Growing in popularity among manufacturers of consumer products, medical devices, and Automotive components, over-molding enhances functionality, performance, and aesthetics.

However, what exactly is over-molding, and, more crucially, how does it influence product design and profitability? This article will provide you with the answers.

What is Overmolding?

Overmolding is a Special injection molding process used to mold one plastic or rubber (commonly a rubber-like plastic called TPE) over top of another component (substrate) which usually is an injection-molded plastic part. Two materials are joined through chemical (adhesion, dissolution) interactions, forming a single component but with dual materials.

How does overmolding work?

In general, the over-molding process follows the same principles as injection molding but involves molding plastic over another component. The distinction lies in the process execution. In overmolding, the base material is introduced into the mold during each cycle. Moreover, how the base material is introduced gives rise to two distinct overmolding methods:

Manual Overmolding

In manual overmolding, the base material is injection molded using a conventional injection molding process. Subsequently, these base materials are manually loaded into another mold for the overmolding step. Operators are responsible for removing the finished part from the mold, conducting inspections, and packaging the parts. Manual overmolding is the predominant approach for low to mid-volume injection molding

Two-Shot Overmolding

More advanced injection molding machines are capable of processing two different polymers simultaneously. Two-shot molding uses sophisticated molds and robotics to mold the substrate on one side and then transfer it to the other half by rotating it 180 degrees during each cycle. This method requires more advanced injection molding machines and molds, but the cost per part can be lower than the manual process.

Tips for Two-shot Overmolding Materials

Plastics differ in their chemical properties, and their compatibility will directly affect the bonding strength and bonding effect of two-color injection molding. Therefore, when designing two-color injection molded products, the first and most important step is to select the most suitable substrate material and covering material.

  • Choose substrate and covering materials with good compatibility, as shown in the diagram above for the compatibility of common plastics.
  • The difference in melting temperature between the substrate material and covering material should not exceed 30~60°C.
  • The coefficient of thermal expansion (CTE) of the substrate material and covering material should be as close as possible.
  • The shrinkage rate of the substrate material and covering material should be as close as possible.
  • Avoid the combination of amorphous plastics and semi-crystalline plastics.
  • The heat deflection temperature (HDT) of the substrate material should be higher.

Pros and Cons of Overmolding

Pros:

  • Enhanced Product Design Flexibility: Two-color injection molding enables the integration of multiple functions into a single plastic part. This includes features like soft-touch surfaces, ergonomic design, dual-color appearance, branding, performance enhancements, noise reduction, shock absorption, waterproofing, and impact resistance. It can save design space and reduce the number of components.
  • Reduced Engineering Complexity for Cost Savings: The one-step injection molding process can produce multifunctional plastic components, reducing the need for secondary processing.
  • Improved Quality and Strength: Different plastics are chemically bonded together, resulting in higher quality and improved strength.

Cons or Limitations:

  • Consideration of Material Compatibility: Compatibility between the two plastics is essential. Different plastics with varying chemical properties may not exhibit good compatibility, which can directly affect the bonding strength and bonding effect. Not all plastics are highly compatible with each other.
  • Minimize Differences in Shrinkage Rates: Significant differences in shrinkage rates between plastics can affect bonding strength and may lead to product warping.
  • Complex Shapes May Not Be Suitable: Overly complex product shapes may not be ideal for two-color injection molding.

Some Applications of Overmolding

Power switches, mobile phone buttons, Auto lamp lens, etc., to meet partial light guiding or translucency requirements.

Handheld products, such as walkie-talkie casings, toothbrush handles, power tool grips, wrenches, and thermos cups, enhance the tactile feel.

Products with diverse color appearance requirements, such as keyboards, use two-color injection molding for both aesthetics and durability, ensuring that the colors won’t wear off during use.

Products that require plating in specific areas, such as the buttons on feature phones.

In the plastic family, only ABS and polysulfone (PSU) surfaces can be metalized. If metallization is required in specific areas or on one side of the product, the metallization area is injected with ABS, while the areas that do not require metallization are injected with other materials such as PC.

Products that require waterproofing to meet waterproofing requirements.

Design guidelines for 2 shot Overmolding  

Design of Internal Bonding Surfaces

The bonding strength between the substrate material and the covering material depends on the compatibility of the materials, processing temperature, bonding surface, molding sequence, and the design of mechanical interlocking structures on the internal bonding surfaces.

A: Increasing the area of the bonding surface to enhance bonding strength.”

B. When the internal bonding surfaces are too small, on one hand, roughness can be increased on the bonding surfaces through knurling, and on the other hand, mechanical interlocking structures can be designed.

Design of Appearance Matching Surfaces

The design of appearance-matching surfaces during the first and second injections is highly challenging. It is crucial to provide sufficient bonding strength; otherwise, issues such as flashing, burrs, curling, and peeling can easily occur, especially in two-color injection molding with different soft and hard plastics. Common designs for appearance-matching surfaces include step-type and groove-type.

Uniform Wall Thickness and Avoiding Sharp Corners

Two-color injection molding is a form of injection molding, so it must adhere to injection molding DFM design guidelines, such as maintaining uniform wall thickness avoiding sharp corners, and ensuring smooth transitions.

Draft Angle

Draft angle is crucial for two-color injection molding because it determines which mold the part adheres to during the ejection process. During the first injection, the part should adhere to the moving mold, and during the second injection, the part should

adhere to the mold’s ejector side.

OVERMOLDING VS. INSERT MOLDING

Insert molding and overmolding are both methods that utilize the injection molding process to encase something with plastic. However, the primary distinction lies in overmolding, which typically entails molding a plastic atop another plastic component. In contrast, insert molding involves molding plastic around a non-plastic component. To go deeper into the concept of insert molding, you can explore our insert molding overview. 

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