Insert moulding places a component, usually metal, into the tool before the shot so plastic forms around it and captures it permanently. It removes an assembly operation, removes a fastener and produces a joint stronger than anything installed afterwards, and it does all that in exchange for a slower cycle and a much less forgiving process.
What gets moulded in
Threaded inserts and studs, electrical contacts and terminals, bearing bushes, shafts and pins, stiffening plates, and magnets. The common thread is a component whose function needs it to be fixed rigidly and concentrically in the plastic, and which would otherwise be pressed, glued or screwed in as a second operation.
Retention comes from geometry
Plastic shrinks onto an insert, which grips it, but shrinkage alone is not a reliable retention feature. Knurls, grooves, flats, holes and undercuts on the insert give the plastic something to key into, resisting both pull out and rotation. Specifying an insert without retention features and hoping shrinkage will hold it is the classic failure of the process.
What the process demands
The insert must be located precisely and held while the shot arrives at high pressure and temperature, so tooling carries pins, pockets or magnets to hold it. Inserts are often preheated so the plastic does not freeze against cold metal and form a weak weld line. Cycle time rises because the tool has to be opened and loaded, which is why automation is common at volume.
Where it goes wrong
Inserts that move under injection pressure and end up off position. Thin plastic sections around a large insert that crack from the stress of shrinkage onto an unyielding metal core. Contamination from oil or plating that stops the plastic bonding or keying. And tolerances on the insert itself, since the moulder can only locate a part as accurately as the part is made.
Questions people ask about insert molding services
What is insert moulding?
Placing a component, usually metal, into the mould before the shot so plastic forms around and captures it, removing a later assembly operation.
How is the insert held in place?
By features in the tool such as pins, pockets or magnets, and retained in the finished part by knurls, grooves or undercuts that the plastic keys into.
Is insert moulding better than pressing an insert in afterwards?
It gives a stronger, more concentric joint and removes an operation. It costs cycle time and process robustness, so volume and criticality usually decide.
Why did the plastic crack around my insert?
Usually too little plastic around a large rigid insert, so shrinkage stress has nowhere to go. Increasing the wall around the insert or softening the resin choice are the usual fixes.