The program control of the injection speed of plastic machining is to divide the injection stroke of the screw into 3~4 stages, and use respective appropriate injection speeds in each stage. For example: slow down the injection speed when the molten plastic first passes through the gate, use high-speed injection during the filling process, and slow down the speed at the end of the filling process. Using this method can prevent flashing, eliminate flow marks and reduce residual stress of products.
The flow rate of plastic machining is stable when filling the mold at low speed, the size of the product is relatively stable, and the fluctuation is small. The molded parts have a tendency to crack, and the low speed does not crack).
Under relatively slow mold filling conditions, the temperature difference of the material flow, especially the large temperature difference between the material before and after the gate, helps to avoid the occurrence of shrinkage holes and dents. However, due to the long filling time, it is easy to cause the parts to appear delamination and poorly bonded weld marks, which not only affects the appearance, but also greatly reduces the mechanical strength.
During high-speed injection, the material flow rate is fast. When the high-speed mold filling is smooth, the molten material fills the cavity quickly, the material temperature drops less, and the viscosity drops less. Lower injection pressure can be used, which is a kind of hot material filling. Modal situation. High-speed mold filling can improve the gloss and smoothness of the parts, eliminate the phenomenon of seam lines and delamination, with small shrinkage and concave, uniform color, and the larger part of machining uhmw plastic can ensure fullness. However, it is easy to produce fat, bubbling or yellowing of the product, or even burn zoom, or cause difficulty in demolding, or uneven filling of the mold. For high-viscosity plastics, it may cause melt fracture, causing cloud spots on the surface of the part.
High-speed and high-pressure injection can be considered in the following situations:
(1) The plastic has high viscosity, fast cooling speed, and low pressure and slow speed for long process parts that cannot completely fill all corners of the cavity;
(2) For parts with too thin wall thickness, the molten material is easy to condense and stay at the thin wall. A high-speed injection must be used to make the molten material enter the cavity immediately before a large amount of energy is consumed;
(3) Machining uhmw plastics, or plastics containing a larger amount of filling materials, due to poor fluidity, in order to obtain a smooth and uniform surface, high-speed and high-pressure injection must be used.
For high-end precision products, thick-walled parts, parts with large wall thickness changes and thicker flanges and ribs, it is best to use multi-level injection, such as second, third, fourth or even fifth.

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