Plastic injection moulding is used in many industrial applications, from toys and model parts to furniture and building materials. You can practice injection moulding on a small scale at home by creating your own plastic objects. A homemade mould will not be as durable as a professional one, but will give you a clear understanding of how the process works.
Sculpt a figure in oil-based clay. It is available in several hardnesses at art supply stores, a medium-hard clay is good for this project. Your figure can be a toy, mechanical part, random shape or any other object that you like.
Lay the figure down on a block of water-based clay and press it slowly and carefully halfway in. You want the edges of the water clay to be as close to a 90-degree angle against the figure as possible to minimise seam lines in the finished product. If needed add more clay and sculpt a good angle with a sculpting tool.
Press a few bamboo skewers halfway in the clay. The skewers should each lead away from the figure, with the blunt end of the skewer touching it. These will eventually form channels that allow air and excess plastic to escape the mould during injection, leaving behind plastic sprues to be trimmed off of the finished copy.
Press a stick the same diameter as the injection syringe into the clay in the same fashion as the bamboo skewers. This should be positioned in an out-of-the-way area on the figure as it will leave a larger sprue to be trimmed off.
Press your thumb a quarter inch into the water clay in a few areas. This will form keys, which will fit into holes on the other half of the mould to help it line up properly.
Press pieces of corrugated cardboard into the clay, forming a box around the figure. It should be at least an inch away from the figure on all sides. Notches should be cut for the sticks, which must extend past the cardboard. Tape the joints in the cardboard to prevent leakage.
Spray two thin coats of acrylic enamel spray into the mould, then paint the cardboard with a thin coat of petroleum jelly as a release agent.
Mix a batch of cement according to the instructions on the cement package.
Pour the cement into the mould until it is an inch higher than the highest point on the figure. Allow the cement to harden for a few hours.
Remove the cardboard and water clay from the mould, leaving the figure and sticks embedded halfway in the cement.
Brush the cement surface with a thin coat of petroleum jelly to prevent the second half of the mould from bonding with the first.
Build another cardboard box for the second half of the mould, extending a few inches above the surface of the first half. Glue the cardboard to the sides of the first half with hot glue and seal the joints.
Prepare the second mould with acrylic spray and petroleum jelly.
Mix a second batch of cement and pour it into the cardboard box, forming the second half of the mould.
Allow the cement to dry.
Open the two halves of the mould and remove the sticks and figure. If any of the channels leading away from the figure have become blocked with cement, scrape them clear with a sharp sculpting tool.
Spray both halves of the mould with a few coats of acrylic spray to prevent the plastic from seeping into the stone.
To cast a plastic part out of the mould, paint a thin coat of hand soap into it as a release agent and duct tape it shut. Mix a liquid plastic compound and pour it into the injection syringe. Inject the plastic into the mould, plugging the air escape holes with wads of clay as they begin to leak plastic.
The air escape channels must always be clear every time you use the mould. If they are not you will have air bubbles and deformities in the finished plastic part.