1. Residual stress is too high
In terms of process operation, it is the easiest way to reduce the residual stress by reducing the injection pressure, because the injection pressure is proportional to the residual stress. In terms of mold design and manufacturing, the direct gate with minimum pressure loss and high injection pressure can be used. The forward gate can be changed into multiple needle point gates or side gates, and the gate diameter can be reduced. When designing the side gate, the convex gate can be used which can remove the broken part after molding.
2. Residual stress concentration caused by external force
Before demoulding the plastic parts, if the cross-sectional area of the demoulding ejection mechanism is too small or the number of ejector rods is not enough, the position of ejector rods is unreasonable or the installation is inclined, the balance is poor, the demoulding slope of the mold is insufficient, and the ejection resistance is too large, the stress concentration will be caused by the external force, resulting in cracks and cracks on the surface of the plastic parts. In case of such faults, the ejection device shall be carefully checked and adjusted.
3. Cracks caused by metal inserts
The thermal expansion coefficient of thermoplastic is 9-11 times larger than that of steel and 6 times larger than that of aluminum. Therefore, the metal insert in the plastic part will hinder the overall shrinkage of the plastic part, and the resulting tensile stress is large. A large amount of residual stress will accumulate around the insert and cause cracks on the surface of the plastic part. In this way, the metal inserts should be preheated, especially when the cracks on the surface of the plastic parts occur at the start of the machine, most of them are caused by the low temperature of the inserts.
4. Improper selection or impure raw materials
Different raw materials have different sensitivity to residual stress. Generally, non crystalline resin is more prone to residual stress and crack than crystalline resin; The resin with high recycled material content has more impurities, higher volatile content, lower strength of the material, and is prone to stress cracking.
5. Poor structural design of plastic parts
The sharp corners and notches in the plastic part structure are most likely to produce stress concentration, resulting in cracks and fractures on the surface of the plastic part. Therefore, the outer and inner corners of the plastic part structure should be made into arcs with the maximum radius as far as possible.
6. Cracks on the mold
In the process of injection molding, due to the repeated effect of injection pressure on the mold, fatigue cracks will occur at the edges with acute angles in the cavity, especially near the cooling holes. In case of such crack, immediately check whether the surface of the cavity corresponding to the crack has the same crack. If the crack is caused by reflection, the mold shall be repaired by machining.
Common plastic products in life, such as rice cookers, sandwich machines, food containers, plastic lunch boxes, storage cans, plastic pipe fittings, etc.,can effectively avoid surface cracks.
Post time: Aug-09-2022