substrate material for pvc overmold to produce bonding

[bdeuell]

Hello,

I am working on a product that requires a PVC (shore 70) overmold. The product requires a air tight bond between the substrate and the overmold and is subject to small mechanical loads. The product is also exposed to outdoor environment (UV and water). Unfortunately the molds have already been produced and limit my options for adding a interlocking mechanical feature to only a few points but not along the entire interface. I am looking to develop a bond between the substrate and overmold materials. Currently the substrate is POM ... im sure you all know that this will not form a good bond. Does anyone have recommendations for a different substrate material I could use that would produce a good bond with the PVC overmold material.

Also I would appreciate any recomendations on processing or design to promote good overmold bonding. For example substrate surface texture, cleaning, molding machine settings, etc.

Any help would be greatly appreciated.

[rickbatey]

PVC and POM are a bomb so time to try another over mold resin; I know mixed in the barrel they are supposed to explode due to synergistic reaction or something to that effect. Look at DuPont's website as they list the only soft resin I know of that can bond to POM. The POM is the issue, you can't hardly glue or stick anything to it.
Rick.

[bdeuell]

Thank you, for the response.

I am aware of POM's very poor adhesion properties. That is why I am looking to change the POM material to something else. I would like to avoid changing the soft PVC ovemold material as it used to form an RF bond in a later process.

Unfortunately it appears PVC overmolding is not very common and I have been unable to find any substrate material compatibility information. I am looking specifically for a new substrate material to replace the POM.

I wasn't aware of the violent material incompatibility of POM and PVC in the barrel, I will look into that.

[rickbatey]

Since POM was picked, it's going to be hard to replace it as its near perfect for the applications and environments it ends up in. Maybe a nylon or polyester first resin?
Rick.

[jnewmanco1]

Most problems I have encountered were due to the widely varied temperatures of the two different plastics involved. I overmolded a nylon substrate with Santoprene but had very hard time getting the required adhesion between the two. It was a rotational 2-shot process and by the time the mold rotated and was ready for the overmold, the nylon had cooled too much and the Santoprene would not stick as it needed to. Ended up having to run the Santoprene at a much higher melt temp than normal to get it to work.

[JayDub]

The surest way to get good adhesion is to use the same or similar materials. Any chance you could use, say, a hard and a soft grade of Urethane?

[bdeuell]

Thanks for the tips everyone.

I am really trying to avoid changing the PVC overmold material so that I do not affect other processes.

I have decided to trial PVC-rigid and ABS for substrate materials. It will be a couple weeks before I am able to test the overmold adhesion, so if anyone has process recommendations to promote overmold adhesion they would be appreciated.

In the meantime I will conduct some testing to see how these materials hold up for the other aspects of their use mechanical wear etc. I know they are not as good as POM but I am hoping they will still work well as the POM was outperforming our requirements.

[pjhall]

Rick and bdeuell,

Another no no is POM (acetal) and elastomer. Mix delrin or celcon with santoprene and BAD things will happen. I had a material handler put a tray of santoprene into a hopper running delrin....Big opps!!!! resulting fumes are almost worse than overheated acetal (POM). "It was a nose and eye watering event"

How about a base substrate of Kynar pvdf flouro-polymer (low coefficient of friction similar to acetal)? I've never overmolded a flouro-polymer with PVC and am not sure if a reaction will occur. Suhas ever done this? I can't find any warnings online..

[rickbatey]

On Dupont's website they list the only resin I know of that can bond to their POM. I found it while doing some research for f a friend. I think the issue with that is its not PVC and it sounds like the next step is an RF weld to the PVC. Maybe the PVC is the resin that really should be replaced?
Rick.

[bdeuell]

Thank you, everyone who responded to this thread.

I just wanted to follow up with my results. I shot trials of both PVC and ABS as substrate materials with a PVC (plasticized shore 70) overmold. The lower melting point of the PVC substrate material resulted in softening and deformation during the overmold process so it was quickly abandon. However the ABS substrate with PVC overmold preformed well and eliminated all of our bonding issues.

We ended up still using POM for the components exposed to sunlight so we were able to avoid the UV requirement on the ABS.