How to Make a Urethane Mold

How to Make a Simple Urethane Mold

Urethane molding is an effective way to create simple block molds for casting resins, concrete, and plaster. Unlike latex molds, urethane molds do not require a rigid mother mold, making them quicker and easier to produce. Follow this step-by-step guide to create your own urethane mold successfully.

Step 1: Prepare the Base and Object

Choose a smooth, nonporous base such as acrylic to prevent adhesion issues.
Secure the object to the base using a hot glue gun to ensure it does not float when urethane is poured.

Step 2: Create a Mold Container

Select a container that fits your object’s shape. A simple paper cup works well for small parts.
Use hot glue to seal the container’s base, ensuring there are no gaps or pinholes where the liquid urethane could leak.

Step 3: Apply Mold Release

Urethane sticks to most surfaces, so a mold release agent is essential.
Use PA 2300 mold release and apply it lightly in a fogging motion. Avoid over-application, as excessive mold release can degrade the urethane’s quality.

Step 4: Determine the Mold Volume

To measure the amount of urethane needed, fill the container with rice to the desired level.
Transfer the rice to a measuring cup to determine the exact volume required.
Avoid using water, as it requires drying time and can cause contamination.

Step 5: Mix the Urethane RTV

Urethane RTV consists of Side A and Side B, which must be mixed in a 1:1 ratio by volume.
Side A is a thick, clear liquid. Take precautions:
- Apply petroleum jelly to the can’s groove and lid to prevent it from permanently sealing.
- Wipe excess material from the can’s groove after pouring to avoid hardened residue.
- To extend shelf life, use Poly Purge dry gas before sealing the can.
Side B is a thin, brown liquid that must be stirred before use.
For accurate mixing, pour equal amounts of Side A and Side B into separate cups, then combine them in a third container.
Stir vigorously for two minutes, scraping the sides to ensure thorough mixing.

Step 6: Pour the Urethane

Pour the mixed urethane into one corner of the mold to allow it to flow naturally over the object, pushing out air bubbles.
Stirring creates some bubbles, but they will release during the 24-hour curing process.
The trouble-free bubble release makes urethane an easy-to-use molding material.

Step 7: Demold and Use

After 24 hours, the urethane mold is ready for use.
Urethane molds are durable and can cast all resins, concrete, and plaster.
Note: Mold release will remain on the cast parts, making them difficult to paint. If painting is required, consider using silicone RTV instead.

By following these steps, you can create high-quality urethane molds efficiently and effectively. Happy molding!