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Introduction to Mold Making & Casting

Welcome to mold making and casting! This class will teach you the ins and outs of how to make easy molds, enabling you to cast unlimited reproductions of almost any object in a variety of materials.

You will finish with a solid foundation of mold making and casting fundamentals that will empower you to tackle new and exciting mold making projects moving forward!


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.


About Me

Made in Canada, I grew up crafting, making, and building. Out of this love for creating things, I got a BFA in Product Design from Parsons School of Design in NYC.

I've done work for Martha Stewart Living, Sunset Magazine, Fossil, Crate & Barrel, and my own ceramic design company. My company specialized in plaster mold making and ceramic slip casting = mold making and casting are my jam!

I now have the most fun ever as a full time designer and content creator for Instructables, making a wide range of projects from food recipes to furniture. You can check out all my Instructables here and my ceramic work here. The following are a few of my favorite Instructables projects I've made so far:


Twin Socket Pendant Light


Easy Desk Organizer


24 Carrot Cake


Terrarium Table


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!


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.


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).

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


Mold Boards

A mold board is a piece of rigid material that is larger than the dimensions of your mold with a smooth and impermeable surface. The benefits of making your molds on a mold board, as opposed to your counter or work surface, is:

  • You can glue your model in place so it doesn't float up once the mold material is poured.
  • You can use hot glue to seal your mold walls to its surface without hurting your permanent work surface.
  • You can easily turn your piece/mold as your working on it if necessary.
  • Once your mold is poured, you can move it to an out of the way location to dry/cure.

The materials I recommend using for mold boards are:

  • Sheet acrylic / plexiglass (1/8" to 1/4" thick)
  • Melamine (3/16" to 1/2" thick)
  • Hardboard* (3/16" to 1/4")

*Do not use hardboard for plaster molds.

The reason you want to build your molds on a rigid surface is that if you used a flexible plastic for instance, like a roll up cutting board, there is more of a chance of a portion lifting up, resulting in a break in the seal and the mold material spilling out.

In the next lesson, I provide sourcing info.


Mold Walls

The reason I like to make my own mold walls (also known as forms or boxes), as opposed to using old food containers or the like, is that I can better customize how the walls fit around the model. The better and more customized the fit, the less mold material you'll need to use, which means substantial savings!

There are four different kinds of retaining wall materials I like to use to create custom mold walls:

  • Corrugated plastic
  • Sheet acrylic / plexiglass
  • Styrene & polycarbonate sheets
  • 5/8" Melamine Board

I'll go over these and when to use them in detail in the next Lesson!


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.


Testing, Testing

A show of hands. Who was listening? :)

{
    "id": "quiz-1",
    "question": "What is another word for model?",
    "answers": [
        {
            "title": "Core",
            "correct": false
        },
        {
            "title": "Pattern",
            "correct": true
        }
    ],
    "correctNotice": "That's correct!",
    "incorrectNotice": "That's incorrect! Try again."
}
{
    "id": "quiz-2",
    "question": "Can you cast a rigid one part mold of an in-the-round peanut shape?",
    "answers": [
        {
            "title": "Yes",
            "correct": false
        },
        {
            "title": "No",
            "correct":true
        }
    ],
    "correctNotice": "That's correct! The undercuts would prevent the model from coming out of the mold.",
    "incorrectNotice": "That's incorrect. The undercuts would prevent the model from coming out of the mold."
}

What's Next?

Now that you've been briefed on the heavy hitter components of mold making, it's time to start collecting the tools and supplies you'll need to try your hand at putting them all together!

In the next lesson, I go over everything you'll need to get a project started and provide links for where to get what you don't already have.

Mold making ho!

CLASS PROJECT

Share a photo of your finished project with the class!

Nice work! You've completed the class project