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Urethane Rubber Mold for Plaster or Concrete Casting

April 17, 2017

Our 75 Shore A Urethane Rubber is a pourable material great for making molds that stand up well to repeated castings of cement, concrete, Hydrocal, plaster and other gritty and/or abrasive materials. It is a 1:1 by volume mix ratio, so no scale is needed.  For this project, I will be making a one piece block urethane rubber mold and casting in plaster.

Materials needed:

See All Urethane Molding Products

Your work area:

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

Project steps:

  • If you don’t have a plastic mold box, go ahead and make one from cardboard and hot glue. This post has specific details on this process, https://www.aeromarineproducts.com/two-part-pourable-silicone-pumpkin-mold/   After you have made the mold box, paint it with any paint you have laying around. I used some brown spray paint I had left over from another project and let the paint dry for several hours. The purpose of the paint is to seal the cardboard, which may have retained some atmospheric moisture. Urethane reacts poorly to even small amounts of moisture, causing bubbles and a weak finished product. Because of this, you don’t want to work with urethane on a rainy day or when the humidity is over 50% or use wood paper or waxed mixing cups and utensils.
  • Once you have the mold box made, affix some of the clay to the back of your original and press it into the bottom of the mold box. This will keep the piece from moving around during the molding process.

 

  • Next, use a rag or paper towel or paint brush to apply the mold release to the original and the inside of the mold box. Urethane likes to bond to most things, so we need a mold release to make sure we can remove the urethane rubber mold once it is cured.

  • Since the 75A Urethane Rubber is a 1:1 by volume mix ratio, I use plastic solo cups to measure out equal parts of the A side and the B side. Then, pour both sides into a clean plastic container and stir vigorously, taking care to scrape the sides and bottom of the mixing container. Because this product only has a 5 minute pot life, you want to mix quickly. Fortunately, it is a very low viscosity product which mixes easily.
  • Pick a corner of the mold box and pour in the urethane. Don’t pour directly on the piece.
  • Let the urethane rubber mold cure for 12 hours.
  • After 12 hours, remove the urethane rubber mold from the mold box and wash with dish soap and warm water to remove any mold release residue.

  • I’m casting with plaster of Paris, which needs to mixed 2 parts powder to 1 part water. I measure out the water and powder and then slowly add the powder to the water, stirring the whole time to avoid lumps.
  • Pour the plaster into the urethane rubber mold and let set up for 20 to 30 minutes.
  • To remove the casting, I lay a folded towel down on my work space and hold the mold over it, bending and flexing the sides until the casting pops out and lands on the towel.

That’s a urethane rubber mold and a plaster of Paris casting! You can paint the plaster casting however you choose. I’m still deciding on how I am going to paint this piece. If you have any questions about this project or others, please email us at [email protected]

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