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.

What You’ll Need

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.