1. Member Since
    Dec 2017
    Dec 26, 2017, 12:07 PM - Plaster of Paris Slush Casting Troubleshooting Question #1

    I am working on slip casting plaster of paris (as opposed to rotomolding). I am using an unopened box of plaster that i'm not sure how old it is. I am trying 3d printed molds and oomoo smooth on molds. I am casting parts slightly smaller than a golfball. I am having several issues that google searches did not help with.

    1. The plaster is extremely watery at 1:1 mix. I have some casts that are still damp after 36 hours.
    2. At thicker mixes, the plaster loses it's liquidity at smaller ammounts. So in the mixing cup it's like batter but in the mold it's like jam. Thicker mixtures still take more than a day to be dry to the touch.
    3. Smooth on oomoo seems hydrophobic. The plaster fails to coat the walls thicker than ~.5 mm.
    4. Even watery mixtures tend to pool up and congeal hen I dump the excess plaster, leaving me with half a solid golfball and the other half is a super thin shell.

    I would like to be able to coat insides enough to get 3 mm thickness. I would like to be able to eject the part after an hour to reuse the mold. Is plaster not the right thing for this? Should I be using hydrocal or something else?
  2. Montagar's Avatar
    Member Since
    Mar 2004
    Dec 28, 2017, 1:49 PM - Re: Plaster of Paris Slush Casting Troubleshooting Question #2

  3. ChickenHaunt's Avatar
    Member Since
    May 2010
    Los Angeles
    Dec 28, 2017, 7:26 PM - Re: Plaster of Paris Slush Casting Troubleshooting Question #3

    A few possibilities here...

    Old plaster tends to take on moisture in the air (even in a sealed bag after a while), and it will take a lot longer to kick off. But consider using hydrocal or ultracal if you want a stronger product.

    1:1 is going to be much too watery. Typically it's more like 2 powder to 1 water. The technique is to fill your mixing bucket with water (warm water will set off faster) a little less than half the total volume you want. Then sift in the powder, allowing the individual particles to absorb water. Keep going until the surface resembles a dry lake bed. Let it all absorb as much water as it's going to. Mix thoroughly. When you think you've mixed enough, keep going. You're looking for the plaster to "cream," that is, transform to a thicker state. You'll know it when you see it. Now you have a mix that will sit where you put it a little better. The proper ratio will give you the strongest final product.

    All silicones are going to have wet plaster bead up on them. The trick is to catch it at the right stage when it's thickened enough to stay put. You may just have to brush it around until it stays.

    Expect to do more than one coat if you're trying to reach a thickness. Oftentimes, you'll pour some in, coat the surface, then pour it back out. Repeat and repeat until the plaster is setting, building up the thickness each time. Then do a second batch, if needed.

    I would not expect success with plaster in a 3D printed mold, as you've got a hard part in a hard mold. You'll have an easier time removing the part from a silicone mold (like the Oomoo). It's entirely possible with the right mold design, though.

    Is it necessary to have hollow parts? At the size you're using, you might consider using a core mold, if possible, to maintain the thickness of the casting, or simply pouring it up solid.

    In summary, it sounds like you've got old product that you need to work more to get the results you're seeking. It's all doable. Hope that helps.
  4. Member Since
    Dec 2017
    Dec 29, 2017, 5:23 PM - Re: Plaster of Paris Slush Casting Troubleshooting Question #4

    Thank you Jasper.

    I will try a thicker 2:1 plaster mix.
    I will also try new plaster and new hydrocal.

    You say my part size might be causing an issue. What is a good size to start with?

    I chose hollow parts because this is part of a larger artwork. Right now I am exploring what it's going to take, cost and energy wise to make the full work. Lots of small ceramic parts are going to be made in a somewhat automated fashion. Slush casting seemed to fit the bill for this and hollow parts more of an aesthetic choice than a necessity. You are very right about solid parts. It would be much easier, and now that I think about it, I WILL get started on most of my work without worrying about the need for the part to be hollow.

    That said, it is very important to me that the parts be hollow because the result is more pleasing. I also, ultimately, aim to make parts the size of a coffee mug. If you've heard of Dunny, I want to make something very close to that, but using plaster instead of vinyl.
  5. ChickenHaunt's Avatar
    Member Since
    May 2010
    Los Angeles
    Dec 29, 2017, 8:16 PM - Re: Plaster of Paris Slush Casting Troubleshooting Question #5

    Don't measure out 2:1, though. Just go by eye using the dry lakebed technique.

    Not that the size of the part would be an issue. Just that it's just easier and faster to cast solid parts, and plaster being as cheap as it is, there's not much difference besides weight at the scale you're working at.

    I looked up Dunny. If you have elements as small as the ears, I might consider going to resin, as plaster (espeically plaster of paris) will be pretty delicate there. More expensive, though.
  6. RPF Premium Member cavx's Avatar
    Member Since
    Oct 2011
    Brisbane, Australia
    Jan 2, 2018, 4:53 PM - Re: Plaster of Paris Slush Casting Troubleshooting Question #6

    What about a cheap 1:1 polyurethane? Mix up small amounts and layer it up until you get the right thickness.
  7. Member Since
    Nov 2016
    Jan 8, 2018, 8:00 AM - Re: Plaster of Paris Slush Casting Troubleshooting Question #7

    Search US Gypsum and you'll get exact recommendation for which plaster to use and the proper mix ratios for all of them. Even if you don't have a brand, etc on yours, going by type, you'll get all the info you need.

    ChickenHaunt's method is sound, back in the day when I used to run very large molds for foam latex, there were short cuts, Hydrocal sifted until a dry bed, Ultracal you built a small mountain, etc. but if you are casting small pieces, want detail and consistency, you're better off using the exact ratios. It's been a while, but I don't remember any that was as straight forward as 2:1. I think they were along the lines of 38:100, etc.
  8. Mystery Chunk RPF Premium Member PoopaPapaPalps's Avatar
    Member Since
    Oct 2010
    IA, USA
    Jan 13, 2018, 1:49 AM - Re: Plaster of Paris Slush Casting Troubleshooting Question #8

    I don't know about mixing ratios of plaster, I tend to do it by eye (something a little thicker than pancake mix is where I typically like it), but like ChickenHaunt says, warm water will help it set off quicker. I've also found an addition of salt also helps it kick faster, too.

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