About Foundry Patterns
To understand the importance of a pattern, first you need an basic understanding of casting.
Casting
Casting is act of pouring molten metal into a hollow cavity in the mold. The mold is then cooled until the metal has solidified, leaving a finished part.
Casting can be used to create metal parts that are either too complex for fabrication, or if a large number of parts are needed. Casting is one of the oldest forms of manufacturing (civilizations have been casting for over 6000 years).
In this manufacturing method, the pattern is not the mold, but rather the tool to make the mold. Using the match plate, a flask is placed on either side (the Cope and the Drag) and filled with sand. The hard sand is then taken off the pattern, and the pattern is removed. What’s left is a 2 part sand mold with a negative of the pattern. The 2 halves of the sand mold are then put back together creating a pattern shaped cavity. Metal is then poured into the cavity creating the casting.
Once cooled, the sand is broken up, leaving the casting. From this point, depending on the casting, machining, cleaning, or coating may take place.
The Pattern
Patterns are necessary prerequisites in the making of molds for casting all metals and other materials. The art of pattern making came into being when man first began to cast molten metals into molds to produce castings for utilitarian purposes as well as objects of art. These were cast of copper or gold, then in bronzes, iron, steel, and many other alloys. Today, pattern making is essential to the manufacture of all types of machinery for modern industry.
A pattern maker must be able to read a drawing, sketch or blueprint correctly and visualize the finished article. He must have a general knowledge of foundry methods and techniques in order to construct the pattern required to make the mold for casting the molten metal into the desired shapes and sizes. He must also have a comprehensive knowledge of the principles of foundry practices, molding, and core making, be able to estimate the shrinkage of metals and visualize every process of manufacture from the drafting table to the finished article. He should have a knowledge of the equipment and limitations of the machine shop that will machine finish and erect the castings. He must be adept in the use of plastics, woods, and metals, these being the materials from which patterns are commonly fashioned.
Pattern Making can be divided into the following units:
- Layout – laying out full size from plans showing necessary draft, shrinkage, finishes, partings, cores, and core prints.
- Model Making – manufacturing scale models, mock ups, templates, jigs of wood, metal and plastics.
- Metal Work – grinding, filing, machining, polishing, soldering, welding, brazing, and assembling, mounting on wood, composition or metal plates.
- Woodworking – selecting, shaping, joining, fitting, gluing, machining, and assembling wood patterns.
- Plaster/Resins – mixing, molding, shaping, and pouring patterns and molds.
- Plastics – casting, fabricating, and mounting of plastic patterns. Forming, shaping and machining of sheet plastic and shapes.
- Finishing – coating and marking for preservation, identification and molding code.
Before selecting materials and equipment from which to make a pattern, the use intended for that pattern must be considered for primary use and expected life of primary patterns are of fundamental importance.
*Definition is from the Pattern Maker’s Manual, printed by the American Foundrymen’s Society.