What is the difference between holding pressure and injection pressure in injection molding?

Holding pressure and injection pressure are two crucial and often discussed technical parameters in injection molding. Although they are both related to the flow and molding of the plastic melt in the mold cavity, the roles they play, the mechanisms by which they work, and the way they are regulated are fundamentally different. Next, this article will take a closer look at the differences between holding pressure and injection pressure in injection molding.

Injection pressure is the pressure exerted when the plastic melt is pushed into the mold cavity by the screw or plunger of the injection unit. This process occurs mainly during the filling phase of the injection cycle, and its purpose is to ensure that the plastic melt fills every nook and cranny of the mold cavity sufficiently to form the desired shape of the product.

The holding pressure, on the other hand, is the pressure that continues to be maintained after the mold cavity has been filled with plastic melt in order to prevent defects such as dimensional instability or shrinkage of the product as a result of shrinkage of the material as it cools.

In short, the injection pressure is concerned about how to "push" the plastic, while the holding pressure is concerned about how to let the plastic "hold".

In terms of the mechanism of action, injection pressure mainly affects the flow and filling speed of the plastic melt. Higher injection pressures allow the melt to fill the mold cavity faster, but may also cause an increase in molecular orientation, affecting the mechanical properties of the product. In contrast, the holding pressure plays more of a role in the compaction and compensation phase after the melt has already filled the mold cavity. Appropriate holding pressure helps to further compact the melt, reduce internal stresses, and avoid defects caused by material shrinkage due to cooling.

Injection pressure and holding pressure also have different considerations in the way they are adjusted. Injection pressure is usually controlled by adjusting the settings of the injection unit (e.g. screw speed, back pressure, etc.), while holding pressure depends more on the pressure-time control system of the injection molding machine.

Finding the right combination of injection pressure and holding pressure is often the key to improving product quality in the actual production process. For example, for thin-walled or long-flow products, a higher injection pressure may be needed to ensure fast filling, while products with complex structures or high dimensional accuracy requirements may require finer holding pressure adjustment to prevent deformation and shrinkage. Through experimentation and experience, engineers can develop standardized process parameters for different situations, thus improving production efficiency and product quality.