risky 2 shot molding! ADVICE PLEASE

[subsearobot]

Hi All!
We are considering overmolding a lower melt temperature polymer over a high temp polymer substrate. they will be in cylindrical form- imagine we are molding a cap on the end of a tube. This investigation is a result of a long series of factors that I can't elaborate too much on, but suffice it to say that we are also looking at single material solutions...

Because of the dissimilar melt temperatures, the two materials do not bond during the 2nd shot. We are considering features (little barbs or something) on the substrate that would mechanically restrain the two parts.

Ok. so, assuming there is any feasibility there, we need the interface between the 2 materials to be "leak tight" (lets just say 4psi delta pressure). oh, if that's not enough, we need to cycle this part 50 to 100 times to 121degrees C (250F), and when back at ambient, no leaks.

I'm taking both: Ideas and votes. anyone think this can work? any ideas for success of this investigation would be greatly appreciated!!

thanks!

 

[Pud]

One of the materials usually has a bonding agent as an additive unless they have a natural affinity. I am guessing the substrate is a semi-crystalline material?
Imho, I would pursue the one material - cheaper all round anyway.


www.tynevalleyplastics.co.uk

Politicians like to panic, they need activity. It is their substitute for achievement.

 

[subsearobot]

Pud, the substrate is amorphous, while 2nd shot is semi-crystalline. would this affect bond capability?

agreed- single material would be preferable, and we are pursuing a couple options. but we have been unable to find a material that delivers our long list of requirements. I am unsure if we are pursuing magical thinking or just being persistent...

thanks regarding a bonding agent- I was unaware.

 

[dgallup]

For sealing I have always done it the other way around. Use a slightly lower melting point first part and over mold it with a slightly higher melting point material. They should be chemically similar to get good bonding. You will want some kind of a labyrinth interface with thin points to encourage partial melting and bonding in the overlap. We have never used bonding agents, application is expensive and inconsistent. Done right, these will survive 50 thermal cycles from -40C to +135C and not leak at 3 bar.

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