Autoclave Steam Permeability through Ultem Enclosure

[obttim]

I am designing a small plastic enclosure (.65" dia. x 3" long) that houses a metal transducer. Both ends of the shell are sealed with o-rings and there are small amounts of moisture present inside the shell after only a few autoclave cycles. The autoclave is 135deg C for 10 minutes. The pressure is 38psi and the material is Ultem HU1000 at a .100" wall thickness.

Is it possible that the steam is permeating through the Ultem? I have made similar enclosures from Radel without this problem. Any help is appreciated.

 

[DesignerGuy16]

http://www.idex-hs.com/materials/guide/Ultem.aspx

Water absorption is only .25% so I have doubts the steam blew through the material. I'd be more worried it blew through the o-ring seal.

It's coefficient of thermal expansion is also comparable to carbon steel so it shouldn't expand THAT much for the size you're talking about.

It is really good material. Have you inspected the O-rings for damage, or the o-ring grooves for proper tolerancing to be liquid tight?

Just curious, let me know how it works out.

James Spisich
Design Engineer, CSWP

 

[patprimmer]

I don't have data in front of me, but I would strongly suspect that the co-efficient of thermal expansion for Ultem is at least 5 times and mabe 10 times that of steel as all plastics I know are about 8 to 10 times higher expansion than steel.

At 135°C the moisture absorption and permeability might be a lot higher than at 25°C.

The temperature gradient and relative humidity gradient may very well be transporting water through the plastic.



Regards
Pat
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[obttim]

Thanks, I am certain that I have a good seal on the o-rings as the outer shell is translucent and the surface finishes are very smooth as they are injection molded parts. I will look into the material more.

 

[btrueblood]

Ultem is pretty porous to steam (water vapor) as compared to many other plastics.

See

http://www.matweb.com/search/PropertySearch.aspx

 

[DesignerGuy16]

Learn something new everyday. Thanks.

James Spisich
Design Engineer, CSWP

 

[btrueblood]

Sorry, Jspisich, I just followed my link...thought it would show you a search that I did...

Click on polymer materials on the left hand side, then pick "water vapor permeability" from the drop-down list on the right side.

Problem - you get a lot of generic data "up front" from this search, and so have to dig thru several pages to get to specific polymers you might be looking for. Also, a lot of the published data is for film grade materials, not molded/molding grade, so YMMV. But, it's a starting place.

 

[obttim]

So by their "steam autoclaving" designation, is that implying their porosity which allows for effective sterilization? I have a polysulfone part that also fails the same test.

 

[btrueblood]

"So by their "steam autoclaving" designation, is that implying their porosity which allows for effective sterilization? I have a polysulfone part that also fails the same test. "

Um, no (I don't think so, but don't know what 'designation' you mean), it just means the material won't break down due to exposure to hot water or steam. Acetals and some polyesters will (they get brittle and crumbly due to hydrolysis of the polymers), and so should be avoided in those applications.

 

[obttim]

The datasheets "designation" highlights this material as a steam autoclaving material.