Machinery's Handbook, 31st Edition
SAND CASTING
1499
Fig. 7. Cope-and-Drag Pattern Cope-and-Drag Patterns: These are similar to match-plate patterns. Except each half of the split pattern is assembled with separate plates, so separate patterns, and possibly separate machines, can be used to make the mold halves. Cope-and-drag patterns (Fig. 7) include a gating and riser system. Cores.—Many cast parts have interior holes (hollow parts) or other cavities in their shape that are not directly accessible from either piece of the mold. Such interior surfaces are formed using inserts called cores , which are placed in the mold cavity before casting and removed from the finished part during shakeout and further processing. Like molds, cores must possess strength, permeability, collapsibility, and ability to withstand heat. Hence, cores are made of sand aggregates. Cores are anchored and supported by core prints , recesses added to the pattern that also provide vents for gases to escape. To keep cores from shifting, metal supports ( chaplets ) may be used to anchor the cores. On pouring and solidification, chaplets are integrated into the casting. The portions of the chaplets protruding from the castings are cut off. Fig. 8 illustrates how a core is held in the mold cavity with and without chaplets.
Fig. 8. Core Held in Place in the Mold Cavity: a) Core Held in with Chaplets; b) Core Held without Chaplets Sand Molding Techniques.—These are the steps involved in the traditional technique of making a simple sand mold: 1) The drag flask is placed on the board. 2) Dry facing sand is sprinkled over the board to provide a nonstick layer. 3) The drag half of the pattern is located on the mold board. 4) Molding sand is poured in to cover the pattern with the fingers; the drag is filled completely. 5) Sand is tightly packed in the drag using hand rammers , peen hammers (used first close to the drag pattern), and butt hammers (used for surface ramming). 6) Ramming must be proper (neither too hard nor too soft). Too soft ramming will generate a weak mold and poor imprint of the pattern. Too hard ramming will not allow gases and air to escape, creating bubbles in castings, resulting in defects called blows . Also, the making of runners and gates will be difficult.
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