x CASTING Copyright Goodheart-Willcox Co., Inc. Chapter 12 Sand Casting 211 Vacuum Molding Process 1. The pattern with vent holes is placed on a hollow carrier plate. 2. A thin plastic film, which has been softened by heat- ing, is draped over the pattern. 3. A vacuum pulls air through the carrying plate and through the vent holes in the pattern, drawing the plastic film around the pattern. See Figure 12-31A. 4. The flask is placed over the film-coated pattern. The flask walls also form a vacuum chamber. 5. The flask is filled with riddled sand, leveled and smoothed. 6. A sprue hole is formed in the sand. 7. The back of the mold is covered with an unheated plastic film. 8. The vacuum in the flask wall is drawn, and pressure hardens the sand. See Figure 12-31B. 9. The vacuum is released on the carrier plate and the pattern is removed, leaving a cavity in the mold. See Figure 12-31C. 10. The second half of the mold is formed using the same process. 11. The cope and drag are assembled forming a plastic lined cavity. The sections are kept under vacuum during pouring. 12. After cooling, the vacuum is released and the free- flowing sand drops away from the casting. The sand is cooled for reuse. Vacuum molding offers many advantages over conventional sand casting because the casting has a smooth surface finish and patterns with zero draft can be used. Some machining operations are also elimi- nated. In addition, the sand can be recovered with less work because no binders are used. GREEN METALWORKING Recycling Sand Sand is one of the primary materials used in large quan- tities for sand casting processes. Most foundries recon- dition and reuse their sand as much as possible. This helps reduce their costs and produces less waste. At some point, the sand can no longer be used and must be discarded. The sand can be screened to remove unwanted debris before sending it off for reuse. Screened discarded foundry sand can be used in other industries and applications. A good example of this is construction applications where discarded foundry sand is used for support layers beneath roads, concrete slabs, and other foundations. The discarded sand can also be used to make construction products such as grout, mortar, and precast concrete. Goodheart-Willcox Publisher Figure 12-31. The vacuum molding process. The sand used contains no water or organic binder and is kept under a vacuum during the mold making and casting cycles. A Vent holes in pattern Pattern Plastic film pulled tight to pattern by vacuum Carrier plate Vacuum Vacuum Vacuum B C Plastic film pulled tight by vacuum Flask wall Sprue hole Finished mold half held by vacuum Vacuum released, pattern and carrier plate removed Illustrations have been designed to clearly and simply communicate the specific topic. Photographic images have been updated to show the latest equipment. Modern Metalworking Copyright Goodheart-Willcox Co., Inc. 14.2.3 Finishing Some parts can be used as they come from the furnace. However, shrinkage and distortion caused by the heating operation often require pieces to go through a sizing or coining operation, Figure 14-6. Coiningg, or g sizingg, is used g to restore or improve the finished dimensions of the piece. 14.2.2 Sintering To transform the briquette into a strong, useful unit, it must be sintered at temperatures just below its melting point. Sintering is the heating of metals to high temperatures, g ranging between 70% and 90% of the melting point of the metals. During sintering, the powdered metal particles become bonded. Sintering is done in a sintering furnace with a carefully controlled atmosphere. See Figure 14-5. GKN Sinter Metals Figure 14-5. Briquettes are sintered in a controlled atmosphere furnace. Goodheart-Willcox Publisher Figure 14-6. Finished pieces that emerge from the sintering furnace may require additional operations to make them usable. Coining restores or improves the finished dimensions of the piece. Loose powder Double-Punch Die Single-Punch Die The Library of Manufacturing Figure 14-4. The metal powder is compacted in a die by a double or single punch. 148 Modern Metalworking Copyright Goodheart-Willcox Co., Inc. A press fit, or force fit, describes two parts that t are held together by friction, such as a pin or a bush- ing forced into a slightly undersized hole, Figure 8-44. The greater the difference in size between the diam- eters of the two mating parts, the greater the friction and the tighter the fit. A tighter fit requires more pressure to assemble, and will place greater stress on the two components. The components need to be machined very accu- rately. The difference between the pin and the hole is usually limited to only a few thousandths of an inch. Bearings, bushings, gears, and pulleys are all press fitted together, Figure 8-45. Goodheart-Willcox Publisher Figure 8-44. Hardened steel bushings are press fitted into a drill jig to guide the drill as it machines a hole in the part. Holes drilled using jig Template T Base plate Pin Drill bushing (2) Work Vladnik/Shutterstock.com Figure 8-45. Rolling-element bearings are press-fitted into assemblies to reduce rotational friction. Ram am A B C Tabble Single lever Frame e Handwh he el Slotted t table plate Ratchet lever Compound lever Dake Corporation Figure 8-43. Examples of three different types of arbor presses. A—A single lever arbor press. B—A ratchet lever arbor press with a handwheel, slotted table plate, and counter-weighted lever. C—A compound leverage arbor press can be used for heavier press work that requires greater pressure. Green Metalworking notes highlight key items related to sustainability, energy efficiency, and environmental issues.