Semi-crystallinity injection moulding

Polyolefin foams are easier to model than polyurethane (PU) foams, since the polymer mechanical properties does not change with foam density. An increase in water content decreases the density of PU foams, but increases the hard block content of the PU, hence increasing its Young s modulus. However, the microstructure of semi-crystalline PE and PP in foams is not spherulitic, as in bulk mouldings. Rodriguez-Perez and co-workers (20) showed that the cell faces in PE foams contain oriented crystals. Consequently, their properties are anisotropic. Mechanical data for PE or PP injection mouldings should not be used for modelling foam properties. Ideally the mechanical properties of the PE/PP in the cell faces should be measured. However, as such data is not available, it is possible to use data for blown PE film, since this is also biaxially stretched, and the texture of the crystalline orientation is known to be similar to that in foam faces. 

Qayyum, M.M. White, J.R. The effect of weathering on residual stresses and mechanical properties in injection-moulded semi-crystalline polymers. J. Mater. Sci. 1986, 21, 2391-2402. 

If the diol chain extender is used in exact molar proportion to the unreacted isocyanate, then a linear polyurethane elastomer is obtained. The resulting thermoplastic elastomer may be extruded or injection-moulded, and the properties arise from the ability of the hard and soft segments to form semi-crystalline domains that act as virtual crosslinks in the polymer and give it elastomeric properties, as shown in Figure 1.17. 

Polypropylene (PP) is semi-crystalline polymer used in many industrial applications, including extruded pipes, films and fibres, and injection-moulded and thermoformed parts, as a result of its physical, thermal and chemical properties (low density, high melting... 

The polymers used in injection moulding can be divided into three main classes, depending on their structure and properties. These classes are amorphous thermoplastics semi-crystalline thermoplastics rubbers. 

All injection-moulded thermoplastics are prone to sink marks and voids in areas where sudden changes in section thickness occur, or over ribs and bosses. Semi-crystalline polymers such as PP are more prone to sinks and voids. Voids occur when the external skin of the moulding is rapidly cooled and becomes sufficiently rigid to support the contraction of the underlying melt. Sinking or surface depression occurs in localised thick sections where internal mass contains sufficient heat to keep the polymer in molten stage and erystallises slowly producing sink marks. 

The use of partial-body nozzles is therefore recommended (Figure 4.36a) to inject amorphous plastics, and full-hody nozzles (Figure 4.36b) to inject semi-crystalline plastics. In the case of partial-body nozzles, the gate is located in the mould cavity, and is consequently relatively cold, so that premature freezing could occur during processing... 

Similar to any semi-crystalline polymer, PE undergoes a greater volume increase than an amorphous material during melting. Hence the compression rate required for injection moulding of PE is lower. 

The main differences in behaviour between the amorphous and semi-crystalline materials observed during injection moulding are ... 

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