Optimizing Injection and Holding Pressure in Plastic Injection Molding

In plastic injection molding, the purpose of injection is to apply enough pressure on the molten plastic to fill up an injection mold's cavity up to the smallest detail, without causing any defects or voids in the finished product.

In this context, there are two critical parameters that are related to pressure: injection pressure and holding pressure. Injection pressure helps in filling the mold cavity, while holding pressure ensures that the cavity is completely filled, avoiding shrinkage-related defects.

Together, these two parameters are some of the most important factors that determine the efficiency of the injection molding process and the quality of the final product. Here, they are described in more detail.

What is Injection Pressure?

Injection pressure is the pressure and speed applied to the molten plastic to fill the mold cavity. It should be high enough to fill 95% of the cavity, while not breaking the mold or degrading the molten plastic. It is measured in pounds per square inch (psi) or bars (1 bar = 14.5 psi).

The hydraulic and injection units of the injection molding machine control the injection pressure. Pressure is transferred to the injection cylinder, through to the screw and the machine’s nozzle, and into the mold. Injection pressure must overcome the viscosity and flow resistance of the molten plastic to fill the mold cavity.

After filling the mold cavity, injection pressure transitions to holding pressure.

What is Holding Pressure?

After the molten plastic fills the mold cavity, it shrinks due to cooling. To compensate for the shrinkage, the screw of the machine keeps moving, applying pressure to ensure a continuous flow of molten plastic into the mold’s cavity. This continuous flow fills the space that is created by shrinkage.

The pressure required to fill this remaining 5% of the mold's cavity is known as the holding pressure, and it is usually 50–65% of the injection pressure. The hydraulic and injection units of the injection molding machine also control the holding pressure.

Last Words

Injection pressure and holding pressure are influenced by the mold design, the type of plastic material used, and the dimensions and shape of the product. All these factors are interconnected, and together, they dictate the efficiency of the injection molding process and the quality of the final product. As a result, understanding these two critical pressure parameters would benefit anyone aiming for a seamless injection molding process and optimal results.

At MOLDEX, we consider every aspect in the design of our injection molds to ensure a seamless production process for our clients. You are welcome to request a quote or contact us, and we'll be pleased to assist you in any way possible.