Even when the injection speed is constant when the melt […]
Even when the injection speed is constant when the melt enters the mould, its flow rate changes. When entering the mould gate area, the melt flow rate is high, but after entering the mould cavity, ie the mould filling stage, the melt flow rate begins to drop. This change in flow rate at the front end of the melt stream can cause product surface defects.
Reducing the injection speed is one way to solve this problem. In order to reduce the speed of the melt flow front at the gate, the injection can be divided into several steps, and the injection speed is gradually increased, the purpose of which is to obtain a uniform melt flow rate throughout the filling phase.
Low melt temperature is another cause of dark spots on the product. Increasing the barrel temperature and increasing the back pressure of the screw can reduce the chance of this happening. In addition, too low temperature of the mould can also produce surface defects, so increasing the mould temperature is another feasible way to overcome the surface defects of the products.
Die design defects also produce dark spots near the gate. The sharp corners at the gate can be avoided by changing the radius. Care should be taken in designing the position and diameter of the gate to see if the gate design is appropriate.