Polyamide to EPDM rubber bonding

[Henry Chiansky]

We have been overmolding rubber cables (specifically SJOOW and H07RN-F types) using a low-pressure molding process. The material used for molding is polyamide, which is typically the go-to polymer for overmolding applications. However, since we are working with rubber cables, achieving optimal adhesion between the polyamide and the rubber surface is crucial for our application.

I would appreciate guidance on the process. Are there any specific primers that should be used to enhance adhesion? Should we treat the surface of the rubber cables with IPA alcohol, or would roughening the surface improve the bond? Any tips or advice on surface preparation, treatment, or process optimization would be highly valued!

 

[btrueblood]

I would talk to the cable manufacturer about the specific compounds used in their outer jacket.

If they are unresponsive, the next place to look for help would be Lord Chemicals - they sell adhesive primers for rubber, which in general will result in bonding that is stronger than the parent material.

One issue you will likely still have is matching the stiffness of the cable and jacket with the overmolding you are doing. Hopefully your compound has a low stiffness to better match the cable flex, and you have plenty of axial length to taper out (strain relief) the part you are overmolding. You are also probably using a low temperature polyamide, to avoid reversion in the cured rubber cable jacket and wire insulation?

 

[Henry Chiansky]

Thank you for your response.

We will definitely explore this opportunity and reach out to the cable manufacturer. However, I would still prefer to approach this from a more general perspective, as the cable being used is standardized, and the compound of the outer jacket must remain consistent (within tolerances, at least).

So, your suggestion is to avoid using primers and instead focus on matching the stiffness (Shore rating) of both materials, while ensuring the mechanical properties promote better adhesion—correct?

 

[btrueblood]

No, my suggestion is to do both (get a good primer, but also look for decent strain relief). Strain relief because otherwise you end up fatiguing the rubber and having it split right where the stiffer overmolded part ends.

 

[Henry Chiansky]

Thanks for the suggestion!

We did another round of injection molding, focusing on roughening the surface and thoroughly cleaning the cable jacket. The results were promising, and the cable passed the testing.

Unfortunately, adding a strain relief isn't feasible for this product due to space constraints. However, since the cable will remain stationary once installed, a strain relief may not be necessary.

I'll update you when we receive the primer and test it with the rubber and PA.

What makes this project particularly interesting (and challenging) is that the product is designed for permanent submersion, which adds an extra layer of complexity.