cheek

In sand casting, a cheek is the central section of a three-part flask, sandwiched between the cope (top) and drag (bottom). It slides laterally on pins or bearings, allowing the pattern to be withdrawn from the mould cavity without damaging the sand impressions left in the cope and drag. The cheek carries part of the mould cavity itself, typically the widest section of the pattern, and its horizontal movement creates an undercut that would otherwise trap the pattern.

Flasks with cheeks are essential for patterns with pronounced horizontal projections or non-vertical external features. A simple pattern with a vertical parting line needs only a two-part flask; a pattern with side pockets, ribs, or flanges extending perpendicular to the parting plane requires the cheek's lateral withdrawal to release it cleanly. Once the cheek is withdrawn, the pattern can be lifted vertically from the drag without risk of sand collapse or pattern breakage.

The cheek rides on guide pins or rails that are fixed to the cope and drag frames. These must be precisely aligned and kept free of sand and debris, or binding will occur during the critical withdrawal stroke. Some foundries use a separate cheek box that sits between the cope and drag during moulding, while others integrate the cheek as a structural part of the flask itself. Pressure and vibration from the ramming process can cause creep, so cheeks are often secured with pins or wedges during moulding.

Wear on cheek surfaces is common and can be accelerated by abrasive sand or inadequate lubrication of the sliding surfaces. Scoring and sticking will force longer withdrawal times and increase the risk of pattern breakage or mould damage. The cheek must be extracted in the correct sequence: typically the pattern is withdrawn horizontally with the cheek, the cheek is then slid away on its pins, and finally the pattern is lifted clear.

The term 'cheek' likely derives from the observation that the section bulges outward from the main body of the flask, much like a cheek on a face. In modern automated casting, three-part flasks with cheeks remain common in jobbing foundries and small-batch work, though high-volume production often uses patterns designed for two-part flasks or expendable moulds to reduce handling time and tooling complexity.