VFI®-3181 80 A MDI Molding Rubber

The best durometer for your project will depend on your unique project requirements. For cast stone applications, we recommend using one of our 25-40 A rubbers. For manufactured stone applications, we recommend using one of our 30-60 A rubbers. For advanced detail formliners, we recommend using one of our 50-70 A molding rubbers. For simple formliners and stamps, we recommend using one of our 60-90 A molding rubbers. Projects that require extreme detail, such as architectural restoration, will want to use a lower durometer rubber. Projects that require a large amount of rubber to hold heavy, abrasive casting material will want a harder rubber that can withstand the weight.

To determine how much mold rubber you need, you’ll first need to calculate the cubic volume (formula: length x width x height) of your mold box and models. Once you have those volumes, you’ll subtract the model volumes from the mold box volume. This will be the remaining space in the mold box that needs to be filled.

All VFI’s molding rubbers list specific volumes on each technical data sheet (TDS), so once you find that number, divide your volume by it to convert it into weight (formula: volume / specific volume = total weight (A side + B side)). Always weigh up more product than you might need to account for loss in the container or for any math errors.

Pour thickness will depend on the hardness of the rubber chosen. Because softer rubbers are more flexible, they’re more prone to tearing. Generally, the softer the rubber, the thicker your mold walls should be, and vice versa. For example, VFI-2123 is a 25 Shore A rubber and should be poured at a minimum of 1 inch thick. On the other hand, VFI-2180 is an 80 Shore A rubber and can be poured at a minimum of 3/8 of an inch thick. Check out VFI’s TDI specification sheet or any individual TDS for recommended pour thickness minimums.

When making a mold, use clay to seal the mold box walls and models so the liquid material cannot slip through or underneath. When using urethane rubber, you’ll want to use a non-hardening, sulfur-free clay to prevent unwanted adhesion or cure inhibition. Do not use water-based sulfurous clay, as urethane is extremely moisture-sensitive and will stick to the clay once it is cured.

Non-porous surface to urethane: When working with urethane rubber, you must use a release agent to prevent the material from bonding with the casting surface. VFI has had great success using Chem-Trend MR-515 Aerosol with our urethane molding rubbers. You can also utilize a similar silicone-based release agent.

Porous surface to urethane: When working with a porous surface, it must be properly sealed before you begin pouring the molding rubber. VFI recommends creating a mixture of 80% mineral spirits to 20% petroleum jelly by weight to seal the surfaces. After applying several layers with a chip brush, spray a light mist of an aerosol release agent over the entire surface. VFI recommends Chem-Trend MR-515 Aerosol or a similar silicone-based release agent.

Concrete or stone to urethane: When casting concrete into a urethane rubber mold, VFI recommends utilizing one of Chem-Trend’s water-based releases specially designed for releasing concrete from urethane. For example, the CR-19597 is ready to use and can be diluted.

Check out our tech piece if you are having issues removing your castings from a urethane rubber mold. 

Also, VFI recently released a new series of Max Release Molding Rubbers that enhance the releasing characteristics for sensitive projects. Check out our press release for more information.

Do not use an oil-based or biodegradable concrete release like soy bean form oil when molding with urethane. These materials can cause your mold to continuously expand as you apply the release.

Using too much release may affect the surface of your castings. The surface may have a shiny or gloss-like appearance, which may not be desirable for your application. We recommend spraying light mists of release over your molding surface and allowing it to dry before casting.

All edges of the mold box must be sealed as well as the edges of your models, so the molding rubber is not able to seep out of the box or underneath the models. To seal the edges, you can use a sulfur-free clay or acrylic caulk. See answer #5 for information about sealing and releasing the molding surface before pouring the urethane.

When sealing your mold box, you should use acrylic caulk because silicone can inhibit the cure of urethane.

Whether you should pour by weight or volume depends on your unique molding process. If you pour rubber in smaller quantities, either method works. To measure your material by weight, you will need an accurate scale. Since most of our liquid urethane rubbers are 1A:1B by volume, some users find it easier to measure their material that way. If you have a proportioner that automatically mixes large quantities of your material when needed, following the mix ratio by volume is usually the way to go.

VFI recommends a specific double-mix method for mixing our urethane molding rubber. First, you will determine how much material you need and measure that out using the mix ratio by volume or weight. Once you have the A side in one container and the B side in another, thoroughly premix the B side and then combine it with the A side. Mix them until uniform using power mixing or meter mixing equipment. Be sure to scrape the bottom and sides of the container for uniformity. Transfer the material to a new, clean mixing container and mix again. This will ensure a thorough mix before it is poured into the mold box.

Yes, the temperature of the environment, material, and models affects liquid urethane when making molds. If the temperature is too cold, it can slow down the pot life and curing process, and if the temperature is very high, it can speed up the pot life and curing process. If it is cold enough, the material may not cure at all. We recommend using urethane rubber at room temperature (77°F).

If you cast in a really warm environment and then use the mold in a very cold environment, you might notice your mold temporarily shrink. You must cast the material in the same conditions you will be using it in when it has cured. We recommend casting against a rigid backing material so the rubber can grip onto it to prevent shrinkage from occurring further.

If you cast in a really cold environment and then use the mold in a very warm environment, you might notice your mold temporarily expand. You must cast the material in the same conditions you will be using it in when it has cured. We recommend casting against a rigid backing material so the rubber can grip onto it to prevent expansion from occurring further.

VFI always recommends pouring in the same conditions you will be using the cured product. If you expect to work in warm conditions, pour the rubber in an environment at similar temperatures. The process is the same if you anticipate that you’ll be using the cured product in cold temperatures. The rubber should then be cast in cool temperatures. This will prevent unwanted shrinkage or expansion of the mold that could compromise the casting process. Note: the cure will be affected depending on the temperature that you pour at.

Most VFI molding rubbers must sit for a minimum of 16 hours at room temperature (77°F) before they can be demolded. After demolding, they must sit for another 3 days outside of the mold to develop the necessary green strength for molding purposes. Even though the mold can be used after 3 days, keep in mind that the material requires 7 days to develop full physical properties. Note: during the 3 days, fully support your mold to prevent it from forming into an unwanted shape.

The amount of pulls you can get from VFI urethane rubber depends on the properties, what you’re using it for, and how you take care of it. To prolong the life of the mold, you’ll need to use adequate release after each cast so as not to damage the mold or casting. When you store the mold when it’s not in use, you’ll want to keep it in a clean, room temperature-controlled environment. Choosing the proper hardness will also lengthen the life of the mold.

Because urethane will adhere to anything, apply release after each casting. The release will have worn away during the demolding process, so it’s important to reapply and negate that adhesion. Allow the release to dry before pouring the urethane on top.

Urethane molds can be wiped down with soap and water before they are used again or stored for future use.

Do not store your molds outdoors or in direct sunlight. Outdoor conditions can cause the mold to deteriorate before its typical lifespan. UV rays will also change the color of the mold and can cause chalking. Store the mold in a cool, dry environment, out of direct sunlight, where temperature is controlled.

Clean the mold before you plan to store it. We recommend storing them indoors where there is no chance of UV exposure. The environment should be clean, dry, and temperature-controlled. Also, make sure the mold is stored flat as this will help it from permanently distorting in storage.

VFI has been manufacturing high performance thermoset polymers for almost 30 years. While our origins were a little different to what we currently do, we have always been in the molding material market for industrial applications.

We have a vast customer base in the concrete industry of manufacturers who use our urethane rubber for precast panels, manufactured stone, cast stone, architectural restoration, hardscape, décor, and more. There’s no limit to what kind of concrete castings you can make with these materials.

VFI strives to offer competitive prices for our molding rubbers, but it will ultimately depend on a handful of factors, including the availability of raw materials, the raw materials used in the formula, and the properties of the final product. We also pride ourselves on manufacturing our own products, which keeps the price down and gives us full control of the manufacturing process.

Bubbles usually indicate that there is trapped air in the mix. One reason bubbles form on the rubber’s surface is due to moisture. Urethane is moisture sensitive and may have reacted with moisture within the mold box or environment. Moisture can come from humidity, a wet or porous model, or wet mixing containers and sticks.

Another reason air bubbles may have formed could be from using too much mold release. While it’s important to use release to ensure the material will demold from the molding surface, using too much will affect the cured surface as well.

Check out VFI’s tech piece to troubleshoot your bubbling issues.

To combat trapped air, you’ll want to work in a temperature-controlled environment. Also, mix the material slowly and pour it into the lowest part of the mold box, allowing the material to spread naturally as you pour. It is also important to give air a way to release when doing molds that are not open backed molds or have severe incuts.

There are a few reasons urethane rubber might not cure. If you’re seeing spots or streaks of uncured material, it’s likely that the material wasn’t thoroughly mixed before it was poured. The best way to ensure your material is fully mixed is by using a double-mix method. Once your components are measured by weight or volume, combine them in one bucket and mix. Before pouring into your mold box, pour the material into another container and mix again. This reduces the likelihood of seeing these streaks or spots of uncured material.

Another reason urethane rubber might not cure is due to an off-ratio mix. It is essential to follow the mix ratio of each material precisely, whether using volume or weight. These ratios are what help the material cure. If the mix ratio is not followed, it can result in a sticky, unusable mess.

One last thing to note is temperature. If your work environment, model, or material is too cold, the material will have a hard time curing. Urethane rubber is a room-temperature curing material, so we generally recommend using the material at 77°F for the best results.

Check out VFI’s tech piece for more information on curing issues.