How to make a negative mold for fiberglass

Written by melissa j. bell
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Fibreglass is a common casting material, often used to make exact backup copies of props in stage and film, since a prop can be easily damaged. The best material for a casting fibreglass from a negative mould is plaster, made from mixed gypsum and water, and the best method is a two-part mould, so that the fibreglass halves are attached at the last stage. To make your own negative mould for fibreglass, read the instructions in this article.

Skill level:

Things you need

  • Template object or sculpture
  • Mold release agent
  • Modelling clay
  • Protected, well-ventilated working area
  • Gypsum
  • Bucket
  • Paintbrush
  • Burlap strips
  • Paint

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  1. 1

    Coat the object that you would like to make fibreglass copies of in a mould release agent. If you are working from a soft clay sculpture, coat the sculpture in Krylon Crystal Clear or another clear sealer first, then let it dry before applying a mould release agent. This will protect the clay.

  2. 2

    Mark out the dividing line of the sculpture or object, from one side of the base to the other. Along this line, build up a wall of modelling clay that is at least as tall as you would like your mould to be. This way, you can pull your plaster mould apart with minimum damage.

  3. 3

    Mix together a small amount of gypsum and water in a bucket until you have a creamy texture. Paint this plaster mixture directly onto one side of the sculpture or object from one end of the modelling clay to the other, creating an impression coat. Let the impression coat cure for six hours.

  4. 4

    Add another layer of plaster on top of the impression coat and, while it is still wet, add strips of burlap over the top, soaking them in the plaster. Let this coat dry, then add another layer of plaster and burlap over it. Continue on in this way, layering plaster mix and burlap strips, until your mould is as thick and strong as you would like it to be. The larger the mould, the stronger it needs to be, so you may not have to do many layers for a small object mould.

  5. 5

    Repeat Steps 3 and 4 for the other half of the object, then let the entire mould cure for six hours.

  6. 6

    Separate the mould halves by pulling them away from the modelling clay, carefully removing the halves from your original object. Clean any clay residue from the inside of the mould. You should now have two sides of a negative mould of your original object.

  7. 7

    Prepare the mould for fibreglass casting. Paint the inside of each mould half, making a smooth coating that will aid in the separation of the mould from the cast. When the paint is dry, apply a mould release agent like polyvinyl alcohol. Two coats of polyvinyl alcohol is recommended. When the mould release is dry, your mould is ready for laminated resin and fibreglass casting.

Tips and warnings

  • To attach both casting halves of your two-part mould after your fibreglass has cured, once you have removed the fibreglass from the mould, put the fibreglass halves together and seal up the seam with more resin and fibreglass.

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