Moulds and Molds

**Scott** · 23rd February 2012, 09:14 PM

The title of this thread depends on which side of the Pacific you live on however considering they're both one and the same thing, I'm going to bore you with yet another mould thread. And yes, I realise this topic has been flogged ad-infinitum. Stick with me on this though.

I have access to a considerable amount of engineering plastics and one of the products which sticks out in my mind for moulds is UHMWPE (Ultra High Molecular Weight Polyethylene). This is a fancy plastic which is an up market cousin of high density polyethylene. It's strong, easy to machine, takes high temperatures and has a high coefficient of friction (slippery).

Does any of you have any idea if plastics (and in particular, UHMWPE) can be used as moulds for polyester resin? Will it react with the plastic? The only down side I can see is that it's not flexible. But that shouldn't be an issue if you give the mould a good shake?

I'm asking this because I've just researched silicon moulds and have nearly keeled over when I've looked at the price. What I'm suggesting maybe a good alternative.

Over to you...

**Crunchie** · 23rd February 2012, 09:47 PM

I have successfully used Polyethylene as a mould for casting Polyester resin but I haven't used UHMWPE. I don't see why it would not work as none of the adhesives work for the Polyester / Polypropylene plastics. The only time I found Polyester hard to remove from the moulds was when they became scratched.

**justturnin** · 24th February 2012, 11:16 AM

I use 1/2" cutting boards for my Molds held together w/ sheet metal screws. I use Pkumbers putty to keep them from leaking. I am not sure if it is the same material though but they sure are cheap and I can get 4-5 out of one board.

**gawdelpus** · 24th February 2012, 06:23 PM

If you think the molds are expensive look up the price of the raw silicone here in australia

. You can make molds from almost anything including wood , you dont say if you have access to the material in casting form or already solid ,in which case you need to be handy with a router or otherwise at fabricating the shapes of the molds you want ,I have only used silicone and so called "bread board" for casting in ,oh and also pvc electrical pipe about 20 mm dia .Cant see any reason you cant use what you suggest as long as its smooth ,you can use mold release if uncertain , you sometimes get slight shrinkage of the casting so that can work for you as well ,in the age old wisdom ( suck it and see) hehe, cheers ~ John

**Scott** · 24th February 2012, 07:06 PM

Cheers John (and others who have responded), thanks for the info. Trust me, I have seen the price of the silicone. Insane.

**stevo81** · 25th February 2012, 11:47 AM

Like others i have been using "cutting boards" to make my casting molds. it also makes good slides on the table saw for home made jigs. hot melt glue works pretty well to hold it all together and to seal it from leaks. get about 5 casts before needing to reseal or reassemble.

**BoomerangInfo** · 27th February 2012, 06:58 AM

I've only used HDPE as it's cheaper than UHMWPE, but if you've got a cheap source of it, give it a try. From what I can see, it has some more desirable characteristics for casting, but may machine slightly differently. Certainly it's higher melting point will help with machining, but being denser, whether it is more brittle and takes as clean a surface as easily, I'm not sure.

Here's some specs comparing the the grades of POlyethylene, if you can understand all the numbers.

Boedeker Plastics : Polyethylene Datasheet

Russell.

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