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Materials engineering Edited by Jean Hoffman A better way to cast aluminum Gravity casting aluminum in permanent molds beats conventional sand and pressure die casting. Arun Gupta Gupta Permold Corp. Pittsburgh, Pa. The mold for this 17-lb, 48-in.-diameter fan was reverse engineered from a sand casting. The fan used in an agricultural application is cast in one piece. The high oday, gravity-fed permanent metal molds can repeatability of the permanent-mold casting process produce near-net-shape parts from a variety of helps minimizes secondary machining operations and aluminum alloys. But it’s up to the designer to spin balancing processes. Tmakes sure it’s both possible and profitable to use permanent molds to produce the part. Knowing the limits of this casting process can help designers create Casting comparison parts that take full advantage of this proven manufac- turing process. Permanent mold The process Piece Permanent molds produce large numbers of dimen- I (variable n Tooling sionally repeatable parts using molds machined from $$ v cost) cast iron or steel. In contrast, investment and sand-cast e S (fixed s a cost) molds are destroyed during part removal and during die t m n D casting, molten metal is injected into dies under ex- e d i tremely high pressures. Consequently, dies must be de- n e signed to withstand these pressures which drastically t boosts cost compared to gravity-filled permanent molds. Permanent-mold castings can be made with uniform 1,000 100,000 nonporous microstructures, but these qualities are Quantity highly dependent on solidification rates and foundry tooling designs. Molds must be carefully designed with Sand casting involves temporary molds made from sprues, vents, and risers all working in tandem so the metal or wood patterns. Consequently, up-front metal completely fills the mold under smooth con- investment for tooling is low, but per-part prices are trolled flow. usually higher than permanent-mold castings. Design and placement of sprues and gates are criti- Conversely, pressure-die casting has shorter cycle cal to help ensure controlled laminar flow of the metal times which lower the per-part price, but tooling can into the mold and adequate feed to all sections of the cost up to 10 times that of permanent-mold tooling. casting. Laminar flow also minimizes the amount of gas entering the melt. Inordinate amounts of dissolved gas in the melt produce voids in castings. Risers act as reservoirs of metal to supply a con- Permanent mold designs stant flow to areas of a part that otherwise may be- come isolated. Thin sections freeze faster than thick Designers unfamiliar with permanent-mold processes ones. Thus, carefully placed risers are needed to con- should consult a casting house early in the design. This tinually feed the cavity as metal contracts during will help ensure that the molds will repeatably form cooling or else an area of the casting may not have parts to the correct tolerances. Here’s a few tips for de- enough metal to fill in behind the shrinking metal. signing permanent molds that will help designers get The void formed as a result of this phenomenon is a started: casting defect known as shrinkage porosity, a major Use uniform wall thickness. Parts of the mold with the nemesis in the casting world. Ideally, risers and smallest cross-sectional area tend to cool and solidify sprues solidify last, leading to what is referred to as first. Thick sections often act as reservoirs for molten “directional solidification.” metal, feeding material into thin sections as they solid- 67 MACHINE DESIGN OCTOBER 25, 2001 www.machinedesign.com Materials engineering ify and shrink. However, most parts have varying cross sections and thinner sections will freeze before thicker Cylinder cross section for sections. Feed paths should account for solidification aluminum alloy from thinnest to the thickest sections. Making the walls of the finished part all the same R thickness simplifies feed-path design. Progressive solid- 0 r Die casting ification is easier to maintain in designs with uniform cross sections. It will also make part microstructure and mechanical properties more consistent. Permanent mold Use the proper alloys. There are aluminum alloys tai- lored for permanent-mold casting including 319, 356, } A356, 413, and 535. In general, silicon (Si) is the most Ultimate tensile important alloying element for any aluminum casting Sand 10 to 20% process. Its high specific heat means it holds heat longer than aluminum. During solidification this results in a 0 r (distance out from center) R uniform freezing of the casting. Alloys also should have a short liquid-to-solid trans- Permanent-mold castings cool faster than sand formation (freeze) range which helps promote strong castings, giving them a much finer, more uniform mechanical properties. Phase diagrams illustrate the microstructure. This boosts mechanical properties liquid-to-solid transformation. Alloys that go from liq- by up to 20%. In comparison, pressure-die-cast uid to solid within about 50°C are best suited for per- parts have a much stronger skin, but weaker manent-mold casting and are commonly known as eu- interior sections. tectic alloys. Pay attention to part details. Use fillets instead of sharp corners. Differential shrinkage at sharp corners will result in persistent shrinkage porosity, which may appear during the machin- ing process. Ribs and gus- sets should be used in place Phase diagram Al versus Si of massive sections. Grad- ual blending of light into Aluminum alloy phase heavy sections is recom- 1,400 diagrams are quite mended. When possible, complicated, but for parts should be tapered for Liquid illustrative purposes, a easy part ejection — 2° ta- simple two-component per or draft is recom- 1,200 diagram illustrates the mended for most parts. And “freezing” or liquid-to- consider coring techniques solid transformation for complex shapes and C 1,000 ° range best suited for even undercuts. It can elim- castings. An alloy with inate secondary machining Liquid + Liquid + solid Si a composition of 88% operations. solid Al and 12% silicon (Si), Don’t forget the inserts. 800 Al for example, has a Forms of all shapes, sizes Temperature, Temperature, short freeze range — and materials are easily on the order of 20°C. molded directly into perma- 600 The 12% Si freezing nent mold castings. Brass range is the shortest on thread inserts, for example, the chart and is are more durable than ma- Al + eutectic 400 referred to as the chined aluminum thread. eutectic composition. Likewise, steel and stain- Si + eutectic For permanent-mold less-steel inserts can pro- casting, eutectic alloys vide the extra-hard surfaces 200 usually make ideal only where needed, keeping casting alloys. the rest of the part as light- Al 10 20 30 40 50 60 70 80 90 Si weight aluminum. I Composition, weight % Si 68 MACHINE DESIGN OCTOBER 25, 2001 www.machinedesign.com Materials engineering An example of mold tolerances A design guide helps uninitiated designers determine permanent- mold casting tolerances for dif- ferent sized parts. o visualize how design elements affect a casting’s tolerances, consider a simple enclosure lid. For simplicity, the wall thickness is a uniform T0.25 in. and the lid consists of a 10 ϫ 10-in. top with 2-in. walls. The casting’s parting line, where the mold splits, divides the casting in half. From the table, Designing for permanent-mold casting, minimal linear tolerance starts at ±0.015 in. for a dimension up to 1 in. in length. For larger dimensions, the tolerance increases by ±0.002 in. for each additional inch. This housing for a heavy- Permanent-mold casting Linear tolerances duty portable magnetic is said to easily mold this Because the 2-in. dimension of the lid is split by the drill weighs 7.16 lb and is complicated double helix, parting line, they can’t be kept to tolerances as tight as highly cored to reduce the 83-lb auger mechanism dimensions not bisected by a parting line. The addi- material removed during for a juicing machine. tional tolerance will be influenced by the projected area secondary machining. Secondary finishing gives of the parting face, in our case 10 ϫ 10 in. = 100 in.2 Con- the auger its bright luster. versely, tolerances on the lid top, which is not bisected by the parting line, can be tighter. additional 9 in. of length. Total tolerance for the 10-in. In our example the tolerance for the 10-in. dimension dimension is thus ±0.033 in. will be: Because the 2-in. dimension crosses the parting line, ±0.015 (for the first inch of length) + 9 (± 0.002) for the calculating the tolerance is slightly more in- volved, taking into account not only part length, but also projected parting-face surface A Sample lid area.