How to Mold Hard Rubber Parts

Written by caroline adams
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How to Mold Hard Rubber Parts
You can make a plaster mould and cast a dupilcate of the statue in hard rubber. (small statue image by Victor M. from

Duplicating any item, such as a hard rubber part, can easily be done by creating a simple two-piece plaster mould. Hard rubber parts can be successfully cast if you use a mould made from Plaster of Paris and this type of mould will last longer. Once you have made a good mould, you will be able to make as many hard rubber parts as you would like.

Skill level:

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Things you need

  • Plaster of Paris
  • Water
  • Mixing Bowl
  • Wax crayon
  • Cardboard box (deep enough to hold the item to be moulded)
  • Petroleum jelly
  • Marbles, 4
  • Butter knife

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  1. 1

    Examine the piece to be used to make a mould and determine the location of the "parting line." The parting line is the place or the point on the piece to be moulded where a two-section mould should come apart without damaging the mould or the piece being moulded within.

  2. 2

    Combine the Plaster of Paris with water and allow to set up according to the manufacturer's directions. Pour the plaster into the box that will make half of the mould. Fill the bottom of the box to a depth of 3/8 of an inch or more depending on the size of the piece being moulded.

  3. 3

    Coat the piece to be moulded with petroleum jelly for easy removal from the new mould. Place the item, with the parting line marked in wax crayon, in the bottom of the box. Continue filling the box with plaster up to the parting line marked on the piece to be moulded.

  4. 4

    Place a marble in each corner before the plaster sets so that it is half buried in the wet plaster. These marbles will act as markers to line up the two mould pieces when they are put together.

  5. 5

    Grease the entire exposed surface of the lower half of the mould with petroleum jelly after the plaster has completely hardened, including the piece that is being moulded. Completely fill the rest of the box with wet plaster. Allow to dry completely before attempting to release the mould.

  6. 6

    Remove the outside box from the new mould by peeling it away. Separate the two pieces of the mould by gently sliding a butter knife between the two pieces of the mould. Remove the piece that has been moulded. The new mould should be an exact replica and ready for you to cast new hard rubber parts.

Tips and warnings

  • Always wear a protective face mask and eye gear when pouring and mixing powders like Plaster of Paris.

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