Packing and Solidification

Once the material is in the tool, filling must be completed (tool packing), the part cooled and finally ejected. The purpose of the packing stage is to add extra material to compensate for the shrinkage caused by the decreasing density of the solidifying polymer. If the additional polymer were not injected the component would shrink and warp due to nonuniform cooling. Ideally the packing and cooling stages should be such that the final dimensions are maintained as close as possible to design tolerances. Variables during this stage are packing pressure, packing time and the mould temperature. Bad mould design can lead to inconsistent cooling along the dimensions of the mould surface which can cause increased residual stresses. Once the material has cooled sufficiently, the component can be injected and the injection cycle continues. The cycle does not occur sequentially, while one part is cooling, plastication of the next cycle has already begun.

A breakdown of the cycle and the relative time for each stage is shown in Figure 1.11.

The injection moulding process and the parameter effects will be further discussed in Chapter 8.

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