NET-SHAPE metal parts in one shot

Senior Editor

Investment casting melds the benefits of several manufacturing technologies, including mechanical fasteners, machining, forging, powder metallurgy, roll forming, and welding. Parts frequently are cast net shape and need little or no finishing steps. For example, a component comprised of multiple parts that are shaped in an NC mill and subsequently joined by welding or threaded fasteners, can instead be investment cast as a single piece. This significantly reduces wasted material, especially important when making parts from expensive alloys.

Investment or lost-wax casting works like this: Wax patterns of parts to be cast attach at the part gate(s) to a wax runner. The assembly is then repeatedly dipped (invested) in ceramic slurry and sand to form a thick shell. An autoclave melts away the wax, leaving behind the shell. A kiln fires the hollow shell mold and molten metal is poured into a ceramic cup molded into the runner. When cool, a mechanical vibrator breaks off the shell and a grinder removes the gates from the parts.

Investment casting is one of four basic casting methods; sand, die, and permanent mold are the others. Part accuracy, complexity,aesthetics, and production volume dictate which method makes sense.

Sand casting can economically make just one part at a time, in some cases. Patterns may cost as little as $1,000. Sand casting holds tolerances of about ±0.0625 in./in. and gives a surface finish on the order of 250 to 500- in. rms. Parts made by the process tend to have simpler geometries because it's difficult to pack sand around complex patterns. Die and permanentmold casting, in contrast, produce highly accurate and complex parts, but need large volumes to justify high tooling costs.

Investment casting can as well turn out highly accurate, intricate parts. Molds similar to those used for plastic make the wax patterns. However, unlike production plastic-injection molds that are constructed of tool steel, wax molds are typically aluminum. The special wax may contain a small amount of polymer to boost structural rigidity, but it is not abrasive as are certain plastics. Injection pressures are also much lower than for plastics so aluminum molds can last millions of shots. Molds that contain slides for making more intricate wax patterns may need periodic refurbishing, however. Manual molds make sense for low volumes of, say, 10 parts annually, and for parts with highly complex geometries. Runs are typically 5,000 to 10,000 parts annually, and individual order quantities, 500 to 1,000 parts.

Molds, depending on complexity, cost about $5,000 and take roughly a year to amortize. "In terms of volume production, investment casting is midway between die and sand casting, and can be a stepping stone to die casting," says David Klie, a spokesperson with Avalon Precision Casting Co., Brook Park, Ohio.

TOLERANCES
As with any manufacturing process, the cost of investment casting scales with dimensional tolerances of parts. Foundries control most process variables that affect tolerances and shrink rates. These include wax and mold temperature, the pressure at which wax is injected, firing temperature, shell composition, and cooling rates. Tolerances of investment-cast parts are on the order of ±0.005 in./in., and surfaces finishes, 125 in. maximum. Shrink rates are typically about 2%, which includes both the wax and cast metal. Proper gate design minimizes centerline shrinkage. Gates go from thick to thin, so metal flows toward cooler areas. The thick portion of the gate stays liquid longest and forms a pressure head to completely fill the cavity. Otherwise, incomplete fills, voids, and porosity are possible.

Volumetric shrinkage of wax and metal during cooling mostly determine the degree of flatness in an investment casting. Shrinkage at the part center is termed "dish" shrinkage, drip, or outofflat. Dish is controllable with special techniques but cannot be eliminated. General flatness tolerances aren't quoted because they vary with part configuration and alloy. In any case, customers should specify the method by which flatness will be measured. A simple surface plate and feeler gage may suffice for normal tolerances, while full layout with equalization and dial indicators may be employed to measure premium tolerances.

Straightness can be an issue with certain casting shapes. A relatively thin, short part may bend, while a long, heavy part

may not. Engineers experienced with investment casting usually can predict that a particular design will bend, but not to what extent. As a rough estimate, axial bow is about 0.005 in./in. for constant cross sections. Ribs and gussets inhibit warpage, but also make it harder to mechanically straighten warped parts.

DESIGN TIPS
Engineers who design investment castings can do their part to boost product quality and consistency which, in turn, lowers costs and scrap rates. For best results, Avalon engineers suggest following these basic guidelines:

• Make sections uniform thickness and avoid thin sections. When casting steel, keep walls at least 0.080-in. thick. For aluminum, that number is 0.060 in. Aluminum has better flow qualities than steel.
• Avoid sharp corners and keep external and internal radii at least 0.020 to 0.030 in.
• Eliminate back locks that can hang up wax patterns in molds. Draft angles aren't needed in most cases.
• Design cores into wax molds, if possible. Else, you'll need two die — one to make the core from soluble wax or ceramic, and the other for the part pattern. Cores made this way can double piece price. The upside of soluble cores: They permit back-lock features that are not possible with wax cores.
• Cast, blind holes should be no deeper than twice their diameter. It's easier to hold tolerance on circular-shaped hole patterns than wide-ranging patterns. It's typically more economical to drill holes than cast them.
• Have design, production, and casting engineers meet at the onset of a new project. Collaboratingin this way may determine that it's possible to relax certain part tolerances, for example.
  
  Steve Santa, co-owner of Savanna Tool & Mfg., North Ridgeville, Ohio, agrees such collaboration is the best approach. Savanna has extensive experience with the finish machining of investment castings.
  
  "Companies these days want turnkey parts from the caster, so it's important for the caster to have a good working relationship with the shop doing the secondary machining," says Santa. " Ideally, engineers who design castings will have had prior machineshop experience. Those who don't may neglect to include features that simplify the machining process." Here are some design tips that help streamline secondary machining operations:
• Parts with irregular surfaces tend to be difficult to index and fixture in a machine tool. Add locating and holding tabs or flats to parts whenever possible, especially for large production runs. Locating tabs act as a datum surface. Otherwise, the machine each cycle must use a touch probe that locates a part relative to the machine spindle, a time-consuming step. Machinists may also have to "tram in" each part, which also adds cycle time and costs.
• Each time a part is fixtured for a machining operation, removed, and held again for another, adds cost. Multiple setups can also hurt accuracy and repeatability. If possible, design parts such that all machining operations can take place in one fixture, in a single setup.
• It's easier to leave edges sharp when building wax-mold tooling. But parts made from the molds are typically tumbled and abrasive blasted after casting, which peens sharp edges and raises burrs. Have the toolmaker put at least a 0.020-in. radius on both the outside and inside edges of parts. Parts look better, need less deburring, and are less likely to cut workers handling them.

MAKE CONTACT
Avalon Precision Casting Co.,
www.avalon-castings.com