One cause of insufficient bonding between materials relates to the timing of the injection of the materials and the temperature of the first material when it is joined with the second. Too much cooling of the first material tends to weaken bonding.

On the other hand, the first shot must be cooled enough not to be deformed or displaced when you shoot the second one. If the second shot comes too soon, while the first material is still soft, the second material can compress and flash over the first one, causing “splash marks.”

When running parts on two injection machines (molding the first shot on machine one and inserting it into the mold of the second machine), bonding is not apt to be as good as on a two-shot machine with rotating table. Even when using compatible materials, the delay time between the two shots is relatively long and the first shot is likely to be too cold. A higher part temperature is recommended for better chemical/mechanical bonding. Also, if the first shot picks up dust while being transferred to the second mold, bonding will also be negatively affected.

Apart from process conditions, material choice can greatly affect bonding. Some materials naturally tend to adhere better than others, and resin suppliers–particularly makers of TPEs—have been working hard to optimize certain grades for overmolding by increasing their range of adhesion to other polymers.

Additives and pigments can affect bonding. Glass fibers in one material can enhance bonding with the second. Fibers on the surface of the material promote a mechanical bond with the second shot.
Note that materials containing fillers like talc or calcium carbonate should be dried adequately. These fillers hold a lot of moisture, which can detract from bonding.