Amorphous plastics, shrinkage

Distortion in mouldings can be worse in crystalline polymers than in amorphous plastics. This is because additional stresses may be set up as a result of varying crystallinity from point to point in the moulding so that the shrinkage on cooling from the melt also varies from point to point. This uneven shrinkage sets up stresses which may lead to distortion. 

Different plastics have different amount of shrink upon cooling, higher for semicrystalline resins (eg, polypropylene can shrink 0.030 in./in.) and lower for amorphous plastics (eg, polystyrene can shrink 0.006 in./in.). Further, shrinkage, especially for semicrystalline materials, varies due to thickness, cooling rate, and often color. Whatever stays hotter for a longer time will shrink more. Therefore, changes in nominal wall thickness cause differential cooling, which causes... 

Plastics shrink in molds during the cooling process. Shrinkage for amorphous plastics is less than for semi-crystalline plastics. Shrinkage is larger in the thickness direction than other directions. Orientation of plastic occurs in the direction of flow. 

Volume shrinkage of a plastic in the mould cavity is 5-7% for amorphous plastics, but... 

Amorphous plastics are basically transparent (exceptions styrene copolymers containing butadiene (B), such as SB and ABS). Amorphous plastics have lower shrinkage values than semi-crystalline compounds, so with amorphous compounds higher levels of accuracy can be obtained at lower cost than with semi-crystalline materials. 

As Carfagna et al. [61] suggested, the addition of a mesophasic polymer to an amorphous matrix can lead to different results depending on the properties of the liquid crystalline polymer and its amount. If a small amount of the filler compatible with the matrix is added, only plasticization effect can be expected and the dimensional stability of the blend would be reduced. Addition of PET-PHB60 to polycarbonate reduced the dimensionality of the composite, i.e., it increased the shrinkage [42]. This behavior was ascribed to the very low... 

Fig. 13.13 Shrinkage distributions of injection molded amorphous PS (a) at two injection rates longitudinal direction, (b) solid curves, longitudinal direction broken curve, transverse direction. [Reprinted with permission from G. Menges and G. Wiibken, Influence of Processing Conditions on Molecular Orientation in Injection Molds, Soc. Plastics Eng., 31st Annual Technical Conference, Montreal, Canada, 1973.]...

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. 

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