What is Overmolding

What is Overmolding ?

It is an injection molding process of creating one single part using two or more  materials. This process is also known as two shot injection molding. During overmolding, additional material layer are molded over and around the initially molded part.

Based on manufacturing process, overmolding can be classified into 3 categories : 

1. Insert Overmoulding

This is most commonly used overmolding process and a cheaper alternative compared to any other overmolding process.

As illustrated in image, metal screw is insert molded with plastic knob.

Insert overmolding Process:

Step-1 : Relatively harder and higher temperature resistant plastic part is injection molded. This part is also known as insert. For example if overmolding of ABS and PC is done, PC is injection molded first. 

Step-2 : Already injection molded part used as insert is placed inside injection molding tool.

Step-3 : Secondary material (usually thermoplastic or elastomers ) is injected inside molding tool.

2. Two Shot Molding

This is also known as 2 material and 2K molding. Parts manufactured with two shot molding can also manufactured with insert molding and vice-versa. Advantage of 2 shot molding over insert molding is ; during two shot molding both injection operations are performed in one cycle.

Two Shot Molding Process : 

Double injection molding machine is used for two shot molding. Molding machine performs two injections cycle. During first cycle, nozzle injects plastic into a mold. The mold is then automatically rotated and a different type of plastic is injected into the mold from a second nozzle.

3. Co-Injection Molding

This process involves injection of two dissimilar materials into one mold. Co-Injection molding process is very expensive and difficult to control. However, because both materials are in a fully molten state, it provides the best bonding between materials.

Co-Injection Molding Process :

Step 1 : As illustrated in image, Skin or outer material is partially injected into mold.
Step 2 : Core material is injected until cavity is nearly filled, which displaces the skin material outside.
Step 3 : Skin material is injected to purge the core material from the sprue.
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