Analysis and solution of dent problem in injection molding of automotive parts

During the injection molding process of HDPE plastic, dent phenomenon is a common and troubling problem, which usually occurs after the gate is closed or due to local internal shrinkage caused by insufficient injection. Dents appear as depressions or tiny depressions formed on the surface of injection molded products, which are mainly caused by multiple factors.

Causes

The formation of dents is mainly related to the change in wall thickness of plastic products. When the wall thickness of certain parts increases, the shrinkage rate at that place will increase. It usually occurs at external sharp corners or sudden changes in wall thickness, such as bulges, ribs or the back of supports, and sometimes in uncommon areas. The root cause lies in the thermal expansion and contraction characteristics of the material. The thermal expansion coefficient of thermoplastics is high, and the degree of expansion and contraction is affected by many factors, such as the performance of the plastic, temperature range, cavity holding pressure, etc. In addition, the size, shape, cooling rate and uniformity of the injection molded parts are also important factors affecting the formation of dents.

During the plastic molding process, the expansion and contraction of the material is directly related to its thermal expansion coefficient. During mold cooling, the molded part loses close contact with the cooling surface, resulting in a decrease in cooling efficiency, while the continuous shrinkage of the molded part depends on the combined influence of multiple factors. The sharp corners of the molded part cool the fastest and harden earlier than other parts; relatively speaking, the thick part in the center of the molded part becomes the last area to release heat because it is far away from the mold cooling surface. As cooling proceeds, the plane between the sharp corners can only be cooled on one side, and its strength is relatively low. When the central area shrinks, the cooled surface will be pulled inward, forming a dent.

The appearance of the dent indicates that the molding shrinkage rate at this location is higher than that of the surrounding area. If one section shrinks significantly more than other parts, it will cause mold warping. At the same time, the residual stress in the mold cavity will also reduce the impact strength and heat resistance of the plastic part.

Solution

Adjusting the injection molding process conditions can help reduce the occurrence of dents. For example, during the holding process, additional plastic material can be injected into the mold cavity to compensate for the molding shrinkage. However, because the gate is usually thin, when the molded part is still at a high temperature and continues to shrink, the solidified gate has little effect on the holding pressure of the molded part in the cavity.

When it comes to material types, the shrinkage of molded parts of semi-crystalline plastics is relatively high, which makes the sink mark problem more serious. The molding shrinkage of amorphous materials is lower, which effectively reduces the formation of sink marks. In addition, the use of fillers and reinforcements can reduce the shrinkage rate, thereby reducing the probability of sink marks.

As a manufacturer of HDPE plastic injection molding, understanding and solving the sink mark problem is crucial to improving product quality. By optimizing the design and adjusting the process conditions, we can effectively improve the appearance and performance of injection molded products.