Push-pull injection molding

Push-pull injection molding

What Is push-pull molding?

Used for producing parts with varying thickness levels, it involves two sets of molds that move in a synchronized manner to allow varying wall thickness.

In many injection molding applications, consistent wall thickness is crucial to avoid defects like warping or sink marks. However, there are cases where a product might benefit from having varied thickness in different sections. 

Push-pull injection molding addresses this need.

1. Process

The process utilizes two mold sets. The first mold defines the overall shape and the thicker sections of the part. The second mold is designed to create the thinner sections.

As the molten plastic is injected, the two mold sets move in a synchronized manner. This movement, in conjunction with the flow of the plastic, allows for the formation of areas with varying wall thicknesses.

The “push” typically refers to the forward movement (or closing action) of one mold set, ensuring the formation of thicker sections. Simultaneously, the “pull” refers to the retraction or opening of the other mold set, facilitating the thinner sections.

2. Advantages

Allows for the creation of parts with different wall thicknesses in a single molding cycle.

By using a synchronized system, it’s possible to achieve complex geometries without resorting to secondary molding processes or assemblies.

By designing parts with thinner sections where full thickness isn’t required, material usage can be optimized, leading to cost savings.

Varying the thickness can be used to strategically enhance the strength, flexibility, or other properties of a part, depending on its intended application.

3. Limitations

The process might be more complex than standard injection molding, requiring specialized equipment and mold designs.

The initial investment for the molds and machinery may be higher than traditional methods.

Manufacturers familiar with conventional injection molding might face a learning curve when adapting to the push-pull technique.

4. Challenges

The mold design for multi-color injection molding can be complex, requiring precise engineering to ensure accurate color placement.

It’s essential to select materials or colors that bond well together to prevent issues like delamination or weak interfaces between the different colors.

Not all injection molding machines are equipped for multi-color molding. Special machinery that allows for rotation of the mold or has multiple injection units might be required.

The initial setup, especially the mold design and machinery, can be more expensive than standard single-color injection molding.


In conclusion, push-pull injection molding appears to be a method that offers designers and manufacturers a higher degree of flexibility in part design, especially concerning wall thickness variation. As with any specialized technique, the decision to employ it would depend on the specific requirements of the project, budget considerations, and the capabilities of the manufacturing facility.

Have Questions about Plastic Injection Molding?

Please contact us, and we shall be happy to answer them!