How to Make Molds

Step 1: Types of Molds

There are a variety of mold types for different applications:

· Pour mold - Mold made by pour silicone over an original inside of a containing box

· 1-part pour mold (block mold) - a pour mold with only one side (simplest type of mold)

· 2-part pour mold - a pour mold with two or more halves and registration keys to align the parts

· Brush-on mold - a mold made by brushing on thin layers of silicone to build up a skin that is then encased in a rigid mother mold to give structure while casting

· Glove mold - A brush-on mold that does not require a parting line (sometimes uses a small slit)

· 2-part brush-on mold - A brush-on mold that is split into two or more parts

· Lifecasting - Mold made of a living human

Diagrams of the cross section of several mold types

Step 2: Vocabulary (helpful Terms)

Some helpful mold making terms:

· mold - Overall system of parts used to replicate parts

· casting (cast) - replicated part made of final material

· flashing (flash) - extra unwanted material left on casting as a result of casting process. Usually along part line

· original (prototype/part) - What is being replicated

· box - container for pour molds (mold walls)

· mother mold (shell) - hard exterior covering to give brush-on molds structure while casting

· runner (sprue) - hole that casting material is poured into

· key - registration points for lining up sides of multiple part molds and holding them in place for registering mother mold to silicone and silicone to silicone

· part (split) line - borders between two sides of multiple part molds

· mold release - release agent applied to original and mold to facilitate release of original and casting from mold

· undercut - area of original that overhangs and can create difficulties casting

· lifecast - mold made with a living original. Usually a person.

· air vent - passage for air to escape mold chamber during casting process helps avoid trapped air in casting

· rotational casting - rotating mold while casting to create hollow final model

Diagram of a basic 2-part pour mold

Step 3: 1-part Pour Mold (block Mold)

This is a 1-part pour mold.

These type of molds require a box or mold container.

It is created by pouring silicone over an original leaving one side fused to the bottom of the box.

This fused area will later serve as an opening in the mold to pour casting material into.

Step 4: 2-part Pour Mold

A 2-part pour mold is similar to a 1-part mold however instead of leaving one side open it is sealed off with another half of silicone. A runner is placed in one end to allow an opening in the mold for casting material to be poured into later.

The two halves of the mold are aligned using registration keys placed along the parting line.

Step 5: Glove Mold

A glove mold is a type of brush-on mold best for molding complex shapes where the silicone needs to be flexible to allow parts to be removed. (note: a glove mold does not necessarily need to be on a hand)

Several thin coats of silicone are applied to the original until a skin of silicone is built up (thickness depends on the type of mold usually 3/16" - 1/2" thick) a mother mold is then built around the silicone mold to give it structure while casting.

Occasionally a glove mold will require a partial parting line in order to get the part out. (this parting line does not cut the mold into two parts it's more like the buttons at the top of a polo shirt)

Step 6: 2-part Brush-on Mold

A 2-part brush-on mold is a type of brush-on mold similar to a glove mold but split into multiple parts. It is best for molding extremely complex shapes where the silicone needs to be flexible to allow parts to be removed or to account for undercuts.

Several thin coats of silicone are applied to the original until a skin of silicone is built up (thickness depends on the type of mold usually 1/2" thick) a rigid mother mold is then built around the silicone mold to give it structure while casting. A 2-part brush-on mold requires a partial parting line in order to get the original out. Sometimes these molds will require more than one parting line and will be split into more than 2 parts.

Here is a 2-part brush-on mold

Step 7: How to Select a Mold Type

Now that you have a basic understanding of what mold making is all about you should put this knowledge to use by making your own mold!

How do you know you're picking the right type of mold for your part?

As we discussed earlier there is a family of mold types to choose from.

The type of mold you choose should be driven by the geometry of the part you're molding.

Start by examining your part from all angles. If this is your first time molding I would suggest picking something very simple until you get a hang of the process.

Is it flat or one sided with a shallow height? (like a medallion or coin)

A 1-part pour mold is probably best

Is there more than one side and it is more or less one main block of material? (like a head)

A 2-part pour mold might work

Is it really complicated with undercuts or no good place for a part line (a deer head with antlers)

A glove mold or 2-part brush-on mold is probably your best choice

The more molds you make, the better you will get better at picking mold types and placing your part line

Don't get discouraged if your first mold doesn't work exactly right. I've messed up countless molds and casts for every one I've gotten right.

Learn from your mistakes, keep practicing and remember to think through all the steps before you start mixing your silicone. It might help to draw out each step or write a list. There's nothing worse than forgetting to put mold release between the two halves of your 2-part silicone mold (speaking from experience)

Mold making materials are expensive and can be difficult to handle. Make sure everything is in place and your environment is prepared before you start. You don't want to run around looking for materials or move your mold once you've started. (remember molding and casting materials require a warm environment to cure properly if you're working in a garage or basement set up a space heater)