Pultrusion

Pultrusion is a simple technique which is employed mostly for fabrication of products based on unsaturated polyester resins (or other resins like epoxy, vinyl ester) and continuous strand mats (fibre or fabric). A pultrusion machine consists of creel for supplying fibre, a resin tank, forming dies, machined dies with a temperature control facility, a puller and a saw for cutting the product from a continuous composite product. The continuous strand mats are passed through a bath containing a mixture of resin and curatives. In the resin bath, the fibre is passed through a series of rods to remove entrapped air and excess resin to ensure complete wetting of the fibre. 

Pultrusion is a process which involves pulling resin-impregnated rovings and/or fabrics through one or more heated dies to provide the profile shape and cure the composite. A wide variety of shapes can be produced. 

Because of the continuous nature of manufacture, process conditions have to be optimized thus good and reproducible components and properties can be attained. Good fibre alignment is achieved through complete automation of the reinforcement lay-up. A variety of fibre orientations can be achieved by using a suitable combination of rovings and fabrics. 

There is a considerable amount of data available principally from a few suppliers and moulders. Consequently, designers are able to incorporate the information directly into the design provided that a suitable profile and fibre orientation/property level are available. 

As very little mechanical data have been quoted in the text, some typical properties follow. 

Properties in the form of data sheets are available from some moulders. A set of properties for a structural profile is given in Table 11.17. 

In contrast to extrusion, in pultrusion a combination of liquid resin and continuous fibers is pulled continuously through a heated die of the shape required for continuous profiles. Shapes Include structural I-beams, L-channels, tubes, angles, rods, sheets, and so on, and the resins most commonly used are polyesters with fillers. Other resins such as epoxies and urethanes are used where their properties are needed. Longitudinal fibers are generally continuous rovings. Glass fiber material (mat or woven) is added for cross-ply properties. 

Control devices must be used to ensure that the proper resin impregnation occurs and is held within the required limits. Simple devices, such as doctor rolls or squeeze rolls, are usually sufficient. It is important to control the resin viscosity. The most difficult part to set up is the shape of the opening in the die/mold. Experience and/or trial and error are required. 

This technology is characterized by a continuous production when the system is equipped with an automated flying cutting saw, production only requires a reduced human presence, limited to system startup and control of possible power interruptions to reinforcement or to the level of resin in the impregnation basin. Only recently has this technology found relevant industrial applications, but, actually, its first application dates back to 1948 and the first patent goes back to 1951. The first products manufactured by pultrusion were high-precision bars, which are stiU the most popular product today. 

Quality-control tests and important process variables 

The important process-control parameters in pultrusion manufacturing are the pull speed, fibre volume fraction, viscosity, temperature settings for die heating zones, and the preform plate area ratio (compaction ratio). Key process variables include die pressure and temperature. 

To produce pultruded products with consistent and high quality, it is important to tailor and control the pultrusion process. To achieve a uniform degree of cure in the cross-section of a product, control of the temperature profile inside the pultrusion die is an essential aspect. Also, to achieve consistent fibre wetting, controlled flow and pressure build-up must 

Cure modelling of polyester pultrusion systems was carried out by Ng and Manas Zloczower (1989). A mechanistic model that couples free-radical polymerization and diffusion control (Section 1.2.3) was used for the cure kinetics and is shown here  

Initial modelling on predicting velocity profiles in pultrusion dies was carried out by Gorthala et al. (1994). Here a two-dimensional mathematical model in cylindrical co-ordinates with a control-volume-based finite-difference method was developed for resin fiow, cure and heat transfer associated with the pultrusion process. Raper et al. 

In reaction injection molding, fillers and the reacting mass of matrix may each participate in the reactions, forming systems with a combination of features derived from the high adhesion between components. Numerous variations of this process 

Entec Composite Machines Inc. (ECM), (now Zoltek Ltd.) Salt Lake City, Utah, USA 

Sales channel Martin Pultrusion Group Twinsburg, Ohio, USA 

Morrison Molded Fiberglass Co.,(MMFG), Bristol, Virginia, USA 

Pultrusion Dynamics, (a Division of Creative Pultrusions Inc.) Oakwood Village, Ohio, USA 

1 able 6.2 Fibre arrangements pttssihle in fibre-reinforced thermoset polymers 

Pultrusiun Filament winding Hand lay-up Hand spray-up Compression moulding 

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The Automatic processes are those such as pultrusion, filament winding, centrifugal casting and injection moulding. 

Rosato, D.V. ed. Rosato, D.V. (ed.) Blow Moulding Handbook, Hanser, Munich (1989). Hepburn, C. Polyurethane Elastomers (ch 6-RIM) Applied Science Publishers, London (1982). Martin, L, Pultrusion Ch 3 in Plastics Product Design Handbook - B ed by E. Miller, Dekker Inc, New York (1983). 

Pultrusion is also used to make structural shapes from composite materials. The incoming material is generally unidirectional and must be pulled through the pultrusion die because the uncured composite material is entirely too flexible to push (as in extrusion processes). The incoming material can be preshaped by various guides and rollers as in Figure 1-18. 

For composite stiffeners, all shapes are builtup from individual layers of material. Of course, some stiffener shapes can be produced by roll forming or pultrusion, for example, and then fastened to panels. Or, the stiffened panel could be made in a single operation involving the placement, usually by hand, of individual laminae of various dimensions in positions such that a builtup structure results. Stiffeners can be fastened to panels by bonding, stitching, or mechanical fastening. 

Process used provides different control capabilities. As an example closed molding (injection, compression, etc.) provides fine detail on all surfaces. Open molding (blow molding, thermoforming, spray-up, etc.) provides detail only on the one side in contact with the mold, leaving the second side free-formed. Continuous production (extrusion and pultrusion) yields products of continuous length. Hollow (rotational or blow) produces hollow products. These processes can be used creatively to make different types of products. For example, two molded or thermo-formed components can be bonded together to form a hollow product, or they can be blow molded. 

Unidirectional construction Refers to fibers that are oriented in the same direction, such as unidirectional fabric, tape, or laminate, often called UD. Such parallel alignment is included in pultrusion and filament winding applications. 

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