Casting is a centuries-old technique, with evidence of iron casting dating back to roughly 3000 years before the common era. In the business of iron casting, pattern making is critical. If the pattern for the mold is flawed in any way, every casting that comes out of it will be defective as well. Therefore, taking the time to create the perfect frame is vital for any decorative piece, tool, or mechanical part.
A pattern in iron casting is merely an exact replica of what the design will be. They can be made of wood, fiberglass, resin, polystyrene, or plastic. Since they are formed individually, wood is the most common material, as it can be easily carved. Patternmaking is a difficult task which can take years of training or apprenticeship to master, and is very similar to fine woodworking.
Once a pattern is created, a sand mold is formed around it and left until hardened. The sand mold must be packed tightly to prevent any air bubbles from forming. After solidified, the pattern is removed, carefully and without any breakage, from the mold, leaving behind an imprint of the design. After that, molten iron is poured into the mold and hardened, if all is done successfully, into the same shape as the original pattern.
While this may seem simple enough, there are many variables in the molding process which need to be taken into account when creating a pattern. These are called allowances.
- For example, when the metal cools, it often times shrinks slightly. This means that the patternmaker will have to create a pattern little larger than what the design calls for. How much shrinkage should be compensated for depends mainly on the type of metal being used as well as the sand casting method, for depending on the material of the mold, the metal may expand more or less.
- Another thing which needs to be taken into account is the finish on the metal. After the metal is removed from the mold, it still needs to be ground into the perfect texture. This means the patternmaker will need to make the pattern slightly larger to cover for what will be ground off.
- Also, when the pattern is being removed from the mold, it is tapped lightly on all sides to make the removal easier. When this is being done, the model is pressed down on the mold, expanding it slightly. To compensate, the pattern will be made somewhat smaller. There is no exact formula for this allowance and depends mostly on the patternmaker’s instincts.
- Finally, when the iron is being cooled, it often distorts and bends. This problem is avoided by creating the pattern curved in the opposite direction of the distortion.
One thing which all patterns need is a gating system. A gating system is a way for the metal to enter the mold cavity. It also regulates how fast the metal enters the cavity. It needs to enter the mold cavity at the perfect speed; otherwise the shape might be deformed. The gating system includes the cup from which the metal is to be poured from, a spruce and runner which leads into the gate, and a gate which runs into the mold.
Not all patterns are the same. For different designs needing to be created, there are different patterns which each serve their own purpose.
- The first one is the single piece pattern or loose pattern. This is the most basic type of pattern, and they are the simplest to make. Single piece patterns are easily created and work well as long as the castings do not need to be manufactured quickly. Loose patterns are a single representation of the cast being made. The feeding system, (the tube which allows for the metal to be injected into the mold) is usually hand carved.
- The next is the gated pattern. The gated pattern is a series of patterns, usually simple patterns, all connected to the same feeding system. This allows for multiple molds to be filled at once and increasing the quantity of irons castings being made. If all the pieces are parts in the same machine, once removed from their individual molds, they will all be attached to wherever they belong.
- Match-plate patterns are two halves of the same pattern, split in half and pressed into two different sand molds. After each half of the mold is created, they are put together, creating the full design of the iron casting.
Molds are used to create thousands of the exact same design at an incredible pace. However, none of this would be possible without a single piece used to shape the mold. Without any mold to guide them, patternmakers are forced to rely on years of experience in the age-old craft of iron casting.
For more information about cast iron restoration projects or historic preservation of cast iron, please feel free to reach out to us at Antique Cast Iron, LLC.