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Toughening a mold

Old 11-13-2004, 07:31 AM
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Default Toughening a mold

Made a few molds over the past year from polyester and then taken both polyester and epoxy parts from them and had reasonable success. So far for most oif my molds I've only taken one or two parts off the mold.

But I've recently made a cowl mold that I now want to take at least a dozen parts from and have found that after two or three pulls the mold breaks down and the edges go. I've read that maybe I should be using a "tooling gel" in the original mold, presumably this is harder and allows more than one part to be taken off it.

But I've not been able to find "tooling gel" in the UK. Is there something else I can add to the mold gelcoat to make it harder? I'm quite happy to take another mold off my cowl plug and make a new mold, but don't really want to have to make a new mold for every 4 or 5 parts I pull from it.

Any ideas or advice appreciated.

Thanks
Old 11-13-2004, 01:38 PM
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Default RE: Toughening a mold

Colin,
I have made a few molds in my day and have found that epoxy molds hold up better. If you are willing to make another mold, use epoxy this time. make sure you mix in a little black or dark blue with your first application of epoxy( it makes the chasing of bubbles easier as you can see them when you layup a part)as well as some colloidal silica to thicken it(so it won't run)(you my choose to put two coats of this concoction on). When the first coat gets tacky, apply a layer of 3/4oz cloth over it and another coat of epoxy. This will keep the cloth weave from showing in the finished mold. When that coat becomes tacky, begin putting your layers of approx. 6oz cloth on (I usually put on two at a time). Four layers of 6oz. cloth will yield a good 1/16" thickness. I usually use (depending on the actual size of the thing) 12-16 layers of cloth. under the last layer of cloth, I usually put a typed slip of paper describing my name and which aircraft it goes to. If you have the time, let it set for some days( I like a week) then pop it loose from the plug, wash out the PVA with dishwashing liquid and water, rinse with cool or cold water, put it back together(presuming you made two halves) and let it rest for a couple more days. when you make a part, just paint the mold with PVA, let it dry, paint one layer of epoxy, let it get tacky, put in one layer of 3/4oz. cloth let it get tacky, then however many layers of cloth you intend to use for your part(one or two at a time). let it set for a couple of days,and then pop out our part. It should come out fine. Like I said, I like to use black for my first coat on the mold so I can see through the layers of cloth to chase out the bubbles for a better finished product (no pin holes). I also always paint a coat of epoxy in the mold first, then let it get tacky before I put the layer of 3/4 oz cloth( this becomes the gel-coat you were inquiring about). Remember, choose a time when you can devote a good length of time to making the mold (or layup) without stopping until your done. If you allow the epoxy to fully cure between applications you run the risk of delamination and wearing a not happy face.
Let me know how things turn out.
good luck,
Rob

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