Castcraft mold making and casting FAQS

Mold Making and Casting FAQS

Book & Video subjects:

What is a model? What is a mold? What is a casting material?

How come I hear about "molded products" all the time?
Is it difficult to make a mold?
Do I need a lot of equipment and a lot of space?
What is the difference between cement and concrete?
What is Pecan Resin, Porcelain Resin, Cultured Marble...?
Can I make plastic foam?
Can I cast metal?
Can I do injection molding, blow molding, rotational molding...?
What are Backup Molds, Mother Molds, Shells, and Keepers?
What is a release agent?
What is RTV rubber?

What is a model? What is a mold? What is a casting material?
A model is a 3-dimensional object you use as a pattern so you have something to make a mold of. Without specialized equipment, you can't make a mold of an idea or a drawing. You must first turn the idea or drawing into a 3-dimensional object by sculpting, carving, construction, and so on.

Specialized pieces of equipment called "rapid prototyping machines" are able to take computer CAD drawings and create a mold or a part directly from a 3D computer model, but this is not yet something the home craftsperson would find affordable.

A mold is something that will give a certain shape to a casting material.

A casting material is a material that will take on and keep the shape that a mold gives.

You can make a very accurate mold in just a few seconds as a demonstration. Dip a coin (your model) in some water, press one face into a lump of "Silly Putty" (your moldmaking material), and remove it. You have just made a mold! You will notice that every feature of the coin is "reversed" in your mold. Now if you have some plaster (a type of casting material), you can pour it into your mold, let it set, and remove a reproduction of the coin face - your casting. You go through all these steps so you can end up with a copy of your original model. The Silly Putty mold will probably distort as you remove the coin and as you remove the casting. Permanent molds made with materials intended for moldmaking hold their shape so you can use them many times and get the same results each time.

How come I hear about "molded products" all the time?
People who make plastic products commercially using various types of machinery don't usually talk about "castings" or "cast products". They refer to things made in molds as "molded products" - for instance, "injection molded" model airplane parts. To make it even more confusing, they refer to materials that go into molds to make parts as "molding" materials, such as "injection molding resins".

Artists and craftspeople usually use "moldmaking materials" when talking about materials to make a mold, and "casting materials" when talking about materials to make a part in a mold.

If you hear someone talk about a "molding material", you need to ask if they are using it to make a mold, or using it to make something in a mold.

Is it difficult to make a mold?
If you use modern rubber materials, moldmaking is usually quite easy. The easiest type of mold to make is for an object like a plaque. You can just put the plaque in a box, pour rubber moldmaking material over it, let the rubber cure, and you're done. With new clear rubber materials, you can even make molds of very complicated objects in the same way - by putting the object in a container, and covering it with the clear rubber. Once the rubber cures, you can cut the rubber into two or mores pieces to release the pattern.

If you insist on using "old fashioned" ways of making molds, it can be very difficult and time consuming. If you make plaster molds of rigid objects, for instance, you may need to make the mold in many different pieces so that you can get the plaster off the model. The Castcraft Guides show how to use and where to get the modern materials, as well as how to use the older materials.

Do I need a lot of equipment and a lot of space?
The materials and techniques we show how to use in the Castcraft Guides are specifically chosen because they do not require a lot of space or special equipment. Making molds and castings for personal use, such as for your hobbies, can be done on a tabletop or workbench. For most projects, the only equipment you need is disposable mixing bowls and mixing sticks. For some casting materials, such as plastic resins, you should do your mixing out-of-doors in fresh air. Plaster and concrete should also be mixed outside, because of the dust.

If you get into making larger items such as concrete molds and concrete castings, you will probably want a larger workspace such as a carport or garage.

The only significant equipment you need is a kiln if you want to do ceramics; a double boiler if you want to make candles; a simple electric melting pot if you want to cast low-melting temperature metals; and a cheap glass microwave dish if you want to work with hot melt vinyl.

What is the difference between "cement" and "concrete"?
Cement correctly refers to Portland Cement by itself - a dry powder. Concrete is something made by mixing Portland Cement with aggregates such as sand and gravel and adding water. When you buy materials, you can get sacks of Portland Cement, sand, and gravel, and mix your own concrete. Or you can buy a sack of concrete mix, which has all the dry ingredients so that you can just add water.

In everyday use, most people use "cement" and "concrete" to refer to the same thing - "cement blocks" and "concrete blocks" for instance. Castcraft offers a Cement Birdhouse Project Guide. We called it "cement" because the mix doesn't contain any sand or gravel aggregates. "Concrete" sounds like a very heavy material to most people (and it usually is).

What is Pecan Resin, Porcelain Resin, Cultured Marble...?
Plastic resins are usually used with various types of fillers. The fillers make the cured resin stronger, save cost because less resin is used, and impart a desired look to the cured plastic.

Pecan resin is plastic resin in which very finely ground up pecan shells ("pecan flour") has been used as a filler. The pecan flour is the same consistency as ordinary baking flour. Cured pecan resin has a dark woody look, similar to walnut wood. Porcelain resin is plastic resin in which powdered clay has been used as a filler to make an imitation porcelain. Aluminum Trihydrate can also be used for a more translucent look. Cultured marble is plastic resin in which calcium carbonate (also called marble dust) has been used as a filler. In addition to fillers, colors can also be added to plastic resins to further enhance the look. Various pigments are almost always added to cultured marble resins to give a natural "streaks-of-color" look.

Many, many terms have been created for plastic resin castings in an attempt to make them seem more desirable. All the terms refer to the same basic thing: plastic resin with some type of filler in it. Common terms include "wood resin", "cold-cast resin", "cold-cast bronze", "imitation stone", "marble resin", "indoor/outdoor resin", "cast marble", "bonded marble", "bonded bronze", etc. etc.

The terms "cultured stone" or "cast stone" usually refer to products made from cement or concrete, however.

Can I make plastic foam?
Most commercial plastic foam products are made with specialized equipment that meters out exact amounts of special resins, mixes the resins, and dispenses the mixed resin into a mold. This type of equipment is beyond the needs of most small scale operations, both in cost and space required, and is not covered in the Castcraft Guides.

However, you can easily make plastic resin castings that have many of the same qualities as foam, but are not actually foam. This is done by using extremely lightweight (hollow) fillers in a plastic resin to make very lightweight but durable products. Many smaller businesses making duck decoys, taxidermy mounts, and floating fishing lures use these materials. The Castcraft Guides do cover these materials.

Also, some of the polyurethane suppliers listed in the Castcraft Guides now sell resins that will foam after they are mixed by hand.

Can I cast metal?
With the techniques we show in the Castcraft Guides, you can cast what we refer to as "low melting temperature" metals, which include lead, tin, pewter, "woods metal", and similar alloys. This type of metal casting is easy and requires a minimum of casting equipment and safety equipment. Plus, you can use special heat resistant silicone rubber to make your molds.

Other types of metal casting, including brass, bronze, aluminum, gold, silver, and cast iron, require very different moldmaking materials and techniques, and much more in the way of equipment, space, and safety precautions. If you are interested in this type of casting, check out metal melting.

Can I do injection molding, blow molding, rotational molding...?
These are all industrial techniques usually requiring very expensive, very large machinery and very expensive metal molds. Injection molding forces molten plastic into a mold and is used to make thin plastic parts, such as CD "jewel cases". Blow molding is used to make hollow plastic products, such as plastic soft drink bottles. Rotational molding is used to make large hollow products such as double-walled plastic garbage cans.

The Castcraft Guides do not cover these industrial techniques. We do have books on small-scale injection molding if you are interested.

There is a technique called slush casting that can be done with casting plaster or plastic resins to make hollow products. The Castcraft Guides do show how to do slush casting. Slush casting is usually done by hand. When you use a machine of some sort to do slush casting it is usually called rotational casting.

What are Backup Molds, Mother Molds, Shells, and Keepers?
If you make a mold from a flexible material such as latex rubber, it may distort out of shape when you put a casting material into it. All of the above terms refer to a rigid mold made around the rubber mold, so the rubber mold will maintain the proper shape while you use it. The Latex Video demonstrates how to make backup molds from plaster, polyurethane, and fiberglass.

The rubber mold is what forms the size, shape, and details in your casting. The backup mold simply supports the rubber mold.

What is a release agent?
A release agent is a substance that prevents one material from sticking to another. When you make molds or castings, release agents may not be necessary, or they may be merely helpful, or they may be absolutely necessary.

In the example given in the first faq, about making a mold from "Silly Putty", the coin is dipped in water to help it release from the Silly Putty easily. The coin would still come off OK if you didn't do this, so the water is a helpful release agent. An example of an absolutely necessary release agent is if you pour fresh plaster against plaster that has already set up. Without a release agent, you would never be able to separate the new plaster from the old plaster. The Castcraft Guides show what release agents to use with each moldmaking and casting material.

What is RTV rubber?
The first rubber materials, developed over 100 years ago, had to be "vulcanized" (heat treated) before they would become a permanent rubber. Without vulcanization, the rubber would stay somewhat sticky or gummy and would not be durable. Certain types of rubber still used today need to be vulcanized also. Those rubbers are usually used for industrial applications or where the rubber is going to be exposed to high temperatures.

Most mold making rubber today is RTV. RTV stands for "Room Temperature Vulcanizing" and refers to a rubber material that cures completely at room temperature. This type of rubber is the most common for making poured molds. After you mix the rubber and pour it over your model, it sets up at room temperature, without needing any sort of heat treatment. Examples of RTV rubber are polyurethane rubber and silicone rubber.