A major overmolding pitfall to steer clear of is additives in thermoplastic resins. Many thermoplastics will have a letter “R” in their compound name, ie 123R, the “R” signifies that a release agent has been added to the plastic chemical compound to promote flow through the barrel of the injection molding machine. Resin distributors should also carry 123 (without the “R”), this is the same plastic compound but without the additional release agent. A certificate should be obtained from the resin supplier stating there are no releases in the plastic resin. While this resin may not run as smoothly and as quickly as resin with release built in, that release agent makes it impossible to overmold liquid silicone onto. It should also be noted that the addition of release agents to plastic resins has become so common place, that not everyone at the resin distributor that you are dealing with will know what the “R” stands for and that there is a difference between the material with and without the “R”.
Stand alone, spray on mold releases of any kind should be avoided (if at all possible) in both the plastic molding and liquid silicone overmolding steps in the process, as they can weaken the adhesion strength if it is permitted to contact the bonding interface.
Tests have indicated that bond strength of the liquid silicone to the overmolded part increases over time, ie. the level of a liquid silicone adhesion measured when the part is removed from the molding machine will be inferior to the adhesion measured the following day.