Overmolding Design Guide

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Designing your part correctly for the overmolding process is critical. The design process starts with understanding the function of the part. Here are some good questions to ask upfront:

  • What does your part accomplish?
    • Because the overmolding process has a wide array of applications, you need to understand the core function of your part. For example, if you are molding a seal on a water-resistant case, proper sealing is likely the most important function.
  • What will the part be exposed to?
    • Exposure to UV light from the sun or harsh chemicals can damage certain kinds of plastics. If you understand where the part goes and what it’s exposed to, you can select materials that are more resistant to UV light than others.
  • Why is the part overmolded?
    • If you are adding a TPE (a rubber-like plastic) to a handle, it could be for grip, ergonomics, comfort, or vibration dampening (see above). Answering this question helps you figure out which material is best for your purposes. For example, you may choose a certain grade TPE (there are many) to yield a high grip in wet conditions and another for high vibration dampening.
  • What are the long-term production needs?
    • Part design factors, such as wall thickness, can affect cycle time and piece price. An example is creating a high vibration absorbing grip. The TPE could be thicker to allow the vibration absorption, but that may cause a longer cycle time and it also uses more material. The long-term cost of the added material and cycle time could be huge at scale; however, it could be “worth it” in a lower volume application.

Once you clearly understand the function of your part, you want to look at the design of the part and make sure it’s optimized for the manufacturing process. Other design considerations include wall thickness, gate location, shrink, sealing features, and mechanical interlock features. 

  • Wall thickness– Like any injection molded part, the wall thickness throughout the part should be consistent and thick areas should be avoided. Consistent wall thickness creates even plastic flow. Having plastic too thick can cause shrink marks and voids. 
  • Gate Location– Plastic needs to get from the molding machine into the desired area of the part. When the plastic enters the part (at the gate), a small mark may be formed. For cosmetic parts, it’s essential to plan for that.
  • Sealing Features– The mold must seal against the surface of the substrate properly to obtain clean edges between the substrate and overmold. If the substrate is plastic, some interference can be designed into the overmold tooling to compress and seal against it.
  • Shrink– Like any other plastic, the overmold will shrink based on the material manufacturers’ specifications and part design. Since more shrink is present with larger wall thicknesses, it’s important to make sure that the overmold does not shrink away from the substrate.

Check out this page for more on overmolding materials and processes.

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