Couple of things I usually recommend to get good bonds of TPE (Thermoplastic Elastomer) and PC:
a. Melt temperature is critical for a good bond. The melt temperature for overmolding is usually 390- 440F for overmolding over polycarbonate. The TPE has to be over the minimum melt temperature of 390F from start to finish. If it goes below this melt temperature at any point, the TPE will not bond to the PC beyond this point.
b. The flow ratio is critical as well - this is the ratio of the flow length and thickness of the TPE. Typically it is suggested to have an 80 - 120 flow ratio for overmolding. If the flow ratio is higher than 120, then multiple gates should be used. If flow ratio is too high, then it will be difficult to maintain the right melt temperature across the length of the substrate.
c. Venting is critical to achieve good bond. With overmolding, if the TPE is not vented properly, air traps will occur at the substrate-TPE interface and this will prevent a bond from occurring at this point. The good thing about overmolding is you can drill holes in the substrate to get venting - these would get covered up by the TPE.
d. Avoid adding texture on the substrate area underneath the TPE. There is a theory that adding texture would add surface area. But in most cases, especially with flow lengths over 2 in, the TPE would flow over the texture without getting into the depths of the texture and the bond is weaker.
e. With certain TPEs, drying is necessary. But this is not required in all cases. TPUs and TPVs require drying.
f. Choose the right TPE. The company I work for sells TPEs which bond well to PC.
g. Two shot does give better bonds. But insert molding works well as well with the right TPE.
h. Do not over pack the parts and do not depend on second stage to fill the parts. Pack pressure or second stage pressures sometimes causes the TPE to fold and lose bond. This is obvious sometimes in the form of lack of bond at the gate.
1) Preheat of the premold - this helps get a better bond. But this is not necessary with the right TPE.
2) Increased texture of surface being over molded - this can hurt the bond most of the times since the TPE bounces over the texture depths and create air pockets. This prevents a bond. So it is suggested to use a polished surface on the substrate.
3) Storage of pre-mold to prevent dust and water absorption - this is important. If the pre mold has grease or lubricants sprayed on the surface, this can hurt the bond. Water absorption can be a problem sometimes. It is typically suggested to mold over the premold within 2 weeks of molding the premold. Older premolds or inserts could hurt the bond.
4) Select alternate material for pre-mold that more readily accepts TPE over mold - PC is pretty easy to bond to. GLS has quite a few grades which can work here. Let me know if you are interested.
5) Use virgin material in pre-mold being careful not to add any impurities - monitor color carrier material - Using regrind is not a problem. Should not affect the bond. Color carrier for the virgin material usually is not a problem unless something very unusual is used.