Pultrusion is a continuous filament-reinforced plastic (FRP) manufacturing process used to produce highly reinforced plastic structural shapes. Unlike filament winding, which places the primary reinforcement in the circumferential (hoop) direction, pultrusion provides the primary reinforcement in the longitudinal direction. The typical pultruded product will exhibit higher mechanical properties in the longitudinal direction (0° ) rather than the transverse (crosswise) direction. The purpose of this document is to describe pultrusion from a processing viewpoint. The goal is to emphasize the practical considerations in manufacturing, although the theoretical issues will be mentioned. The process described in this article applies to a thermoset polymer; this section will not discuss the pultrusion of thermoplastics.

THE BASIC PROCESS

    Figure 1 depicts the basic components of a pultrusion machine. The standard pultrusion process combines longitudinal reinforcements (roving doffs in the figure) and transverse reinforcements (mat creels) to form a composite by impregnating these reinforcements with the polymer and curing the composite in a die. Roving resembles a glass rope while mat resembles a glass sheet.

    The term roving is used for glass reinforcement, but other longitudinal reinforcements such as carbon fiber or Kevlarä could be placed on the racks containing the roving doffs. The roving doff design will change as the form of the reinforcement changes because various generic reinforcements are packaged differently. For example, the glass reinforcement (roving) will be delivered from the inside of the cylindrical package while the carbon fiber reinforcement (known as tow) will be delivered from the outside of the cylindrical package. Thus, the roving reinforcement can be placed standing on a rack while the carbon fiber reinforcement requires a mandrel system because of the outside pull (off axis). The mat adds the transverse reinforcement to the system and is pulled from the outside of a roll. Only two mat stations are shown in Figure 1, but multiple mat layers could be added to the pultruded profile at different points within the composite. Adding more than two layers of transverse reinforcement would require alternating it with layers of the roving or other longitudinal reinforcement, which in turn would require a more complex arrangement than shown. A continuous roving layer which is too thick may cause processing problems in the form of cracks.

    The reinforcements are pulled through a guide plate that aids in locating the reinforcing materials correctly in the final pultruded part. The aligned materials are then passed through a resin impregnation chamber which contains the polymer solution. The polymer solution impregnating the reinforcements acts as a glue connecting the various components of the reinforcement. The polymer solution (sometimes known as resin mix) contains the polymer resin in addition to filler, catalyst, and other additives that enhance the performance of the pultruded structural shape.

    Surfacing material is generally added to pultruded structural shapes after the impregnation step. The surfacing material has insufficient tensile strength to withstand pulling in the wet impregnation area and also tends to wet out (become completely saturated with the resin mixture) more easily than the reinforcements. It should be noted that the reinforcements generally do not pass directly through the impregnation chamber in a straight line, but over transverse breaker bars, which spread the reinforcement layers so that the solution can better impregnate the reinforcement.

    The saturated reinforcements exiting the resin impregnation chamber area are generally flat and shaped into an approximation of the final configuration imparted to the product in the preformer to reduce stress in the part. The curing of the product (changing from a wet saturated reinforcement to a solid part) occurs in the curing die. Most pultrusion processors use an electrical or oil-heated system with one to four separate heating zones on the die surface. The number of heating zones is dictated by the speed of the process, type of resin to be cured, length of the die, and type of heating source. More heat zones increase speed.

    After exiting the die, the part passes through a pull system. Figure 1 shows the caterpillar type of puller in which a series of rotating blocks pull the product. Unlike extrusion, which pushes the part from the entrance of the die, pultrusion pulls the part from the exit of the die. Another common type of pultrusion puller is the hand-over-hand pull system (see Figure 2). In the hand-over-hand system, puller number-one drives the part exiting the die and transports it some distance to puller number-two. Puller number-two will then grab the part, and puller number-one releases and returns to its original position; puller number-two moves the part to the saw. The arrows beneath puller number-one and puller number-two in the figure indicate the motion of the two pullers, which pull the part consecutively (one at a time) instead of operating simultaneously. During the return step, the puller not functioning is not in contact with the part. The choice of pull system is one of the possible variations in basic pultrusion machine design. The caterpillar system supplies more pull force but may crush weaker pultruded shapes.

    Two other potential variations are the use of a coolant system and the use of radio-frequency (RF) heating. Some pultruders will utilize air and/or water cooling in the space between the curing die and the pull blocks to cool the hot part (from the exothermic reaction) before it enters the pull blocks. The pull block material is generally some form of urethane, and the pressure applied by the pull blocks could distort the pultruded shape if the product enters the pull blocks at too high a temperature. If the area between the pull blocks and the curing die is sufficiently long, no supplemental cooling is required, but this may not be possible for some industrial layouts. In the other potential variation, a radio-frequency heating unit is placed after the resin impregnation step and before the curing die. This is a form of preheating the composite and is very useful in accelerating the curing reaction for pultruded composites consisting of only longitudinal glass reinforcement. Carbon fiber reinforcement cannot be heated in a radio-frequency unit because of potential fire hazards.

RESIN

    Some thermoset resins are processible using pultrusion. Polyesters, vinyl esters, epoxies, and phenolics are currently processed commercially, and each of the resins have particular characteristics advantageous for processing and performance. Each of the four generic classes of resin can be further subdivided for other attributes.

    Polyester resins processed in pultrusion are generally isophthalic polyester, terephthalic polyester, or orthophthalic polyester. Orthophthalic polyesters offer a cost advantage in pultrusion but are not as corrosion-resistant as isophthalic or terephthalic polyesters; there is a substantial amount of pultrusion performed with the isophthalic polyester resins. Besides cost and corrosion resistance, factors affecting resin selection are resistance to high temperatures and the compatibility of the polyester resin with the glass. A high-heat distortion temperature (HDT) or glass transition temperature (T_g) indicates a resin with higher temperature resistance. However, such resins tend to be more brittle and are generally processed in thinner shapes. The compatibility of the polyester resin with the glass is measured by the wet out (degree of saturation of the reinforcement). Strengths will be lower if the wet out is deficient.

    The composite arrangement also influences resin selection. Roving is a glass strand composed of many individual, very small filaments that must be individually wet out to optimize the strength characteristics; lower viscosities enhance roving wet out. The continuous strand mat that is often used in pultrusion requires a thicker resin to prevent the resin mix from bleeding through the porous continuous strand mat.

    The cure properties of the polyester (reaction rate) must be selected in conjunction with the heating and catalyst systems. A common measure of reaction rate that has been utilized for years in the pultrusion industry is the 180° F (82° C) SPI gel time; newer techniques using a differential scanning calorimeter (DSC) can be used. The material having the lowest gel time is not always the fastest curing resin. In addition to the usual catalyst and heating system adjustments to resin curing, some pultruders will heat the resin bath to accelerate the cure reaction. In this case, the catalyst must be selected with special care to prevent the resin mix form gelling prematurely.

    Vinyl ester resins tend to wet out (saturate) reinforcements more efficiently and produce higher test results. Vinyl ester resins tend to have a higher temperature capability with improved flexibility (toughness) and improved corrosion.

    Epoxy resin systems are a further extension of vinyl ester resins for strength and can be produced with an amine or anhydride curing system; the vinyl ester and polyester resin matrices are peroxide-cured. The epoxy resin system requires a significantly higher reinforcement content than either the polyester or vinyl ester resin matrices. In addition to their higher reinforcement content, epoxy resins also have a significantly shorter pot life than polyester or vinyl ester systems increasing the difficulty in processing epoxies. Pot life is the amount of time that the resin mix maintains stable properties (viscosity, curing) in the resin impregnation area. A 24 hour resin mix pot life is required for pultrusion although shorter pot lives can be accommodated with special provisions.

REINFORCEMENT

    The reinforcement material is selected by chemical type and form. The chemical types include glass, carbon fiber, aramid, and polyester fibers; other types of reinforcements may be selected for special applications. The forms of reinforcement include rovings (tows, for carbon fiber), stitched rovings in different orientations, continuous strand mat, chopped strand mat, woven rovings, and bulk rovings. The effects of resin and reinforcements are discussed in "Composite Selection" which follows.

    Glass is the typical reinforcement in pultrusion available in roving, continuous strand mat, and stitched rovings (0° , 90° , and ± 45° orientations). Roving is typically E-glass while mat can be "E" or "A" glass. The difference between "E" and "A" glass is a chemical composite with "A" glass containing more soda. Glass is coated with a binder which enhances compatibility with the resin matrix. The glass weight is generally expressed as yield (yards per pound), the mat weight is generally expressed as ounces per square foot, and the stitched fabric weight is expressed as ounces per square yard.

    Carbon fiber increases the stiffness of the composite significantly in the lengthwise direction. It is difficult to find a continuous strand mat type of carbon fiber product, and carbon fiber products requiring transverse reinforcement are either mixtures of transverse glass reinforcements with lengthwise carbon fibers or stitched carbon fiber products. A stitched carbon fiber product is significantly more expensive than the all longitudinal carbon fiber product. Carbon fibers produce a stiffer composite than glass fibers which produce a stiffer composite than polyester fibers.

    Mixing reinforcement types may cause warpage problems because of differential shrinkage; carbon fiber has a negative coefficient of thermal expansion. Reinforcements compatible with one type of resin matrix may not be compatible with another type.

OTHER RAW MATERIALS

    Other raw materials include filler, catalyst, anti-ultraviolet additives, and release agents in the standard pultrusions. Filler acts as a resin extender and reduces porosity in the surface of the pultruded composite. The filler must be selected judiciously because of the wet out requirements described previously. A small amount of filler is used with all roving composites while larger amounts of filler are used with the mat/roving composite. The particle size of the filler is an important design characteristic, as smaller particles will absorb more resin on the surface and tend to increase the resin mix viscosity. Calcium carbonate filler is used where cost is a consideration while clay filler (kaolin) is preferred for corrosive applications. Aluminum trihydrate filler can enhance the performance in some flame retardance tests, but once the water of hydration is consumed, the remaining aluminum oxide does not prevent flaming.

    Pultruders commonly call the peroxide initiators catalysts, although these additives are not catalysts in the true sense of the term, since they are consumed in the thermoset reaction. Various catalysts can be utilized in pultrusion, and the choice is determined by the thickness of the part and the heating package available for the pultrusion process. Thermoset pultrusion is an exothermic reaction, and parts will generate higher levels of heat within themselves; systems running too hot can cause a crack in the center of a thick part. Many pultruders use a multiple catalyst system, although many successful commercial pultrusions have been produced using only a single type of catalyst system, benzoyl peroxide (BPO). Catalysts are classified as high, low, or medium temperature catalysts by their 10 hour half life; catalysts with low 10 hour half lives initiate at lower temperatures. Thicker parts tend to have higher levels of low temperature catalysts and lower levels of high temperature catalysts, and the reverse is true for thinner parts.

    Ultraviolet (UV) inhibitors are often placed in pultruded parts to enhance discoloration resistance. There are two forms of weathering damage in pultrusion: discoloration and fiber exposure. The UV inhibitor will help to mitigate discoloration while a synthetic surfacing veil will help to prevent glass fiber exposure. Glass fiber exposure gives a prickly feel to the surface of a part. If the surface veil degrades, it gives a powdery appearance to the part surface.

    Release agents are added to the pultrusion resin mix to enhance part separation from the surface of the metal die. Release agents, both in powder and liquid form, may be processed successfully. Most pultruders favor a liquid release agent; powders can cause a problem if the die surface is not hot enough to melt the powder. Acidic release agents (pH less than 7) tend to function very well in pultrusions, although they have a negative effect on the die surface. Steel dies exposed to acidic release agents must be cleaned thoroughly between uses.

    Cycle time depends on the choice of the catalyst system, resin matrix, and heating package for the die. Working in conjunction, these factors determine the cycle time for the pultrusion process. These factors are extremely important if the cycle time is equated to line speed and inches per minute (meters per minute).

    One of the more subtle effects on cycle time is the choice of composite. Some reinforcement materials can be processed substantially faster than others, and some shapes can be processed faster than others. Complicated shapes requiring substantial bending of the reinforcements will be processed slower to prevent folds during the bending operation. Pultruded parts that process very well at 36 in/min (91 cm/min) may not process well at all at 60 in/min (152 cm/min) with the same tooling (preformer and guide plates); the tooling must be optimized for a particular speed. Composites containing higher reinforcement levels will process slower because of the increase in pull force. Reinforcements holding too much resin must have the excess resin stripped at the die entrance. Stitched fabrics may inhibit line speed because of their tendency to deform. Continuous strand mat which tears easily will also reduce line speeds.

COMPOSITE SELECTION

    The selection of resin and reinforcement will be driven by the desired product performance. Some rules of thumb for selecting a composite are:

Systems exposed to caustic chemicals will require a vinyl ester resin.

Some mechanical properties are enhanced by using epoxy resin rather than vinyl ester resin, and vinyl ester resin is superior to polyester resin for mechanical performance.

The binder on the reinforcement must be matched to the resin.

Epoxy resins tend to offer more fatigue resistance than vinyl ester or polyester resin.

Epoxy resins tend to be significantly more difficult to process than polyester or vinyl ester resins. Epoxy resins require higher reinforcement levels.

Phenolic systems can process easily.

Stitched reinforcement improves the transverse mechanical properties but also increases the cost.

Chopped strand mat will reduce the mechanical property performance in coupon and structural testing.

Higher roving levels in composites tend to have a reduced 24 hour water absorption rate when compared to a mat/roving composite (ASTM D-570).

The ± 45° stitched reinforcement enhances torque or twist properties within the composite but is not an efficient way to add a 90° orientation coupon for tests.

The higher the roving content in the pultruded shape, the better the lengthwise mechanical properties.

Phenolic resins require higher reinforcement levels.

TOLERANCES

    The tolerances for pultruded structural shapes are occasionally governed by ASTM D-3917 and ASTM D-4385; these are only generalized tolerances, and the individual pultruder and designer may require other tolerances. A common mistake is for the designer to expect the steel tolerances to be maintained in pultruded shapes. Different pultruders have different techniques for adjusting to tighter tolerances from the customer.

    Transverse reinforcement is shipped to the pultrusion manufacturer on rolls and must be slit to the appropriate width of the part. The variation in the slitting widths will cause some variation in the localized reinforcement contents within the part. This localized variation will have some impact on mechanical property performance and will also impact the dimensional performances as reflected by part shrinkage.

    Hollow shapes are formed by wrapping the transverse reinforcement around the hollow shape. If the transverse reinforcement is wrapped so that the two ends butt together, a localized weakness may occur at the butt joint. If the transverse reinforcement is wrapped so that the two ends overlap to prevent the butt joint weakness, then some differential glass content will occur causing some dimensional variation.

    The weights of reinforcements received from different reinforcement suppliers vary. This weight variation can reflect itself in localized variations of the part shrinkage. There is also a variation in the individual shrinkage rates of the received resin which can have a similar effect. The use of low shrinkage additives in the resin reduces this variation but at the cost of mechanical properties and possibly corrosion performance.

    The most confusing tolerance aspect of pultruded structural shapes is straightness. There are many different definitions of straightness, and it is incumbent on the pultruder and customer to verify that they are using the same definition. Straightness is probably the single largest area of confusion between pultruders and their customers.

TROUBLESHOOTING

    The troubleshooting procedure for a particular pultrusion process depends primarily on the composite, mode of processing, and pultrusion machine. However, a few general statements can be made about troubleshooting.

    Many problems arise from excessive die wear which shows up as an increased amount of scales on the pultruded structural shape. If it is not possible to change the heating profile and/or add wax to the pultruded structural shape to improve this situation, the best solution is to change and/or repair the die. This problem will most often appear in the tips of flanges. The choice of the filler and release combination is sometimes an occasion for concern. An acidic release (pH less than 4) should not be used with a calcium carbonate filler.

    Many problems occur when the guide plates and/or preformers begin to wear from the abrasion of the glass reinforcements on the metallic or plastic surfaces. Improperly cut transverse reinforcement widths are often a cause for many scaling problems. This would have the same effect as a worn die.

    An improperly specified polyester or vinyl ester resin can be the cause of some pultrusion problems. The resin supplier often specifies the 180° F (82° C) gel time, peak temperature, and peak time. Another specification is to define the 180° F (82° C) gel time, peak temperature, and time interval between the gel time and the peak time. The time internal between the gel time and the peak time is a better reflection of the inherent reactivity of the resin.

    Contamination can be found in the incoming resin, especially if tankwagons are used, and in the continuous strand mat or stitched reinforcements. The contamination might take the form of an off colored (black or brown) speck, but contamination has also been seen as specks of other colors. Contamination can also be more subtle in that the continuous strand mat product may have been "overcooked," giving a darker appearance on the pultruded part; this will be a problem with lightly shaded profiles.

    Pigment and catalyst suppliers are generally very consistent in delivering quality products; however, catalyst stored at very cold temperatures may lose some of its potency in the pultruded product.

    Common causes for lack of straightness (camber) are misalignment of the pultrusion machine and unequal tensioning of the reinforcements. Unequal tensioning of the reinforcements may occur when the structural shape contains many rovings that must travel significantly different distances into the pultruded part. Low profile additives can aid straightness but at the cost of other properties. An improperly aligned preformer can cause mat and/or roving shifts, producing scales, etc. Bulk roving can be used for parts that are designed with sharp edges. Although the polymer resin is often thought to be the cause of a problem, in reality, few problems are caused by this raw material.