Urethane Molding and Casting Materials

Industry professionals can greatly benefit from various molding and casting materials when it comes to making structural and architectural concrete elements. Among some of the best materials to use are urethane rubbers.

They are very versatile two-component kits that can be used to make molds, formliners, and stamps for sturdy, functional, and artistic concrete projects. Their versatility comes from the ability to be formulated with varying properties, including a range of Shore hardnesses, which allow the materials to be used for small, detailed projects as well as large structural projects.

What Is a Concrete Mold?

Concrete molds, also called forms, are a type of molding and casting material used to shape fresh, liquid concrete. Once hardened and left to strengthen, these concrete pieces are demolded to be used as building components, decorative elements, or artistic displays.

A concrete mold must be sturdy and abrasion-resistant to ensure it won’t deform during the casting process and the concrete will demold without damage. Molds can be made of various materials, including rubber, plastic, silicone, latex, wood, and metal. Urethane rubbers are among one of the most robust materials for concrete casting. These molding and casting materials can be made into a range of shapes and sizes with various designs and textures. These details are transferred into the wet concrete to influence the final finish of the piece.

The hardness of the mold is an important factor to consider when casting concrete and depends on the piece you intend to produce. The softer the rubber, the easier it will release from a complex, detailed concrete piece. However, softer rubber is not as durable as harder rubber. As urethane’s hardness increases, it is less flexible but more durable. This is why more detailed stone molds (ones with more incuts and the need to compress) use 20-50 A material and large form liners and stamps use 70-90 A material.

How Is It Used to Cast Concrete?

A single urethane mold can be used to make repeated concrete castings that mimic natural materials. It captures high surface detail and can be reused over and over, making it the material of choice for high-volume production. These molds are used to cast concrete in many different ways, including:

• Precast panels – Harder rubber is typically used to make larger structural elements. The concrete material is poured into massive flexible form liners off-site, in a controlled environment. These detailed liners can then make decorative panels, fences, retaining walls, and more.
• Manufactured Stone – Rubber ranging from 30-50 A is great in general assembly lines to make molds for lightweight concrete stone facades and/or decorative exterior elements. Urethane rubber’s dimensional stability is perfect for the repeated casting process required. These stones are used on homes, restaurants, and more.
• Cast Stone – Urethane rubber is made primarily for wet cast stone and is used to create corbels, wall caps, crown molding, and other advanced shapes. Urethane rubber is mainly used in cast stone when compression of the mold is required.
• Hardscape – Rubber stamps or rollers are capable of turning wet concrete into natural-looking, textured pieces that mimic real brick, flagstone, slate, etc., for outdoor decks, patios, walkways, and more. Rather than using expensive natural stone, stamping concrete with urethane can create pavers, steppingstones, small retaining walls, and curb walls for less.
• Décor & furniture – Concrete can be cast into molds to make large statues or art pieces for decoration or even functional elements like benches, countertops, planters, fireplaces, and outdoor kitchens. These creations can utilize stone veneer as a finish or cast stone for complete pieces.
• Architectural Restoration – Softer urethane rubbers are great for restoring old stonework on buildings. The lower durometer makes it easier to demold around complex shapes and undercuts without damaging the original piece.
  Whatever your project is, it is important to use an appropriate mold release when casting concrete. While urethane is an extremely durable material, it can also create strong bonds with many surfaces.

Not using release can have disastrous effects on the mold and concrete pieces. Even with a release, you may have to use a tool to pry the piece out of the mold if it’s stubborn. VFI, however, has developed new easy-releasing urethane rubbers for ease of use and reduced breakage when used with release.

Are there Alternatives to Urethane?

Choosing the right material is essential to ensure it will meet the needs of your project. And while urethane does come with great benefits, there are alternatives that might be better for your application. Other materials must be non-porous, non-reactive with concrete, and rigid enough that the mold won’t change shape once the concrete is cast.

1. Silicone rubber molds

Silicone is another flexible molding and casting material that produces intricate designs and textures on concrete elements. It has outstanding demolding characteristics that don’t require mold release. It works well for architectural restoration since it is able to keep the original model intact without damaging it.

However, polyurethane has more variety when it comes to hardness so it can be used in a variety of concrete applications that silicone may not be able to. Silicone is also more expensive and not great for large-scale, high-volume concrete casting. Shrinkage is another major concern with silicone when repeatedly casting concrete.

2. Plastic forms

If you desire a more affordable material, various plastics like ABS have been used to cast concrete for decorative and artistic pieces. They can offer high detail in the cast part, producing complex shapes and fine textures. They are also relatively easy to use due to their lightweight nature. However, making these molds requires certain skills and techniques you don’t need with urethane. Most plastic molds will need to be purchased from a manufacturer and will have a preset design. They also will not last as long as other materials, producing only about 10 castings per mold.

3. Wood forms

Wood molds can be custom-made and are easy to construct for large-scale structural elements. They are versatile, tough, and strong, and because wood is readily available, they can be more affordable for certain projects. However, these molds are not as durable as other materials and are not suitable if you want intricate designs or curves in the concrete. To increase the usability of these forms, a form coat epoxy can be used to protect the casting surface from abrasion.

4. Metal forms

Most metal molds are made from either steel or aluminum and can be used for structural or decorative concrete purposes. They are durable, made to last, and, like wood, produce smoother finishes. Similar to making plastic molds, metalworking requires specialized tooling and skills for precision. Metal is ideal for repeated, high-volume use when making concrete pieces for larger industrial projects that require strength and durability over aesthetics. Most metal molds will be small because of cost restrictions. A form coating epoxy can be used for long-term protection of these forms as well.

5. Latex molds

Because of its low viscosity, latex is applied to a model by brushing on multiple thin layers until the desired thickness is met. Having to apply multiple layers is a big drawback to using this material. Some latex molds can take as long as 2 weeks to finish because they don’t cure as quickly as silicone or polyurethane. So, if you require quick production, this is probably not the material you want to use. Also, due to latex being used as a thin film, you will always need a rigid backer mold for casting. However, it is probably the strongest and most resilient material, leaving you with long-lasting molds for years.

Why is Polyurethane Rubber Better?

• It is a long-lasting, reusable molding and casting material. Due to its high abrasion resistance and strength, it is more durable than other materials.
• It creates highly detailed and accurate concrete pieces that mimic natural materials. With high flexibility, the material is able to form around complex undercuts and other details to make perfect copies of the original.
• It is easy to work with. While it requires a release agent for demolding, it requires little effort to separate it from delicate details once the concrete has set. There are also easy-releasing urethanes available at VFI, which makes using these materials more desirable when heavy release cannot be used.
• It can be a cost-effective option compared to silicone and metal molds. Silicone has a few benefits that urethane lacks, but they’re fairly equal in their abilities. While it is more expensive than wood or plastic, it makes up for that cost in its longevity, flexibility, and reusability.
• It is an extremely versatile material that can come in a range of hardnesses to complete both large and small projects. There’s really no limit to what urethane molds can be used to create.

VFI manufactures a variety of urethane rubbers for various concrete casting projects. Contact us today if you need help finding the best material for you.