Patterns define metalcasters' skills.

Several millennia ago, one of our brilliant ancestors figured out how to melt metal. He also learned that he could contain the molten metal in a mold to form a shape like a knife, an ax head or a spear point. It obviously was much better than chipping flint stones or looking for fire-fused silica or sharp volcanic rock. No doubt that first foundryman/pattern maker did very well; at least it's nice to think that he made a fortune turning out swords, bowls and religious totems.

Melting, his successors soon discovered, was only part of the process. With enough heat, they could melt any ore or combination of the metals they knew. How to shape the molten metal proved to be the trick. Thus, early molders gradually became skilled patternmakers who began to develop an unique body of molding knowledge that continues to be expanded even today.

Lost wax molding was discovered by early Egyptian and Chinese artisans. Intricate gold castings of great beauty were made by the Ircas, who never got the hang of making a wheel, and ancient Chinese dynasties were gifted with clever iron and bronze founders who are famous for their castmetal art and armaments.

Early on, sand was recognized as a good molding material. It withstood heat well, could be compacted and be removed from the casting rather easily. It also was plentiful and could be reused. All that was needed with it was a good pattern.

Every casting requires a pattern. The patternmaker's task is to translate the part designer's ideas into tooling that will make the part accurately. The quality of the pattern directly influences the quality of the casting. Over time, patterns have become much more precise, more complicated, foundry tools. The ancient molders learned to use two-part molds, not unlike the upper (cope) and lower (drag) sections of modern molds. They also used cores, or sand shapes, to form the cavities within the finished casting.

Although a pattern is identical to the casting, its dimensions are different. A shrinkage allowance is added to patterns to correct for the shrinkage that takes place when molten metal solidifies. It may vary from an eighth to a quarter inch per foot, depending on the type of metal being poured, pouring temperature and mold resistance to molten metal expansion.

Metal expands as its temperature rises then contracts during cooling. The patternmaker increases the dimensions of the pattern in an amount equal to the expected shrinkage to provide for the total contraction. Individual shrinkage rates are known. Patternmakers can consult a table to calculate the specific rate of the metal with which his pattern will be used and build his pattern accordingly.

Patternmakers use a shrink rule," a special linear scale different from a regular ruler. It accommodates varying shrinkage rates by uniformly increasing each linear graduation by a uniform amount. It obviates the necessity for the patternmaker to calculate the percentage of shrinkage for each pattern dimension.

There are several types of patterns used in the production of castings. They include loose patterns, matchplate patterns, cope and drag patterns and special patterns or pattern devices.

Loose patterns are usually used for a one-off or low production casting.

A matchplate has the pattern fastened to a wood or metal plate. The plate forms the parting line which separates the two pattern halves. Sand molds are made of the patterns mounted on the matchplate, and they in turn are mated to form the mold cavity. Like match plates, cope and drag (upper and lower) patterns are used in higher volume production runs. The upper and lower sections of the pattern are placed in flasks, open-ended metal forms, and sand is rammed around the halved patterns. The patterns then are removed and the cope and drag forms are precisely aligned, or kissed," so that the two mold halves form the completed pattern shape ready to receive molten metal.

Special patterns are those constructed for extremely large castings, or use a symmetrical shape involving the use of a profiled "sweep" to form the mold surface, such as for large bells or circular castings. Others use polystyrene casting models in the Expendable Pattern Casting (EPC) system, or use the lost wax process.

in the latter two casting processes, the pattern is destroyed, but both are capable of producing exceptional fidelity to the pattern. EPC castings are used extensively in the automobile industry while the lost wax process is ideally suited to the dental appliance and jewelry industries where fine detail is more important than precise measurements.

Cores are important to many molds and, generally, have two parts: the core prints, which are used to locate and support the core(s); and the body portion, which actually contacts the molten metal and controls the shape of the casting cavity. Cores form the contours in a casting that is not molded. They can be very simple, like the inside of a short, straight tube, or they can be very complex, such as those used to define the inside water jacket surfaces of an internal combustion engine or the internal and external configurations of an air-cooled engine.

The patternmaker builds coreboxes to create a casting cavity of desired form and size, control casting wall thickness, reduce casting weight, eliminate or reduce machining and handle hot spots and draft problems. In sum, the pattern defines the spaces for the molten metal to go; the core restricts where the metal is allowed to flow. The pattern invites the metal, the core bars it. The patternmaker's skills facilitate the most economical way to make both work to the metalcaster's ultimate advantage.