Overmolding for creating a hermetic seal between metal housings

[spiridij]

Does anyone have any experience with creating a hermetic seal on metal housings that are sandwiching a thin metal membrane? I'm told overloading will have its challenges since there will likely always be a fluid leak path between the metal and the plastic.

 

[dgallup]

Unless the over molding can be done in such a way as to completely enclose the parts the probability of a leak is high. If there is a seem the fluid can follow it will due to differences in thermal expansion. A series of ridges or grooves can improve sealing if the requirement is not too strict. Everything leaks, you need to define the requirement.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[btrueblood]

What dgallup said about specifying what you mean by hermetic (how much, and of what, is allowable?). I'd add that you can do a lot by being careful about what the substrate and plastic materials are. Some plastics and some metals work very well together, others not so much. We used a 2-layer coating plus overmold for one part, with the coating being a specifically hydrophobic compound that reduces moisture intrusion.

 

[Tmoose]

Pressure?
Fluid contained?

As others said, define "leak."

Some pressure balanced seal designs can be self energizing to the point of being visually leak free whens subjected to many hundreds of PSI.

https://www.tractorbynet.com/forums...42d1336089121-best-hydraulic-top-link-tnt-jpg

 

[mighoser]

If it’s a flaw sealing surface use grafoil. McMaster sells it.

 

[btrueblood]

" I'm told overloading (overmolding?) will have its challenges since there will likely always be a fluid leak path between the metal and the plastic."

Technically not correct. A good sealant, or elastomer, can flow to fill minor surface flaws and resultant in smaller leak paths, otherwise why would we use gaskets/orings/sealants? But: all elastomeric compounds have a much higher permeability to most gases and liquids than does pretty much any metal or metal alloy. So that's why you need to be specific - some elastomers are less permeable to some gases than others. Silicone, which seems like everybody's favorite "sealant" is one of the worst for moisture permeability.

One fun fact: stranded leadwire is usually used for connection to electrical components (sensors, switches) exposed to the elements (because it is flexible) - but moisture can wick down between the wire strands over time and temperature cycling. Getting a good seal for stranded leads means you need a solid wire (or wire/solder joint) somewhere to bond the overmolding to, to create a gap-free seal.

 

[MintJulep]

"Hermetic" is one of those words that sounds technical but has no inherent quantified meaning.

Set some quantifiable requirements, then research the ability of solutions to achieve them.

 

[btrueblood]

"her·met·ic
/hər' medik/
adjective
1.
(of a seal or closure) complete and airtight.
"a hermetic seal that ensures perfect waterproofing"
synonyms: airtight, tight, sealed, shut; More
2.
relating to an ancient occult tradition encompassing alchemy, astrology, and theosophy."


I think the two defintions need be combined for engineers: "hermetic: making an attempt to seal an enclosure, and praying it stays that way".

 

[spiridij]

Thanks for the replies, let me clarify. The device is small (roughly 1" x 0.5" x 0.25"), single use only, medical device and has two chambers separated by a thin metallic membrane. I'm looking for a way to seal the two outer housings to each other and sandwiching the membrane between them. I am limited by size and materials since it is a medical device and the two chambers contain very different materials. One is a drug product that contains water, vegetable oil and a surfactant. The other chamber contains a propellant. By hermetic, I mean the device cannot lose more than 25% of its propellant mass over 18 months. I was thinking about over molding a PET or similar plastic over the housings, which are currently Ti in order to create the seal, I realize PET is not hermetic, but if I limit the area of exposure of PET to the propellant, I think I'll be okay. My question was, does anyone have similar experience and were they able to create a good enough bond between a metal and a plastic so that a gas (or liquid) couldn't find a fluid path through the interface. I can't really use an o-ring or seal without compromising the size constraint I have. Hopefully that clarifies my question.

 

[btrueblood]

Propellant, to an ex rocket engineer, probably means something different than you mean here. Are you talking about a pressurized gas? If so, you need to look at the fluid and/or gas barrier properties of the plastic and see if it suits your needs. You probably will end up needing a multi-layer film, and will be talking to specialist film manufacturers for help.

Bonding of plastics or elastomers to rubber is also a pretty specialized science. Usually, a surface activation or primer layer is required on the metal to achieve good adhesion. Lord Corporation is a good resource for bonding primers and activators, I'd be talking to them.

 

[Compositepro]

Perhaps you should consider using metallized plastic films films or moldings rather than over-molding metal. Metallizing addresses the permeability issue of plastics, and because the metal is so thin there are no stresses caused by CTE issues that result in delamination.

https://www.sorbentsystems.com/mylar.html