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Old 19-05-2009, 22:16   #1
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Join Date: Aug 2006
Location: Whangamata. New Zealand
Boat: H28
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Casting Resins

I wish to make some small parts for my windvane self steering gear - these parts would normally be made of cast aluminium or bronze - but since we're hundreds of kilometeres away from home (and my forge) my simplest option is to make them from cast glass reinforced resin. (Believe it or not, I have a few litres of epoxy casting resin on board.) What I need to know is, how does a block of cast glass reinforced resin compare with a same sized block of cast aluminium with regards to compressive, tensile, sheer and every other sort of strength??? I have trawled the internet for an answer to this and found all sorts of highly technical information which means altogether dick to a poor ignorant sailor, so any simple to understand comparisons between the two would be highly appreciated.
Thanks, Chris
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Old 19-05-2009, 22:31   #2
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Chris,

I have used cast epoxy for things like mounting my primary winches (50 - 80mm thick epoxy pads). They see upwards of 1100 kg of load and handle it quite nicely.

I don't have any figures for what is typical but here is a link for one supplier.

Casting Resin Epoxy Casting Resin

8500 psi is about 1/4 of unwelded aluminum, and 1/2 of typical welded aluminum properties.

But that is probably for "neat" resin with little filler added. If you can, add glass milled fibers; you will increase the strength considerably. Longer fibers are stronger but make the part fuzzier until you sand it. Making it "chunkier" than the broken part if possible would be good too if dimensional issues don't prevent you from doing that.

I use cast epoxy for lots of serious load bearing applications but it won't be as strong as alum. or bronze so you just have to make it a bit bigger when possible.
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