Mixing 2-part Polyurethane Mold Rubber

For this project we used Polytek 75-60 Polyurethane Rubber. The number 60 refers to the shore hardness of the rubber, which is a mid-range hardness. A lower number means a softer rubber, and a higher number means a harder rubber. In this case, we need the rubber to bend but still hold it’s shape, so a 60 series should work well.

This mold rubber is 2-part in a 1:1 ratio. Mix 1 part A to 1 part B, by weight.

Step 1 – Prepare for Mixing

Mixing Mold Rubber - Step 1 | CHENG Concrete Exchange
  • Have (2) clean 5-Gallon Buckets ready.
  • 1/2″ Corded drill with paddle mixer and extension chord.
  • Clean work space.
  • Scale to weigh out rubber.

Step 2 – Weight Out Part B

Mixing Mold Rubber - Step 2 | CHENG Concrete Exchange
  • Part B of the Polytek 75 series is more fluid than part A. Weighing it out first will make it a little easier to scrape the sides of the bucket after Part A has been added.

Step 3 – Add Part A

Mixing Mold Rubber - Step 3 | CHENG Concrete Exchange
  • Add an equal amount of Part A to the same bucket.

Step 4 – First Mix

Mixing Mold Rubber - Step 4 | CHENG Concrete Exchange
  • Blend Part A and Part B together using the corded drill.
  • Scrape the sides and bottom of the bucket with the spatula and continue blending for a few minutes.

Step 5 – Second Mix

  • Pour the rubber into the second 5-gallon bucket and resume blending.
  • The double-bucket mixing technique will help ensure any unmixed material on the sides of the first bucket won’t make it into the mold.
  • Scrape down the sides and bottom of the bucket just like before and continue mixing.
  • Total mixing time shouldn’t be longer than 7 minutes.
  • After the rubber is mixed, begin pouring it in the form. There’s no time for standing around or cleaning stuff!

Step 6 – Dry Gas Blanket and Cleanup

  • Using a product like Poly Purge Dry Gas Blanket will help extend the shelf life of the unneeded rubber.

    •     Spray the dry gas blanket inside the buckets containing Part A and Part B and then screw the caps back on.
  • Cleanup is easy after the rubber has cured. It can usually be peeled out of the mixing buckets and scraped off of the tools with a trowel.

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