How Much Liquid Rubber Do I Need to Make a Mold?

Before making a mold, the most important thing to consider is how much liquid rubber you’ll need. Several factors will affect the amount, including the complexity, depth, width, and length of your model(s). Also, consider how thick you want to make your mold to ensure longevity and reusability.

If you use too much rubber, you’ll end up wasting material, but if you use too little, you may run out during the mold-making process and have to scramble to mix up more.

Before you can begin estimating, the first step is making a mold box or form. You must determine how far apart to place your models if you’re molding over multiple objects and the overall mold wall thickness. Then, you’ll have a better understanding of the empty cavities that need filling.

Why Mold Thickness Matters

Mold wall thickness is a critical element in the design of your urethane mold. It considers the thickness of the sides and bottom of the mold. How thick you should pour the rubber is determined by if the mold is going to be supported in a mold box and the number of incuts and intricate details that are present.

All VFI molding rubbers have a minimum pour depth that can be found on our TDI specification sheet or the individual technical data sheet (TDS) for each product. Follow these specifications to have the necessary tear strength of flexibility for proper demold and use of the mold.

Lower durometer rubbers (20-60 A) are softer and more flexible, which is why they must be poured thicker. If they are poured too thin, they can be weak and prone to tearing. They are recommended to be poured ¾ of an inch to 1 inch thick. Molds made of these rubbers are typically used for cast stone and manufactured stone projects.

A thicker mold will prevent the mold from deforming during the casting process and increase tear resistance. Thinner walls might make a flexible mold bulge from the pressure of the casting material. The castings created will not be exact replicas of the original. To prevent this, you would need to build a mold box to keep the mold from deforming.

As the rubber’s durometer increases, the minimum pour thickness decreases. The material is less flexible but will have good tear strength. These higher durometer rubbers (70-90 A) are typically best for larger precast concrete projects like formliners, stamps, or rollers.

Thinner pours (between 3/8 of an inch to ½ an inch) use less material, which also keeps material costs lower. They can be very large, which is why mold makers want them to be as light as possible for easier handling during the casting process.

Note: Thinner pours of urethane can take longer to cure. When less material is used in certain areas, it creates a smaller exothermic reaction. The less heat generated, the slower the material will cure.

Also, as a rule of thumb, we generally recommend pouring at least half of an inch above the tallest point on your master. This will create a thick enough mold bottom that withstands the abrasive effects and weight of materials like concrete.

How To Calculate Mold Size

Once you’ve determined how thick you need to make your mold walls, you can assemble your mold box and find out how much material you need to fill it.

There are a few calculations used to determine how much material you need. First, you need to find the volume of the mold box in cubic inches. Most molds are square or rectangular in shape, so we use the simple math formula length x width x height to determine the volume.

• Formula: L x W x H = volume (cubic inches)

Ex.) Box dimensions: 11 ½ inches x 5 ¾ inches x 2 inches = 132.25 cubic inches (in3)

Once you find your mold box volume, you also have a model(s) inside that will take up space. This will reduce the amount of material you need to fill the space. You can calculate the volume of your model(s) using the same formula.

Ex.) Stone dimensions: 9 ½ inches x 3 ¾ inches x 1 inch = 35.625 in3

If you’re using multiple models, you’ll need to determine the volume of each. You can then add each model volume together to get an overall total model volume.

Once you have both volumes for your mold box and model(s), you’ll need to subtract them from each other to figure out how much of the space needs to be filled.

• Formula: Box volume (cubic inches) – model volume (cubic inches) = total volume of material needed (cubic inches)

Ex.) 132.25 in3 – 35.625 in3 = 96.625 in3

The next thing you’ll need to do is convert your volume into a weight measurement, like pounds (lb). The information you’ll need to do this is specific volume. VFI provides this property for all our products under liquid properties on each product page and technical data sheet. We typically recommend using 26 in3/lb. because it will accommodate some extra material in the event that waste occurs.

You’ll need to divide the cubic volume by the specific volume to determine the total weight of material needed, which means the combination of both A and B side materials.

• Formula: cubic volume ÷ specific volume = weight (pounds)

Ex.) 96.625 in3 ÷ 26 in3 = 3.7 lb. (Part A + Part B)

We would then round that number to a whole number (4 pounds). You would then look at mixed liquid density, also known as weight per gallon, to determine how much material to order. Most of VFI’s rubbers, with the A and B sides combined, are 8-9 pounds per gallon. Because VFI’s rubbers come in 1-gallon kits, 5-gallon kits, drums, or totes, we’d recommend a 1-gallon kit of material for this specific project.

Note: Many people have poured water into their mold box to determine how much material they need. However, this can be risky because urethane is sensitive to moisture. Getting your mold box and models wet can cause adhesion, curing, and bubbling issues. Some models will be very porous, so ensuring they are dry and sealed properly before pouring the rubber is essential.

Accounting for Waste

Material can be lost through accidental spills or as it clings to the sides of mixing containers and mixing equipment. Since waste occurs, it is recommended to add 5-10% more material to your estimate.

In most cases, having more material saves time and money when accounting for this waste. If you don’t have enough material, it might disrupt the mold-making process and create more waste than anticipated.

Contact VFI if you need further assistance determining how much rubber you need to make your molds.