The program control of the injection speed is to divide the injection stroke of the screw into 3 to 4 stages, and use the appropriate injection speeds in each stage. For example, the injection speed is slowed when the molten plastic first passes through the gate, high speed injection is used during filling, and the speed is slowed at the end of filling. With this method, flashes can be prevented, flow marks can be eliminated, and residual stress of products can be reduced.
The flow velocity is stable at low speed filling, the product size is relatively stable, the fluctuation is small, the internal stress of the product is low, and the internal and external stress of the product tends to be consistent (for example, immersing a certain polycarbonate part in carbon tetrachloride, using high-speed injection molding Parts have a tendency to crack, low-speed cracking). Under relatively slow filling conditions, the temperature difference of the material flow, especially the temperature difference between the material before and after the gate, is large, which helps to avoid shrinkage and depression. However, due to the prolonged mold filling time, it is easy for the parts to have delamination and poor weld marks, which not only affects the appearance but also greatly reduces the mechanical strength.
When high-speed injection, the flow rate is fast. When the high-speed filling is successful, the molten material fills the cavity quickly, the temperature of the material drops less, and the viscosity decreases less. The injection pressure can be lower, and it is a kind of hot material charge. Modal potential. High-speed filling can improve the glossiness and smoothness of the parts, eliminate the phenomenon of joint lines and delamination, shrink small depressions, uniform color, and ensure that large parts of the parts can be full. However, it is easy to produce products that cause blisters or blisters or yellow parts, and even burn the zoom, or cause mold release difficulties, or the phenomenon of uneven mold filling. For high-viscosity plastics, it can cause melt fractures and produce cloud spots on the surface of parts.
High-speed high-pressure injection can be considered in the following cases: (1) High plastic viscosity, fast cooling speed, and long process parts can not fully fill the corners of the cavity with low pressure and slow speed; (2) Parts with too thin wall thickness, melting material When it reaches the thin wall where it can easily condense and stay, a high-speed injection must be used to immediately enter the cavity before the melt energy is consumed in large quantities; (3) Glass-fiber-reinforced plastic, or plastic with a relatively large amount of filler material, due to poor fluidity In order to obtain smooth and uniform parts, high-speed and high-pressure injection must be used.
For high-precision products, thick-walled parts, parts with large wall thickness changes, and parts with thick flanges and ribs, multi-stage injections such as secondary, tertiary, quaternary, or even five are preferred.
