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How to Make a Rubber Mold of a Corbel

This mold making tutorial features the making of a poured block mold of a plastic corbel. It was one of the projects worked on at our October 2014 Mold Making & Casting Workshop.

A polyurethane mold rubber will be used to the make the mold because the intent is to eventually cast concrete (although we do show some resin casting in this tutorial). Polyurethane mold rubbers work very well for a variety of concrete applications.


Polytek Products Used

  • Poly 74-20 Liquid Polyurethane Rubber
    • this mold rubber is especially good at releasing concrete and, in many cases, does not require release agent
  • PolyPoxy® Quick Stick Adhesive
  • Plasticine Clay
  • Pol-Ease® 2300 Release Agent


Other Supplies & Tools

  • Melamine-laminated particle board (or other material to make mold box)
  • C-Clamps
  • Drill
  • Wood screws
  • Dry Brushes
  • Putty knife
  • Digital Scale
  • Mixing Pail
  • Mixing Tool (Poly-Paddle)


Step 1: Prepare the Model & Containment Area

Because this corbel is plastic, the only preparation for the surface of the model is the application of a release agent. This will be applied once the mold box is constructed around it.

When working with a porous model (e.g., unsealed wood, concrete), it must be sealed prior to the mold making process. Some sealer options include paint, lacquer, paste wax and pre-packaged options from Polytek, including water-soluble Poly PVA Solution, Pol-Ease® 2350 Sealer & Release Agent and PolyCoat Sealer & Release Agent. View the Sealer & Release Agent Selection Guide for more information.

Corbel for Mold Making



Secure the corbel to a baseboard. We use PolyPoxy® Quick Stick Adhesive, a two-part, fast-setting epoxy adhesive to secure this model to a melamine-laminated particle board. It could also be screwed to the baseboard.
When working with porous models, the baseboard should be vented from beneath to prevent trapped air from forming bubbles in the rubber.


Epoxy Adhesive for Mold Making







Adhere Corbel to Baseboard

Construct mold box walls around the model. They should be spaced at least 3/4″ beyond the model on all sides and the height should be at least 1/2″ above the model.
We use melamine-laminated particle board as the mold box wall material and secure them together with C-Clamps. The walls are also secured to the baseboard with screws.







Build Mold Box Walls







Corbel in Mold Box

Because there is quite a bit of empty space surrounding the thinner, bottom-area of the model, some block-outs could be added to take up space (pictured below in red) and reduce the amount of rubber needed to make the mold.







Mold Box Diagram

Seal the inside edges and corners of the mold box with clay (we use warmed plasticine clay). Hot glue or caulking could also be used.
Also seal the edges of the model that meet the baseboard (all clayed areas are marked with a dotted line below). Sealing the edges of the model is easier to do before the mold box walls are in place.
This step helps to prevent mold rubber from seeping beneath the model and out of the mold box.







Seal Edges of Mold Box and Model with Clay

Spray Pol-Ease® 2300 Release Agent onto the model, baseboard and mold box walls and then brush it out to ensure even coverage and no puddling. If a silicone mold is being made, use Pol-Ease® 2500 Release Agent instead.







Pol-Ease 2300 Release Agent







Step 2: Determine How Much Mold Rubber is Needed

To determine the amount of rubber needed for the mold, first start by estimating the volume of the original corbel.

Measure Corbel Height



Measure Corbel Width







Measure Corbel Bottom Width







Measure Corbel Depth

Based on the measurements, we estimate 88 in³.



Then, determine the volume of the mold box. This mold box is approximately 506 in³.

Next, subtract the volume of the original corbel from the volume of the mold box.

506 in³ – 88 in³ = 418 in³.

Divide the result by the specific volume of the mold rubber. The specific volume of Poly 74-20 Liquid Rubber is 27.5 in³/lb.

418 in³ ÷ 27.5 in³ = 15.2 lb

~15.2 lb of Poly 74-20 Liquid Rubber is needed to make this mold.


Step 3: Measure, Mix & Pour Poly 74-20 Liquid Rubber

Poly 74-20 Liquid Rubber is the softest (Shore A20) rubber in the 74-Series line of polyurethanes. It is selected because it is flexible enough to easily demold from the intricate sides of the original corbel.

The Basics of Poly 74-20

  • Mix Ratio: 1A:2B
  • Shore Hardness: A20
  • Pour Time: 30 minutes
  • Cured Color: Yellow
  • Mixed Viscosity (cP): 800

Measure Part A & Part B on a digital scale at the proper ratio (1A:2B) and then combine them (we recommend adding Part B to the pail first because it’s lower in viscosity). Mix thoroughly, scraping the sides and bottom of the mixing container several times.

Mixing Poly 74-20 Liquid Rubber

Pour the rubber into a low spot in the mold box and allow the rubber to rise.


Pour Poly 74-20 Liquid Rubber







Pour Rubber into Mold Box

Fill the mold box to at least 1/2″ above the highest part of the corbel.







74-20 Liquid Rubber in Mold Box

If needed, slightly tilt the box and baseboard back and forth to help dislodge any air bubbles that may be trapped against the model.







Release Bubbles

Allow the rubber to cure at room temperature for ~16 hours before demolding.







Step 4: Demold

Remove the mold box walls and then loosen the edges of the mold (a putty knife or stainless steel spatula are useful tools for this).




Loosen Edges

After loosening the edges, carefully remove the entire mold.







Finished Poly 74-20 Mold

The mold is now ready for casting.







Step 5: Casting

There are many casting materials that can be poured/brushed/sprayed into polyurethane molds, including:

  • Concrete
  • Plaster
  • Wax
  • Resin (polyurethane, epoxy, polyester), with the proper release agent
  • Foam, with the proper release agent

NOTE: Although it’s not necessary in this case, there are times when the mold box should be placed back around the rubber mold when casting so that the mold walls do not deform. Use of the mold box is most often needed when casting deep concrete objects.
In the example below, EasyFlo 60 Liquid Plastic, a fast-setting polyurethane plastic, is measured out by volume (1A:1B). Pol-Ease 2300 Release Agent is sprayed into the mold and brushed out and then the EasyFlo 60 is mixed and poured.

Measuring EasyFlo



Release and Pour EasyFlo

Although the mixed liquid is a translucent yellow color, this plastic cures to a white color. The demold time is 15-30 minutes.







Allow EasyFlo to Cure







EasyFlo Casting from Mold

The picture below shows the EasyFlo 60 casting next to the original.







Copy and Original Corbel

To make a cold cast bronze part, mix up a layer of EasyFlo 60, Brown PolyColor and Bronze Powder and slush/brush a thin layer into the mold. Again, Pol-Ease 2300 Release Agent must be applied prior to casting.







Cold Cast Bronze Process







Cold Cast Bronze Layer

Once this layer gels, it can be backed with EasyFlo 60 and Brown PolyColor (exclude the bronze powder this time) or a less expensive material like rigid polyurethane foam.
Once it cures, remove it from the mold.







Cold Cast Bronze before Burnishing

Burnish the casting with steel wool to expose the metal particles.







Cold Cast Bronze Corbel

Pictured below from left to right: EasyFlo Clear Liquid Plastic, EasyFlo 60 Liquid Plastic and EasyFlo 60 + Brown PolyColor + Bronze Powder.







Plastic Corbels - Mold Making

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