The Pultrusion Process

Dateline: 10/13/97

The next major feature I am working on is about a composite pedestrian bridge. The bridge is made almost entirely from stock fiberglass pultrusions. This week, as an "introduction" to that feature, we'll take a look at the pultrusion process itself.

Composites have many properties that make them attractive for structural projects, but manufacturing is often difficult. Traditionally, composite manufacturing methods are very labor intensive. Manufacturing development has focused on automated processes that can produce large quantities of finished product.

One of the most efficient manufacturing techniques is pultrusion. Thousands of yards of finished structure can be made in a continous process. The major limitation is that the finished product must be straight and of a constant cross section.

Reinforcements

The first step in the pultrusion process is to setup the materials for pulling. Roving (see Fabric Terminology) may be used for unidirectional pultrusions, or fabrics may be used to add off-axis fibers. Because the entire cross section is pulled at once, instead of building it up as in a traditional layup, a large number of spools are required. Figures 1 and 2 show the creels required for pultruding a 1/2 inch diameter solid rod.

Figure 1: Overview of creel racks at start of pultrusion line

Figure 2: Closeup of creel racks holding spools of graphite fiber

With such a large number of strands, tangles could easily form. Immediately after leaving the creels, the fibers are passed through an alignment card (Figure 3). This card also prevents twisting of the roving.

Figure 3: Rovings passing through an alignment card just before entering the resin bath

Almost any type of fiber may be used, but fiberglass is by far the most common. Graphite is also used, but presents additional challenges in the design of the die (see below).

Resin Impregnation

Once all of the fibers are aligned, they are passed through a resin bath (Figure 4). To make sure the fibers are fully wetted, the strands are passed through a series of rollers which flatten and spread out the individual rovings (Figure 5). This process is similar to impregnation during filament winding, but the baths are usually much longer.

Figure 4: Close up of fibers passing through a resin bath

Figure 5: Rolling stations for working resin into fiber bundles (note die entrance at far end)

Resin choices are just as varied as for other manufacturing methods and include polyesters, vinyl esters, and epoxies. Property requirements, however, restrict the choices somewhat. Pultrusions resins must have a fairly low viscocity (for good, rapid impregnation), a long pot life (for continuous processing), and a short cure time (for full cure during the time the pultrusion is in the die).

Pultrusion Die

The key to any pultrusion process is the die. Loose fibers enter the die (Figure 6) and exit as a cured part (Figure 7). The die must maintain fiber alignment, compress the fibers to the desired volume fraction, and cure the composite in a relatively short period of time. This must be done without damaging the fibers. Die design is probably the most difficult part of establishing of pultrusion run.

Figure 6: Aligned and impregnated fibers entering a die for making rods

Figure 7: Cured rod exits from heating platens located on the far end of the die

Pulling and Cutoff

By necesseity, the pulling station is located at the end of the pultrusion line. A set of padded clamps grips the cross section (Figure 8). The grips are translated horizontally to pull the section. In order for the process to be continuous, two sets of grips are used, with one pulling while the other travels back to its initial position. Typical pulling forces are on the order of six to eight tons, but can go higher.

Figure 8: Pultrusion is pulled along by the hydraulic pulling station

Finally, a cutoff saw is located beyond the pulling station. This are usually some sort of radial saw. The saw must be mounted on a table that moves with the pultrusion, so it can make clean cuts as the pultrusion continues to travel.

Additional Information

The above description of the pultrusion process is meant as a brief introduction. Many variations on the process are possible, and certain materials and cross sections pose additional problems.

For slightly longer introductions to the process, see the ASM Engineered Materials Handbook or the Composites Engineering Handbook.

Many pultrusion manufacturers have Web sites (see my library listing, Manufacturers: Pultrusions). These sites usually have pictures of the machines and some information on the process itself. In particular, Pultrusion Dynamics specializes in die design.

All photos courtesy of the Air Force Phillips Laboratory.

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