Plastic Overmolding Process: Advantages Disadvantages & Applications

What is Overmolding Process?

The Overmolding process is a type of injection molding process where a secondary material is molded over the first material known as substrate.

Example
Insert molding process involves overmolding plastic over metal substrate.
In the above image, the thermoplastic material is insert-molded over metal screws.

The primary condition for over-molding is that the melting temperature for substrate material should be higher than the second molding material. 

For example, we can not over-mold polycarbonate material over ABS. Because during over-molding PC over ABS, higher PC molding temperature will damage the ABS part. Click this link to know material selection guidelines for injection molding.

Plastic Overmolding Process Types

Following three types of plastic over-molding processes are widely used in the industry.

  1. Insert molding.
  2. Two-shot Molding.
  3. Co-injection molding

Insert Molding

Insert molding is a widely used low-cost over-molding process. It involves positioning an insert (plastic or metal) in an injection mold. Afterward, the thermoplastic or elastomer material is injection molded over the insert.

Here are the four different ways to insert mold plastic parts.

  1. Thermoplastic over metal substrate.
  2. Thermoplastic over plastic substrate.
  3. Elastomer over plastic substrate
  4. Elastomer over metal substrate.
Insert molding process involves overmolding plastic over metal substrate.
Insert Over-Moulding Process

We can use single-shot injection molding machines for insert molding. Insert molding tool cost is low compared to two-shot and co-injection molding tools. Insert molding process involves the following steps.

Step-1:

Position the substrate inside the injection mold. For example, place a screw inside the injection mold.

Step-2:

Inject the secondary material inside the injection mold.

Step-3:

After cooling, remove the part from the injection mold.

Click this link to know the factors affecting injection molding tool price.

Two-Shot (Multi-Shot) Molding

Two-Shot Injection molding is also known as Multi-Material or 2K molding. The multi Material molding requires an injection molding machine consisting of multiple barrels and tool rotating arrangements. Multiple barrels allow us to inject different materials into the same mold in one injection cycle.

This image shows the example of two shot overmolding part.

You can manufacture insert molded parts using multi-shot molding and vice-versa. But multi-shot injection molding has the advantage of superior part quality, low cycle time, and cost. The following options are available for two-shot overmolding.

  1. Thermoplastic over thermoplastic substrate.
  2. Elastomer over thermoplastic substrate.
Two Shot injection Overmolding Process
Two-Shot injection over molding process is also known as Multi-Material or 2K molding.
Two Shot Molding

Here is the list of steps for two-shot injection molding.

Step-1

Firstly, the machine injects molten plastic into the injection mold from barrel-1. 

Step-2

The core and cavities separate after the cooling of molten plastic.

Step-3

The injection mold is rotated and brought in position with injection barrel-2.

Step-4

Afterward, the mold is closed. And machine injects the molten plastic from barrel-2. 

Step-5

After the material is cooled, the part ejects out of the injection mold.

Plastic Co-Injection Molding

Co-Injection overmolding Process involves injection molding of two dissimilar materials in one mold. During co-injection, both materials become in a fully molten state. Therefore high bonding strength between materials is achieved. Co-injection molding is an expensive process and difficult to control.

2K injection molding Process

The Co-injection or 2K injection molding process involves the following three steps.

In Step 1 during co-injection molding. Skin or outer material is partially injected into mold.
Step-1: As illustrated in the image, Skin or outer material is partially injected into the mold.
Step-2: Core material is injected until the 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

Over molding process involves additional tool and machine costs. But the over-molding process has the following advantages:

Improved Product Quality and Performance

Over-molding ensures proper alignment and rigid assembly of mating parts. It improves product quality and performance.

Eliminates Fasteners and Adhesive

Overmolding parts eliminate the need for extra fasteners or adhesive. For example, a toothbrush rubber grip is two-shot molded with a plastic handle. Therefore overmolding eliminates adhesive requirements.

Reduced Part Cost

Plastic overmolding eliminates the use of secondary operations to reduce labor costs, time, and requirements of fasteners.

Improve product ergonomics and User Experience

Overmolding can provide rubber grips to tools, toothbrushes, and other handheld products. Rubber grips help in improving product ergonomics. Two-shot molding and co-injection molding have applications in manufacturing parts in multiple colors. Multiple colors improve product aesthetics.

Enhanced Design Flexibility

Complex assembly design and manufacturing of multi-material parts are feasible with overmolding. For example, car headlights are manufactured using insert or two-shot molding.

Applications of Plastic Overmolding

Overmolding in plastic and metal parts improves product aesthetics or eliminates the assembly operation. Following are the applications of over-molding in various industries:

  • Manufacturing parts in multiple materials.
  • To manufacture multiple color parts such as car tail-lights.
  • Provide a soft grip around a component.
  • Manufacture waterproof products such as fitness trackers.
  • To eliminate the assembly of parts.
  • Assembly of parts without using fasteners or adhesives.
Application examples of Plastic overmolding Process

Following are the examples of over-molded plastic products that we use in our day-to-day life.

1. Over-Molding of plastic over plastic part

Overmolding of different thermoplastic materials is required to manufacture parts in a different color, finish, or material. Here are the examples of over-molding plastic material over plastic.

To manufacture car tail lights in multi-materials and colors (Clear and Red tinted glass) two-shot or insert injection molding.
Smartwatches and fitness devices screen is manufactured in Clear PMMA whereas the remaining housing in ABS or PC. To eliminate the need for an adhesive and higher product IP rating, insert molding is used.
2. Over-Molding 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 the product rugged.

Here are the examples of over-molding rubber over thermoplastic

TPU or silicone material 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.
3. Over-Molding Plastic Over Metal Parts

Metal parts can be insert molded during injection molding:

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

We will keep updating more information on various overmolding processes. Please add your suggestions, comments, or questions on plastic over-molding in the comment box.

We suggest you also read this article on Plastic Part Design Guidelines for injection molding.

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