Machined 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, shape complexity becomes an expense only at the tool stage, which 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.

The 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 gets introduced. Therefore, gates should be placed in the largest sections of parts.

Using investment casting, a shaft support bracket that was once made using welding and several drilling operations is now made in a single operation. The change saves over $100,000/year for the company.

As molten metal freezes, it shrinks. How much it shrinks and what this does to part shape is a property of the metal and its 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 good 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.

Designing parts in a thickest-to-thinnest arrangement machined investment casting is not always practical. It may be necessary to place tubes or chambers with relatively thin walls between thicker end sections, like flanges or mounts. In these cases, follow similar aspect-ratio guidelines that apply to the overall part:

  1. Keep the mass or dimensional ratios between adjoining sections to 4:1 or less where possible.
  2. Avoid abrupt changes in section thickness.
  3. Make gradual transitions between them.
  4. Where thin walls are a must, consider adding short ribs to support the walls.
  5. Add fillet radii 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. It 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 the shells can support as metal is poured. Parts weighing 150 lb or more can be produced, but it’s advisable not to exceed 100 lb. The handling becomes an issue on the lower end, and the minimum weight of investment castings is typically 1 oz.
  • Wall-thickness limits are governed by aspect ratio machined investment casting limitations, i.e., thick walls are easier to pour than thin walls. An absolute minimum limit of 0.030 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 Parts

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, many designers mistakenly believe that combining 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, cleats, 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 with machined investment castings, 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 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 they do a single task when they’re assembled. Try to replace them with a single piece that does the entire job.
  • Be prepared to change the overall shape or forming material. Machined 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 a good flow into all sections.
  • Design parts to maximize the number of units that fit on an investment-casting tree. Visualize the part as 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.

Conclusion

Machined investment casting offers several advantages over other production methods. But to make full use of the technology, designers must be aware of its limitations. Also, consider aspect ratios between sections of a part. Ideally, investment castings are constructed like a carrot, with the thinnest sections placed farthest from the gate. It is because thin sections freeze first. If a thin section lies between two thick sections and only one gate, the material flow will be interrupted, and shrinkage voids.

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Rosalind is a passionate guest blogger who writes on trending topics related to machined investment castings, metal forging techniques, defense casting etc.

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