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Injection molding process parameters

Holding pressure and time

At the end of the injection molding process, the screw stops rotating and only advances, at which point the injection molding enters the pressure holding phase. During the pressure keeping process, the nozzle of the injection molding machine continuously feeds the cavity to fill the volume vacated due to the shrinkage of the workpiece. If the cavity is filled without holding pressure, the part will shrink by about 25%, especially if the tendon is too large to form a shrinkage mark. The holding pressure is generally about 85% of the maximum filling pressure, of course, it should be determined according to the actual situation.

Back pressure

Back pressure refers to the pressure that the screw needs to overcome when reversing the backstock. The use of high back pressure facilitates the dispersion of the colorant and the melting of the plastic, but at the same time prolongs the screw retracting time, reduces the length of the plastic fiber, and increases the pressure of the injection molding machine, so the back pressure should be lower, generally not exceeding the injection molding. 20% of the pressure. When injection molding foam, the back pressure should be higher than the pressure formed by the gas, otherwise the screw will be pushed out of the barrel. Some injection molding machines can program the back pressure to compensate for the reduction in screw length during melting, which reduces the heat input and slows the temperature. However, as the result of this change is difficult to estimate, it is not easy to adjust the machine accordingly.

Injection molding process parameters