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New investment casting method combines CAD and CNC.

Direct Shell Production Casting (DSPC), a new manufacturing method for producing cast metal parts, combines the advantages of CNC machining and investment casting. The process is based on Three Dimensional Printing|TM~ (3DP), a technology developed at the Massachusetts Institute of Technology (MIT). DSPC reportedly allows users to produce ceramic molds for metal parts directly from computer-aided design (CAD) models created on a computer screen.

This technology promises to reduce the time and effort required to make metal parts through conventional investment casting methods. DSPC eliminates several steps from the conventional investment casting process, including manufacturing of core and wax tooling, core and wax molding, assembly of patterns to a gating system, shell dipping and dewaxing.

The DSPC machine translates a CAD visual representation directly into a physical object using a process similar to ink-jet printing. Objects made with 3DP are built up in thin layers. First, a layer of powder is spread by a roller mechanism. Then, a printhead incorporating one or more inkjet-type nozzles moves across the layer, depositing liquid binder in regions corresponding to the cross section of the part geometry. The binder penetrates the pores between the powder particles and causes them to adhere to one another.

Using ceramic materials and binders nearly identical to those used by investment casting foundries, this technique can directly fabricate a ceramic shell with integral cores. Virtually any molten metal can be poured into the shell to yield a functional part. Already, castings have been produced in such alloys as cobalt-chrome and Inconel.

The apparatus used to create shells for DSPC includes a bin containing powder. The bin is fitted with a piston that can be lowered in small increments by computer control. A small amount of powder is released from a hopper above the bin and dropped onto a shelf flush with the bin's rim. A roller then moves across the bin, spreading the powder into a thin layer.

Above the powder, an inkjet-type printhead supplied with liquid binder moves across the powder surface in a television-like raster pattern of parallel lines. The inkjet nozzle continuously sprays small drops of binder at the powder surface. However, when a voltage is applied to electrodes beneath the nozzle, the drops become charged and are deflected into a catcher before reaching the powder. The voltage on the electrodes is varied by computer to deposit binder only in specific locations corresponding to the layer cross section.

The process of spreading powder and printing is repeated for each cross section of the object, until the entire geometry has been constructed. Shells are then dried and kiln fired for increased strength. Finally, unbound powder is removed, usually by brushing and shaking.

Among the benefits claimed for DSPC are that it allows the designer to design parts with no geometry constraints, to receive functional parts made from any castable metal within days, and to pay in proportion to the weight of metal contained. Orders for DSPC machines have been received from United Technologies-Pratt & Whitney, Johnson & Johnson and Sandia National Laboratories.
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Title Annotation:direct shell production casting utilizes computer-aided design and computer numerical control
Author:Cohen, Adam L.
Publication:Modern Casting
Date:Sep 1, 1992
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