By J. Michael Simmons
Investment casting offers several advantages over other production methods. But to make full use of the technology, designers must be aware of it limitations. Here are several tips for getting the most out of investment casting.
SHAPES AND ASPECT RATIOS
Investment casting can probably produce a greater array of shapes than any other method of making parts. In many cases, shape complexity is almost not a factor in the cost of a part. If inserts such as ceramic cores or soluble waxes are avoided, then shape complexity becomes an expense only at the tool stage, and this is a one-time cost in the process. Many internal passages, through holes, concentric cylinders, and curved and tapered holes are as simple to cast as external forms such as bosses, flanges, or shoulders. And no draft angle has to be factored in because wax shrinkage makes it easy to remove injection dies.
Part shape does affect how parts solidify or "freeze." Ideally, freezing begins at the part's extremities and proceeds backward toward the gate where liquid metal is introduced. Therefore, gates should be placed in the largest sections of parts.
As molten metal freezes, it shrinks. How much it shrinks and what this does to part shape is a property of the metal and the part's design. Obviously, the longer a dimension, the greater the shrinkage in that direction. Shrinkage makes it difficult to control dimensions in parts long in one direction and short (or thin) in the transverse direction. The dimensions of stocky shaped parts are easier to control than those of long, slender parts. It is a good idea to keep overall size ratios (either mass or dimensional) less than 4:1. Secondary sizing and straightening operations can improve parts that deform due to solidification.
Also consider aspect ratios between sections of a part. Ideally, investment castings are constructed like a carrot, with thinnest sections placed farthest from the gate. This is because thin sections freeze first. If a thin section lies between two thick sections and there is only one gate, then material flow will be interrupted and create shrinkage voids.
Obviously, designing parts in a thickest-to-thinnest arrangement is not always practical. It may be necessary to place tubes or chambers with relatively thin walls between thicker end sections, such as flanges or mounts. In these cases, follow similar aspect-ratio guidelines that apply to the overall part: Keep the mass or dimensional ratios between adjoining sections to 4:1 or less where possible. Avoid abrupt changes in section thickness. Make gradual transitions between them. Where thin walls are a must, consider adding short ribs to support the walls. Also, add fillet radii where possible while avoiding sharp edges that can lead to tears and crack as parts solidify.
A few software tools work with current CAD packages to analyze metal flow. They can help designers minimize solidification problems. The software predicts where shrinkage will exceed design limits and lets engineers add or move gates to prevent problems. This can be done before running costly, time-consuming prototype tests.
SIZE AND TOLERANCE LIMITS
Perhaps the only upper-size limit on investment castings concerns the weight that can be supported by the shells as metal is poured. Parts weighing 150 lb or more can be produced, but it's advisable not to exceed 100 lb. On the lower end, handling becomes an issue and the minimum weight of investment castings is typically 1 oz.
Wall-thickness limits are governed by aspect ratio limitations, i.e., thick walls are easier to pour than thin walls. An absolute minimum limit of 0.030 in. is possible, with 0.094 (3/32) in. being common. Overall external dimensions typically stay within a 30-in. cube. Special wax injection and shell equipment may be needed for larger components.
Combining several parts into a single unit and producing it in one forming operation saves hours of assembly and finishing in the automotive, aircraft, and appliance industries. However, a lot of designers mistakenly believe that combinant thinking should focus on plastic parts, such as headlight nacelles and power-tool housings.
The truth is that investment casting offers the same capability in metal. Tubes, bosses, fins, cams, flywheels, gear racks, handles, bowls, chambers, detents, hooks, cletes, snaps, and housings can all be combined in a single unit in one operation. A jet-engine maker, for example, rolled 88 metal pieces into a single compressor part. And the more pieces that are combined, the greater the manufacturing savings.
Combining parts also boosts an item's utility. A good example involves a food-dispensing system. Designers combined four parts of a plunger return system into a single device. Now, instead of welding, riveting, or threaded fasteners. Sheet-metal items, stamped parts, and machined parts that are subsequently joined by welding or mechanical processes are good candidates for assembly-by-investment casting. Redesigning parts as a single item eliminates fixturing, assembly operations, straightening, and scrap.
- Reconsider the functions of individual parts. A fitting, tube, indexer, collar, brace, and mounting bracket serve individual functions, but when they're assembled, they do a single task. Try to replace them with a single piece that does the entire job.
- Be prepared to change the over-all shape or forming material. Investment casting may require a fillet or rounded corner to replace sharp angles. (Sharp angles may tear the cast metal as it cools.) If possible, keep the thickness of various sections constant. If this is not possible, make gradual transitions from one section to another. Avoid having many sections meet at one point, and stagger meeting points to avoid large mass points. Also, thin walls may need ribs or shoulders for the required strength. Finally, if the overall shape of the design is complex, you may have to use an alloy that is more easily cast to get good flow into all sections.
- Design parts to maximize the number of units that fit on an investment-casting tree. Visualize the part as being one of a cluster of parts on a casting tree. The casting process becomes more efficient and less costly as the number of parts attached to a single tree increases and the number of gates per part decreases. These are the most important factors in the final per-unit cost for investment casting.
- Investment-casting engineers should work concurrently with product engineers in the earliest stages of design to minimize casting problems and reduce overall costs.
INVESTMENT CASTING PRIMER
Chinese and Egyptian artisans used investment casting as early as 4000 BC to produce intricate sculptures and figures. Over the centuries, this "lost wax" concept of casting has retained its reputation for turning out finely detailed parts. World War II brought a resurgence of interest in the process, with the need for intricate and precisely shaped aircraft components. Today, the productivity of investment casting has been improved so much with digital process controls and robotics that is more often associated with precision processes such as CNC machining and injection molding than with conventional casting. And quality continues to improve. The technology can produce as-cast parts with normal tolerances of ±0.005 in./in. and surface finishes of 125 in., rms. Premium tolerances can be held even closer. The concept of "near-net" shapes has become the mantra of the industry.
Investment casting also lets engineers combine multiple metal shapes into one piece in a single operation. This reduces manufacturing costs by eliminating fixturing, machining, finishing, assembly, and scrap.
Investing casting begins with machining a negative-image tool (die) of the part. This is done by reverse engineering the tool from an existing part or model, machining it from design drawings, or using CAD files. The tool is placed in a wax-injection machine to form wax patterns, one for each part being manufactured. (Tools are normally made from aluminum and the only material injected into it is wax, so they tend to last for many years.) Patterns are mounted on wax "trees" in a way that enhances pouring and controlled solidification. Clusters of trees are dipped into a ceramic slurry then coated with sand to build up a rigid shell around the patterns. The shells are dried, transferred to a boiler clave, and the wax is melted out (or "lost"), leaving behind the shells and wax residue. Firing the shells removes any residue. Shells are preheated to about 1,800°F and filled with molten metal. Once clusters cool, individual parts are cut off the tree.
Almost any metal that can be melted and air poured can be investment cast. However, most parts are cast from moderately to expensive metals. The most commonly used metals are austenitic and martensitic stainless steel, Hastalloy and other nickel-based steels, tool steel, cobalt-based alloys, brass, bronze, and copper.
Not all metals cast the same, however. Flow and freezing characteristics vary, and what can be produced in a 300-Series stainless steel cannot easily be produced in low-carbon steel. Thus, sometimes a part's complexity determines if an alloy can be economically cast.
Typically, investment casting's per-unit cost lies between that of conventional forming (casting, machining, and welding) and automated forming (die casting and CNC machining). Low-volume parts are typically cast, machined, or welded; high-volume items are die cast or CNC machined.
Investment casting is economically suited for small to intermediatesized lots (100 to 10,000 units or more). This is because investment casting uses material efficiently, a benefit when raw stock is expensive. Scrap rates run in the range of 3 to 5%. Tooling costs also tend to be low, particularly when compared to fixturing needed for welding or molds needed for die casting.