Mould filling flow patterns

The process of mould filling is followed by means of a series of short shots . In these, deliberately, insufficient melt is injected to fill the mould. The resultant short mouldings reveal flow patterns and help towards a programme for satisfactory mouldings. 

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. 

An example of computer-aided prediction of mould-filling is shown in Figure 8.4. The component is a radiator fan cowl for a motor vehicle. It is to be moulded firom 10% talc-filled pofypropylene. As can be seen, the shape is three-dimensional and an anafytical solution for the flow pattern is impossible. The first step in the computer simulation is to approximate the s pe of the real component by the mesh of triangular elements shown... 

The direct visualization of the flow behavior inside of the mould cavity under processing conditions close to industrial set-ups can be used for experimental investigation with several aims. The main one is to have direct access to the details of the flow patterns developed under different processing conditions. Visualization is also used to benchmark computer modeling codes, namely of filling and holding stages. 

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