Mouldability index

The flow process in an injection mould is complicated by the fact that the mould cavity walls are below the freezing point of the polymer melt. In these circumstances the technologist is generally more concerned with the ability to fill the cavity rather than with the magnitude of the melt viscosity. In one analysis made of the injection moulding situation, Barrie showed that it was possible to calculate a mouldability index (p.) for a melt which was a function of the flow parameters K and the thermal diffusivity and the relevant processing temperatures (melt temperature and mould temperature) but which was independent of the geometry of the cavity and the flow pattern within the cavity. 

Some typical data for this mouldability index are given in Figure 8.8. One limitation of these data is that they do not explicitly show whether or not a mould will fill in an injection moulding operation. This will clearly depend on the thickness of the moulding, the flow distances required and operational parameters such as melt and mould temperatures. One very crude estimate that is widely used is the flow path ratio, the ratio of flow distance to section thickness. The assumption is that if this is greater than the ratio (distance from gate to furthest point from gate)/section thickness, then the mould will fill. Whilst... 

Figure 8.8. Mouldability index of some common moulding materials (after Barrie)...

Criteria of extrudability, mouldability, spinnability and stretchability as used in practice are described (melt index, spiral length, mouldability index, melt strength, ultimate thread length, etc.). These criteria are based on the dimensionless parameters (numerics) of the processes. 

The mouldability index is the length of the elongated flow path that is filled before solidification of the injected molten polymer composition at specified moulding conditions, i.e., volumetric injection rate, mould temperature, polymer melt temperature. The mouldability index of the moulding compositions of this invention is characterised by spiral flow. 

Weir (1963) defined the mouldability index, aSTv (the index STV stands for shear-temperature-viscosity), as follows ... 

FIG. 24.6 Evaluation of four different polypropylenes by means of their mouldability index (after Weir et al., 1963). 

For a number of other polymers Deeley and Terinzi (1965) have confirmed that there is an unambiguous relation between asrv and the spiral length, which indeed permits a good polymer selection. This does not mean that the mouldability index is beyond criticism. It is found, for instance, that the critical values mentioned do not apply to high-melting aromatic thermoplastics. 

Nevertheless, the mouldability index is a valuable criterion, which provides a rational basis for the construction of the moulding area diagram. 

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