Suitable pressure control points

A general rule should be observed when selecting a suitable pressure control point.

The switch-over pressure can be higher or lower than the fill pressure, but in general the following rule should be observed:

Remember: If the fill pressure is close to the pressure/clamp ceiling for the machine, then the switch-over (packing) pressure should be less than the fill pressure to prevent the mold from flashing. If the fill pressure is substantially less than the pressure ceiling for the machine, it may be possible to use a packing pressure which is greater than the fill pressure, to ensure a product with a good surface finish while not exceeding clamp force limits.

Selecting a transition point for small, thin-walled parts

If a part is small and thin walled, such as a connector part, the molder can set up the machine to fill the cavity quickly by using the ram displacement control, and then switch over to pressure control either just before or just after the part has filled volumetrically. This can cause a rapid escalation in pressure, but because the machine may have a reserve of clamp force, the molder does not have to worry about the resulting high clamp opening forces.

Selecting a transition point for large, thick-walled parts

By contrast, if the part is a thick-walled, large-area molding, such as a large rubbish container, available clamp force may be the critical factor and so the molder will set an injection time (ram forward velocity) that the molder knows will not fill the part. When the mold is only partially filled, the molder can switch over to pressure control, to prevent the part from flashing. In this case, if the switch-over point is set close to the instant of fill, the pressures and clamp forces calculated in the analysis may appear unreasonable. This is because the software has not been run in the way the molder sets up the machine.

The choice of the transition point can influence the calculated values of pressure at the injection node, and the clamp force prediction. Consequently, if these are of interest, the analysis must be set up in a realistic way to represent what happens in an actual injection molding machine. This is best done by thinking how the molder is likely to set up the machine in practice.