Thermoplastics shrinkage

The pressure of the extmder forces uniform plastic distribution throughout the mold. Cooling the mold solidifies the plastic with slight shrinkage. The mold is maintained closed by mechanical or hydraulic pressure while the thermoplastic is injected and solidified. 

Noryl is a rigid dimensionally stable material. Dimensional stabiUty results from a combination of low mold shrinkage, low coefficient of thermal expansion (5.9 x 10 per° C), good creep resistance (0.6—0.8% in 300 h at 13.8 MPa (2000 psi)), and the lowest water absorption rate of any of the engineering thermoplastics (0.07% in 24 h at room temperature). Noryl resins are completely stable to hydrolysis. They are not affected by aqueous acids or bases and have good resistance to some organic solvents, but they are attacked by aromatic or chlorinated aUphatic compounds. 

Crystalline polymers undergo a discontinuous decrease in volume when cooled through (Fig. 4). This can lead to nonuniform shrinkage and warping in molded objects. On the other hand, it also causes the polymer to "lock on" to reinforcing fibers, eg, glass (qv), so that crystalline thermoplastics benefit much more than amorphous thermoplastics from fiber reinforcement. 

The sliced sheet will still contain large quantities of alcohol and it is necessary to season the sheet at elevated temperatures. This may only take three days at 49°C for 0.010 in (0.025 cm) thick sheet but will take about 56 days for 1 in (2.5 cm) thick blocks. The removal of alcohol, as might be expected, is accompanied by considerable shrinkage. Fully seasoned sheet has a volatile content of 2%, the bulk of which is water but there is some residual alcohol. The sheet may be fully polished by heating in a press between glazed plates under pressure for a few minutes. Because the material is thermoplastic it is necessary to cool it before removal from the press. 

Low shrinkage - all thermoplastics are processed in the amorphous state. On solidification, the random... 

Alterations by moisture exposure are weak the shrinkage and coefficient of thermal expansion are rather low, typical of amorphous thermoplastics creep resistance is good, the more so as the glass fibre content increases. 

The softened or melted thermoplastic is forced into a mould cavity, and then cooled to solidify it and acquire its final performances. Apart from a coefficient of shrinkage and possibly warpage, the part has the shape of the cavity. 

The primary resin of interest is epoxy. Carbon-fiber-epoxy composites represent about 90% of CFRP production. The attractions of epoxy resins are that they polymerize without the generation of condensation products that can cause porosity, they exhibit little volumetric shrinkage during cure which reduces internal stresses, and they are resistant to most chemical environments. Other matrix resins of interest for carbon fibers include the thermosetting phenolics, polyimides, and polybismaleimides, as well as high-temperature thermoplastics such as polyether ether ketone (PEEK), polyethersulfone (PES), and polyphenylene sulfide. 

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). 

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