How to Make Liquid Rubber Molds for Decorative Concrete Casting

Making concrete molds is a process that requires careful planning and preparation. The materials used will vary, but liquid rubber is commonly chosen for various projects. These rubbers have been used to produce castings, such as stepping stones, pavers, stone veneers, concrete furniture, architectural elements, and more. Knowing how to make a concrete mold is the first step to successful, continuous production, so VFI put together this guide to make the process easier.

Note: This guide explains how to make a single cavity block mold, which would be best for prototypes or low-volume reproduction. For high-volume production, you would want to use multiple models to create a multi-cavity mold.

What Mold Making Material Is Right for You?

Liquid rubbers are used because their flexibility allows for easier removal from original objects and future castings. When choosing a material, consider the size and complexity of the object you need to make, the desired finish, and the number of copies you need to make. The following liquid rubbers are recommended:

• Silicone: This rubber is popular for its ability to create detailed replicas of original objects without the use of a release agent. Silicone rubbers can range from very soft to medium in hardness. Platinum silicone is the most desirable for mold making. However, these rubbers are the most expensive and may be sensitive to other materials, resulting in cure inhibition. They will also wear more easily in concrete casting applications, especially if no release is used.
• Latex: While durable, latex is a natural rubber that can only be brushed onto original models, which increases mold production time. It can take days or weeks to make a mold, which extends the casting process. These rubbers will be the most affordable option, especially with their longevity, but they will shrink in production and exude an unpleasant ammonia smell.
• Polyurethane: Urethane rubber is highly desirable for its reusability and ability to produce high-quality replicas of original objects. While your model needs to be well-prepared to make the mold, they are relatively easy to use and release bubbles well without degassing. VFI also offers a line of urethanes with best-in-class release characteristics. They are the best choice for concrete casting due to their high abrasion resistance, and they will make many copies before wearing down. Since the same mold yields many castings, they are also cost-effective. If coloring the concrete is desired, the casting will be more vibrant from improved color transfer.

This guide will explain the process of using urethane rubber to make a concrete stone mold.

What Materials Do I Need?

Before making a concrete mold, make sure you have all the supplies you need to complete the process from start to finish. Essential materials you’ll need include:

• Urethane Rubber. Select a urethane rubber that works best for your project. Base your selection on the Shore hardness and properties the material provides. Most concrete stone molds are made of rubber with a Shore hardness between 25-55 A due to the undercuts or details of the original object.
• Model / Master. This is the original object that you are basing your mold on. The mold will produce exact replicas of the model when concrete is cast into it. Select the most intact model. Wash off or dislodge debris from the object, as the rubber will pick up every detail.
• Melamine board. While this isn’t a necessary choice for creating a mold box, it is easier to use than wood. It’s also cheap, readily available, and works for several applications.
• Hot Glue. Hot glue is typically recommended to adhere the model(s) to the base of the mold box. It’s easier to remove from the model(s) after making the mold if needed.
• Screws & Drill or Nails & Nail Gun. These materials are used to fasten the mold box together. The most durable option will be to screw or nail the mold box together, but other methods can be used.
• Clay or caulk. These materials are used to seal the mold box to prevent the rubber from leaking out once poured. Non-sulfur-based clay or paintable caulk works best to avoid cure inhibition.
• Release & Sealing Agents. When working with urethane rubber, a release agent is required to demold the rubber from other surfaces. Sealing agents may also be required when making the mold if the original object is porous.
• Chip Brushes. Some release and sealing agents are brushed onto the model(s), so it’s nice to have paintbrushes handy to help with the process.
• Measuring Scale. Urethane rubber can be measured by volume or weight. When measuring by weight, you will need an accurate scale to ensure you use the correct ratio of the A and B side materials.
• Mixing Containers & Stir Sticks. These materials are used to combine and mix the two components before the rubber is poured into the mold box.

Importance of Using Personal Protective Equipment

Be aware that you are working with chemicals. Reading the Safety Data Sheet (SDS) for the material you are using is crucial for the safe use of the product. The SDS will also provide information on personal protective equipment. It is always recommended to use gloves, safety glasses, and long sleeves when working with urethane rubber to avoid eye and skin contact. An approved respirator may be required if you are not working in a well-ventilated area.

Preparing the Work Area

When preparing your workspace, make sure it is clean and well-ventilated. Avoid working on or near surfaces you don’t want to get messy. Cover surfaces with plastic sheets or throw-away materials to protect them from spills.

Be aware of the temperature of your work environment. The ambient temperature, materials, and surfaces should be at or close to room temperature (77°F). A colder environment will cause a longer cure time for the rubber or a failed cure. Extreme temperatures can also cause the mold to expand or shrink. Humidity should be kept at a minimum, as urethane rubber is sensitive to moisture.

Making the Mold Box

Step 1: Arrange your model on a piece of melamine board. Trace around the model with a pencil to mark where you plan to adhere it.

Step 2: Measure and cut the board, accounting for the desired thickness of the mold walls. Measure at least ¾” to 1” away from your model on all sides to create a thick enough wall that prevents deformation when casting. Check the technical data sheet (TDS) for thickness requirements. If you have multiple models, leave at least 1” of space between each one.

Step 3: Apply a generous amount of hot glue or adhesive across the bottom of the model and adhere it to the baseboard where you traced the outline.

Step 4: Using a caulk gun, apply caulk around the base of the model on all sides. Alternatively, you can roll out thin strips of clay and press them into the gaps as well. This will create a seal to prevent the rubber from slipping underneath the model.

Step 5: Clean up any excess clay or caulk along the edges. This can be done using a utility knife or similar tool.

Step 6: Measure and cut more melamine boards to form walls around the baseboard, accounting for the desired height and overlap. Overlapping your walls may help with demolding later on. Wall height should be at least 1 inch higher than the highest point of your model.

Step 7: Use a nail gun or drill to fasten the other cut melamine boards to the baseboard, forming the mold box. Sometimes, a clamp is used to hold one of the walls so it can be removed easily for better access when demolding.

Step 8: Use more acrylic caulk on all the seams and corners where the boards meet. Alternatively, you can roll out more clay and press it into the seams as well. This helps ensure that the mold box is completely sealed.

Step 9: Use a level to ensure the mold box is level with the surface.

Note: When making a multi-cavity mold, if there is negative space, you can create wooden blockouts, so less rubber is used. This only works if the negative space does not need to be filled to prevent the mold walls from tearing.

Preparing the Model (If Applicable)

This step is required for porous surfaces like wood, plaster, stone, masonry, and more. Sealing pores prevents the rubber from mechanically locking to the model.

VFI recommends using a semi-permanent sealer like a mixture of petroleum jelly and mineral spirits. Weigh and mix 80% mineral spirits with 20% petroleum jelly. Let the mixture sit overnight.
Brush the mixture onto the model for better release and to preserve the texture of the model. Allow it to dry, and then apply two more coats.

Apply a Release Agent

Release agents are required when working with urethane rubber. The release will prevent the rubber from interfering with the model(s) and mold box to prolong the life of the mold.

There are different types of mold release agents available, including aerosol sprays and brushable solutions. VFI recommends a silicone-based release like ChemTrend’s MR-515 Aerosol. Only use a release agent from a reputable company such as Chem-Trend, Stoner Molding Solutions, or Cresset Chemical Co. Bad options for release agents include spray vegetable oil, mineral oil, water-based detergents, motor oil, and spray lubricants like WD40. Do not use any solvent-containing release, as this may shrink or distort the rubber.

Apply a light misting of release on the mold box baseboard and walls in sweeping motions. Shake the can well and hold it 8-10” from the surface. Spraying at different angles will ensure all side walls are coated. Avoid applying excess material, as this can affect the surface finish. Allow it to dry before pouring the rubber.

Mixing & Pouring the Urethane Rubber

Once your mold box is prepared, it’s time to mix and pour the mold material. Use a level to ensure the mold box is level with the surface. Calculate the amount of material you’ll need. Check out this article to help determine how much your specific project will need.

Quick formulas to remember:

• Find Mold box and model volume: L x W x H = __in3
• Subtract the volumes to get the total Volume: __in3 – __in3 = in3
• Divide total volume by specific volume (found on TDS) to get the total pounds of material needed: __in3 (volume) ÷ __in3/lb (specific volume) = __lb

Always check the material’s technical data sheet for the proper mix ratio. Urethane rubber consists of two parts (A side and B side). When mixing by weight, use an accurate scale to weigh the components into an appropriately sized container. We recommend measuring by weight for accuracy. When mixing by volume, mark an appropriately sized bucket.

It’s important to know how much time you have to mix and pour the material before you start. First, premix the B side (Poly) material with a flat mixing stick.

Pour the measured A component into the measured B component. Mix them with a flat mixing stick or power mixing equipment until uniform, making sure to scrape the sides and bottom of the container. There should be no lumps or streaks in the material. Transfer the mixture to a clean container and mix again to ensure the material is homogenous.

Pour the rubber in a thin, even stream in a single spot at the lowest point of the mold box. Fill until the desired height is reached, allowing the material to flow across the model and seek its level. Avoiding pouring on the model will help with air entrapment. We recommend pouring about 1 inch above the highest point of the model(s).

Curing & Demolding the Urethane Concrete Mold

Allow the rubber to cure at room temperature for at least 16 hours before demolding. Demolding too soon can result in deformation.

If needed, remove the walls of the mold box for easier access to the mold. Slowly remove the mold to avoid tearing it or damaging the model. Removing the mold may require leverage in some scenarios.

Let the mold sit at room temperature for 72 hours on a flat surface before casting.

Cleaning & Finishing the Mold

Once your mold is out of the mold box, clean it up to ensure your castings are high quality. With a paper towel or clean cloth, wipe off the release from the mold after it has been demolded. If the residue is hard to remove, dip the towel in solvent to wipe off the rest. If you used clay on the mold box, it may pull off onto the rubber, so you’ll want to remove that residue, too.

Inspect the mold for weaknesses. Sometimes, small pieces of the model can break off and get stuck in the mold. They can be removed with a soft-bristle brush.

Note: The longevity of the mold will depend on how well you maintain it. Proper care can extend its life.

Common Mistakes When Making a Mold

• The mold did not cure: this typically occurs if the wrong mix ratio was used, OR the room temperature, material, or surface was too cold during the application and curing process.
• The mold partially cured: If your mold is sticky in spots, this is typically due to using too much of one of the components, OR not thoroughly mixing the material before pouring it.
• The mold stuck to the model or mold box: This means that you didn’t use a release or sealing agent, OR used too little, OR didn’t use the right one for your application.
• The mold is uneven or warped: This is usually a consequence of demolding the mold too early OR casting into the mold too soon after cure.

Casting Concrete into the Urethane Mold

You always want to use a release agent before casting concrete into the mold. Use one from a reputable manufacturer that is suitable for concrete casting. We recommend Chem-Trend’s water-based release CR-19597. It should be applied after each cast to ensure demolding and prevent breakage.

Also, make sure you’re not using too much release, as this could cause defects in the final casting, such as shininess.

This is also the point when you would add dry pigments to the mold to color the concrete casting. Some people add the pigment to the concrete mix if the color needs to be throughout the casting.

Follow the manufacturer’s instructions for mixing the concrete. The mold should always be on a flat, level surface before the concrete is cast. Do not overfill it, as this could result in the creation of air bubbles. Some people use a vibrating tool to remove these bubbles and ensure a smooth, void-free cast.

Contact VFI if you need more tips when making larger, multi-cavity molds.