Metal overmoulding moulds plastic around a metal component so the two become one part: a stamped lead frame in a connector, a metal core in a handle, a bearing in a housing. It removes an assembly step and produces a joint that cannot loosen, and it introduces one problem that has to be designed for from the start, which is that metal and plastic expand at very different rates.
Retention comes from geometry
Plastic shrinks onto metal as it cools, which grips, but shrinkage alone is not reliable retention. Holes through the metal that plastic flows through and forms a rivet, knurled surfaces, slots, tabs and undercuts are what actually hold. On a stamped insert those features cost nothing to add at the blanking stage and are the difference between a part that lasts and one that works loose.
Differential expansion is the recurring failure
Plastics expand several times more than steel with temperature. A part moulded around a rigid metal core is therefore in tension around the metal as soon as it cools, and every thermal cycle in service works that stress. Thin plastic sections around large inserts crack, sometimes months later. Sufficient wall around the insert and a resin with some ductility are the mitigations.
Preheating and the weld line
Injecting hot melt against cold metal freezes it instantly at the interface, producing a weak skin and poor filling around the insert. Preheating the metal inserts before loading solves it, and on production tooling it is a standard part of the cycle. It also reduces the residual stress from differential contraction, so it helps the cracking problem too.
Loading, contamination and tolerance
The insert must be located precisely and held while the shot arrives at pressure, so tooling carries pins, pockets or magnets. Inserts must be clean, because oil or plating residue prevents the plastic keying properly. And the insert's own tolerance limits the finished part's, since the moulder can only position a component as accurately as it was made.
Questions people ask about metal overmolding
What holds plastic onto a metal insert?
Geometry: holes the plastic flows through, knurls, slots, tabs and undercuts. Shrinkage alone grips but is not reliable retention.
Why do overmoulded parts crack around the metal?
Differential thermal expansion. Plastic expands several times more than metal, so it is stressed around a rigid insert and cracks over thermal cycles if the wall is thin.
Should inserts be preheated?
Yes, on production work. It stops the melt freezing against cold metal, improves filling and reduces the residual stress from differential contraction.
Does insert cleanliness matter?
Very much. Oil, coolant or plating residue prevents the plastic keying and bonding, and is a common cause of inserts working loose in service.