The programmed control of the injection speed involves dividing the injection stroke of the screw into 3-4 stages, using the respective appropriate injection speed in each stage.

Characteristics of high and low speeds：

1. low-speed mold filling when the flow rate is smooth, the product size is more stable, less fluctuation, the product internal stress is low, the product inside and outside the stress tends to be the same (for example, a polycarbonate parts immersed in carbon tetrachloride, high-speed injection molding of the parts have a tendency to crack, low-speed does not crack).

In the slower mold filling conditions, the temperature difference between the material flow, especially before and after the gate material temperature difference is large, to help avoid the occurrence of shrinkage and depression. However, due to the mold filling time continues longer easy to make the parts appear layered and poor combination of fusion marks, not only affects the appearance, but also make the mechanical strength is greatly reduced.

2. High-speed injection, the material flow speed, when the high-speed mold filling smoothly, the melt quickly filled with the cavity, the material temperature drops less, the viscosity drops less, you can use a lower injection pressure, is a hot material mold filling situation. High-speed mold filling can improve the gloss and smoothness of the parts, eliminating the seam line phenomenon and delamination phenomenon, shrinkage depression is small, color uniformity and consistency of the larger part of the parts to ensure fullness.

However, it is easy to produce products fat blisters or yellow parts, or even burned and scorched, or cause demolding difficulties, or the phenomenon of uneven mold filling. For high viscosity plastics may lead to melt rupture, so that the surface of the parts produced cloudy spots.

The following cases can be considered to use high-speed high-pressure injection:

(1) high viscosity plastic, fast cooling, long process parts using low pressure and slow speed can not be completely filled with all corners of the cavity;

(2) The wall thickness is too thin parts, the melt to reach the thin wall is easy to condense and stagnation, must be used once the high-speed injection, so that the melt immediately into the cavity before a large amount of energy consumption:

(3) with glass fiber reinforced plastic, or plastic containing a large number of filler materials, due to poor fluidity, in order to get a smooth and uniform surface of the parts, must use high-speed high-pressure injection.