How to Make a One-Piece Silicone Rubber Block Mold

This step-by-step mold making tutorial shows you how to make a simple, one-piece silicone block mold of a small, decorative object. 

The Basics:
  • Block molds are generally the simplest type of mold.
  • They usually require more rubber than other methods (like blanket molds).
  • They are made by pouring liquid rubber into a containment area around the model (or “master”).
  • These containment areas can be made from a variety of objects (melamine-laminated particle board, PVC pipe, plasticine clay sticks, UHMW polypropylene, plywood, Sonotubes®, foam core, plexiglass, and much more).
  • In our product line-up, you can select from one of the following mold material options: polyurethane rubber, tin-cured silicone rubber, or platinum-cured silicone rubber. 
 Supplies & Tools Needed to Complete this Project:

Always review product safety data sheets before use.
Step One: Select Containment Area
In this case, melamine-laminated particle board was used as the “mold box” material. The mold box walls were screwed together and placed on top of a base board. Sometimes, a mold strap is used to hold all of these pieces together.
If the mold box is not going to be used to support the mold during the casting process, make sure that the rubber is thick enough between the model and mold walls so that it doesn’t deform when filled with casting material. Generally, it should be at least ½” on all sides [Figure 1]. Also, make sure that the mold box is tall enough to accommodate the appropriate mold height – at least ½” of rubber above the model.
Step Two: Secure Model to Base
Securely fasten (e.g. screw or glue) the model to the base board.
If the model is not secured, the poured rubber may leak beneath it and it may begin to float.
NOTE: If the model is porous (e.g. plaster, wood, stone, masonry, pottery), drill a vent hole in the base board underneath the model to allow air to escape [Figure 2].
Step Three: Seal the Mold Box
To help prevent rubber from leaking out of the mold box, seal the interior edges (between walls and between walls and base board). Warmed plasticine clay was used for this process [Figure 3]. Hot melt glue is another option.
Step Four: If Necessary, Apply Sealer and/or Release Agent
Sealing Porous Models
Generally, porous models need to be sealed before a release agent is applied and before rubber is poured. The sealing agent should be determined based on the model material and rubber mold material.
For many porous models, several coats of paste wax, petroleum jelly, shellac, paint, melted paraffin, lacquer, or PVA (polyvinyl alcohol) solution work well. Some rubbers contain ingredients that dissolve paint and lacquers; wax or shellac may work in this case. (Polyurethane rubbers will bond to shellac, so a release agent needs to be carefully applied in this case).
For bare plaster, lather potter’s soap or Murphy’s Oil Soap onto the damp surface with a soft brush. Rinse and repeat this process several times and then polish the surface with a soft cloth. Do not try this method on materials that are not plaster.
In this case, a sealing agent was not necessary.




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