Counter pressure molding

Counter pressure injection molding

How Gas counter pressure molding works

Counter Pressure Injection Molding (CPIM), sometimes referred to as Gas Counter Pressure (GCP) molding, is a variation of the traditional injection molding process. This method is primarily utilized to achieve a higher surface quality and to reduce defects commonly associated with the standard injection molding of thick-walled parts or parts with intricate geometries.


1. Process:

Similar to traditional injection molding, the process starts with the mold being closed.

Before injecting the molten polymer, an inert gas (often nitrogen) is introduced into the mold cavity, creating a counter pressure against the upcoming injection of the molten plastic. This gas pressure is controlled and maintained throughout the molding cycle.

With the gas counter pressure in place, the molten polymer is then injected into the mold cavity. The counter pressure from the gas ensures a slower, more controlled filling of the mold.

After the mold is filled, the gas counter pressure is gradually reduced, allowing the plastic material to fully occupy the mold cavity and complete the part formation

The molded part is cooled and solidified within the mold.

Once the part is sufficiently cooled and solidified, it’s ejected from the mold.

2. Advantages:

The controlled mold filling process reduces the chances of surface defects, such as jetting, weld lines, or flow marks, resulting in parts with smoother and more aesthetically pleasing surfaces.

The controlled filling and pressure equalization reduce internal stresses within the molded part. This is particularly advantageous for transparent parts, where internal stresses can affect clarity or lead to defects

The counter pressure process allows for better replication of intricate mold details, producing parts with sharper features and more precise geometries.

The controlled molding environment results in more uniform cooling, which can reduce the issues of shrinkage and warpage in the finished parts.

Thick-walled parts are more prone to surface defects and internal stresses. CPIM can effectively address these issues, making it an ideal process for such components.

3. Limitations:

Due to the added steps of creating and releasing counter pressure, the molding cycle times might be slightly longer than standard injection molding.

Implementing counter pressure requires additional equipment and sophisticated control systems to manage the gas pressure throughout the molding cycle.

4. Applications:

    Where clarity and defect-free surfaces are critical.

    Where precision and surface quality are essential.

    For components that require intricate details and high aesthetic standards.

    4. Summary

    In summary, Counter Pressure Injection Molding offers a solution for producing high-quality, defect-free parts, especially those with intricate designs or thick walls. The process, while more complex than traditional injection molding, delivers benefits in terms of surface quality, structural integrity, and detailed reproduction.

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