Casting is the process of pouring liquid metal into a mold, where it cools and solidifies. The casting process can produce everything from art pieces to engine parts. The shape is determined by the mold cavity, but something needs to shape the mold – that’s where the pattern comes in.
Patterns are a model for the object to be cast. A pattern makes an impression on the mold, liquid metal is poured into the mold, and the metal solidifies in the shape of the original pattern.
Patternmaking isn’t as simple as the definition suggests. In addition to shaping the mold cavity, a pattern must (1) provide accurate dimensions, (2) have means of exiting the mold cavity without breaking it, (3) compensate for solidification shrinkage and distortion, and (4) include a feeding system of gates and risers to deliver liquid metal into the mold. Any flaw can result in a failed casting.
Getting all those factors right requires precision and more than a few calculations. Each pattern is carefully designed and constructed.
Patterns include allowances
Patternmaking is the art of designing patterns. It is the first and most essential part of the casting process.
There is much more to patternmaking than making an exact replica of the shape you want to cast – the patternmaker must account for the mold type and casting metal characteristics. These allowances are built into the pattern:
DraftThe pattern needs to be removed from each mold it shapes without breaking or distorting it. The draft is a taper that facilitates pattern removal. The exact angle of the taper depends on the complexity of the pattern, the mold type, and surface type.
Shrinkage allowanceLike all materials, metal contracts as it cools. If the pattern were made in the exact dimensions specified for the end-product, the casting would be smaller than required. Shrinkage allowance compensates for the amount that a metal will shrink during cooling. The precise allowance depends on the metal being cast.
Distortion allowancePatterns may be intentionally distorted to compensate for expected cooling distortion.
Machining allowanceSome castings are finished by machining. The patterns for machine-finished castings intentionally include excess material to compensate for material that will be lost in the finishing stage.
All patterns need a gating system
Every pattern includes a gating system that delivers liquid metal to the mold cavity. The gating system also regulates the speed with which metal enters the mold – too fast, and the turbulent liquid metal can erode the mold; too slow, and it may cool before completely filling out the cavity.
The system includes several interconnected parts:
Pouring cupLiquid metal is poured directly into the pouring cup/basin. It helps separate slag from metal, reduces turbulence, and helps maintain the correct flow rate.
Sprue & runnerMetal flows from the pouring cup into the tapered sprue, then through the runner, which in turn feeds into the gates.
Gate(s)Metal flows through the gates to fill the mold cavity. Small gates are used for castings that solidify slowly, while larger gates are used for castings that solidify rapidly. The gates need to be placed carefully to promote directional solidification.
RiserThe riser is a reservoir that prevents shrinkage cavities. For a riser to work properly, it needs to cool more slowly than the casting.
Gated patterns incorporate the gating system into the main pattern body. Alternatively, the gating system can be added by hand cutting or with separate pattern pieces.
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There are different pattern types
Patterns come in many materials, including wood, metal, plastics, or wax. The pattern material is chosen based on the casting volume and process used. Wood and metal patterns are usually used with sand casting, while wax is rarely used for anything but investment casting.
A pattern is often a solid replica of the object to be cast, but that isn’t always the case. Match-plate patterns have the cope and drag (top and bottom) portions mounted on opposite sides of a plate. Match-plate patterns speed-up the molding process, making them most suitable for large scale production.