How to Solve the Cracks on the Surface of Injection Molded Products?
Failure analysis and troubleshooting methods
The residual stress is too high.
For the mold design and production, we can adopt direct gates with minimal pressure loss and higher injection pressure. We can change the forward gate to multiple pin point gates or side gates, and reduce the gate diameter. When designing a side gate, we can use a tab gate that can remove the broken part after molding.
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. We should increase the temperature of the barrel and the mold appropriately. What’s more, we should reduce the temperature of the melt and the mold. Then, control the cooling time and speed of the mold in the mold, so that the orientation molecules have a longer recovery time.
External force causes concentration of residual stress.
Under normal circumstances, such failures always occur around the ejector pin. If this kind of failure occurs, we should carefully check and adjust the ejection device. Besides, the ejector pin should be set at the position with the greatest resistance to demolding, such as bosses, reinforcing ribs, etc. If the number of ejector pins cannot be expanded due to the limitation of the pushing area, we can adopt the method of small area and multiple ejector pins. If the demolding angle of the mold cavity is not enough, the surface of the plastic part will be scratched and then form a wrinkle pattern.
There are differences in the thermal expansion coefficient between the molding material and the metal insert.
The metal inserts should be preheated, especially when the cracks on the surface of the plastic parts occur just after the machine boots up. Most of the cracks are caused by the low temperature of the inserts.
In addition, in the selection of insert materials, we should use the materials with a linear expansion coefficient close to the characteristics of the resin. When selecting molding materials, we should use high-molecular-weight resins as much as possible. If low-molecular-weight molding materials must be used, the thickness of the plastic around the insert should be thicker.
The raw materials are improperly selected or impure.
Practice shows that low-viscosity loose resin is not easy to crack. Therefore, in the production process, appropriate molding materials should be selected based on specific conditions. During the operation, pay special attention not to mix polyethylene and polypropylene and other resins together, which will easily cause cracks. In the molding process, the release agent is also a foreign body for the melt. If the amount is not appropriate, it will cause cracks. Thus, the amount should be minimized.
Poor structural design of plastic parts.
The sharp corners and gaps in the structure of the plastic part are most prone to stress concentration, causing cracks and breaks on the surface of the plastic part. Therefore, the outer and inner corners of the plastic body structure should be made arcs with the largest radius. Experiments show that the best transition arc radius is the ratio of the arc radius to the wall thickness at the corner of 1:1.7.
The cracks on the mold are reflected on the surface of the plastic part.
During the injection molding process, due to the repeated action of the injection pressure on the mold, fatigue cracks will occur at the edges of the cavity with acute angles, especially near the cooling holes.
When the cracks on the surface of the mold cavity reflect on the surface of the plastic part, the cracks on the surface of the plastic part always appear continuously in the same part with the same shape. When such cracks occur, we should immediately check whether there are similar cracks on the surface of the cavity corresponding to the crack. If the cracks result from the re-imaging effect, the mold should be repaired by mechanical processing.