Why Can’t I Remove My Casting from My Urethane Mold?

Some urethane rubber users have run into issues with being unable to remove their concrete casting from their mold. Major reasons this occurs is because a release agent was not used or not enough of it was used to prevent unwanted adhesion.

The casting will stick to the mold, which may lead to the piece breaking or the mold tearing if you have to use excessive force during the demold process. Overall, this results in increased breakage rates, material waste, production loss, and possible loss of the mold.

Reasons Why Your Casting Isn’t Demolding Properly

Reason #1: Not Using Mold Release

Urethane rubber is a unique molding material because of its adhesive characteristics. Unlike silicone or latex, it will adhere to anything. You need to create a barrier by using a release agent so it won’t bond with the surface of the casting.

If you do not use a release agent, you probably won’t be able to get your concrete casting out without breaking it. The adhesive effects of the concrete will also wear or tear your mold making it unusable.

Reason #2: Not Using Enough Mold Release

There’s a fine line between using too much and too little mold release. If you give your surface a light coat or mist of release, it might not be enough to prevent adhesion from occurring. If the mold has complex details or undercuts, you might miss some spots, which allows certain parts of the concrete piece to get stuck.

A torn mold can result from not using enough release because the concrete sticks to and pulls on the mold during removal. If your mold tears when you are demolding, you’re using too much force and stretching it beyond its limit to break the bond.

It’s also important to wait for the release agent to dry before you start casting. Once it is dry, it creates a smooth, effective barrier on the mold surface. If you don’t wait, the release agent might mix with the concrete, preventing the release agent from forming a barrier. Waiting also prevents residue from transferring onto the concrete, which could cause imperfections.

Reason #3: Using the Wrong Mold Release

If the specific release agent you have chosen is incompatible with your polyurethane mold or the concrete you’re casting, it can cause demolding issues. The casting may stick to the surface, tear the mold, or cause imperfections on the surface. Most release agents specify which materials they are compatible with, so it’s important to read the manufacturer’s instructions. Most solvent or oil-based releases will cause issues with urethane.

Reason #4: Choosing the Wrong Mold

Some key things to consider for your concrete casting project are the rigidity or flexibility and the complexity of the mold, including shape, size, details, etc.

You will typically have an easier time removing concrete castings from flexible, less complex molds. If you choose a firm rubber for a project with intricate details or undercuts, you might have trouble removing the casting, resulting in breakage. While most liquid rubbers stretch, over-stretching can cause tears in your mold.

Reason #5: Demolding Too Soon

Sometimes, issues with demolding are as simple as not waiting long enough for the concrete to cure. If the concrete doesn’t have enough time to gain its inherent strength before you demold it, it may be fragile and could break in the process. It also might still be sticky and leave residue in the mold that needs to be cleaned out.

Reason #6: Pigment

Depending on the casting, you may need to use pigment, and depending on the pigment, it can make it harder to demold. There are two styles of pigment, a powder and a liquid pigment. VFI always prefers the use of powder pigment over liquid pigment, because the liquid pigment is usually used for stamps and contains solvents that will attack the urethane rubber. Once the liquid pigment attacks the urethane rubber it will bind the concrete to the mold surface.

Note: Liquid pigment in concrete does not cause the same issue as applying liquid pigment to the surface of the mold.

Solutions

Always read the manufacturer’s instructions before using a product. They can be found on any product page or technical data sheet.

• Make sure you apply a release agent and use the correct kind. Using one will extend the life of your mold and decrease breakage rates. When casting concrete into a urethane mold, VFI recommends using one of Chem-Trend’s water-based releases specifically designed to release these materials from each other. We’ve seen great success with the dilutable CR-19597. The release agent should be applied before each cast to ensure the next one will demold without issue. Do not use an oil-based or biodegradable release, as these can cause the rubber to expand.
• Make sure you’re evenly coating the surface with a release agent, especially in complex and detailed mold areas. You need to find a balance where there’s enough on the surface to make the casting demold with ease but not too much so that it will make the surface shiny. VFI recommends spraying a few coats of a light mist over the mold. Allow the release agent to dry in between each coat and before casting. Consistent reapplication will also help prevent underuse.
• Use a Max Release urethane molding rubber. VFI released a series of low-durometer rubbers that have enhanced release abilities comparable to silicone. The benefit of using these rubbers is that you don’t need as much mold release on the rubber, and less effort is required to demold your casting. Since their hardness ranges from 25-50 A, they also provide better flexibility, allowing you to make more detailed castings without worrying about breakage. Check out the press release for more information.
• Allow the concrete to cure fully before removing it from the mold. The longer you let your material sit in the mold, the longer it has to gain strength. It can typically be removed within 24-72 hours.
• Go slow when removing your casting. People want the demold process to be quick and easy, but depending on the size of your casting, it might take more time and effort to get it out. Additional care should be taken if your casting is extremely detailed because you don’t want those details to break off when you pull the rubber away. Carefully work the edges of the mold to reduce the stress put on the rubber. Do not pull or twist the mold harshly.

Note: For increased reusability of your concrete mold, clean it after each use. Warm, soapy water is typically the best way to clean a urethane mold. It will remove residue and particle buildup, so you start with a clean surface when you’re casting next.

Contact VFI if you’re still having trouble demolding your concrete project so we can assist you.