There is a bit you can do to help improve adhesion between PC and TPE; two materials that don’t typically love each other. I am working on a fix right now on one of our products having just this problem.
- Make sure the material has been dried properly to obtain the best adhesion possible (which may not be great).
- I believe you should expect better results in a two shot process as the initial shot is still warm when it receives the TPE attaining a better bond. In an insert mold operation, the part is rarely warm and sometimes has had the opportunity to take on moisture if it has been sitting around.
- If you don’t need the impact strength of PC, you might want to try a PBT/TPE or a PC/PBT and TPE overmold. We have seen much better results with this formulation.
- Check with your vendor. Different vendors offer different compounds within the TPE that will allow the elastomer to achieve better adhesion to PC. Sometimes they work and sometimes they don’t.
- Make sure that your second injection or overmold temperature is set correctly.
- Finally, make sure that your color carrier resin in your materbatch is the appropriate base material. Many times, the base material in a masterbatch is just some inexpensive material that the company uses because it is being used at a 50:1 ratio.
I always try to use PC as a base carrier resin in case. Immiscible carrier resins can inhibit adhesion. The important still to point out is the gate location and type of gate consideration for insert molding:
- to ensure a flow path that will yield optimum adhesion
- At the heaviest cross section, to facilitate part packing and minimize voids and sink
- to minimize obstructions in the flow path (flowing around cores or pins)
- to minimize jetting (> 0° angle injection in thin-walled components)
- part packing and gate removal or vestige depending on different gate types
At least provide mold venting (0,01-0,025mm) to avoid air trapping between TPE and OM component.
Looking now to many different kind of TPE materials (Santoprene from Exxon Mobil is an EPDM/PP chemical cross linked matrix), you will definitely target a huge varity of material combinations. Differences between TPE-V and TPE-S compounds occure in
- Flow length/viscosity is very shear dependent on SBC compounds, testing with spiral flow before
- Melt temperature
- good adhesion properties to various thermoplastic substrates/EPDM-PP compounds are limited due to chemical X-linking.
Basically the fundamental bond of GLS TPE over the rigid substrate can be provided by a combination of molecular adhesion.