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Understanding Tooling, Patterns and Core Boxes for Green Sand Metal Casting

In order to produce castings utilizing an automated mold machine and green sand casting process, tooling must be created. This article provides an overview of the types of tooling currently in use.

Patterns

Skilled Pattern makers build or make patterns as per the design provided by an engineer or designer. The pattern could be made using wood, metal, or plastic such as expanded polystyrene. They can be hand cut from wood, molded from aluminum or CNC cut. Most modern patterns are CNC machined.

The pattern is slightly larger than the finished product to accommodate the “contraction” or “shrink allowance” of the metal to be cast and the product size. Depending on the size of each part to be cast, and the size of the Flask (sand surface available), one or dozens of impressions of each part will be on the pattern. In the instance where there are impressions of different parts on one pattern, that is commonly called a ‘family’ pattern.

Depending on the configuration of the part, patterns sometimes also have core prints that create registers within the molds into which the sand cores are placed. Such cores, sometimes reinforced by wires, are used to create under cut profiles and cavities that cannot be molded with the ‘cope and drag’ (top and bottom of mold).

Paths for the entrance of metal into the mold cavity constitute the runner system including the sprue, various feeders to maintain a good metal ‘feed’, and in-gates that attach the runner system to the casting cavity. Gas and steam generated during casting exit through the permeable sand or via risers that are added either in the pattern itself, or as separate pieces. 

Molding box and materials

A multi-part molding box (known as a casting flask, the top and bottom halves of which are known respectively as the cope and drag) is prepared to receive the pattern. Molding boxes are made in segments that may be latched to each other and to end closures. For a simple object, flat on one side, the lower portion of the box, closed at the bottom, is filled with casting or green sand. The sand is packed in through a vibratory process called ramming. The pattern is then placed on the sand and another molding box segment is added. Additional sand is rammed over and around the pattern. Finally a cover is placed on the box and it is turned and unlatched, so that the halves of the mold may be parted and the pattern with its sprue and vent patterns removed. 

Coreboxes

To produce cavities within the casting-such as for liquid cooling in engine blocks and cylinder heads – negative forms are used to produce cores. Usually sand-molded cores are inserted into the casting box after removal of the pattern.

Coreboxes are made using wood, metal, or plastic such as expanded polystyrene. They can be hand cut from wood, molded from aluminum or CNC cut. Most modern coreboxes are CNC machined. 

Chills

To control the solidification and metallurgical structure of the metal, it is possible to place metal plates or chills in the mold. The associated rapid local cooling will form a finer-grained structure and may form a somewhat harder metal at these locations. In ferrous castings, the effect is similar to quenching metals in forge work. The inner diameter of an engine cylinder is made hard by a chilling core. It is possible to prevent internal voids or porosity inside castings by controlling the way a casting freezes.

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