What is the role of holding pressure in injection molding processing?

Injection molding processing, as a common plastic molding process, is widely used in the modern manufacturing industry for its high efficiency and precision. In the injection molding process, holding pressure is a crucial parameter that plays a decisive role in the quality of the product. So what is holding pressure and what role does it play in injection molding?

Holding pressure is the pressure that is continuously applied to the molten plastic after it has been injected into the mold cavity in order to ensure that the material fills and makes up for the contraction. Unlike the injection pressure, the holding pressure is mainly used to maintain the filling pressure of the molten plastic on the mold for a certain period of time, to ensure that the molded plastic products are dimensionally stable, shrinkage rate is uniform, and shrinkage defects are avoided.

The first important role of holding pressure is to push the molten plastic to continue to fill the mold cavity. Even after the main injection process is complete, the holding pressure stage continues to push the material flow, filling the spaces created by the cooling shrinkage of the plastic. This significantly reduces the formation of internal voids and improves the denseness and mechanical strength of the product.

The second important effect of holding pressure is to help improve the appearance of the product. Without adequate holding pressure, plastic parts may warp and distort due to uneven shrinkage. Proper holding pressure allows the plastic to cool evenly in the mold, resulting in a flatter, smoother finish.

A third important function of holding pressure is that proper holding pressure can also help improve the dimensional accuracy of the product. As the plastic cools and solidifies, the presence of holding pressure ensures dimensional stability in all parts of the plastic part and reduces the problem of dimensional deviations caused by inconsistent shrinkage. This is particularly important for precision parts, and is directly related to the final product's assembly performance and functional realization.

The holding pressure stage also involves a key factor - time. The control of holding time is as important as the amount of holding pressure. Too short holding time may lead to the plastic is not completely filled with mold cavity that stops the pressure, resulting in shrinkage; while too long holding time may result in over-compaction, increasing the internal stress of the product, or even cause product cracking. Therefore, in practice, engineers will adjust the holding time and pressure value according to the specific material characteristics and product requirements to obtain the best molding results.