Casting a Polyurethane Rubber Mold

Here’s a great polyurethane rubber project that’s been featured on instructables.com and concreteexchange.com as part of our Bent Concrete Side Table how-to guide. We needed a flexible rubber mold to allow bending of the concrete while it was still in a pliable state but didn’t want the mold to be too soft to prevent distortion of the mold during the casting.

Step 1 – Pouring Rubber

  • First make sure the table / work surface is level. If the table is not level, the rubber will settle to one side and be uneven.
  • Pour from one place and let it flow outwards. Pouring from multiple places will create more air bubbles.

Step 2 – Continue Pouring and Spreading Rubber

  • Use a spatula to move the rubber to the edges of the form. The whole time, you’re trying not to create extra air bubbles, so try to minimize any turbulent motions.
  • Look for any large air bubbles that are coming to the surface and use the spatula to drag them up and pop them.

Step 3 – Knock Down Air Bubbles and Allow to Cure

  • When the rubber is at the full height, quickly mist the back side with Pol-Ease Mold Release to knock down any bubbles on the surface.
  • The rubber will start to set up relatively quickly, in about 10 minutes, so be efficient when filling the form. It needs to cure at room temperature and will be ready to remove the next day. If you try to remove the mold too quickly, the rubber can change shape because it hasn’t fully cured.

Step 4 – Removing Rubber Mold

  • Remove the form walls first.
  • Slice the silicone seal with a razor blade if necessary.
  • Loosen the rubber by pulling on it, and gently pull it off of the positive.
  • Store the mold flat when not in use.
  • Keeping it out of direct sunlight and away from extreme temperatures will help maximize the life of the mold.

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