How to cast polyurethane resin (I don't think it is worth a new topic Tony). I’ve edited this post to include more lessons I’ve learned.
The first thing I realized is that this is a messy business, do not try it on your wife's kitchen table. Once you mixed the two ingredients it turns into a thin liquid, that pours easily but spills easily too.
The stuff I used comes in one big container and one small one. You start by adding 2/3 of the volume you need from the larger container. The larger one contains sticky stuff that doesn't want to flow and once you get it going, it doesn't want to stop. The end result was that I poured out more that needed. Then you add 1/3 from the second container. This contains black, watery stuff that pours easily.
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According to the instructions you have to stir it for 3 minutes. I was worried that I would not be able to get it out of the container so I used a shallow, wide container for mixing. It was a mistake; I had a hard time preventing the mixture from spilling. For the second attempt I used a large tin.
The company I bought the stuff from sells a "release-agent" that you apply to the surface before pouring mixture into your mold. I tried grease, Vaseline and paper to prevent the polyurethane from sticking to the mold.
The paper became soaked and didn’t help much when I tried to extract the end product from the mold. The grease worked well but I think it may react chemically with the polyurethane so I think the Vaseline is the ideal "release-agent". Of cause you are welcome to buy the real thing from the suppliers. In the end I realized that polyurethane doesn’t stick well to either bare or painted metal (which is just as well because I spilled a lot).
I spend a while preparing the surface etc. but still slipped up. I used clay (the type you will buy in a toy store) to form a platform for the backing plate, making sure there were no cracks that the rubber could seep into.
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However, I assumed in my ignorance that I didn’t need to seal the joint between the backing plate and the metal ring that was simply lying on top of it. Seal ALL joints and cracks. The mixture runs like water and will leak out through the smallest crack.
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Curing takes about 3 hours. By then the polyurethane will be hard enough to handle but soft enough to get out of a mold etc. However, don’t think that because this is rubber you will simply squeeze your creation out of a rigid mold. I had to butcher my first attempt out with a knife. You must be able to split the mold in order to get it out.
It takes 24 hours for the polyurethane to completely settle and be ready for use. I experimented a couple of times before I was confident enough to tackle the real thing, but in the end I think it went well.
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