Machine Design

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.

Edited by Jean Hoffman

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 repeatability of the permanent-mold casting process helps minimizes secondary machining operations and spin balancing processes.

Sand casting involves temporary molds made from metal or wood patterns. Consequently, up-front investment for tooling is low, but per-part prices are usually higher than permanent-mold castings. Conversely, pressure-die casting has shorter cycle times which lower the per-part price, but tooling can cost up to 10 times that of permanent-mold tooling.

Permanent-mold castings cool faster than sand castings, giving them a much finer, more uniform microstructure. This boosts mechanical properties by up to 20%. In comparison, pressure-die-cast parts have a much stronger skin, but weaker interior sections.

Aluminum alloy phase diagrams are quite complicated, but for illustrative purposes, a simple two-component diagram illustrates the freezing or liquid-tosolid transformation range best suited for castings. An alloy with a composition of 88% Al and 12% silicon (Si), for example, has a short freeze range — on the order of 20°C. The 12% Si freezing range is the shortest on the chart and is referred to as the eutectic composition. For permanent-mold casting, eutectic alloys usually make ideal casting alloys.

Today, gravity-fed permanent metal molds can produce near-net-shape parts from a variety of aluminum alloys. But it's up to the designer to makes 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 parts that take full advantage of this proven manufacturing process.

The process
Permanent molds produce large numbers of dimensionally repeatable parts using molds machined from cast iron or steel. In contrast, investment and sand-cast molds are destroyed during part removal and during die casting, molten metal is injected into dies under extremely high pressures. Consequently, dies must be designed to withstand these pressures which drastically boosts cost compared to gravity-filled permanent molds.

Permanent-mold castings can be made with uniform nonporous microstructures, but these qualities are highly dependent on solidification rates and foundry tooling designs. Molds must be carefully designed with sprues, vents, and risers all working in tandem so the metal completely fills the mold under smooth controlled flow.

Design and placement of sprues and gates are critical to help ensure controlled laminar flow of the metal into the mold and adequate feed to all sections of the 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 constant flow to areas of a part that otherwise may become isolated. Thin sections freeze faster than thick ones. Thus, carefully placed risers are needed to continually feed the cavity as metal contracts during cooling or else an area of the casting may not have enough metal to fill in behind the shrinking metal. The void formed as a result of this phenomenon is a casting defect known as shrinkage porosity, a major nemesis in the casting world. Ideally, risers and sprues solidify last, leading to what is referred to as "directional solidification."

Permanent mold designs
Designers unfamiliar with permanent-mold processes should consult a casting house early in the design. This will help ensure that the molds will repeatably form parts to the correct tolerances. Here's a few tips for designing permanent molds that will help designers get started:

Use uniform wall thickness. Parts of the mold with the smallest cross-sectional area tend to cool and solidify first. Thick sections often act as reservoirs for molten metal, feeding material into thin sections as they solidify and shrink. However, most parts have varying cross sections and thinner sections will freeze before thicker sections. Feed paths should account for solidification from thinnest to the thickest sections.

Making the walls of the finished part all the same thickness simplifies feed-path design. Progressive solidification is easier to maintain in designs with uniform cross sections. It will also make part microstructure and mechanical properties more consistent.

Use the proper alloys. There are aluminum alloys tailored for permanent-mold casting including 319, 356, A356, 413, and 535. In general, silicon (Si) is the most important alloying element for any aluminum casting process. Its high specific heat means it holds heat longer than aluminum. During solidification this results in a uniform freezing of the casting.

Alloys also should have a short liquid-to-solid transformation (freeze) range which helps promote strong mechanical properties. Phase diagrams illustrate the liquid-to-solid transformation. Alloys that go from liquid to solid within about 50C are best suited for permanent-mold casting and are commonly known as eutectic 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 machining process. Ribs and gussets should be used in place of massive sections. Gradual blending of light into heavy sections is recommended. When possible, parts should be tapered for easy part ejection — 2° taper or draft is recommended for most parts. And consider coring techniques for complex shapes and even undercuts. It can eliminate secondary machining operations.

Don't forget the inserts. Forms of all shapes, sizes and materials are easily molded directly into permanent mold castings. Brass thread inserts, for example, are more durable than machined aluminum thread. Likewise, steel and stainless-steel inserts can provide the extra-hard surfaces only where needed, keeping the rest of the part as lightweight aluminum.

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