The success of an overmoulded part is decided almost entirely by the material pair, and the choice is not made by picking a nice feeling elastomer. It is made by starting from the rigid substrate and asking which soft grades are formulated to bond to it, because a thermoplastic elastomer that bonds to polypropylene will peel straight off polycarbonate.
Start from the substrate
Styrenic elastomers bond well to polypropylene and other polyolefins. Polyurethane and copolyester elastomers bond to polar substrates such as ABS, polycarbonate and their blends, and to nylon. Silicone bonds to very little without a primer or a specifically bondable grade. Material suppliers publish compatibility tables per grade, and those tables are the specification rather than a suggestion.
Chemical bond against mechanical interlock
A chemical bond forms at the interface and makes the two materials behave as one part, which is what you want. Where no compatible pair exists, the fallback is mechanical: holes through the substrate, undercuts, grooves and captured edges that the soft material flows into and grips. That has to be designed into the substrate from the beginning, not added when the bond fails.
Hardness and what it does to the process
Softer grades flow more easily and fill thin sections better, and they are also more likely to flash. Harder grades are stiffer to grip and need more pressure. The hardness is a product decision about feel and function, and it changes the tooling requirements, so it should be settled before the tool is designed rather than adjusted afterwards.
Testing the pair before you tool
Bond strength is measurable by peel test on moulded plaques, and material suppliers will usually provide or run one. Doing that on the actual grades, at the actual processing conditions, before the tool is cut, is the cheapest possible insurance. Colour, additives and flame retardants in either material can all affect adhesion, so test the exact formulations you will ship.
Questions people ask about overmolding materials
How do I choose an overmoulding material?
Start from the rigid substrate and choose an elastomer grade formulated to bond to it. The supplier's compatibility table is the specification.
What if no material bonds to my substrate?
Use mechanical interlock: holes, undercuts and captured edges designed into the substrate so the soft material grips it physically.
Does hardness affect the process?
Yes. Softer grades flow better and flash more easily; harder grades need more pressure. Settle hardness before the tool is designed.
Should I test the bond before tooling?
Always, on the exact grades and colours you will ship, at the actual processing conditions. Additives and pigments can both affect adhesion.