Liquid Rubber for Concrete Stone Molds

Ensuring you have the right liquid rubber material to make concrete stone molds is important to the success of your project. If you choose just any rubber to make your mold, it may result in damage to your casting or even the mold itself.

Artificial stone castings are not like any concrete project. You wouldn’t use the same rubber for a large formliner or stamping project. Because these materials are more delicate and detailed, they require material that can accommodate the complexity of the original design.

Polyurethane rubber is one of the best molding materials for these applications because of its abrasion resistance. It will hold up to repeated concrete casting without deteriorating quickly. However, as a versatile material, there are many formulas to choose from. You’ll need to look into the material properties to determine the best one for your concrete stone project.

What Properties Affect the Rubber You Choose?

Urethane rubber comes with a handful of important properties that you should look into, including:

• Shore Hardness – A standard tool called a durometer is used to determine a material’s resistance to deformation or indentation. The number tells you whether the rubber will be very soft, hard, or somewhere in between. Typically, for concrete stone projects, you will want to look at rubbers with a Shore hardness below 55 A. If you tried removing a delicate piece from a harder mold, you would most likely see some breakage from the force used to remove it. This force could ruin your casting and your mold. It could also break your original model when removing the mold from the mold box. While hardness is an important property, it’s not the only one that affects the function of the mold.
• Tensile Strength – This property determines how much force it takes to stretch a material before it breaks. Concerning Shore hardness, tensile strength tends to increase as the hardness increases. Having high tensile strength is important when making larger molds so the mold will hold up to large pours of abrasive materials. Molding rubbers with a hardness between 25-55 A will have a lower tensile strength. This isn’t as much of a concern for concrete stone molds, as the mold must be made with thicker walls to ensure durability.
• Elongation – This property determines the length the material can be stretched before it breaks. The elongation of a material is closely related to how flexible it is. Typically, lower durometer rubbers will have better elongation than higher durometer rubbers because they have better flexibility and stretchability. Since concrete stone molds need good flexibility, this would be an important property to consider.
• Tear Strength – This property determines the force needed to initiate a tear. It can be important for some molds if they are roughly handled in the demold process. Big molds or formliners that make concrete panels can tear more easily due to the pulling force initiated by the removal of the casting, which is why they require good tear strength. Because smaller concrete stone molds aren’t normally put through as much stress when demolding, tear strength is less important.
• Flexibility – This isn’t a property with standard testing; it is more so determined by the results of other properties, such as tensile strength and elongation. It’s also related to Shore hardness because if the rubber has a lower hardness or is softer, it will be more pliable. The most desirable thing about a flexible rubber is that it is better at capturing finer details that rigid molds might not be able to. The flexibility is also directly related to how thick you are pouring the rubber. A thinner pour will make a harder rubber more flexible. This is an important feature for most stone mold projects.

Why Use Urethane for Concrete Molds?

Urethane is a desirable material when casting concrete because it has a good variety of properties. These molds will typically be the most durable because of better tear strength and elongation. They also take less skill to make since they are a pourable formula.

They’re a more cost-effective alternative compared to making pourable molds out of materials like silicone. They are extremely reusable, as they can handle the abrasive effects of concrete. You can produce exact replicas of hundreds of artificial stone pieces from a single mold as long as the mold is properly cared for. You also don’t have to worry about shrinkage like you would with a latex mold.

Some silicone rubbers are more desired because they can be brushed onto surfaces to produce the highest detail possible and do not require release from stone. The problem is that it takes a lot more time to make these molds. Plus, they will always need to be backed with a mother mold for support when casting. So, if your concrete project doesn’t have certain requirements that prevent urethane and release from being used, a urethane mold is the way to go.

What Can Low Durometer Concrete Stone Molds Produce?

Liquid rubber is prized for being a versatile material because it can meet your design specifications. There’s not really a limit to the kinds of concrete elements you can produce, including:

Cast Stone

These elements produced from urethane molds are architectural building materials made to look like natural stone. They will typically be 3-dimensional decorative blocks, flat pieces, or other ornamental structures. They add beauty and dimension to homes, commercial buildings, gardens, parks, and plazas. Cast stone is only made for decorative purposes and will not usually offer structural support.

The rubber mold is produced from original stone pieces in the shape of quoins, keystones, cladding, window surrounds, corbels, columns, pillars, coping, capping, and more. Liquid rubber allows you to create a number of the exact same casting and is great for highly customized projects too.

VFI typically recommends using a rubber with a durometer between 25-40 A. Using a softer rubber is crucial for these projects because it ensures easy and complete removal of the casting from any intricate details, shapes, and undercuts without tearing or sticking to the mold.

Manufactured Stone

Manufactured stone castings are 3-dimensional, but typically flat-looking architectural elements. They are also called stone veneers as they are a thin layer of concrete used to decorate the exterior or interior of a building or structure.

These are not stand-alone products. They are applied to a surface like a concrete or wood wall, fireplace, or outdoor kitchen to create or match a desired aesthetic. They can easily be pigmented and will mimic the details of slate, limestone, granite, and other natural stones.

VFI typically recommends using a rubber with a durometer between 30-60 A. These molds are best for mass production of artificial stones. Unlike natural stone, these molds create a standard, repeatable pattern for homes, buildings, and other structures.

Architectural restoration

Architectural restoration projects are the replacement of natural stone pieces on historic buildings. They are extremely delicate, which is why most mold makers choose to use latex or silicone. These rubbers can be brushed onto surfaces to make a mold and do not adhere to the surface like urethane would.

However, VFI has developed Max Release Urethane Rubbers for these exact projects. In the past, you would have had to use enough release to ensure the rubber would demold from the architectural element. With this new formula, less release is required, and you won’t have to worry about damaging the original piece.

Apart from best-in-class release characteristics, the Max Release line also offers premium properties. These rubbers range in durometers from 25-50 A and help minimize damage during the casting process. They put less stress on the casting, allowing for breakage rates as low as 2-3%. So, you protect your castings and make the mold last longer for less material waste.

How Do You Prepare Concrete Stone Molds?

Models used to make urethane rubber molds typically need to be prepared differently than silicone or latex due to their release characteristics. Determine if your model needs to be sealed before you begin the molding process. Porous surfaces will need to be sealed with something stronger than release.

Since most models used to make stone molds are porous, you must seal the stone beforehand. VFI recommends using a combination of 80% mineral spirits to 20% petroleum jelly by volume. Several layers of the sealer should be applied before pouring the molding rubber.

Also, a release agent must always be used on any surface to ensure urethane demolds without issue. Apply the release agent to your model, mold box, and mold when casting.

Contact VFI if you need help choosing the best rubber for your concrete stone mold project.