Choosing Plastic for Product with Long Term Steam / High-Temp Exposure

[superart]

I am currently in the process of designing an enclosure for a disposable product that will need to sustain prolonged exposure in an environment that has temperatures of 180-200F with a large amount of steam. Expected exposure time is between 10-24 hours continuous.

My first thoughts were to use either PP or HDPE since they are relatively good for those temperatures, and are not too expensive. I made up some prototype enclosures using some rapid-prototyped tooling and plastic sheets. I used .030" and .040" thicknesses of both HDPE and PP. I can't go into too much detail about the enclosures due to NDA, but they are very low profile, with a square footprint of 2"x2", completely open at the top, with a .125" flange around the opening.

When I tested the prototype enclosures in the oven set to 200F, they were pretty good. They did not deform or slump. However, when the enclosures were tested in the intended environment (the main difference being that the oven was dry and the intended environment has lots of steam) the enclosures deformed and slumped. Not a lot, but definitely not within tolerances.


Some possible solutions that I have come up with is either to change the geometry of the enclosure by adding ribs or similar features. This may help increase stiffness and keep the enclosure from slumping. The downside to this is that it would involve getting new prototype tooling to test the revised design. The client has expressed that he would rather not go down that route at this time due to the associated costs.

The other possible solution I have thought of is to use a different material. My concern with this is that it could increase the cost of the part. Since this is a disposable product, minimizing cost is critical.


I'd like to hear some suggestions and thoughts from those with more experience in these matters. Thanks in advance for your replies.

 

[Pud]

What PP did you try? PP Homopolymer should be fine at your stated temperatures. HDPE only maybe.
If you want stiffer than PP try SAN.

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[superart]

I'm not sure. Like I said, these were prototypes, so I just ordered some PP from McMaster. It is black, temp range of -40°F to +200°F, and they have it labeled as "Flame-Retardant Polypropylene". They were unable to furnish any sort of datasheet.

Do you think the steam would have any impact on how the plastic responds to the high temperature? Given similar temperatures, why would the plastic enclosures be acting so differently in the dry heat of the oven, as apposed to the high-steam environment?

 

[Pud]

Could be the flame retardant additive being affected, rather than the polymer itself. Just a guess.

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[superart]

hmm, I would think the flame retardant additive would be able to withstand higher temps better than anything :-/. Do you think the steam may be somehow reacting with it or something?


I think I'll try the SAN. I'm not really familiar with that material. How is it compared to PP and HDPE in terms of cost? Do you have a suggestion on where I can get some? I would need about 250-300 sqinch of sheets that are .030" thick, and I think I may want to try .020" as well.