Insert Molding (overmolding)

Insert molding is a process by which an object, or insert, is placed into the mold between cycles and the plastic is formed around the insert. By doing so the object becomes a part of the final molded product or a detail of the insert is left on the plastic part. The materials used as inserts include a variety of metallic inserts, cloth, retaining rings, and film. A metallic core can be inserted into the mold to become part of the molded part or to leave its profile in the molded part. These cores can be used to create threads in the molded part and an operator or automation fixture in the post-mold process can back them out. Metallic inserts can also be used to increase the strength of the molded part or to create a metallic threaded hole or through hole.


  1. A custom-built mold is loaded with inserts. Inserts may be loaded robotically or manually.
  2. Molten plastic is injected into the mold.
  3. Upon cooling, the mold opens and the components are removed.
  4. Components are separated from the sprues and inspected.
  5. Post-molding assembly can include a variety of secondary operations, such as:
    • Die cutting of stampings into discrete circuits
    • Bonding
    • Microsoldering
    • Circuit testing


  • Reduced assembly and labor costs
    Because insert molding joins numerous components with thermoplastic, assembly and labor costs are greatly minimized. For example, a single stamping can be overmolded, then perforated to create multiple circuit paths.
  • Reduced size and weight
    By eliminating fasteners and connectors, and by combining the physical strength of resin and metal inserts, insert molding yields smaller and lighter components.
  • Increased reliability
    With every part tightly secured in thermoplastic, an insert molded component prevents part loosening, misalignment, improper terminations, and other problems. The thermoplastic resin also provides improved resistance to shock and vibration.
  • Increased design flexibility
    Designers appreciate the virtually unlimited configurations that insert molding allows. For example, in creating a 3D circuit board, overmolding permits circuitry to move freely through the part, from inside to outside, up walls, down in holes-and the plastic ties it all together.

Typical parts manufactured using this process include fittings, connectors, switches, mechanical assemblies and electrical assemblies.