How to Cast Concrete Molds

Written by john albers
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When attempting to remodel outdoor areas of the house, you may have need of concrete decoratives, such as finished bricks or cobblestones for walkways and small figurines for your garden. But finding something really unique in home and garden stores is difficult and expensive. There is another option. You can make your own concrete mould and use it to produce as many castings as you like for your outdoor projects. This guide will explain how to create and use moulds specifically designed for concrete materials.

Skill level:
Easy

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Things you need

  • Pourable mould rubber
  • Item to be cast
  • Leakproof container
  • Sealing agent
  • Release agent
  • Concrete release agent
  • Concrete
  • Mixing pail
  • Razor blade
  • Drill with wide-gauge drill bit
  • Steel file

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Instructions

  1. 1

    Seal the pores of the item to be cast in a mould. Use a store-bought sealing agent, shellac or paste wax. Use at least two coats and let it dry fifteen minutes between coats. Set it aside to dry.

  2. 2

    Use the sealing agent again on the inside of the leakproof container. A plastic container will work well, though cardboard can be substituted if the mould is very small.

  3. 3

    Use the release agent on the item and the container. They can be found in hardware stores, usually in a spray can. What you specifically want is an agent that acts on latex and rubber polysulfate.

  4. 4

    Take your pourable rubber and mix it up in an expendable jug or pale. It will typically come in the form of a liquid agent and a powdered agent that must be combined before it can solidify.

  5. 5

    Pour an inch or two of the rubber into the bottom of your container, making sure it’s evenly coated.

  6. 6

    Wait a few moments for the rubber to develop a slight skin.

  7. 7

    Press the item to be cast into the rubber, making sure that it does not reach the bottom of the container.

  8. 8

    Pour the rest of the rubber over the top of the item, covering it completely. Allow to dry for the rest of the day.

  9. 9

    When the rubber is hardened, remove it from the container. If you coated the walls of the container sufficiently, it should slide free.

  10. 10

    Use your razor to bisect the rubber. By the end you should have two mould halves, allowing you to remove the original cast item.

  11. 11

    Place the empty mould halves back together and fit them back into the container.

  12. 12

    Use your drill to bore an inch-wide hole down into the core of the mould. Your mould is complete.

  1. 1

    Spray or apply your water-based concrete releasing agent to the insides of the mould halves. It is necessary in order to remove the concrete items you cast from the mould.

  2. 2

    Fit the halves back into their container.

  3. 3

    Mix your cement and pour it into the hole bored into the top of the mould.

  4. 4

    Agitate and shake the mould after the concrete is poured to pull air bubbles to the surface. This will help prevent air pockets ruining the detail of your finished concrete piece.

  5. 5

    Let the concrete dry for at least a day before removing from the container and the mould. It should peal away from the mould easily.

  6. 6

    Use the file to remove any jagged edges or unwanted marks to the finished piece.

Tips and warnings

  • Depending on the level of detail and shape of your mould, you can choose different types of pourable rubber. The stiffer the mould, the more castings that can be made from its before it begins to degrade, though these castings will have less detail than a mould made with soft and supple rubbers.
  • All the necessary materials can be found at any well-stocked hardware store.
  • Most mixable rubbers smell terrible and can cause coughing fits, wear a respirator or learn to hold your breath when mixing them.

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