Using Plaster of Paris to Level Resin Pour Molds

Unlevel OOMOO 30 Silicone Molds

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So, back when I started out making OOMOO 30 molds, I wasn’t always good about making sure the surface I was casting on was perfectly level. (Whoops!)

To fix this, I’d been using pieces of index cards to shim the molds and help offset the lean. It was a bit hit or miss and it took some finagling every time to get it just right.

It finally annoyed me enough to fix it permanently!

NOTE: This will only work if the top of your mold is level. If you’ve somehow managed to end up with an unlevel top (and bottom) then you’ll have to figure out something else.

Level work surface for plaster casting

First, make sure you have a level surface to work on. I made one out of a piece of acrylic and risers (made from glued together index card pieces).

Carving out interlock channels and edges

Flip your mold over so that the flat/level side is towards the table and carve out some small interlocks. I carved a few channels using my linoleum cutter and chopped off the edges. This will make sure we can align the mold the right way on the plaster when we use it later.

Then build walls up around your mold. You want the walls flush against the mold since it’s meant to sit on top of it when we’re done. I cast molds using a mix of real and faux-Lego mold blocks on plates, so I just rebuild the shape, only this time about an inch higher than the mold.

You want this to be at least half an inch thick, to make sure the plaster is sturdy. I didn’t use the same wood glue mixture to make this that I do for casting the magnets… but I did throw a little into the mix for luck.

Mixing plaster to pour the mother mold

Mix up the plaster of paris as normal, but let it sit for a few minutes so that it gets thick before pouring. I waited until it was roughly pudding/ketchup thick before pouring it into the mold. This should reduce spills and any seepage between the silicone mold and the walls.

Please don’t do what I did! Make sure you have a catch basin or paper towelsunder your casting area to catch any plaster that leaks out! I was just very lucky that the Legos are pretty water-tight.

Note: Never pour plaster or water with plaster in it down the drains. The plaster WILL harden in the pipes and over time (or in one really thick pour) you can clog your pipes.

Wait for the plaster to harden, which will depend on the weather. I normally give it 3-4 hours before I check on it. When you can’t push a toothpick into it anymore it should be done.

Demold thing and then let the plaster sit another 24-48 hours before you try and use it for anything. I went back and sealed it up with matte varnish just because, but they are simple enough to make so if it breaks you can make another one.

If you do a bunch, it helps to write the name of the mold it goes to on the plaster. Don’t use a felt-tipped marker for this as the plaster dust can harden the nib!

Now you’re ready to resin cast without the headaches! 😁

Plaster base and OOMOO 30 Silicone Mold parts

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