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Roto Casting Resin in a Two Part Silicone Mold

May 12, 2017

Rotational or roto casting means rotating your mold continually while the resin sets up so that youget a hollow casting. Two of the most common reasons to cast this way are to keep the weight of the piece down and to save money on casting materials. For this post, I’m doing the roto casting by hand. The internet is full of plans and how-to videos for making rotational casting frames, either motorized or hand turned method. If you think roto casting is something you will be doing for a lot of your pieces, it’s definitely worth a Google search to see what’s out there.

Materials needed:

Your work area:

  • Clean, level work surface, covered in paper or plastic for easier clean-up.
  • All materials comfortably within reach.

Project steps:

  • To start, I took my two part mold and taped it shut, leaving the vent and sprue (opening for pouring the product into) unobstructed.


  • I blocked the vent with some clay and then applied more tape to hold the clay in place.

  • Next, I got ready to mix the Urethane Casting Resin. This product is mixed 1:1 by volume. This mold usually takes about 14 ounces to fill, so I figured out that I would use about 6 ounces to do the roto casting. I measured out 3 ounces of Part A and 3 ounces of Part B. I mixed some blue urethane colorant into the Part B and then mixed both parts together. I carefully poured the resin into the sprue, wiped away the little bit that got on the outside of the mold, plugged the sprue with clay and taped it shut.

 Blue colorant mixed into Part B 

  • I marked the bottom of the mold with a star so I know which side to set the mold down on when I’m done rotating the mold. I want to set the mold bottom down so that any excess resin settles at the bottom of the casting. Otherwise, the casting will be heavier on whatever side the excess resin settles, making it prone to falling over.
  • Once everything was taped up, I picked up the mold and rotated it by hand for 10-15 minutes. I turn it every which way, so that the resin will completely coat the inside of the mold.
  • After 10-15 minutesof rotating, I set the mold star side (bottom side) down and let it sit for the remaining 15 minutes of it’s cure time.
  • After 15 minutes, I cut the tape off, opened the mold and popped out my casting. I trimmed a small amount of flash from the parting line and sprue.


This roto casting turned out nicely. As always, if you have any questions, about this or other projects, please send us an email at

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