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prattcm

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  1. I am working on a project where I need to be able to mold the interior walls of a part. Similar to the double wall mug shown in the attached picture. I know this isn't recommended for slipcasting, but I can't come up with a better way to make it. I have only been able to find a few examples of this type of mold and I am trying to see if anyone has any guidelines or recommendations. I am able to make this part of the mold air release, if that is necessary. The interior walls of my part will be a little over 5 inches deep and about 5 inches in diameter, I know having a steep draft angle will help, but will something like 10 degrees be enough? Thanks for any help!
  2. I've had a few bags of no 1 pottery plaster sitting in my garage for about a year. I'm in ohio so it been all sorts of temps, but it has been out of rain and hasn't been too humid. I tried mixing up a small cup of it and it didn't get completely smooth, it had small grainy looking pieces mixed throughout it. Is this normal? I have never worked with it? I poured some and let it harden, about a day later I took a look at it and it didn't seem as hard as I would expect it to be. I got it wet and could scratch away the surface pretty easily. I haven't worked with it before so again I'm not sure if this is normal.
  3. Thanks for the advice, I will give the shellac a try. I am planning on making a working/master mold. I want to test it out one or two times and if everything works as expected I will make a rubber mother mold of it. Is this okay or is there really a difference between a master mold and a working mold? I have a few bags of #1 pottery plaster and a few bags of Hydrocal 11-a and am not sure which I am going to use
  4. I don't have the molds yet, but this gives you a basic idea of what the surface will be. It's styrofoam, liek the stuff often used in packaging, but a little more dense.
  5. I am having mold negatives machined out of 2 lb. EPS foam which I will pour plaster into to make my mold. Have any of you done this? If I have to break the foam away it not a big deal, but it would be nice if I didn't have to. Should I seal the foam with anything? I was thinking about just using house paint to try to seal the foam a little then using foam soap over that. I don't expect the finish of the plaster to come out perfect, but this is kind of a one shot deal and I would rather not screw it up. Thanks!
  6. I am working on making a prototype of a new urinal I designed. I 3d modeled the part I want to slipcast and modeled the molds as well. It's rather complicated and not something I can make by hand. Have any of you gotten a part or molds machined before? I was thinking about trying to find a local place that could machine the part out of foam then pour the plaster mold around that, but if I could find a place that could just machine the plaster molds for me that would be even better. Any feedback would be helpful. Also, are draft angles necessary? If I want to make the mold only 3 pieces I will have spots with no draft that are about 4 inches deep. Can this be done because of shrinkage?
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