Solutions to Cracking and Whitening Defects in Injection Molding and Insert Molding

Cracking is a common defect in plastic products, and its main cause is stress deformation. Factors that cause cracking include residual stress, external stress, and stress deformation caused by the external environment. The following is an analysis of cracking and whitening defects and corresponding solutions.

Analysis and elimination methods of cracking and whitening defects

1. Excessive residual stress on the surface of the part

– Excessive residual stress is the main cause of cracking on the surface of plastic parts. In process operation, the process parameters should be set according to the requirements of reducing the residual stress of plastic parts. Especially when the melt and mold temperature are high and the melt fluidity is good, the injection pressure should be reduced as much as possible. When cracks have appeared on the surface, annealing treatment can be used to eliminate the cracks. The annealing process is to heat the plastic part for about 1 hour at a temperature about 5 degrees lower than the thermal deformation temperature of the plastic part, and then slowly cool it. In order to better eliminate cracks, it is best to anneal the plastic part immediately after molding. However, in mass production, it may be difficult to eliminate cracks by annealing, so it is generally not recommended. In addition, the residual stress in the cracks of the cracks may cause the flux in the coating process to expand the cracks. Therefore, in this case, special attention should be paid to using coatings and thinners that will not cause melting cracks.

2. The surface of the plastic part is subjected to concentrated external forces

– External forces are the main cause of whitening on the surface of the plastic part. This whitening phenomenon usually occurs at the ejection part of the plastic part. For example, during the demolding process, if the demolding is poor and the demolding force is close to the elastic limit of the resin, whitening may occur. For the whitening problem, the injection pressure can be appropriately reduced, the demolding slope can be increased, and chamfers should be avoided especially near the ribs and bosses. In addition, the surface finish of the cavity should be ensured to improve the smoothness of demolding. If necessary, a small amount of release agent can be used.

In summary, for the cracking and whitening defects that occur during the Overmolding and Insert Molding, analysis and control should be carried out from multiple aspects, and the molding process parameters should be reasonably set. At the same time, a reasonable demolding scheme should be considered during the design to reduce the possibility of defects and improve the overall quality of the plastic part.