Over-molding plastic on metal air leakage problem

[Saurabh_J]

Hello, We are working on an over-molding part in which Brass over mold with Nylon 6 30% GF. A design like the cylindrical brass part with knurling or some grooves from over-molding face and plastic covered with open one end like cap design. After over-molding we find some samples having air leakage from the joint between plastic after testing 120 psi pressure. Also, some samples are tested with a maximum 80­° C temperature and after testing with 120 psi pressure find air leakage from the joint between the plastic. How will avoid air leakage? We take lots of trials please advise a solution. 

 

[Tmoose]

Pictures and drawing please.

I am anticipating that a few drops of (wicking) Loctite 290 on the pressure side before pressure testing might be useful.

I speculate the surface finish of the insert might be providing leak paths that the Nylon can not fill.
Are there inserts designed to be pressure tight in addition to just structural? I picture a nicely finished circumferential groove deeper than the knurling somewhere along the insert's over molding face/ OD is required.

 

[EdStainless]

First you shouldn't have designed expecting a seal.
Second there are ways to chemically treat the insert (usually etch and pre-coat) with an adhesion promoter. This also usually requires pre-heating the inserts.
It is usually easier to add a sealant after the molding process. Some parts even leave a small groove around the insert just for this.

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P.E. Metallurgy, consulting work welcomed

 

[Saurabh_J]

Hello #Tmoose, Check following attach sketch files. We already provided groove. We required to both side open type of insert as per application. Also as per your suggestion use Loctite for testing purpose but leakage are find from insert and plastic over moulding part.

Thanks per reply.

 

[3DDave]

Is it correct to say that pressure will increase the diameter of the nylon?

I think an external collar/bushing might be required to resist both that and the thermal expansion of the nylon - both of which are working to break the seal. Perhaps that fitting can be swaged on to provide a preload.

 

[Saurabh_J]

Hello #EdStainless, we also some adhesion for pasting before over-moulding. After take some sample test with 80°c temperature. After temperature adhesion are loose it and also some of them leakage creates.


Thanks for replay and support.

 

[SwinnyGG]

Your groove does nothing if you don't use sealant. Well, it lengthens the path of a leak, but that doesn't mean much.

You need to find a different insert that's designed for this, or start post-applying sealant, preferably in the bottom of the interface.

 

[btrueblood]

Why is the bottom of the insert open, can it be machined in a closed design to prevent leaks?

 

[Saurabh_J]

#btrueblood, insert design as per application so we need hole from other face.

#SwinnyGG, did you mean we required improvement in insert design and material of the insert.

Is it any affected enclosure by mold design. Because we design side by side design of mold ots means it creates partition line in middle portions.

 

[btrueblood]

Pressure tight adhesion of nylon to brass across a 80C temperature cycle is, to me, a non-starter and poor design. Redesign the brass to have a solid end wall if leakage is critical, or incorporate a secondary adhesive or elastomer seal.

 

[desertfox]

Hi
Check o coefficient of expansion for the brass and plastic and check what the stresses are in both the plastic and brass due to heat expansion, you may found that you cannot obtain an air seal with that design.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein