RAPID PROTOTYPING TESTS POINT TO ITS USES AND DRAWBACKS

by
James Mishek
Vista Technologies LLC

The modern age of rapid prototyping (RP) began just 10 years ago, when stereolithography (SLA) was first made commercially available. Stereolithography is the process of building parts layer by layer using a computer-driven ultraviolet laser shining onto a platform that is in a vat of photo-sensitive liquid resin. The ultraviolet light from the laser activates the resin to produce longer molecules, which results in a solid. This process takes only nanoseconds. After the first layer is produced, the platform drops .004 inch (.01 millimeter) into the tank, and the process is repeated until the part is completed.

Since then, the field has expanded with the introduction of other prototyping technologies, including laminated object manufacturing (LOM), fused deposition modeling (FDM), and selected laser sintering (SLS). The plastics industry is the market for these techniques, and LOM has found a niche in the foundry industry.

Why Metal Fabricators Have Not Used RPWhy has solid modeling been used more extensively in metal fabricating? One reason is that these technologies make three-dimensional (3-D) models. The sheet metal industry has always been classified as a two-dimensional (2-D) industry. Even the design software for sheet metal fabrication is in two dimensions. However, the finished sheet metal product is almost always 3-D because of bending and forming. Therefore, sheet metal components are candidates for solid-modeling technologies.

Another major reason that RP has not been developed for metal fabricating is that most of the people who dveloped it had backgrounds in plastics. Accordingly, their initial push was into the areas in which they were most comfortable.

The third reason for this lack of development is that the sheet metal industry was not crying for help. Because so many changes were going on with the development of faster lasers and turret punch presses and tooling abilities, shop owners had a hard time keeping up. These reasons explain why RP technology, even though it emerged 10 years ago, was not considered for use by the sheet metal industry until recently.

SLA's Progression into Fabricating

Some of the first work in RP was done by experimenting with hydroforming. At the time, the thought was that although the die could be made quickly using SLA, the material was too brittle to withstand the forming pressures. However, the SLA material was able to withstand the wear and pressure to successfully hydroform sheet metal. At last report, material had been tested up to pressures of 950 tons.

Because few people have hydroforming presses for plates, the technology was not very practical. However, it would be more practical if tools could be made to withstand the force of straight punching, either in a conventional die set or in a turret punch press.

Using SLA on Sheet Metal RP can be used in the form and fit development of sheet metal parts. The following example of a motor housing for a small electric motor illustrates this point. In production, this part is produced by a progressive die that double hits the metal to produce the draw form, and then stamps the holes. An investment in a progressive die is high, so fabricators do not want to make costly changes in the tool. A Catch-22 develops because this style of tooling is too expensive to prototype. However, without a prototype, the production run is a crap shoot, and reworks may be required.

Two options are available for prototyping the part. The first option is to machine the part from a solid. According to a toolmaker who was asked to quote a prototype for this motor housing, it would take a minimum of one week to produce and would be difficult to create because of the housing's thin walls (see Figure 1).

The second option is to make a model using RP. In this case, an SLA prototype was made in 24 hours. The prototype was used to prove fit and form, and the housing was mounted on the motor to test airflow. SLA Technologies in Tooling

Tooling is a crossover area in which RP begins to move into rapid manufacturing. The first tooling example is a forming punch and die made using a proprietary powder metal process.

In this process, a master was produced using SLA and then a negative mold was made. A slurry of powdered A6 tool steel and a binder was poured into the mold and cured. Upon curing, the mold was pulled, and the resulting piece was heated to burn out the binder material. The part was then packed in copper, and a copper impregnation occurred. This process took two weeks. The resulting part was at a hardness of 30 Rc. The part was machined and heat treated to a maximum hardness of 50 Rc.

A test was run by placing a simple emboss on the tool (see Figure 2). In reality, there would have been more practical ways to make a simple tool such as this, but the purpose of the test was to see how the material held up under punching conditions.

The tool inserts were fitted into a punch holder. The tool was inserted into a 30-ton turret punch press and run. Since the operator had doubts as to how it would run and thought it would break early, he chose to run it in 18-gauge (1.1-) stainless steel and to end the test quickly. To his surprise, the tool did not break and kept running. He ran 1,400 forms in the stainless steel. At the end of the run, measurements of the punch and the die were taken and compared to measurements taken before the tool was used. No measurable difference was noted.

Figure 2
An emboss was placed on a tool to determine how the material would hold up during punching.

Although the metal tool worked, it was decided to determine how the SLA material would hold up on its own. Therefore, a second test was designed. In the second test, a simple emboss was again chosen as the test shape. The SLA forming tools were punched using a 30-ton turret punch press. A high shape was chosen because it offered the option of changing the tool during the run in the event of tool breakdown. The SLA material held up, but the forming material could not stretch far enough and continually tore.

The first run was done with the full-size tool. The punch pad form height was .492 inch (12.5 millimeter), with a .040-inch (1-millimeter) radius added to the punch and die pads. The first two runs included three pieces of .06-inch (1.5-millimeter) aluminum, followed by five pieces of .06-inch (1.5-millimeter) cold rolled mild steel. The first punch pad had slight abrasions on the tip from the torn metal stripping from the punch (see Figure 3).

The punch pad shape was modified for the second run to try to reduce metal tearing. The second tip was .375 inch (9.5 millimeter) high, with a .031-inch (.8-millimeter) radius added to the punch tip. This tip suffered no wear from the forming, despite tearing the metal again.

The final run was made with the emboss height at only .250 inch (6 millimeter) and a .031-inch (.8-millimeter) radius added to the punch tip. The run began with three pieces of aluminum, all of which tore. The next five pieces of cold rolled mild steel, followed by two pieces of galvanized mild steel, all ran forms with no measurable tool wear.

Forms with no damage to the SLA tooling were produced when running the aluminum, mild steel, and galvanized steel. When stainless steel was used, the tool was able to produce the form, but after the first hit, the tool began to degrade. From the second to the sixth hit, the punch and die pads bulged so that the punch pad finished with a radius of .06 inch (1.5 millimeters), and the die pad radius had grown to .125 inch (3 millimeters).

The stainless steel forms were domed by the sixth hit, but still within industry specifications for fabrications of ±.005 inch (.12 millimeter). The emboss tool took only48 hours to produce.

This part was taken one step further. The emboss was laser cut and marked to show how fast a stamped part with forms could be produced.

Additional Applications Tested

Additional tools were tested to further define the parameters for tool design, applications, and wear.

Titanium Shell. A prototype of a shallow titanium shell was needed. The shell had some critical blending radii on the base that needed to be developed. The approach was to make a simple forming tool to run in a hydraulic press. The tool was deep enough to keep the resulting wrinkling above the needed shell height.

The resultant wrinkling did mar the sides of the SLA tool, but the set successfully formed 15 shells of the .012-inch titanium (see Figure 4). The tooling took three days to produce.

Progressive Die. A stamper wanted to test one segment of a progressive forming die. The design was tested and adjustments were made to the tool before building the final tool. A tool design and material flow analysis on this part would have taken at least a week, and there were no guarantees of the final result.

Press Brake Tool. A press brake tool was developed to form a large radius in stainless steel. The stainless steel was .060 inch (1.5 millimeters) thick. The tool was coined at a pressure of 10 tons. The end result was properly formed stainless steel with no wear on the tooling. The press brake operator was excited about the tool's success because he envisioned a single, custom-built tool that contained the various shapes needed to make the multiple bends required to produce a complete sheet metal part, and the tools would be lightweight.

The press brake operator envisioned building the tooling with multiple bends, so that there would be one tool per job, and the tools would be lightweight.

Emboss and Louver. To test the longevity of an SLA tool, an emboss and louver were made (see Figure 5). As with many of the tools used on the computer numerically controlled (CNC) presses, the forming tools were made with SLA forming inserts.

The test was done on a CNC press. The result on the emboss showed a doming of the emboss from the first hit to the 120th. The edges of the tool compressed and there was no noticeable wear, but a greater radius of about .030 inch (.08 millimeter) was noted on the edge.

After 350 hits, the louver showed no wear. The sheet metal was sliced beforehand, so the tool was only forming.

Logo. A tool designed to emboss a logo was run on a 30-ton CNC turret punch press. The tool successfully ran a dozen logos at a 20-ton loading. The details of the logo were not as crisp in the center as they were at the edges. However, when a 30-ton load was supplied, a crisp logo was created. Each of these prototypes was produced at a cost between $500 and $2,000, with a turnaround time of approximately one week.

When SLA Will Not Work

The edges of SLA tooling may compress during stamping and may deform upon repeated hits. In addition, the tooling must be properly supported. The SLA material has a compressive strength of 13,000 pounds per square inch (PSI). If this is exceeded, the tool will break. Because of this risk, proper shielding must be used.

SLA tooling should not be used in environments that have temperatures above 120 degrees Fahrenheit (F) or in applications in which the tools experience high abrasion or high friction. The testing outlined in this article was limited to forming applications because the edge strength of the SLA material will not withstand piercing and shearing operations. Summary

Rapid prototyping technology can be used in the fabricating industry. Prototypes for fit and form can be produced quickly. The technology can also be used to produce inexpensive prototypes of forming tooling in a short amount of time. Rapid prototyping may also be used in a stamping press, turret press, press brake, or a hydroforming press.

James E. Mishek is President of Vista Technologies LLC, 4457 White Bear Parkway, Suite D, White Bear Lake, Minnesota 55110, phone 612-653-0400, fax 612-653-0900, e-mail prototype@vistatek.com. Vista Technologies is a rapid prototype service bureau specializing in stereolithography and its applications.