How to Make Lightweight Parts & Molds by Backfilling with Foam

An alternative to making solid rubber or plastic parts and molds is to make hollow or thin castings that you backfill with a urethane expanding foam. Doing this makes them lighter, easier to handle, and cost-efficient to manufacture.

The type of foam you choose will depend on the desired density of your part, as well as other properties such as pot life and volumetric expansion. Using a flexible foam can help your part maintain flexibility, while a rigid foam can provide more structure to it.

What is Backfilling?

Backfilling is a common technique used to fill a hollow or thin casting to add strength or adjust weight. It uses less material than making a solid part or a thick mold, which is beneficial if the material is costly. Even if it is more time-consuming to make a backfilled part or mold, the material savings are typically worth the extra labor.

Rigid, semi-flexible, and flexible foams are frequently used. VFI recommends our rigid 5100 series or our semi-flexible 5200 series. They are closed-cell foams that provide a uniform structure, high strength, and reinforcement to hollow castings. Backfilling is a great technique for a variety of projects, including precast, part manufacturing, theming and fabrication, and more. Only foams that can be poured or injected are recommended, not spray foams.

Something to note about using pourable expanding foams is their relatively short working time. It’s important to have all your materials prepared so you can quickly mix the foam and pour it into the part or mold before it begins expanding. Be ready for it to expand in under a minute after you’ve thoroughly mixed the components.

Methods of Backfilling

There are three distinct methods of backfilling, but all of them begin with a moldable surface.

• Rotocasting. To make a hollow part, a small amount of liquid plastic or rubber is cast into an open mold or an opening in a closed mold. It is then either rotated by hand or by machine till the material coats all the walls of the mold in a thin layer. It’s important to note that this method only works with a material that has a long enough pot life to be applied evenly before it starts to thicken and solidify. The empty space in the hollow part can then be filled with urethane foam. This method is great for making light, cost-effective props, art displays, industrial parts, and prototypes.
• Pouring. To make a mold, enough rubber is poured onto a moldable surface with models to sufficiently cover them, and then the rest of the mold is filled with foam. This method is great for making light, cost-effective manufactured stone or cast stone molds.
• Spraying. To make a mold, a thin layer of a rigid or flexible coating is sprayed into a mold, typically over a model, and then backed with foam. This method is great for making light, cost-effective concrete formliners and stamps.
  It’s recommended that the initial wall thickness of the part or mold be at least ¼”. This may take a few pours of material depending on the method chosen. When backfilling, leave the part in the mold. This will help eliminate distortion that the heat and pressure from the foam may cause.

Why Should I Backfill My Parts & Molds?

Backfilling is desirable for:

• Adjusting the overall weight of molds and parts to ease handling, movement, and transportation where desired.
• Adding stability and structure to thin molds or hollow-core parts to prevent them from warping or collapsing when used in extreme environments.
• Cushioning the mold or part with a flexible material to prevent damage from impact or constant handling.

Estimating How Much Foam You Need

To calculate how much foam you’ll need to fill your hollow part or add more structure to your mold, you’ll need to calculate the volume of the space you are trying to fill. Since most parts are small, we’ll use inches.

• Formula: Length x width x height = volume
  • Example: 12 inches x 12 inches x 6 inches = 864 cubic inches

Once you have the volume you need to fill, you will need to use that to determine how many pounds of material you need. It is much easier to measure foam by weight than by volume because your measurements could be off due to various factors, including trapped air.

You’ll then need to determine the density of foam you’ll be using. Lower-density foams will expand more, while higher-density foams will expand less but are stronger and heavier. Expansion may be affected by ambient temperature, humidity, and the mix ratio. If your desired density is 10 pounds per cubic foot, you would use a 10 PCF foam like VFI-5110 10 PCF Rigid Polyurethane Foam. What this means is that 10 pounds of foam will fill 1 cubic foot.

• Formula: 1 cubic foot = (pounds / 1 cubic foot) = pounds
  • Example: 1 cubic foot = (10 pounds / 1 cubic foot) = 10 pounds

To determine how many pounds of the 10 PCF foam you need to fill 864 cubic inches, you will need to convert that number into pounds. Note: a 12 in x 12 in x 12 in space equals 1728 cubic inches. You can do this using the following formula:

• Formula: Cubic inches (1 cubic foot / 1728 cubic inches) x (pounds / 1 cubic foot) = pounds
  • Example: 864 cubic inches (1 cubic foot / 1728 cubic inches) x (10 pounds / 1 cubic foot) = 5 pounds

So, you would need 5 pounds total to fill the part or create a uniform backing for a mold. Make sure to add a little extra foam to account for short filling or uneven expansion, especially if you have a large empty cavity. We usually recommend adding between 10-15% of material to provide a little overflow. Most molds are able to withstand this pressure. Then you can shave off the excess for a flat, smooth surface.

Note: If you underfill the part or mold, you can mix more foam to fill the rest of the space. It has good adhesion to itself, so adding layers is acceptable if needed.

You can also use extra material for overpacking purposes.

What is Overpacking Foam?

Overpacking is the process of adding more foam to the mold or part than is required and applying back pressure, preventing the foam from free-rising, to create a tighter, uniform cell structure. This results in a better surface finish and a stronger part.

When foam is overpacked, it will increase the density of the foam due to the additional pressure. Because the foam isn’t allowed to free-rise, the only place for it to distribute pressure is on itself, so it compresses itself into a higher density. The higher the initial foam density, the higher it will be when it is overpacked.

The mold and walls of the casting need to be able to handle the pressure the foam will exert. If the wall thickness is too thin, the pressure of the foam could distort it. Also, overpacking without providing proper vent holes can distort it as well. Typical mold materials used include silicone rubber, plaster, epoxy-coated fiberglass, urethane rubber, metal, and wood with thick enough walls to withstand pressure.

How to Overpack Foam

To properly overpack foam, the mold will typically have an open top. A back pressure plate, like a melamine board with vent holes, is made to cover the opening. The size and placement of the vent holes control the pressure of the foam and its true density. The plate must be released to prevent the foam from sticking to it once it rises. It also must be securely clamped during the foam process.

The plate creates a more uniform piece, which reduces the need for sanding or cutting if you let the foam free-rise. When done right, minimal foam leakage through the vent holes is expected and is trimmed after the foam has cured.

Contact VFI if you need help finding the right foam for your backfilling project.