Overmolding means moulding a second material over a first one so the finished part behaves like two materials at once: a rigid frame with a soft grip, a metal insert in a plastic body, a seal moulded directly onto the part it seals. Done well it removes an assembly operation and a failure point. Done badly it produces a part that delaminates in the customer's hand.
Two shot against insert overmoulding
Two shot moulding runs both materials in one machine with a rotating or shuttling tool, so the substrate never leaves the mould. It is fast, repeatable and expensive to tool. Insert overmoulding moulds the substrate first, then loads it into a second tool by hand or robot. Tooling is much cheaper, cycle time is longer, and handling introduces the variation. Volume decides which one is right.
The bond is chemistry, not pressure
Compatible material pairs bond chemically at the interface and behave as one part. Incompatible pairs rely on mechanical interlock alone, which means undercuts, through holes and captured features designed into the substrate. Choosing the elastomer to suit the substrate at the start is the difference between a part that survives and one that peels.
What the design has to allow for
The substrate is reheated by the second shot, so it must be stiff enough not to distort and thick enough not to melt through. Gate the second shot where flow will not wash across a thin edge. Keep the soft layer at a consistent thickness, because thin sections may not fill and thick ones sink. And provide shutoff surfaces, because the second material goes wherever the tool does not stop it.
Where the money goes
Tooling dominates, and two shot tooling dominates hardest, because you are effectively paying for two tools that must register perfectly. Add machine cost, since a two shot press is a more expensive machine hour, and add the second resin. The saving comes back in eliminated assembly, eliminated adhesives and a lower field failure rate.
Questions people ask about overmolding
What is the difference between overmoulding and insert moulding?
Insert moulding places a component, often metal, into the tool and moulds around it. Overmoulding moulds one polymer onto another, usually a soft material onto a rigid substrate. Insert overmoulding is the two shot process done in two separate tools.
Which materials bond well?
Thermoplastic elastomers are formulated to bond to specific substrate families, so the grade is chosen from the substrate. Ask the material supplier for the pairing data rather than assuming any soft grade will stick to any rigid one.
Can overmoulding be done without a bond?
Yes, with mechanical interlock: holes, undercuts and captured edges. It is a valid approach when the material pair cannot bond, but the interlock has to be designed in from the start.
Does overmoulding always need two machines?
No. Insert overmoulding can run on one machine with two tools and a manual or robotic load between shots. That is why it is the usual route at lower volumes.