Direct shell sand rapid prototyping: from CAD to casting in days.

Creating functional cast prototypes from CAD data, this new application of rapid prototyping could open more doors for casting applications.

The current science of rapid prototyping (RP) began nearly 10 years ago when fragile, dimensionally inexact "touch and feel" models were produced to aid in the three-dimensional, mental conceptualization of drawings. These models were found to be useful in validating the part for form and fit. Today, the science of RP has advanced to the point where exceptionally accurate, durable parts can be fabricated with a variety of materials using a number of prototyping methods.

As RP methods improved, foundrymen received more durable models and, for some applications, innovative patternmakers had a new way to make a pattern or even a piece of tooling. Unfortunately, from the foundry standpoint, there was not a RP method that could produce a metal casting that emulated the production process in a sand foundry. To the operating foundryman, successful prototyping means making a prototype metal part directly from his CAD file with the metal and casting process to be used in the final production. For the foundryman to be competitive with other metalforming alternatives, a functional cast metal prototype must be quickly produced for testing.

Functional Prototyping

The two most accepted methods for producing functional prototypes of cast metal parts are CNC machining and a variation of investment casting in which various RP models are burned out of the ceramic shell prior to casting. Unfortunately, these relatively high cost manufacturing techniques do not provide a high production foundryman producing castings in sand with sample parts that have physical characteristics similar to those made in a resin-bonded sand casting process.

Another RP process that approximates sand casting is the Soligen Process. In this RP method, an inorganic, colloidal silica binder is sprayed onto aluminum oxide, treated with citric acid. The reaction during the layering/build process forms a gel mold, which is subsequently sintered in a high temperature oven to produce cores and molds. This process produces accurate, complex castings, but the use of an inorganic binder, which is uncommon in foundries, and the tendency of the mold to breakdown further during pouring limits its use.

But a new process - Direct Shell Sand Rapid Prototyping (DSS RP) - has emerged from the prototyping methods that produces metal castings available for accurate evaluation and testing.

DSS RP is a combination of three processes: computer aided casting design; selective laser sintering, which is a close tolerance laser sintering of a fusible powder-like material; and the shell mold process. Although each of these three processes are currently used by metalcasters, uniting them enables foundrymen to produce a prototype casting with physical and metallurgical properties virtually identical to a metal casting produced with sand cores and molds. In addition, DSS RP enables extremely complex prototype castings - manifolds, cylinder heads and even engine blocks - to be produced quickly and accurately for evaluation.

DSS RP

Introduced in Europe in 1995 under the title of Direct Croning Laser-Sintering Process, DSS RP is developed and marketed outside of the U.S. by Electro Optical Systems (EOS)-Zeiss Group, Munich, Germany. In the U.S., DSS RP technology is distributed by DTM, Corp., Austin, Texas under the trademarked name of "Sand Form."

The process begins with a CAD file being converted to a stereolithography (STL) file. The prototyping equipment utilizes the STL format. After the STL file transfer to the prototyping equipment is complete, the prototype mold and/or core is formed via selective laser sintering of the sand grains. DSS RP can make sand cores and/or molds with either silica or zircon sand that has been pre-coated with a B-staged phenolic/novolac resin. (A B-staged thermosetting plastic is one that has been partially cured or polymerized and requires additional polymerization through exposure to heat to reach the final cured stage, or the C-Stage.)

A thin layer of coated sand is applied over the entire surface of the build area within the prototype machine by a spreading device. After deposition, the sand, which will become part of the mold and/or core, is heated with a C[O.sub.2] laser beam [ILLUSTRATION FOR FIGURE 1 OMITTED]. As the coating melts on the outside surface of the sand it adheres to adjoining sand grains. This bonds the grains, one layer at a time, to the agglomerated shape being formed below. As subsequent layers are bonded together and harden, a specific shape is formed within the confines of the build volume. With the application of each successive layer, the build platform is lowered into the machine until the height of the build is completed. The total amount of material in the build volume is comprised of the specific bonded sand shape formed by the agglomerated grains, surrounded and supported by uncured, free flowing resin-coated shell sand.

Upon completion of the building process the build platform is raised, and the partially cured and uncured sand is removed from the machine. The cured sand mold and/or core is then separated from the loose sand by simply brushing and blowing it away from the cured sand part. Upon separation from the uncured sand, the partially cured mold and/or core is post-cured. If sections are to be attached together, this is done after post-curing. Then it is set up to be poured.

It should be noted that any loose, unbonded sand removed from the process is reusable and can be recycled back into the machine. Silica and zircon sand is currently the standard aggregate used in DSS RP. However, nearly any sand used in the "standard" shell process can be employed. Typically, silica sand from about 90-110 AFS/GFN and zircon sand from about 100-120AFS/GFN is employed.

Process Advantages

* DSS RP has virtually no limitations on part complexity;

* almost any geometry can be built within the dimensions of the machine's build volume;

* castings with dimensions larger than the build volume of the machine can be fabricated by assembling the mold sections together;

* cores can be built into the mold at the same time that the mold is fabricated, allowing precise locating of the cores within the mold (prevents finning and locating dimensional inaccuracies);

* undercuts can be built into the core and mold package;

* no draft angles are required;

* casting wall thickness as low as 1.4 mm can be produced without any problems;

* cores can be formed in the unit for use in another casting process;

* any metal utilized in the shell sand process can be used in DSS RP.

The time required between CAD and the first casting using conventional foundry methods, even with the help of standard RP tools, is typically 2-5 weeks. For DSS RP, the build time for a 1-cylinder head is two days, and for a 4-cylinder head it is less than 1 week. Even with the additional design time required for the mold's runner and gating system, the total processing time for a new cylinder head can be significantly reduced.

In addition to the time saving consideration, the DSS RP-produced cylinder heads are virtually identical in all metallurgical and mechanical respects to the sand-cast components that will be produced during the production phase. This means that test engines can undergo evaluation using parts with physical, mechanical and metallurgical properties similar to those parts that will be manufactured in the final production phase of manufacturing.

Limitations

Despite the benefits the technology provides, DSS RP equipment can be costly for a foundry. But an option available to foundries interested in the process is the use of service bureaus. These resources often own the equipment as well as the expertise to operate it. Service bureaus also have various processes and applications at their fingertips, allowing foundries to do some one-stop shopping.

Another limitation to the technology, as is the case with most RP generated curved surfaces, is the apparent "stair-stepping" on the outside of the part as the height increases. Stair-stepping is caused by layers of sand being applied during the z-axis layering operation in such a way that the new layers are not as wide as the ones below it. When the narrower layer is cured, a step is formed between the edge of the previous layer and the top edge of the new layer.

These steps continue along all concave and convex surfaces, but can be virtually eliminated by orienting the part to eliminate stair-stepping during the planning stage, or minimized by building with thinner layers. Unfortunately, thinner layer building extends the build time, and the thickness is limited to the diameter of the thickest coated sand grain. Another solution is sanding the sharp edges, which can minimize jagged appearance of the stair-stepping.

Although DSS RP doesn't have many limitations on part complexity, it does have an apparent limit on mold and/or core size based on build volume. The two-laser EOS machine has X, Y and Z build volume dimensions of 28x15x15-in. respectively. The DTM machine, which operates with a single laser, has build volume dimensions of 13-in. in diameter by 15-in. in height. Restrictions caused by mold build size limitations don't pose insurmountable obstacles. If the goal is to make parts larger than the build volume dimensions, multiple cores and molds sections can be fastened together prior to pouring, as shown by the mold assembly of BMW and VAW Mandl & Berger (see The Results of DSS RP sidebar).

A RP Breakthrough

Prior to the recent introduction of DSS RP, most foundry prototypes involved the need for secondary tooling to produce a prototype part. This resulted in cast metal prototype components that could only be made through a series of steps, which often led to extensive time delays. Other metalforming processes, such as weldments and hand fabrication, could be produced and quickly submitted for evaluation. This put the supplier of a sand casting in a disadvantageous position because once the initial testing program was underway, the customer was generally not anxious to start it over again. The development of DSS RP allows sand casting foundries to compete on a faster level and quickly translate mold and/or core ideas into functional sand cast prototypes available for evaluation of their physical characteristics, metallurgical properties and part performance.

GM's Advanced Development Lab Displays Step-by-Step the Making of a Prototype Casting Using DSS RP

GM Powertrain, Saginaw, Michigan introduced DSS RP to its Advanced Development Lab (ADL) in January. Its goal was to decrease lead times and costs associated with casting product development. The laboratory installed a DTM Sinterstation 2000 to use in conjunction with shell sand material. GM Powertrain selected this new process to avoid some of the previous secondary operations that were encountered with its investment cast prototyping method.

Following is a step-by-step look at the laboratory's development of a compressor housing.

1 Initially, the compressor housing model is developed with a mold within a mold envelope on the CAD system. The mold is split to allow the removal of excess unbonded sand after the mold/core build has been completed. Metal feed paths and cavity vents are added. The STL file from the CAD system then has the mold build, scale and orientation parameters applied as it is sent to the machine for building the mold and core package.

2 Once the mold has been built from the phenolic resin-coated zircon sand by the laser sintering process, it is raised out of the build cylinder. Excess, unbounded sand is then removed from the mold cavity.

3 The mold and core is then put into the curing oven for post-curing. The mold and core is post-cured at 485F (250c) for 15-16 min. Once cured, the mold halves are mounted together and prepared for the molding operation by putting a bead of hot melt glue around the joint to keep the halves from separating or shifting.

4 The build volume of the DTM machine limits the casting size to a 13-in. diameter by 15-in. height, so once the mold is cured, it is put into a room temperature nobake sand mold that contains the standard gating, sprees and runners. The cope and drag mold is closed and prepared in the conventional method prior to pour our.

5 After pouring, the solidified mold enters shakeout. The portion of the mold that was laser-sintered has cured even further due to exposure to the molten metal. It is, therefore, harder than the sand encompassing it.

6 With the removal of the shell sand, the casting is finished in a conventional way. The key benefits of this technology for the lab are the blending of DSS RP into its current casting processes, materials and products. There isn't any need for special tools (other than the DSS RP equipment), and the customer receives an accurate casting in a short amount of time. The ADL's use of DSS RP has produced experimental castings that surpass the competition in casting quality and responsiveness.

Formation of a BMW Cylinder Head with DSS RP

Following are three stages of a DSS RP mold assembly of a cylinder head for an air cooled, aluminum motorcycle engine. This project was developed by BMW, Munich, Germany, and VAW Mandl and Berger, Linz, Austria. The mold, as shown in Step A, is very complicated due to the fins for the air cooling that are 2 mm thin and must be formed by the horizontal walls of the outside mold segments. The entire mold assembly consists of six segments plus one small core for an oil channel. The gating and risering system is integrated into the mold segments. As shown in Step B, both the inlet and outlet channel cores are solidly incorporated into the adjacent outside mold segments, allowing for closer tolerances due to the lack of an additional gap between channel core and outside mold segments.

The partial assembly of the mold in Step C shows how the mold segments fit together. An additional large segment from above, which contains inlets and risers, then closes the mold. Two complete molds ready for casting were delivered 10 days after the CAD date was received.