Crystalline thermoplastics, shrinkage

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 thermoset (TS) plastics and reinforced thermosets (RTSs) are more suitable to meet tight tolerances. With amorphous and crystalline thermoplastics (Chapter 1) reinforced thermoplastics (RTPs), and particularly unreinforced thermoplastics (UTPs) can be more complicated tolerance-wise if the fabricator does not understand their behavior. Crystalline plastics generally have different rates of shrinkage in the longitudinal (melt flow direction) and transverse directions when injection molded. 

The primary motivations for blending a crystalline thermoplastic polyester such as PBT with other polymers are (a) to improve the solvent resistance and process-ability of amorphous polymers such as PC, styrenics, PPE, etc. (b) to reduce the mold shrinkage of the thermoplastic polyesters associated with their crystallization (c) to increase the DTUL of the unfilled polyesters and (d) to improve toughness. 

ASTM D955 describes a method to measure mold shrinkage. Both the mold and the molding are at room temperature when the mold shrinkage is measured. The values of 0.022 in./in. in the flow direction and 0.018 in./in. in the transverse direction are typical for nnfilled crystalline thermoplastics. 

Rimplast materials from Petrarch are silicone/thermoplastic IPNs that combine the warpage and wear resistance properties of silicone with nylon, thermoplastic polyurethane, or styrene/butadiene block copolymers. The combination of properties these IPNs possess make them suitable for high quality, high tolerance gear and bearing applications that can benefit from the addition of internal lubrication and isotropic shrinkage of silicone to the fatigue endurance and chemical resistance of crystalline resin, while processability still remains the same. 

Typical thermoplastic binders which are found in literature for injection molding of ceramic bodies are, styrene-butadiene, polyethylene, polypropylene, polybutene, ethylene vinyl acetate, polymethylmethacrylate and polyoxymethylene. When selecting one of these binders for thermoplastic extrusion of ceramic bodies, it should be noted that the shrinkage of par-tially-crystalline polymers is higher than for amorphous polymers, and hence warping during cooling is more critical in the former case. This is, however, not the only criterion for selection price and processability at adequate temperatures are also important factors to consider. 

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