When a Ford supplier produced sample parts of a polypropylene injection moulded internal trim panel for the bottom lip of a vehicle’s boot, the cosmetic top surface showed sink marks directly above the internal ribs on the underside. These depressions were caused by shrinkage at the junction of the rib and the main skin.

To solve the problem, technicians initially thought of enlarging the gates where material flowed into the cavity. This idea was based on the assumption that the gates freeze before the cavity, preventing effective pressurisation during the packing phase of the injection moulding cycle which, in turn, leads to high shrinkage.

Plastic materials have volumetric shrinkage in the order of 25% as they cool from the melt temperature to solid at ambient temperature. So more melt must be forced into the cavity as this cooling takes place to compensate for the shrinkage.

This requires that sufficient pressure can be transmitted to the extremities of the cavity to ensure that this compensating flow takes place. The pressure must be controlled so that all regions of the cavity freeze at similar pressures. If a constant pressure is applied, the pressure drop between the feed point and the cavity extremity would lead to lower shrinkage near the gate. So the pressure must be reduced progressively as the part freezes towards the gate.

By adopting a suitable pressure profile, which involves a fixed pressure for a specific time followed by a decay to zero over another time period, variations in volumetric shrinkage can be minimised. By ensuring that sufficient pressure is available in the areas with ribs, sinks will become less visible.

The determination of these times and pressure levels is very difficult in practice, since the effects of changing physical conditions on an injection moulding machine cannot easily be correlated to the resulting shrinkage.

At around the same time, the trim panel used in the corner of the vehicle’s boot had been found to be distorting when ejected from its mould. Several analyses, carried out using Moldflow’s MF/MFLOW software, showed that most of the warpage was caused by the inability to transmit packing pressure uniformly across the part as the various areas froze, which caused non-uniform shrinkage. The problem was resolved by using an additional feed point plus a pressure profile during the packing phase to ensure that all areas of the part froze at similar pressures, thus reducing the warpage to an acceptable level.

Engineers then used the Moldflow software to investigate the sink marks on the back trim panel. Again, a series of packing analyses were carried out using the various packing profiles to assess their effects on sink mark depth. It soon became clear that the part was freezing before the gate, negating the earlier argument that increasing the gate size would have an effect on the pressure distribution within the cavity.

When the recommended solution, which involved higher pressures than previously used, was proposed to the moulder, he expressed concern that the high pressures would cause the part to stick in the mould, thus preventing ejection of the finished part. It was agreed to start with a lower pressure and gradually increase it.

As parts were produced at the staged pressures, the sink marks became less visible and a close correlation could be seen between the predicted sink visibility and the part weight produced by the analysis for each set of conditions.

When the optimised pressure was reached and the correct profile used, the sinks were acceptable and the part ejected more easily, rather than less so. When the parts were measured, they were shown to be some 5mm longer due to the lower shrinkage caused by the higher pressures. Because the part was shrinking less along its length meant that less ejection force was required to move it from the mould as less resisting forces were being generated by shrinkage between the ribs.

The extra length required a mould modification to give a correctly sized part.

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