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The World of Mold Making

By Ian McPherson

Mold making is a necessity in the prop makers world and having a variety of abilities to make different types will help in choosing which application will fit the particular needs the best. 

There are many types of molds including direct, silicone, urethane, injection and extrusion, to name just a few, that we use on a regular basis. Molds are normally made when we need to make multiple pieces of the same design for production or to make an item out of a different material than the original.

The first step to making a mold is to create or have a model or plug that will be the replicated part for casting.

Then you decide which type of mold will best fit the application for the needs of the specific project and understanding the different types will help you make that decision.

Direct molds are typically made from a fiberglass shell that has a high gloss gel coat on the interior surface and it will be waxed or sprayed with a PVA material which essentially is a non-stick barrier so that your casted parts can be released from the mold with ease. Typically we use these for casting GRG (Glass Reinforced Gypsum), GFRC (Glass Fiber Reinforced Concrete) or FRP (Fiberglass) products that do not have undercuts or very much detail so that when the mold is separated it will de-mold easily. Examples of these would be precast fountain statuary or flat panels with angles or low details.

Silicone molds, on the other hand, are normally used for products that have a very high amount of detail and intricate undercuts and reliefs. To make a silicone mold, it requires making what we call a jacket mold previous to making the silicone mold because this is what the silicone will attach to and will be the inner workings of the mold. You can cast many different materials using this type of mold including the ones mentioned above as well as plastics and rubbers. Silicone molds are great for many uses and you see them today in examples such as cast products to kitchenware. The uses are increasing with people’s new innovations and these molds are very similar to the Urethane molds.

The real difference comes down to cost and usage. The cost of a silicone mold is significantly higher than a urethane mold and the reasoning is that it has a longer lifespan and you are able to pull more parts from that mold before needing to replace the mold. Also, it will not leech the oils onto your cast parts like the urethane can. Urethane is a great choice for a small batch of castings and can save you money in the project.

Injection molds are probably the most expensive of the types of molds that you can make but for good reason. These are normally made from steel and can have many chambers so that you can cast at a high production rate. The chambers are injected with different materials and can also have blown air pushed into them to help the parts take form. These are what most plastic products are made from like toys, plastic dishware, and soda bottles. To make these molds they are made from machines with very specific tolerances and take into account the expansion or shrinkage associated with the material of choice.

Extrusion molds are also typically made from steel and they are used for making things like PVC piping, tubular plastics and even softer metals. A mold is a form that the material passes through and then the parts take on that shape coming out the backside of this mold.

There are many aspects of mold making that are not covered here in this brief summary but I hope it gives a general understanding of how mold making can make your props making career take shape.

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