Overmold Engineering and Design

As automotive manufacturers have been driven to reduce vehicle weight, CS Tool Engineering has stepped up to the challenge of developing increasingly complex plastic injection molds. Plastic injection molds that combine strength with minimal materials. Our engineers love the challenge.

The intricacies involved in manufacturing an automotive door handle are hard to imagine. The geometrical complexing of designing a plastic injection mold that produces plastic with the strength of steel is difficult.

Overmolding has proven to be an effective method for achieving this end.

What Is Overmolding?

AdobeStock 60750529 PreviewIn overmolding, a single part is created using a combination of two or more different materials. The first material (the substrate) is partially or fully covered by the subsequent (over-mold) materials during the manufacturing process.

The hard plastic substrate provides structural support. The second material usually provides a soft feel. (For instance, screw driver handles and tooth brushes are common hard/soft overmolded products.) Overmolding allows the use of two different materials without using glue or fasteners.

The substrate could be just about anything: a machined metal part, a molded plastic part, or an existing product (such as threaded inserts, screws or electrical connectors). It is the basis for what will eventually become a single continuous part made from chemically bonded (and often mechanically interlocked) materials of different types.

The overmold materials are often plastic-based and are typically in the form of pellets. These pellets are then mixed with colorants, foaming agents and other types of fillers. Then they’re melted down and the resulting liquid is injected into the mold tooling.

How Does It Work?

The overmold process is fairly straightforward:

  1. The substrate material or part is placed into an injection molding tool
  2. The overmold material is injected into, onto or around the substrate
  3. The overmold material is allowed to cure or solidify
  4. The end product: two materials joined together as a single part.

Typical over-mold combinations include:

  • Plastic Over Plastic – A rigid plastic substrate is molded first, followed by another rigid plastic overmold on or around the substrate. The two plastics often differ in color and/or resin.
  • Rubber Over Plastic – A rigid plastic substrate is molded first, followed by a soft rubber or TPE over-mold. (TPE, or thermoplastic elastomer, is a mix of polymers, typically, plastic and rubber.) This process is often used to create a rigid plastic part with a soft grip area.
  • Plastic Over Metal – A metal substrate is first machined, cast or formed. It is then inserted into an injection molding tool, where the plastic is molded onto or around the substrate. This process is often used to capture metal components within a plastic part.
  • Rubber Over Metal – A metal substrate is first machined, cast or formed. It is then inserted into an injection molding tool and rubber or TPE is molded onto or around the metal. This process used to create a rigid metal part with a soft grip surface.

Why Overmold?

AdobeStock 82651393 PreviewThe most common reasons for using the over-mold process are:

  • To break up color (for aesthetics)
  • To provide a soft grip surface around a rigid part (e.g., hand tools)
  • To add flexible areas to a rigid part
  • To capture one part inside of another part without the use of fasteners or adhesives
  • To reduce production time. No need to assemble two separately manufactured components (such as a metal tool and a rubber hand grip). Overmolding the metal tool with a rubber hand grip eliminates the assembly process.

Replacing Metals with Plastics

AdobeStock 108665382 PreviewFor the automotive industry, replacing metal with high-performance plastic injection or compression molded components is a proven strategy for vehicle weight reduction. On average, vehicle fuel mileage improves 2% for every 100 lbs. of weight reduction.

For example, overmolding with some of the new all-plastic composites is now replacing over-molds using heavier plastic-metal “hybrid” composites. So far, these new overmolded plastics are being used primarily on automotive interiors. But additional applications are expected.

When mass production is combined with vehicle weight reduction, the results are astonishing. Reducing the vehicle weight of the 70 million light-weight vehicles produced annually would not only decrease CO2 emissions; it would save 90 million gallons of fuel - enough fuel to power 180,000 vehicles for a year.

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