There are various methods of making a mold to reproduce an object – from the simple glove, blanket and one-part molds to injection molds and the complex two-part or even multi-part molds.
A multi-part mold is an advanced process for duplicating a complicated model that has intricate shapes, sides and undercuts. As none of the sides are flat, a one-part mold is not feasible. The mold has to actually be made in parts with two or more installments to ensure that all the details are properly captured and can be reproduced. This is why each part of the mold should have appropriate ‘keys’ so that they can precisely join when put together for the casting process.
Multi-part molds can be made from various materials – such as plaster, silicone rubber, polyurethanes or latex – just like their one-part counterparts.
As the name goes, making a multi-part mold is a complex process that requires infinite care, patience and experience to get it right. One of the most important steps is right at the beginning where you have to draw parting lines all around the model to define where the different parts of the mold will meet to form a whole.
Place the model on one side and portion it off along the demarcating line by covering one side entirely with clay. The clay border should be made level with the parting line and should fill all gaps and cracks before being smoothed out properly. Then make the keys either as a depression (with a coin) or a raised surface (with say, acorn nuts).
Now move to making a tall mold box along the perimeter of the model and seal it properly so that the mold making material will not flow out. Provide sufficient airways so that the trapped air can escape easily from the mold.
Once done, you are ready to mix and pour the mold making plaster, polyurethanes or silicone. Do not forget to provide a generous coating of an appropriate release agent to allow for easy remolding.
Allow the mold to set properly before removing the clay edging from the model. Flip the model over and repeat the process of making a mold box once again. Apply the release agent before pouring the mold making material once more. Repeat again if you are making three or more mold parts.
Carefully separate the different parts of the mold to reveal the original model. You need to cut out a big hole at one appropriate end of the mold to provide an opening where the casting material can be poured in later on.
Join the parts of the mold again along the keys and secure them properly (use mold straps) before moving to the casting process. Once done and finished properly, you will have a beautiful casting that is an exact replica of the original model that you started out with. Multiple casts can be successfully made in this manner.