An unconventional approach to producing investment castings.

A new technology developed by MIT is being adapted to investment casting to reduce the turnaround time from design to shipment.

Direct Shell Production Casting (DSPC |TM~) is a recently developed casting process in which ceramic shells with integral cores are automatically fabricated directly from a computer-aided design (CAD) file of the desired part. DSPC produces a ceramic shell that is similar to the shell produced by investment casting, but eliminates the need for tooling, wax and shell dipping.

As with investment casting, DSPC has the advantages of producing near-net shape parts, with complex geometry in a wide choice of alloys with low material waste. Because the process is also highly flexible, design changes are easily incorporated into the part's geometry, eliminating the need to maintain patterns or dies. The flow chart in Fig. 1 compares the various steps in conventional investment casting with DSPC.

The enabling technology for new process is Three Dimensional Printing |TM~ (3DP), which was invented and developed at the Massachusetts Institute of Technology and has been licensed to the author's company for use in metal casting.

Initial applications for the new process are in low-volume manufacturing, such as prototyping complex metal parts, whether or not these parts are intended to be produced with investment casting. Production of one-of-a-kind parts, as well as tooling for plastics molding and diecasting are also gaining interest.

First-generation DSPC machines are in operation at United Technologies Pratt & Whitney, Johnson & Johnson and Sandia National Laboratories. The primary application for these machines is prototyping metal parts. Casting of injection molding tooling is also being explored.

The Process

The production of a cast metal part using DSPC involves several steps. First, a CAD model of the desired part is loaded into the shell design unit (SDU) shown in Fig. 2. Here, it is modified as required for casting. Modifications typically include scaling to compensate for shrinkage, the addition of filets, removal of machined features such as holes and so on.

Next, the number of cavities in each shell is specified and the gating system is designed on-screen. This is done by selecting from a library of basic sprues, runners and gates, joining these together, and modifying them as required.

The SDU then generates an electronic model of the shell, complete with cores, to the specified thickness. If desired, mold flow and solidification simulation can be run at this time and changes can be made to the shell design to correct casting problems if needed. An additional option available is calculating the weight of metal required for the pour.

The second step is the transfer of the electronic shell model over a network from the SDU to the shell production unit. The SPU converts the model into a solid, three-dimensional ceramic shell.

The actual layer formation process is shown in Fig. 3. A thin layer of fine alumina powder is spread over a piston. Next, a printhead moves over the layer, shooting tiny drops of colloidal silica onto the powder surface in a pattern which matches the cross section. Finally, the piston is lowered within the powder bin, making room for the next layer.

This cycle is repeated until all layers have been printed and the entire shell, which is imbedded in a block of loose powder, has been formed. The excess powder is then removed to yield a finished shell.

The process builds shells from materials very similar to those used in investment casting. Once the shell has been generated, the process is identical to investment casting: the shell is fired, then poured with metal. After cooling, the shell is knocked off. The cores are then leached out, the gating metal is cut off, and the part is finished and inspected. A rocker arm cast from a DSPC shell is shown in Fig. 4.

The System

Each turnkey system, expected to sell in the U.S. for about $300,000, will feature a 12x12x12 in. workspace volume and produce shell layers of 0.007 in. thickness and 0.005 in. resolution. The molding shells can include several cavities, allowing more than one copy of a part to be made at the same time. The shells may also contain integral ceramic cores, allowing hollow parts to be produced. The projected build rate will be 90 to 100 |in..sup.3~ per hour, and the total build time for the entire workspace volume will vary from nine to 20 hours, depending on the geometry of the shell design. The cost per molding shell is estimated to be between $250 and $2500.

Internal mold surface finish is still a concern for developers. As the machine's resolution improves, the final part finish will improve. Methods by which the inside mold surfaces could be smoothed or coated to achieve improved part surface quality are being tested.

For the foundry that uses DSPC, tooling (patterns and coreboxes) is eliminated, as are the costs for making, reworking, storing and maintaining them. For investment casting foundries, elimination of wax can improve accuracy and eliminate the disposal of wax and wax fumes. The wax assembly (or tree) can be completed on-screen and, once the shell design is complete, the electronic model of the shell is stored and repeat orders can be executed.

The capability to quickly modify the casting shape to accommodate customer design changes allows a foundry to meet the growing demands for the faster turnaround on metal parts.

In the near future, designs that have no geometrical constraints and do not require expensive tooling will become feasible. Design changes will be easily incorporated and drawing will no longer be needed. The design engineer will be able to send the CAD file via computer modem or network to the foundry, and within days, receive functional parts ready for assembly.

The process can also help meet just-in-time production schedules and concurrent engineering goals. Since no tooling or setup is required, it has the potential to produce parts at a cost that is almost independent of the quantity ordered, and to eliminate the distinction between prototype and production metal parts.

Furthermore, when no machining is required, the cost of parts produced with the process will be determined primarily by the cost of the metal, rather than geometrical complexity.