10. Program control of injection pressure（Injection Molding Technical Information）

The control of injection pressure is usually divided into primary injection pressure, secondary injection pressure (pressure maintaining), or more than three injection pressure controls. Whether the timing of pressure switching is appropriate is very important for preventing excessive pressure in the mold, preventing material overflow or shortage, etc.
The specific volume of the molded product depends on the pressure and temperature of the molten material during the pressure maintaining stage when the gate is closed. If the pressure and temperature during each transition from pressure maintaining to product cooling are consistent, the specific volume of the product will not change. At a constant molding temperature, the most important parameter that determines the size of a product is the holding pressure, and the most important variables that affect the dimensional tolerance of the product are the holding pressure and temperature.
For example, after the completion of mold filling, the pressure retaining pressure immediately decreases, and when the surface layer forms a certain thickness, the pressure retaining pressure rises again. This allows for the use of low molding force to form large products with thick walls, eliminating pits and flash.
The holding pressure and speed are typically 50% to 65% of the maximum pressure and speed when filling the mold cavity with plastic, which means that the holding pressure is approximately 0.6 to 0.8 MPa lower than the injection pressure. Due to the lower holding pressure than the injection pressure, the load of the oil pump is low within a considerable holding time, extending the service life of the solid oil pump, and reducing the power consumption of the oil pump motor.
Pressure injection can not only ensure smooth mold filling of the workpiece, but also avoid weld lines, dents, flash edges, and warpage. It is beneficial for the molding of thin-walled parts, multi headed small parts, long process large parts, and even for parts with uneven cavity configuration and less tight clamping.