The control of plastic machining pressure is usually divided into one injection pressure, two injection pressure (holding pressure) or more than three injection pressure control. Whether the timing of pressure switching is appropriate is very important to prevent excessive pressure in the mold, prevent flash or lack of material, etc. The specific volume of the molded product depends on the melt pressure and temperature when the gate is closed during the pressure holding stage. If the pressure and temperature are the same every time when switching from the holding pressure to the product cooling stage, the specific volume of the product will not change.
At a constant molding temperature, the most important parameter that determines the size of machining uhmw plastic products is the holding pressure. The most important variables that affect the product size tolerance are the holding pressure and temperature. For example, after filling the mold, the holding pressure will immediately decrease. When the surface layer has a certain thickness, the holding pressure will rise again. In this way, low clamping force can be used to form large thick-walled products, eliminating craters and flash.
In China Turning injection molding industry, the holding pressure and speed are usually 50%~65% of the highest pressure and speed when the plastic is filled in the mold cavity, that is, the holding pressure is about 0.6~0.8MPa lower than the injection pressure. Since the holding pressure is lower than the injection pressure, the load of the oil pump is low during a considerable holding time, the service life of the solid oil pump is prolonged, and the power consumption of the oil pump motor is also reduced.
The three-stage pressure injection can not only make the part fill the mold smoothly, but also will not appear welded lines, dents, flashing and warping deformation. It is good for the molding of thin-walled parts, small parts with multiple heads, large parts with long processes, and even parts with uneven cavity configuration and tight mold clamping.