The Principle of Injection Speed Setting
All the injection molding machine may adjust the injection speed through the controller. The program of the controller of the injection speed divide the injection stroke of the screw into 3-4 stages, and use respective appropriate injection speeds in each stage.
Characteristics of high and low speed
1. Low injection speed filling
The flow rate is stable and the product size are relatively regular. Besides, the fluctuation is little and the internal stress of the product is low. The internal and external stress of the product tends to be the same.
For example, a certain polycarbonate part is immersed in carbon tetrachloride, and the part molded by high-speed injection has a tendency to crack. However, the low-speed does not crack.
Under relatively slow mold filling cases, 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 delamination and poor welding weld marks on the part. And it not only affects the appearance of the products, but also greatly reduces the mechanical efficiency.
2. High injection speed filling
The material flow rate is fast running in the mold cavity. When the high-speed mold filling is smooth, the melt will quickly fill the cavity. What’s more, the material temperature and the viscosity will drop less. High-speed mold filling can improve the gloss and smoothness of the parts, eliminate the seam lines and delamination. And it results in small shrinkage depressions, uniform colors, and ensure fullness for larger parts of the parts.
However, it is easy to cause the product to become bubbling or yellow, or even burn the 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 adopted in the following situations:
(1) The plastic has high viscosity and fast cooling speed. Its long process parts can not completely fill all corners of the cavity with low pressure and slow speed, such as cable tie manufacturing .
(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 drive the molten material enter the cavity immediately before a large amount of energy is consumed.
(3) Plastics reinforced with glass fibers, or plastics containing a larger amount of filler materials, have poor fluidity. In order to obtain a smooth and uniform surface, high-speed and high-pressure injection must be used.
