Injection pressure, holding pressure, back pressure, injection speed
For perfect processing, stepless control of the pressures and injection speeds is essential. It should be possible to regulate the injection and holding pressure in a range from 100 to 1,200 bar. The injection speed will depend primarily on the wall thickness. In general, thick-walled articles require slow filling of the mold, and thinwalled articles fast filling.
Apart from the wall thickness and type of gating, the venting of the mold plays an important role for the injection speed. This helps to avoid so-called "burn marks" caused by highly compressed hot air.
The biggest influence on the dimensional stability and demoldability of the component is exerted by the injection pressure/ holding pressure. Excessive injection pressure overloads the molding, while too low a holding pressure produces sink marks.
Overloaded moldings are more difficult to demold. It is advisable to work with staggered pressures, with the holding pressure being set lower than the injection pressure. As a rule, a holding pressure of 50 % of the injection pressure will be adequate. This enables articles to be produced with a minimum of internal stresses.
Fig. 4 shows a typical cycle.