Patterns are often mistakenly referred to as molds. Patterns are required to make the expendable sand molds. Sand from the high-speed sand mixer is compacted around the pattern in a mold box. Once the sand is cured the pattern is removed from the mold to be used again. While a mold can be used only once to make a casting, a pattern can be used multiple times to create molds. Depending on the type of pattern, it can be used to form as few as one mold, or as many as several thousand molds. Patterns can be made from foam, wax, plastic, wood, or metal depending on the number of molds required. If only one simple casting is needed, then a foam pattern may be a good choice, but a high production pattern typically is made from metal. Due to being a short-run production facility, Clinch River Casting typically uses wooden and/or plastic patterns to create our molds.
The first step in producing a new casting is to build a pattern. Clinch River Casting offers new pattern services as well as repair, rigging and gating. Customers typically will have a pattern for an existing casting. CRC’s molding process allows us the flexibility to run several different types of patterns, including loose patterns, without major modification. Depending on the number of castings required, we can remount and gate existing pattern equipment to optimize it for our molding process.
Building a new pattern is not a trivial process and requires a skilled patternmaker. A pattern basically looks like the object to be cast, but the pattern maker has to account for draft, pattern shrink, and machine stock.
Common Pattern Terms
Pattern – A form, generally wood or metal, around which sand is compacted to make a mold for casting. Because patterns are used repeatedly to make molds, the strength and durability of the material selected is directly related to the number of molds that will be produced. Wood patterns are easily repaired and a work well for air set molding.
Core Box – Wood, plastic or metal box containing a shaped cavity in to which sand is packed. Basically, a core box is used to make a core. Core boxes are used similarly to a pattern.
Core Print – Projections attached to a pattern to form recesses in the mold at points where cores are to be supported.
Draft – The taper on the sides of a pattern that are perpendicular to the parting plane, which allows the pattern to be withdrawn from the mold without damage.
Pattern Shrink – Contraction allowance made on patterns to compensate for the decrease in dimensions as a casting cools in the mold. Gray Iron and Ductile iron patterns are usually made with 1/10th of an inch per foot shrink rule. As, white irons or Ni-Hard patterns are made with a 1/4 of an inch per foot shrink rule.
Gating System – The assembly of sprues, runners, gates and risers in a mold through which metal flows to enter the casting cavity. All molds must have a gating system. The system can be a designed dedicated system mounted on the pattern board, or hand cut into the mold in the case of a loose pattern. The design of the gating system controls the flow of metal into the cavity to reduce casting defects.
Sprue – The opening in the mold that molten metal is poured.
Runner – The channels in the system that connect the sprue with the ingate or ingates of the casting.
Ingate – The channel where molten metal enters the casting cavity.
Riser – A reservoir of metal that supplies feed metal to the casting as it solidifies to prevent shrink defects in the casting. Because iron shrinks as it cools additional iron has to added to the casting cavity to maintain the volume of iron in the cavity. For a riser to work properly the riser has to remain molten longer than the casting.