Pultrusion process improvements

One of the desired features of the technical plant stem is improved torsional stability and oscillation damping. As mentioned, the diagonal fiber bundles can be incorporated into the technical plant stem via a braiding machine, which was installed in-line with the pultrusion process (Figs 9.13 and 9.14). 

The selection of a suitable thermoplastic matrix material mostly depends on the desired mechanical properties and the desired long-term service temperature. Depending on the application area, there are other decision criteria known from thermoset matrix materials, which can be of major interest, for example the chemical resistance or the water absorption properties. In contrast, thermoplastic composites are normally featured with an improved toughness compared to their thermoset competitors. Table 8.2 gives an overview on properties of different common polymers which are used for the pultrusion process. 

Kowsika and Mantena (1996) proposed a statistical test pattern in manufacturing unidirectional graphite-epoxy composite beams using pultrusion. The influences of significant variables of the pultrusion process together with their interactive effects on the dynamic mechanical properties were investigated. Mathematical models were subsequently derived to determine the optimal pultrusion process conditions for improved dynamic mechanical properties of the finished product. 

The continued quest for improvement and innovation are expected to bring pultrusion process science to a level whereby newer products can be achieved with cleaner and more efficient energy options. 

Thunhorst K, Goetz D, Hine A and Sedgwick P (2011), The effect of nanosilica matrix modification on the improvement of the pultrusion process and mechanical properties of pultruded epoxy carbon fiber composites . Composites 2011, Fort Lauderdale, FL, American Composites Manufacturers Association. 

The characteristics of the three most common thermoset resin systems used in pultrusion are compiled in Table 11.2 [3]. It is noteworthy that unreinforced polyesters and vinylesters shrink 7-9% upon crosslinking, whereas epoxies shrink much less and tend to adhere to the die. These epoxy characteristics translate into processing difficulties, reduced processing speed, and inferior component surface finish. It is normal practice to use resin additives to improve processability, mechanical properties, electrical properties, shrinkage, environmental resistance, temperature tolerance, fire tolerance, color, cost, and volatile evaporation. It is normally the resin, or rather its reactivity, that determines the pulling speed. Typical pulling speeds for polyesters tend to be on the order of 10-20 mm/s, whereas speeds may exceed lOOmm/s under certain circumstances. Apart from the resins characterized in Table 11.2, several other thermosets, such as phenolics, acrylics, and polyurethanes, have been tried, as have several thermoplastics (as will be discussed in Sec. 11.2.6). 

Some fabrication processes, such as continuous panel processes, are run at elevated temperatures to improve productivity. Dual-catalyst systems are commonly used to initiate a controlled rapid gel and then a fast cure to complete the cross-linking reaction. Cumene hydroperoxide initiated at 50°C with benzyl trimethylammonium hydroxide and copper naphthenate in combination with tert-butyl octoate are preferred for panel products. Other heat-initiated catalysts, such as lauroyl peroxide and tert-huXyi perbenzoate, are optional systems. For higher temperature molding processes such as pultrusion or matched metal die molding at temperatures of 150°C, dual-catalyst systems are usually employed based on /-butyl perbenzoate and 2,5-dimethyl-2,5-di-2-ethylhexanoylperoxy-hexane (Table 6). 

Table 12.1 Aspects of pultrusion that contributed to the improvement of the process...

Various optimization studies on thermosetting as well as thermoplastic pultrusion parameters were conducted (Chew, 2004 Joshi et al, 2003 Kyaw, 2003 Li, 2001 Tat, 2006 Zaw, 2003) using the same FE/NCV method developed by Joshi and Lam (2001). Chew (2004) worked on cooling optimization in thermoplastic pultrusion whereas Tat (2006) studied the impact of mounting locations for the feedback thermocouples controlling heater temperatures. It was observed that if the thermocouples sense either the maximum or the minimum temperature on a hot die surface, the power input to the heater will be rather uneven and the process may not completely optimize. Joshi et al. (2003) looked for improved pultudate quality with the... 

Dub6 et al. (1995) addressed both the issues of volatiles and fibre wetting with reaction injection pultrusion (RIP) process used to produce thermoplastic polyurethane and thennoset polyisocyanurate matrix composites. The low viscosity constituents used in RIP help in improving fibre impregnation, while the small volume of the impregnation bath reduces emissions. 

The pultrusion industry has been growing and the growth is driven by newer applications for pultruded composites. Advances in materials and innovations in the process have enabled manufacturers to design new or improved products (Busel, 2008). 

CompositGS. There has been substantial progress in improving the processability of phenolics in composite materials (104-106) to the point where a variety of materials are available for pultrusion, filament winding, and resin-transfer molding (see Composites, Fabrication). More complicated composite systems are based on aramid and graphite fibers. 

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