After the molten metal is injected into the mold cavity, a thin shell first forms on the cavity wall. When this shell is compressed by subsequent molten material during the filling process, it can cause the melt to fracture.
Once this thin shell is torn or shifted, scratches or wrinkles appear on the surface of the plastic part. For example, on low-density polyethylene parts with a low melt index, alternating light and dark striped areas are often visible on the surface. These areas are generally located some distance from the gate and cover the entire surface. Thin-walled parts are particularly prone to this type of defect, mainly because the molten material is subjected to significant pressure before the small cavity is completely filled, leading to melt fracture and surface defects.
Generally, slowing down the cooling rate of the molten metal during the filling process and the rate of shell formation is the best way to eliminate this type of defect. This can be achieved by appropriately increasing the mold temperature or increasing the local temperature at the melt fracture site. Local heating of the mold cavity surface can be achieved using small tubular electric heaters installed near the gate and at the melt fracture site.
Irregular pulsating flow of molten material occurs within the mold cavity.
The flow characteristics of the molten material are related to its rheological properties and the gate cross-sectional area, which determines the shear rate of the molten material at the mold inlet. When the gate size is small and the injection rate is high, the molten material is injected into the cavity as a thin, tortuous jet. If the molten material cools rapidly, it will not fuse well with the subsequent irregular flow, resulting in surface cloudiness and streaks near the gate. Sometimes, a small amount of cold material may move along the mold cavity surface, causing surface cloudiness and streaks to occur at locations farther from the gate.
Generally, surface cloudiness and streaks generated during the injection of crystalline polymers are more difficult to eliminate because these resins have relatively high melting temperatures. Compared to amorphous polymers, crystalline polymers cure faster, have a narrower processing temperature range, and the irregular flow of molten material generated at points of rapid wall thickness change and sudden changes in flow direction has a shorter time to fuse with the remaining molten material in the cavity, easily leading to surface cloudiness and streaks.
To troubleshoot these types of problems, in terms of process operation, the temperature of the mold, barrel, and nozzle should be appropriately increased, while the screw advance speed during injection should be reduced.
In terms of mold operation, the gate size should be enlarged, with a fan-shaped gate being preferred. If a tunnel-type gate is used, its top size being too small will cause residual material and impurities at the gate to affect mold filling, exacerbating irregular material flow; its top size should be appropriately increased. Poor mold venting will also affect the regular flow of material and should be improved.
Furthermore, the amount of lubricant used should be reduced, and a suitable type should be selected.
