Mold Making and Casting FAQS
Book & Video subjects:
come I hear about "molded products" all the time?
What is a model? What is a mold? What is a casting material?
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.
How come I hear about "molded products" all the time?
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 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.
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"?
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).
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.
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.
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.
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.
The rubber mold is what forms the size, shape, and details in your casting. The backup mold simply supports the rubber mold.
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.
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.