Equipment for the direct molding of the mold cavity in a casting mold. The pattern is a model of the casting to be made with the accurate dimensions and contours.
However, what needs to be taken into account is the fact that the shrinkage of the relevant casting material due to solidification and cooling to room temperature must be compensated by a corresponding expansion of the dimensions (shrinkage allowance). In addition, the subsequent machining of the casting surfaces must be considered when making the pattern and the so-called machining allowance must be added. If cores are needed in the further course of casting production, the respective core prints must also be added to the pattern.
Depending on the number of castings produced from them (single castings, small batches, large batches), patterns are made of wood (see Wooden pattern grade), gypsum, plastics or foams and metal. Lost-wax patterns containing wax to be melted or burned out are used for precision casting processes and lost-foam patterns containing rigid form to be evaporated are used for the full-mold casting process.
Patterns for sand casting are divided into parted and one-piece patterns. A parted pattern consists of two halves whose parting surfaces correspond to the parting plane during molding. If these pattern halves are secured on a pattern plate and designed with the riser and gating system, they can be used for machine molding. In this case, the entire pattern plate is mounted onto the table of the molding machine. The production of a complex casting mold requires at least two mold parts, cope and drag, which are either made with one double-sided pattern plate at only one molding machine or with a one-sided pattern plate at two separate molding machines. With large hand-molded castings, multiple mold partings may be necessary, e.g. two or three partings using three or four mold boxes for a single casting mold.
Large patterns may be designed to be able to be broken down into multiple pieces which considerably facilitates the demolding and lifting of the patterns from the sand mold.