Top 10 Tips on How to Fix Injection Overmolding Defects

When it comes to Overmolding injection molding, the most common two defects that frustrate we see is flash and two plastic material not sticking tightly.

What is overmolding injection then?

It is a process when one material being is molded over a second material.The base layer is molded first and the additional plastic layer(s) are molded over and around the original part – resulting in a single, finished product.


What are the defects of overmolding and tips on how to fix them?

Defect 1.: substrate part deformed

This happens because the substrate part is too thin and it gets deformed under high temperature during the process of secondary molding. Besides, if the overmolding area is too thick and narrowly long, the shrinkage force of soft plastic will lead to the “bump”  or arch deformation of the substrate as well

Suggestion:check gate position and add strengthening rib to the substrate part.


Defect 2.: poor local bonding

It usually occurs on the area where the flow path of the molten plastic is long and far from the gate.

Suggestions: properly increase injection temperature;improve material flow; optimize gate design.


Defect 3.:  Gate peeling and seperation

This is usually seen on the plastic part that with smaller runner gate but flow channel is long. It is usually because the compatibility of both materials is not so good and product structure is easy to form a strong shear effect at the gate location, which leads to the stratification and peeling at the gate position


TPE plays at very important role on the success of a overmolding part as well.

When injection overmolding soft TPE, certain parameters as below need to be consider. Here are 10 tips for your refence.
1. TPE needs to be compatible with the substrate plastic part. Only when the molecular solubility of both plastics are close can they be compatible with each other.

  1. Avoid sharp corners in the part design as possible, so as to make sure the contact between TPE and substrate is good and improve bonding effect.
  2. Keep proper vent to make sure the air left in the mold cavity could be vent out.
  3. Temperature of molten TPE should be kept stable to guarantee boding effect.

5. TPE needs to be dry before being molded to reduce water mark on the surface of finished part and obtain uniform cosmetic look.

6.The chosen color master batch should be compatible with TPE and substrate plastic as well.

  1. For smooth surface, special treatment is required, otherwise the bonding wont work.
  2. TPEshould be of good flowability. Because the thickness and size ratio for TPE layer is quite small, TPE usually needs to flow through a long runner and thin wall area to be injected into the mold.


  1. Use best bonding agent.

10.TPE should give a soft and comfortable feeling while in touch with human body.  This is the main purpose of injection overmolding.

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