Does Styrofoam expand?

[pauljohn]

Hi

I hope this is not a stupid question. I am looking to cover a shaped piece of styrofoam, (Dow Cornings 'Foamular'), with epoxy and am wondering if it will crack the epoxy with changes in temp. of about 30degreesF. From about 40F to 70F. Foam is approx. bathtub size and similar shape.
Thanks for any help.

 

[DJKeng]

CTE is 38.9millionths of an inch / inch * degree F.

So a 30 degree diff would yield:
.01167" expansion for an inch long part double that for two inches triple it for three... i think you can take it from there. This assumes the foam was formed at 40deg F.

Your epoxy shell will also expand (but at a different rate). The shell may be strong enough or flexible enough to accomodate this difference in expansion.

You will likley be ok but i would do a quick prototype.

Hope this helped.

 

[Compositepro]

The stiffness (modulus) of the styrofoam is so low it is very unlikely to significantly stress the epoxy (which I assume has fiberglass reinforcement). The gas pressure in the cells would be a bigger factor, but shouldn't be a problem over this teperature range.

 

[EdStainless]

IF you go real cold the styro may crack since it is trying to contract more, but over you limited temp range range I don't see it being an issue. The bigger problem is, will the epoxy be strong enough to not break? The styro has a low modulus, so it will flex a lot. The exopy is much stiffer. Unless the expoy is thick enough to support itself (as if it were hollow) then you might get cracking.
I also suppose that you will use fibre reinforced epoxy.

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[pauljohn]

Thanks for the replies.

The foam is a plug to make a mold from. I was hoping to simply cover with epoxy and paint so it can be finished to a very smooth, high quality surface similar to a boat hull.

The temp. problem is simply the range of temps I am expecting over the process of finishing the mold. There will be other products eg. 'Awl Fair' by US paint that will be used to help the fairing process but no reinforcement in this piece.

 

[DJKeng]

Ahhh, i was curious what you were up to.
Don't worry about it this sounds like no problem. The epoxy is sticky yes? No problem, as long as what you make off of the mold has an overall tolerance of +/- 1/6". It sounds like you'll have a tougher time controlling the thickness of the epoxy!

GO FOR IT, its a good idea and a cheap way of making the mold.

I'm really curious now. what is the final product? a personnal watercraft?

 

[Compositepro]

Your mold will be very fragile and easily damaged without fiberglass reinforcing, and the dimensional stability will be relatively poor. You may not even be able to release your part without damage.

 

[pauljohn]

Thank you all for your posts, the info has been very informative.