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Question for the resin casters.....

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I occationally cast a few small bits and pieces for my own use. So far, I've only done this using simple one-piece molds. Now, I need to cast a few parts that will require two-piece molds. My second attempt was successful, and the part actually looked great, no pinholes or anything. The problem, however, was the clay. This stuff was a real pain to remove. All the "how-to" videos I've seen, it just peels right away, slick as a whistle. Not so with me, it gets stuck in all the nooks and crannies....my first attempt at this mold I just threw away because I couldn't get all the clay out. Second go-round wasn't much better (see picture), but I finally got it so I could work with it after picking with toothpicks and knives and stuff all morning.

Am I doing this right? Am I using the right type of clay? Would putting it in the freezer for awhile help? This can't be normal....or can it?

Thanks, guys!!


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Which brand/type of clay are you using?

Which brand/type of mold rubber are you using? (appear to be Alumilite Quick Set?)

As long as you use the non-hardening, modelling type clay you should be fine. I use whatever Michael's had, but don't recall the brand.

Try to make the clay base as smooth as possible before setting your part in it, or pouring any rubber. The smoother it is, the fewer nooks an crannies there will be for the rubber to flow into. I use 91% isopropyl alcohol for cleaning molds and cleaning off excess clay, but I try to minimize undercuts and rough edge where the rubber can flow and become trapped once hard. Did I mention smooth is good? :rolleyes:

I'm not sure what type of container/enclosure you're using, but that's also an important consideration. I prfer to use use solid things suach as LEGO block, plastic cups and containers, etc., rather than an enclosure built with clay walls. Again, it goes back to the "smoother is better" principle, and makes cleanup a snap.

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It's best to use RTV silicone for a resin mold. You can use clay for one off stuff, one piece molds. I tried it once and experienced the same issue that you are.

I assumed the yellow-ish object in the picture was mold rubber, but if not, and it's actually hardened resin...what Mark said. :rolleyes:

Edited by Casey
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You have to be careful which clay you use with resin casting, many of the modelling clays are incompatible with silicone and can cause it to do bad things, like not cure.

I use Jolly King which the place I get my casting supplies from carries specifically for use with silicone. A 1 lb block costs around $5 and that is plenty of clay for most any project.

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  • 2 weeks later...

Sulfur is deadly to resin--will keep it from hardening almost permanently, so any clay used must be sulfur-free. For years, I used nothing but AMACO (American Art Clay Company) X-11 moist pottery clay, which is gray in color, is a natural clay, softened with water. One HUGE advantage of this stuff is that any residues can be washed out of a master and mold before making a second pour of rubber for the second part of a 2-part mold.


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