Simple Solutions That Work! Issue 17

26 ADVANTAGES AND DISADVANTAGES Because of the various factors that must be considered when choosing a casting process the decision to use tilt pour permanent molding should be based on thorough engineering and production cost studies. Properly engineered well-made tilt pour castings will have the following advantages: • Dimensional accuracy is superior to sand or shell mold castings, because the mold is rigid and does not allow mold wall movement during solidification. Additionally, dimensional repeatability is improved. The reduction in casting variations allows a reduction in machining allowances, which will lower the downstream cost. • Ferrous and nonferrous inserts can be accurately cast in place. Typical insert materials can be iron, steel, stainless steel, or copper base alloys. In some cases, threaded inserts can be cast in place, eliminating the need for machining and related costs. • Because permanent mold castings are chilled castings, they are generally sounder than sand castings. Permanent mold castings are generally stronger than sand or die castings and are less porous than die castings. Castings produced in the permanent mold process have finer dendritic arm spacing (DAS) and grain structure. The finer structure displays better strength properties than those cast in similar alloys in sand castings. Permanent mold castings have fewer inclusion defects than sand castings. Therefore, the casting designer has the freedom to use thinner sections and lighter weight designs. Permanent mold castings have a higher degree of reliability with regard to pressure applications of fluids and gases. • The tilt pour process allows the molten metal to flow to the bottom of the mold, forcing the air out the top. As the molten aluminum flows through the runner a static skin of aluminum oxide forms which allows clean metal to enter the mold cavity. • Automatic pour machines eliminate many of the variables found in hand pouring. • Generally, permanent mold castings require less finishing than sand castings. Permanent mold castings have a smoother as cast surface finish than sand casting and finishes approaching 100rms can be achieved. In many cases casting buyers can use as cast surfaces without subsequent finishing for cooking utensils, hardware items, automotive parts, and ornamental work. • Studs, nuts, bushings, pipes and other inserts may be cast as integral parts of the casting. The inserts must be held in positive position in the mold to prevent movement during casting process. The inserts should be scored, knurled, or under cut to provide a locking surface. There are limitations to the size of castings produced in the permanent mold process. Most castings weight less than twenty pounds, however castings as large as 350 pounds have been poured. The casting design may be so complex that it is not practical in permanent mold processes. Contact: JOHN HALL [email protected]