After the melt is injected into the cavity, a thin case is formed on the wall of the mold cavity. When the case is pressed by the subsequent melt in the mold filling process, it will cause the melt to break.
Once the thin case is torn or moved, the surface of the plastic part is scarred or wrinkled. For example, on a low-density polyethylene molded part with a low melt index, the surface diameter can often be seen alternately between light and dark. In the bar-shaped area, the generated area is generally at a certain distance from the gate, and spreads over the entire surface. Especially for thin-walled plastic parts, this type of failure is most likely to occur. This is mainly due to the fact that the molten material is less affected before filling the small melting chamber. The large pressure causes the melt to crack and form surface defects.
In general, slowing down the cooling rate of the molten material during mold filling and the rate of formation of the case layer are the best ways to eliminate such failures. This can be ruled out by appropriately increasing the mold temperature or increasing the local temperature of the melt fracture site. Faults. Local heating of the cavity surface can be achieved with a small tubular electric heater installed near the gate and melt fracture.
The flow characteristics of the melt are related to its rheological properties and to the cross-sectional area of the gate that determines the shear rate of the melt at the die entrance. When the gate size is small and the injection rate is high, the melt is finely curved. The jet flow injected into the cavity, if the cooling rate of the melt is very fast, it will fuse with the irregular filling material of subsequent filling, resulting in surface turbidity and marking near the gate.
Sometimes, a small amount of cold material moves along the surface of the cavity, causing surface turbidity and markings to be generated farther away from the gate.
In general, the surface turbidity and markings generated when crystalline polymers are injected are more difficult to eliminate because the melting temperature of these resins is rather high. Compared to amorphous polymers, crystalline polymers cure faster and have a narrow processing temperature range. , And in the sharp changes in wall thickness and melt flow suddenly change the direction of the flow of molten material and the remaining melt in the cavity fusion time is relatively short, it is easy to produce surface turbidity and markings.
For the elimination of such failures, in terms of process operations, the temperature of the mold, barrel, and nozzle should be appropriately increased to reduce the forward speed of the screw during injection.
In terms of mold operation, the size of the gate should be enlarged. The use of a fan gate is preferred. If a tunnel gate is used, the top size of the gate will cause impurities in the gate to affect mold filling and aggravate the irregular flow of the flow material. The size of the top should be increased as much as possible; if the mold is poorly vented, it will also affect the regular flow of the flow material and should be improved.