shorting

the life of the die and potentially compromising part quality. Acidic mold release agents are

often used to ease the separation of the part from the metal die. Steel dies exposed to

acidic conditions must be cleaned thoroughly between uses to maximize the lifetime of the

die.

Puller Clamp System

The pultruded product is cooled prior to the traction unit, which can be a counter rotating

caterpillar unit, or preferably, a hand-over-hand reciprocating clamp type unit, since the

caterpillar unit requires the tracks to be fitted with machined rubber pads to accommodate

each pultruded profile. The hand-over-hand unit grips above and below and while one unit

is pulling, the other unit returns to position, ready to take over the role of pulling. Typical

line speeds vary in the range 1.5–100 mh-

1, depending on the section(s) being

produced. The pulling forces depend on

the type of machine which are available

upwards to some 30 MT.

There are two common puller systems

1. Caterpillar belt

2. Reciprocating clamp puller

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In both systems, pads, typically made of

urethane, must be shaped to match the part

profile in order to apply a uniform clamp

load, which will not cause damage to the

pulled part. The advantage of the caterpillar

belt system is the capacity to provide large

pulling force, spread over a larger part

surface area. The advantage of the

reciprocating clamp puller system is cost, as

it requires only 2 puller pads per clamp vs

10-100 depending on the caterpillar belt

size.

Cut off Saws

Most pultruders utilize what are known as flying

cut off saws. A flying cut off saw moves at the

same speed as the moving part, so the cut edge

of the part is square and straight. Using a non-

flying saw results in cut edges that are not square

and straight. Flying cut off saws are

recommended for part quality. There are two

basic types of flying cut-off saws:

1. Wet saw – A wet saw uses water during the

cutting cycle to cool and lubricate the blade and flush the fiberglass particulate to a filter.

This effectively eliminates dust and airborne particulate.

2. Dry saw – A dry-cut saw uses a continuous rim diamond blade that does not require any

fluid during the cutting cycle, but which gives a good clean cut. A dry-cut saw requires a dust

collection system to capture the fiberglass dust, both to address operator health and

comfort and from a housekeeping standpoint. Using a dry-cut saw without a properly

designed dust collection system is not recommended, due to the dust generated during

cutting, both from an operator health/comfort and a housekeeping standpoint. In most

cases pultruders will utilize an automatic cut-off saw, which automatically cuts the part into

the proper lengths. This enables employees to carry out other duties in order to enhance

productivity.

Pultrusion Process



Basic Processing Steps

The basic pultrusion process can be divided into the following operations:

1. Reinforcement handling

A suitable creel positions the requisite number of tows, with minimal damage, prior to entry

into the resin bath. If tows are supplied from containers, a christmas tree with ceramic

eyelets will be required to direct the tows to the resin bath. In some pultruded sections,

smaller tows are used in parts where the profile shape does not permit the use of larger

tows. Higher size contents (2–5% w/w) will permit easier handling and minimize fiber

damage. Tows can be joined by knotting, but knots must be staggered to ease their passage

through the die. Hybrid constructions can be used (e.g. with glass and aramid) and

transverse properties can be introduced with an In-feed system using woven cloth, knitted

fabrics, braid, or mat

2. Resin impregnation

The dimensions of resin baths are restricted to minimize the volume of catalyzed resin and

can be heated to control the resin viscosity to promote fiber wetting, although this will

reduce the working life of the bath. To facilitate lacing up, the roller assembly in the resin

bath is manufactured in two parts—a lower fixed set of rollers submerged in the resin bath

and a moveable upper set, under which the fiber is positioned. The assembly is then pressed

down to push the fiber into the bath to contact with the lower set of rollers. This system

facilitates an easy lace-up procedure and ensures good compaction to expel all air and

promote fiber wetting. Alternatively, the fiber can be passed over a drum upon which the

correct amount of resin has been metered and adjusted by a doctor blade.

3. Pre-die forming

A preforming die gently shapes the material and removes all but about 10% of the excess

resin prior to entry into the pultrusion die.

4. Heated die to shape and cure the resin

The pultrusion die can be made from polished chromium plated tool steels, or when

pultruding epoxies, a high chromium content tool steel. The die must be accurately lined up

and its length typically 300–1000 mm, which is governed by the size of the section being

pulled, the pulling speed and the resin system. Longer dies require greater pulling forces

due to the increased frictional drag and a die lubricant, such as zinc stearate, can be added

to the resin mix to help reduce frictional resistance, but which may interfere with any

subsequent composite bonding process. The die inlet is tapered at 7–100, with well rounded

edges to prevent fiber fracture. The excess resin exudes from the inlet end of the die,

Pultrusion Process



causing the entering fiber bundle to swell, eventually attaining equilibrium with the process

conditions. Adding this exuded resin to the resin bath will curtail the life of the bath.

Cartridge or plate heaters are preferred for heating the die to a uniform temperature within

±10C and maintaining a temperature gradient along the die to avoid premature gelation,

while taking into account any exotherm. An RF (Radio Frequency wave generator) unit can

be used to either heat the fiber entering the die or the die/resin. The die must be preheated

prior to commencement of pultrusion.

Shrinkage during polymerization reduces die forces and should always be arranged to be

greater than the thermal expansion caused by the temperature rise.

5. Pulling unit to provide traction

The pultruded product is cooled prior to the traction unit, which can be a counter rotating

caterpillar unit, or preferably, a hand-over-hand reciprocating clamp type unit, since the

caterpillar unit requires the tracks to be fitted with machined rubber pads to accommodate

each pultruded profile. The hand-over-hand unit grips above and below and while one unit

is pulling, the other unit returns to position, ready to take over the role of pulling. Typical

line speeds vary in the range 1.5–100 mh_1, depending on the section(s) being produced.

The pulling forces depend on the type of machine which are available upwards to some 30

MT.

6. Cut off saw

Once the pultruded section has left the die and cooled sufficiently, it is clamped and a flying

saw moves along with the clamped section to cut off required lengths. Extra long lengths

can be accommodated by feeding the pultrusion out through a door, window or hatch at the

end of the building.

7. Post cure oven

For optimum properties, all pultruded sections will require post curing and care must be

taken to ensure adequate support along the entire pultruded length to prevent deformation

occurring in the post cure oven.

Pultrusion Process



Following are some of the considerations while manufacturing and designing

pultruded parts.

Wall Thickness

Wherever possible, select uniform thickness in the cross-section because it provides uniform cooling

and curing, and thus avoids the potential of residual stress and distortions in the part. Moreover,

uniform thickness will provide uniform shrinkage in the part and thus will limit the warpage in the

product. Typically, 2 to 3% shrinkage occurs in the pultruded part. Also, maintain symmetry in the

cross section for minimal distortion. For high-volume production, the thickness of the part is critical

because the curing time and therefore the rate of pull depend on the thickness of the part. For

example, a 0.75-in. thick cross-section can be produced at a rate of approximately 9 in./min,

whereas a 0.125-in. thick cross-section can provide a production rate of 3 to 4 ft/min. Therefore, if a

design requires high rigidity in the part, then it can be achieved by creating deeper sections with

thinner wall or by including ribs in the cross-section. Similarly, if there is a choice between selecting

a thick rod or tube, select the tube because it offers a higher production rate, lower cost, and higher

specific strength.

Corner Design

In a pultruded part, avoid sharp corners and provide generous radii at those corners. Generous radii

offer better material flow at corners as well as improve the strength by distributing stress uniformly

around the corner. A minimum of 0.0625-in. radius is recommended at corners. Another important

consideration in the design of corners is to maintain uniform thickness around the corner. This will

avoid the build-up of resin rich areas, which can crack or flake off during use. Moreover, uniform

thickness will provide uniformity in fiber volume fraction and thus will help in obtaining consistent

part properties.

Tolerances, Flatness, and Straightness

Dimensional tolerances, flatness, and straightness obtained in pultruded parts should be discussed

with the supplier. Standard tolerances on fibreglass pultruded profiles have been established by

industry and ASTM committees. Refer to ASTM 3647-78, ASTM D 3917-80, and ASTM D 3918-80 for

standard specifications on dimensional tolerances and definitions of various terms relating to

pultruded products. Pultrusion is a low-pressure process and therefore does not offer tight

tolerances in the part. Shrinkage is another contributing factor that affects tolerances, flatness, and

straightness. The cost of a product is significantly affected by tolerance requirements. Tight

tolerance implies higher product cost. Therefore, whenever possible, provide generous tolerances

on the part as long as the functionality of the product is not affected.

Surface Texture

Pultrusion is a low-pressure process and typically provides a fiber-rich surface. This can cause

pattern-through of reinforcing materials or fibers getting easily exposed under wear or weathering

conditions. Surfacing veils or finer fiber mats are used as an outer layer to minimize this problem. To

create good UV and outdoor exposure resistance, a 0.001- to 0.0015-in. thick layer of polyurethane

coating is applied as a secondary operation.

Pultrusion Process



Advantages of using pultrusion

Pultrusion has a number of benefits over other composite processing systems. Some of the

lowest cost, highest quality composite profiles are created by this process. This is because it

is automated and has very little manual interface. A manufacturer can be assured the 1st

ten-meters of pultrusion will have the same quality and consistency as the 100th ten-meters

of pultrusion. Human interface is eliminated, as required in most other processes, such as

molding and hand-lay-up. Quality is not a function of motivation of factory technicians.

Another distinct advantage of the pultrusion process is cost. It is not unusual to find 80-90%

of the cost of pultrusion profiles are the raw material costs. The amortized machine costs

and the labor to run pultrusion machines is a small portion of the total factory costs. This

has been a primary driver for pultrusion being one of the fastest growing and accepted

manufacturing processes in the composites industry.

Features Description Benefits Applications

Strong

Unit strength in tension &

compression is approx. 20

x that of steel when these

properties are combined

on the basis of unit

density

Optional strength as

desired. Exceptionally

high impact strength

reduces damage

potential

Structural process

equipment support.

Tank supports. Cooling

tower ancillaries.

Flooring supports.

Trusses & joints.

Light Weight

Density of pultruded

components is about 20%

of steel and 60% of

aluminium

Higher performance at

less weight. Lower

shipping, handling &

installation costs. Less

operational energy

demand.

Automotive leaf springs

& bumpers.

Prefabricated walkways

& platforms. Bus

components.

Corrosion

Resistant

Unaffected by exposure to

a great variety of corrosive

environment & chemicals.

Minimum maintenance

costs. Long term safety.

Longer life.

Chemical plant hand

railings, gratings,

walkways & bridges.

Cable trays. Pipe

supports.

Electrical

Insulation

Provides strength &

rigidity with dielectric

properties.

Lesser no. of

components. Non-

magnetic & safe.

Predictable insulation

values for wide range

of frequencies.

Ladders, Cable trays.

Switch gear

components. Mounting

braces and backboards.

Pultrusion Process



Thermal

Insulation

Pultruded components

have a low thermal

conductivity, 1/250 of

aluminium & 1/60 of steel.

Reduces installation

thickness. Eliminates

condensation

problems. Reduces

energy operation

requirements.

Bulk head frames. Walk

in refrigerator door

jams. Window frames.

Insulated roll up panel

doors.

Consolida-

tion

Many individual

components can be

combined into a large

profile.

Reduced assembly cost.

Reduced inventory.

Fewer parts improve

reliability.

Window latch supports.

Roll up door

Dimensional

Stability

Pultruded components are

highly resistant to warping

stretch/swelling over a

wide range of

temperature & stresses.

No permanent

deformation under high

stress. Close

tolerances.

Spring bumpers.

Crossing gate arms.

Scrubber components.

Safety

The pultruded

components are very

strong & safe to work

with. They are microbes

and insect proof.

Many gratings suffer

from the problem of

microbes etc. due to

wet or unhygienic

working conditions.

This property makes

them ideal choice for

pharmaceutical & food

industries.

Pultrusion Process



DISADVANTAGES OF PULTRUSION

1. It is suitable for parts that have constant cross-sections along their length. Tapered and

complex shapes cannot be produced.

2. Very high-tolerance parts on the inside and outside dimensions cannot be produced using

the pultrusion process.

3. Thin wall parts cannot be produced.

4. Fiber angles on pultruded parts are limited to 0°. Fabrics are used to get bidirectional

properties.

5. Structures requiring complex loading cannot be produced using this process because the

properties are mostly limited to the axial direction.

6. Voids may result in parts if excessive opening given at die entrance

7. Standards play an important role in acceptance of new materials. The lack of design

standards is a significant constraint to the use and growth of composites in structural

applications.

8. Shrinkage (commonly 2% - 3%)

Pultrusion Process



References

Pultrusion of Glass Fiber Composites - A Technical Manual (Owens Corning)

PULTRUSION — HIGH PRODUCTIVITY NOW, GETTING EVEN BETTER, By A. Brent

Strong/Brigham Young University

Manufacture by Pultrusion - Dr J M Methven, MACE

Pultrusion of Composites - An Overview, Atul Mittal & Soumitra Biswas

Carbon Fibers and Their Composites - Peter Morgan

COMPOSITES MANUFACTURING - Materials, Product, and Process Engineering,

Sanjay.K.Mazumdar, Ph.D.

pultrusion process - composite manufacturing

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