Patternmaking and Foundry Practice are two of the principle crafts or arts comprising the metal industry. Patternmaking is one of the most important of these skills but is probably the least understood. These two arts date back to the very first time man created implements for his own use by pouring liquid metal into a simple mold. Patternmaking is the process consisting of forming in wood, metal, plaster or plastics, patterns of inventions, tools, machinery parts, and other items. The pattern is used as a tool in the foundry for making a mold of appropriate material for the sole purpose of casting liquid metal into the required size and shape. Founding includes the making of a mold from an appropriate mold material, the selection and melting of the proper metal or alloy, and the pouring and cooling of the liquid metal in the mold cavity so that sound usable castings result.
To define a pattern accurately in all its aspects is difficult, but in simple terms a pattern is a model or guide with which a mold is made. From the molding viewpoint the model (pattern) is an imitation of a part that is to be made from metal. The pattern is constructed with certain modifications in order that making the mold may accurately duplicate it in molding sand. Core prints are attached to the pattern for support of the dry sand cores that will form the interior part of the casting. Additional metal thickness (machine finish) is added to provide for final machining of the casting. The pattern is also made slightly oversize to compensate for the natural shrinkage or contraction of the metal as it cools to room temperature.
The mold is actually a negative print of the pattern in molding sand. The pattern must be removed from the sand, leaving an undamaged cavity that can be filled with molten metal. To remove (draw) the pattern from the mold, a parting line (parting) and draft must be provided on the pattern.
For those of you interested there are books available on the subject of patternmaking and foundry practices. For starters here are two links, one for patternmaking and the other for founding.
Patternmaking & Founding