When filling the mold at low speed, the flow rate and the product size is relatively stable with small fluctuation. What’s more, the internal stress of the product is low, and the internal and external stress tends to be consistent. For example, a polycarbonate part is immersed in carbon tetrachloride, the product using the high-speed injection molding has a tendency to crack. But it does not crack at low speed.
Under the conditions of relatively slow mold filling, the temperature difference of the material flow, especially the large difference between the material before and after the gate, helps to avoid shrinkage and dents. However, due to the long filling time, it is easy to cause delamination and poor bonding weld marks on the part. And this not only affects the appearance, but also greatly reduces the mechanical strength.
During high-speed injection, the material flow speed is fast. When the high-speed filling is smooth, the molten material fills the cavity quickly. Besides, the material temperature and the viscosity drop less. You can adopt lower injection pressure, which is a hot material filling modal situation. Also, High-speed mold filling can improve the gloss and smoothness of the parts. And eliminate the phenomenon of seam lines and delamination, with small shrinkage depressions, uniform colors.It can ensure the fullness of the larger part of the product.
However, it is easy to cause the product to become fat, 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 and cloud spots on the surface of the part.
High-speed and high-pressure injection can be adopted according to the following：
- The long process parts that has high plastic viscosity andfast cooling speed. Low pressure and slow speed cannot completely fill all corners of the cavity.
- For parts with too thin wall thickness, the molten material is easy to condense and stay at the thin wall. You can use thehigh-speed injection to make the molten material enter the cavity immediately before a large amount of energy is consumed.
- Glass fiber reinforced plastics or plastics containing a large 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.
- For high-end precision products, thick-walled parts, parts with large wall thickness changes, and parts with thick flanges and ribs, it is best to use multi-level injection, such as second, third, fourth or even fifth.