Do I Need to Use a Release Agent with Urethane Rubber?

Release agents are required for any application when making a two-part urethane rubber mold to prevent the materials from adhering together. Urethanes have inherent adhesion, which means they stick to just about anything. They will chemically and mechanically lock without release. You would have to use extensive force to break them apart, which could likely tear the mold and break your masters.

A release agent will create a barrier between the liquid rubber and the molding surface. This allows the mold to release easily from the mold box and masters.

What Can You Use as a Release Agent?

The release agent you use when making a mold will depend on the materials you are working with. The release should be specifically formulated for your molding application. Most will be labeled with a list of materials they are compatible with.
When the appropriate material is used, it aids in the demold process, helps produce quality molds, and increases the working life of the mold. There are several types available, including:

• Water-based – environmentally friendly and suitable for a wide range of materials.
• Solvent-based – ideal for use on molds exposed to high temperatures and pressures.
• Silicone-based – versatile and compatible with various materials. When making urethane molds, silicone-based agents are typically used.
• Silicone-free – preferred if silicone interaction is a concern.

How Can Your Rubber Mold Benefit from Using Mold Release?

• Release agents make it easier to remove a newly made mold from a mold box or form. This ease prevents your mold from tearing in the demold process, preventing rework and material waste. An intact mold is important for manufactured stone and advanced detail form liner projects.
• You can also preserve the masters you are using, as these may be more delicate than the material you will be casting to make replicas. The release prevents breakage of the original piece for future use if you have to make new molds.

What Are Some Best Practices for Releasing a Mold Box?

First, use adequate personal protective equipment when making your mold. Gloves, safety glasses, and long sleeves can prevent skin and eye irritation. Follow safety guidelines and instructions provided by the manufacturer.

When making your molds, ensure that your molding surface is clean and dry. Dirt, debris, and other contaminants can affect the function of the mold release and the quality of the mold. The contaminants can transfer over to the finished surface when you begin casting.

Prepare the model(s) you are molding around. Determine if you need to use a sealing agent. Porous surfaces like wood, concrete, stone, plaster, etc., must be sealed first. This will prevent the urethane from penetrating the porous model(s). Allow the sealer to dry before applying release.

Different mold releases can be sprayed, brushed, or wiped onto the mold box or form. Typically, the best method is spraying (aerosol), especially when coating large mold boxes or forms. Brushing on liquid release can be suitable for small projects and detailed areas. However, it’s easy to overapply using this method.

You’ll want to apply several coats to ensure your mold releases from the details on the model(s) with ease. Allow the release to dry before applying another and before you begin pouring the rubber. Also, avoid touching the surface until it has dried, as this could cause defects in the mold.

After demolding, clean the mold with detergent or solvent to remove residue from the mold box or form. This will ensure that is ready for use when you begin casting.

What Are Common Issues When Making the Mold?

When making a urethane mold, do not use a water-based release, as urethane is sensitive to moisture and will bubble. We also recommend avoiding the use of shellac. We’ve seen issues with this material where users will also apply another solvent or mineral spirits, which makes the shellac soft. When the rubber is poured, it will bond with the shellac and fail to demold.

Avoid overapplying the release to the point where it is dripping or pooling. Using more is not better and can affect the quality of the mold. Common problems with overuse include air bubbles and a glossy finish. That glossy finish will then transfer over to your future castings.

What Release Agents Does VFI Recommend When Making the Mold?

VFI recommends the silicone-based MR-515 Mold Release for non-porous surfaces when making urethane molds. Sprayable materials are the easiest and least time-consuming to use.

Shake the can and hold it about 8-10 inches from the molding surface. Spray evenly in a sweeping motion to prevent unwanted adhesion. Generally, you should apply a few coats before pouring the rubber. This will protect and extend the life of your mold.

If your models are porous, use a sealer before the release agent. We recommend a combination of 80% petroleum jelly to 20% mineral spirits by weight to seal those surfaces. Apply several coats of the sealer with a chip brush. Wait for each coat to dry before applying another. Then, apply a light mist of MR-515 over the model(s) and the non-porous areas of the mold box or form.

Some polyurethane rubbers require less release. This is a desirable feature for projects like architectural restoration since using a certain amount can diminish intricate details. VFI offers a line of lower durometer urethane rubbers with enhanced demold characteristics. They are the closest you can get to silicone release properties in a urethane.

Check out our press release for more information.

Contact VFI if you need further assistance with mold release for urethane rubber.