What is Overmolding : Insert Molding, Two shot and Co Injection Molding

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Overmolding is an molding process of creating one single part using different materials such as thermoplastic or elastomers. In a laymen language, overmolding is a process of putting additional layer of material over and around another material. First material is known as substrate. For example when inset is overmolded with plastic parts. Insert is known as Substrate.

Overmolding Process Types

Based on how parts are molded together. Overmolding process is classified in following Categories.

1) Insert Molding

Insert molding is low cost and most commonly used overmolding process. It involves positioning an insert (Plastic or metal) in Injection Mold. After that thermoplastic or elastomer material is molded over insert. Following options are available for insert molding:

  • Thermoplastic over Metal Substrate.
  • Thermoplastic Over Plastic Substrate.
  • Elastomer Over Plastic Substrate
  • Elastomer over Metal Substrate.
Insert Moulding Process

Single shot Injection molding machines can be used for insert molding. Insert molding tool costs is very less compared to two-shot and co-injection molding tool cost.

Step-1: Substrate is positioned inside injection mold. For example screw is placed inside mold. 

Step-2: Secondary material (usually thermoplastic or elastomers) is injected inside injection mold.

Step-2: After Cooling part is ejected out of injection mold.

2) Two-Shot (Multi-Shot) Molding

Two-Shot injection molding process is also known as Multi-Material or 2K molding. Multi Material molding requires special injection molding machine. This machine consist of Multiple barrels and tool rotating arrangement. Multiple barrels allows multiple materials to be injected into the same mold in one cycle.

Parts manufactured with multi shot molding can also manufactured using insert molding and vice-versa. But multi shot injection molding has the advantage of superior part quality, low cycle time and cost. Following options are available for two-shot overmolding:

  • Thermoplastic over Thermoplastic Substrate.
  • Elastomer over Thermoplastic Substrate. 
Two Shot Overmolding Process
Two-Shot injection over molding process is also known as Multi-Material or 2K molding.

Step-1 : Firstly molten plastic is injected into injection mold from Barrel-1. 

Step-2: Molten plastic is cooled and core, cavity are Separated from each other.

Step-3: Injection tool is rotated and bring in position with injection Barrel-2.

Step-4: Mold is closed and molten plastic is injected from Barrel-2. 

Step-5After material is cooled, part is ejected out of injection mold.

3) Co-Injection Molding

It involves injection molding of two dissimilar materials into one mold. This process is very expensive and difficult to control. During co-injection, both materials becomes in a fully molten state. Therefore high bonding strength between materials is achieved. 

Co-injection Molding Process

This process involves following three steps.

In Step 1 during co-injection molding. Skin or outer material is partially injected into mold.
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.
Advantages of Overmolding Process

Overmolding process requires additional investment. But it offers numerous advantages.

  • Improved Product Quality and Performance: Overmolding ensures proper alignment and rigid assembly of mating parts. Therefore product quality and performance is improved.

  • Eliminates Fasteners and Adhesive: Toothbrush rubber grip is two shot molded with plastic handle. This eliminates the need of adhesive.
  • Reduced Cost: Overmolding eliminates use of secondary operation. This helps in reducing labor cost, time and fasteners. Therefore overall product cost is reduced.
  • Improve product ergonomics and User Experience: Overmolding is used to provide rubber grips to tools, tooth brushes and other handheld products. Rubber grips helps in improving product ergonomics. Two shot molding and co-injection molding is used to manufacture parts in multiple colors. Multiple colors improves product aesthetics.
  • Enhanced Design Flexibility: Complex assembly design and manufacturing of multi material parts is feasible with overmolding. For example car headlights are manufactured using insert or two-shot molding.
Applications of Overmolding

Overmolding has applications where two or more different thermoplastic or rubber parts are to joined together. Typical applications of overmolding includes:

Thermoplastic Over Thermoplastic

Different thermoplastic materials are molded together to manufacture parts in different color, finish or material.


Car tail lights are manufactured in multi-materials and colors: Clear and Red tinted glass. This color and material combination is achieved using two shot or insert injection molding. 

Smart watches and fitness devices top screen is manufactured in clear PMMA whereas remaining housing is manufactured in ABS or PC. To avoid use of adhesive insert molding is used.

Rubber over Plastic

Rubber can be Insert-Molded and over molded on plastic and metal parts to:

  • Add soft grip on hard materials
  • Make a product waterproof (Improve Product IP Rating).
  • To make product Rugged.


TPU or silicone soft grip is added over metal or plastic parts to improve user experience.

Silicon Rubber or TPU is inset molded on plastic parts on a tablet to make it more rugged and Waterproof.

Plastic Over Metal Parts

Metal parts can be insert molded during injection molding:

  • To save assembly cost
  • Improve Bonding between plastic and metal parts.


In a know metal screw is insert molded with the plastic part.


To sum up, Overmolding offers various advantages compared to conventional injection molding. It involves high tool investment and machines cost. But this high initial cost can be amortized with lower part cost. 

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