The majority of moulds used for plaster casting are made from either vacuum formed plastic or latex. The plastic moulds are used for objects that have a flat back while the latex are used for casting 3D figurines and similar pieces. The great thing about plastic plaster moulds is that they are re usable. What that means is that you can make as many pieces as you wish without worrying about reproduction costs.
Bags of plaster are available at a fraction of the cost of other casting material and will normally cast from 30 to 50 pieces depending on the size.
Mixing the plaster.
(1) Measure the amount of water required for each mould, by filling it with water and pouring into the mixing container. Use cold water as this gives you a little extra time during the mixing process.
(2) Slowly sift the plaster into the water, spreading it evenly over the surface. Continue sifting the plaster until all the water is filled with plaster and a small mound or plaster about 2-3 cm high shows above the water line. Note: Sift the plaster into water, NOT water into plaster.
(3) Leave untouched for 2 to 3 minutes while the water absorbs the plaster. This process is known as “slaking” and is essential to the art of mixing plaster correctly. Many UTube videos show water being dumped into the dry plaster and mixed immediately, resulting all too often in lumpy improperly mixed plaster.
(4) Now stir gently with a flat bladed plastic or wooden spatula, to avoid introducing air into the mix. You will need to stir for about 30 seconds to get a smooth mix. Over stirring will result in a plaster mix that will set up quickly and make it difficult to pour into the mould.
Preparing the mould for Casting
Rinse the mould with a surfactant such as Aldax Craft Rinse. Good results can also be achieved by using one teaspoonful of dishwashing liquid to 500ml of water. Shake the mould dry before casting. This helps reduce the surface tension on the face of the mould and produces quality surface results.
Casting the Mould
Fill the mould to about 1/3 full and then give the mould a series of light taps to ensure that the plaster has filled all undercuts and areas with fine detail such as faces, hands etc. This action also helps eliminate air bubbles from the surface of the mould.
Complete filling the mould with the plaster and give a final few taps to bring any remaining air bubbles to the surface.
The plaster will harden in about 30 to 40 minutes and will generate quite a deal of heat during the process.
Removing the Plaster from the Mould
Check the plaster is ready to be removed from the mould by feeling the base. It should be dry and firm to the touch.
Cover the table with a cloth or towel to prevent damage to the plaster casting when it is released from the mould.
If using using a plastic mould, we tap the mould edge against a cloth on the table as we work around the mould, flexing the mould gently as we go. Sometimes this may take several repeats, until the vacuum suction, between the mould surface and the casting is broken.
Wipe the outside of the mould with warm soapy water or use dishwashing liquid to enable the latex to slip easily over the outside latex mould surface. The action of release is akin to removing a pair of socks.
Pull as firmly and as rapidly as possible to release the casting from the mould.
The article is now ready for decorating with acrylic paints and finally sealing with an acrylic spray for long life.
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