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Composite Materials Research Group The University of Mississippi The Pultrusion Process
2012-02-08 10:34:59

The pultrusion process is one of the most cost-effective methods for the production of composite materials. It is a continuous process that produces little waste material. In the pultrusion process for thermoset resins, fiber reinforcement is pulled through a resin impregnation area to coat the reinforcement with resin, through preform plates to begin to shape the fiber/resin bundle, and through a heated die to cure the resin. A cured part in the desired shape that requires no further processing exits from the die. Although the process appears to be simple, numerous process variables such as pull speed, die temperature, quality of fiber/resin wet-out, and fiber volume can affect the quality of pultruded composites. In order to take full advantage of the pultrusion process, the effect each process variable has on mechanical properties must be completely understood. Because it is a cost-effective method for the production of advanced composites, the pultrusion process has tremendous potential for traditional composite applications as well as currently expanding applications such as infrastructure revitalizaiton.

The pultrusion process also offers a unique combination of characteristics. Due to the continuous nature of the pultrusion process, composites of any desired length can be produced. The pultrusion process can be used to fabricate profiles with simple or complex geometry; however, the part must have a constant cross-section down the length of the part. Travel through the die results in all surfaces of a pultruded composite being smooth, finished surfaces. Process speeds vary depending on the type of resin and the profile geometry, but faster line speeds are desirable to increase process efficiency and economics. Pultrusion can produce composites with higher fiber volume and, thus, higher mechanical properties than many processes can achieve. The automated nature of the process produces composites with more uniform properties that are not highly dependant on operator skill as they are for some processing methods such as hand lay-up or VARTM. Commercial applications for pultruded composites include standard structural shapes like I-beams and channel sections, nonconductive ladder rails and tool handles, grating, and window framing.

 

 

 

 

products & process: process description

Pultrusion is a manufacturing process for producing continuous lengths of FRP structural shapes. Raw materials include a liquid resin mixture (containing resin, fillers and specialized additives) and reinforcing fibers. The process involves pulling these raw materials (rather than pushing as is the case in extrusion) through a heated steel forming die using a continuous pulling device. The reinforcement materials are in continuous forms such as rolls of fiberglass mat or doffs of fiberglass roving. As the reinforcements are saturated with the resin mixture ("wet-out") in the resin impregnator and pulled through the die, the gelation (or hardening) of the resin is initiated by the heat from the die and a rigid, cured profile is formed that corresponds to the shape of the die.

While pultrusion machine design varies with part geometry, the basic pultrusion process concept is described in the following schematic.

The creels position the reinforcements for subsequent feeding into the guides. The reinforcement must be located properly within the composite and controlled by the reinforcement guides.

The resin impregnator saturates (wets out) the reinforcement with a solution containing the resin, fillers, pigment, and catalyst plus any other additives required. The interior of the resin impregnator is carefully designed to optimize the "wet-out" (complete saturation) of the reinforcements.

On exiting the resin impregnator, the reinforcements are organized and positioned for the eventual placement within the cross section form by the preformer. The preformer is an array of tooling which squeezes away excess resin as the product is moving forward and gently shapes the materials prior to entering the die. In the die the thermosetting reaction is heat activated (energy is primarily supplied electrically) and the composite is cured (hardened).

On exiting the die, the cured profile is pulled to the saw for cutting to length. It is necessary to cool the hot part before it is gripped by the pull block (made of durable urethane foam) to prevent cracking and/or deformation by the pull blocks. Two distinct pulling systems are used: a caterpillar counter-rotating type and a hand-over-hand reciprocating type.

In certain applications an RF (radio frequency wave generator) unit is used to preheat the composite before entering the die. When in use, the RF heater is positioned between the resin impregnator and the preformer. RF is generally only used with an all roving part.


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