Introduction: Mold Making & Casting Basics

About: Made in Canada, I grew up crafting, making, and baking. Out of this love for designing and creating, I pursued a BFA in product design from Parsons School of Design in NYC. Since then I've done work for Martha…

In this lesson, we'll cover the basics and mold making and casting.

Step 1: What Are Molds & What Are They Used For?

Molds are negative forms that are used to shape casting materials, creating duplicates of the model (object) the mold was made from. The resulting cast will be an exact likeness of the shape of the hollow mold form.

The easiest example of what a mold is, that almost everyone has used before, is an ice cube tray. The hollow forms of the mold get filled with water (the casting material) to produce ice cubes. The cubes are released from the mold and voila! Finished cast pieces. (Iced teas rejoice!)

Many industries use mold making as a way of producing their wares.

Commercial ceramic factories use plaster molds and slip (liquid clay) to produce bowls, cups, candlesticks, figurines, and more. Plastic factories use metal molds and different processes like injection molding, rotational molding, and blow molding to produce plastic parts. The glass industry blows molten silica into metal or graphite molds to produce drinking glasses, bowls, vases, etc. Most parts of any car's body and engine are made in molds.

Needless to say, we are constantly surrounded by objects that have come out of a mold!

There are 3 basic types of molds:

  • One part
  • Two part
  • Multi-part (advanced)

As this is a foundation class, we will be focusing on one and two part molds only.

Step 2: What You'll Be Learning

While there are a variety of mold making techniques to accommodate the MANY different model types, mold materials, and casting applications, we will focus on three of the easiest and most commonly used mold types in this class and all the making techniques that go into each one:

  • Simple one part mold – for models with a flat back and no undercuts
  • Flexible block mold – for 3D models with undercuts
  • Two part mold – for models with challenging shapes and undercuts

Lesson 3: Simple One Part Mold - casting a rhinoceros ice cube!

Lesson 4: Flexible Block Mold - casting custom soaps!

Lesson 5: Two Part Mold - casting a donut trophy!

My aim is to provide you with enough information about basic mold making and casting, that you can go your own way after reading through this class and choose your own model and whichever mold and casting material you'd like to use. This class lays the knowledge foundation. It's your own imagination that will bring it to life in a way that excites you.

Just remember, most mold making materials, with the exception of plaster and alginate, will never decompose. So take time and care in choosing what you want to make a mold of so that it's something you will continue to use for a long time!

Step 3: What's a Model (or Pattern)?

L to R: donut model from Lesson 5: 2-Part Molds, plaster cast of model, finished spray painted cast

Before we dive into the world of mold making and casting, we need to have a quick chat about the object you choose to make a mold of, known as a model (for this class I will be using the word model, but it can also be referred to as a pattern).

A model is any type of three-dimensional object that you want to reproduce with the mold making and casting process. In other words, it is the original object from which a mold, and then a cast, are made. It's what is used to make a mold from.

How complicated the model is determines the kind of mold technique that must be used to successfully cast the object.

Step 4: Getting to Know Undercuts

An undercut is a protruding or indented area of a model that prevents the easy and safe removal of the model (and therefore cast) from the mold (see examples in illustration above). These are important to be aware of when determining the type of mold making technique to use and the level of casting difficulty of an object.

As an example of a model with no undercuts, a simple cup (conical) shape is well suited to an open faced, one part mold because it has nothing obstructing it's easy removal (I demonstrate this mold type in the Simple One Part Mold lesson).

Something with a peanut shape would be prevented from coming out of a one part mold due to its 'waistline' or undercuts. This shape would require a more complex mold type like a cut flexible block mold (see Flexible Block Mold lesson) or a two part mold (see Two Part Mold lesson).

IMPORTANT NOTE: When making molds, small undercuts created by surface relief details (like canvas fabric texture for example) aren't an issue if you're using a flexible mold material like RTV silicone or urethane rubber. Though, if you're using a more rigid material for your mold, like plaster, and a rigid model, even the smallest undercut can be an issue when it comes time to remove the model from the mold. It will get hung up on the undercuts and can only be removed by breaking the mold.

This is all to say that the decisions about what kind of mold technique and material to use must come from the shape properties of the model (original object) you wish to reproduce and the material you want your final cast to be.

In each project lesson, I provide some general guidelines for which mold techniques and materials, and casting mediums, work best for each type of model.

My hope is that after reading through this class you will be able to decide for yourself which kind of mold technique to use, but if you're ever in doubt, please fee free to send me a message to my instructables page for a second opinion. :D

Step 5: Sealing Compounds & Application Techniques

In order to connect the mold wall or box to the mold board, you must use a form of sealing compound.

The most common sealing compounds used in mold making are:

  • Hot glue
  • Sulfur-free modeling clay
  • Double stick adhesive sheets or tape

For small molds that use a plexi, plastic, melamine, wood, or foam core mold wall or box, a large bead of hot glue is usually sufficient. If you're making a larger mold that will be holding a higher volume of mold material – and therefore have more downward pressure – I recommend backing up your large bead of hot glue with coils of the sulfur-free modeling clay.

Step 6: What's Next?

Now that you've been briefed on the important basics of mold making, it's time to start looking at what you can do with this knowledge.

We're off to the mold making races!