Creating plastic parts at home can be a relatively complex process for the beginner, but it is not impossible. Whether you want to copy small parts for crafting, model work, toys or merchandise, you can create a simple mould form through which you can cast your plastic copies.
Sculpt a template or model object out of clay or use an existing plastic mould part that you would like to copy. Coat any clay model with a clear sealer, like Krylon Crystal Clear.
Build a lidless moulding box out of Legos or use a wooden box that can be easily taken apart. Flatten a piece of modelling clay for the bottom and attach the Legos or box sides to it. Make sure you can fit your model object in the box with at least a 1/4-inch clearance at the sides, top and bottom.
Mix half a batch of silicone RTV rubber and catalyst, stirring until the mixture turns a pale shade of the catalyst's colour. Follow all recommendations on the product packaging and wear protective latex gloves. Pour this mixture into the moulding box until the box is filled halfway, then press your model object into the box up to its halfway point. Press several wooden craft balls around the object, which will create keying impressions. Let the silicone RTV rubber cure overnight in the moulding box.
Remove the wooden craft balls from the moulding box. Mix another batch of silicone RTV rubber and catalyst, then pour the mixture into the moulding box covering the model object completely. Let this cure overnight again.
Take apart the moulding box and remove the modelling clay bottom. Peel apart both halves of the mould and remove your model object. You should now have a negative impression of each side of your model object and keying impressions that fit together. Strap the mould halves together with a few rubber bands and carve a hole in the top with a razor. Make sure that the hole reaches the cavity inside. You now have a mould that is suitable for casting in liquid plastic.
During the plastic casting process, be sure to leave the plastic enough time to cure before unstrapping the mould and separating the mould halves.