Injection time directly depends upon the gate solidification time. The gate solidifi­cation time is the time when the material stops flowing due to its solidification at the cavity gate. Injection time consists of filling time and holding time. The gate sealing time is determined by measuring the weight of the product at regular intervals while increasing the injection time till the weight of the product becomes constant. If the injection time is less, means the holding pressure is removed before the material is solidified the material in the mold cavities would flow back due to high pressure in the mold cavity leading to various defects like voids, sink, war page and dimension­al mismatch. Therefore in order to optimize the injection time, holding time should be reduced gradually till the optimum hold time is achieved (till gate solidification).

8.3.1.2 INJECTION PRESSURE

Injection pressure consists of the filling pressure and the holding pressure. The fill­ing pressure during injection molding is usually more than the holding pressure. It should be able to inject 90 to 95% of the shot volume into the mold cavity. Ideally the filling pressure should be the maximum pressure that can be achieved without flashing but high pressure would lead to increased stresses in the molded part. Hold­ing pressure is important to overcome the shrinkage during low temperature solidi­fication of the crystalline polymer. The holding pressure should be set according to the material shrinkage properties. High holding pressure also reduces the void and sink defects, but it should be regulated carefully as high holding pressure also leads to burr formation.