For the past several years, rapid prototyping processes have gained use as designers and engineers have come to recognize the benefit of having physical models of the objects that they are developing. It may be that the model serves the purpose of providing those involved in the development process with something that they can touch and examine to get a better idea of what it is than can be gained by gazing at a CAD image. Or in some cases the model is used to check fits and clearances in the proposed application.
Generally, the parts created via rapid prototyping methods are made with a polymer material or a metal alloy with restricted properties. These materials typically result in objects that are close analogs of the real thing.
But a company based in Plymouth, Michigan,Precision Optical Manufacturing (POM), has commercialized research performed at the University of Michigan that allows the production of parts, molds and dies that are made out of the actual end material, such as aluminum or tool steel. In other words, this produces the real thing.
The POM method, named "Direct Metal Deposition" (DMD), utilizes a laser under CNC control to melt metal powder that rapidly solidifies (thereby providing a small heat-affected zone and a fine grain structure). Pass by pass, layer upon layer, the object is built up, finally resulting in objects ranging from aluminum throttle bodies to working injection molding tools. (The layering process isn't blindingly fast: the deposition rate of steel is approximately 1-in3 per hour; the deposition layer is on the order of 0.04 in. wide and 0.0098 in. thick.)
In some ways, the process is competitive with the now-traditional rapid prototyping methods, which it resembles in operation (i.e., a material being processed by a laser that's under computer control). In other ways it is competitive with conventional machining (i.e., for tool making). And in still other ways, it is competitive with electrical discharge machining (i.e., able to create intricate features). But Dwight M. Morgan, POM president and chief operating officer, calls DMD "a game-changing technology," one that has implications for both cost and time-to-market.
Here are some aspects of DMD...
Almost Rocket Science
One might be inclined to describe DMD as "rocket science" due to the paradigm-shifting nature of the process. But there is one reason why that description is not appropriate: much of the work performed on additive metalworking processes (which, in addition to DMD, includes LENS: laser engineered net shaping and DLF: directed light fabrication) was done under a contract from the U.S. Department of Energy, not NASA.
One of the contrasts that Morgan makes between conventional mold and die making processes—the machining away of materials, performing what he describes as a "subtractive process"—and DMD (which is, as the "deposition" in its name connotes, is an additive process) is that DMD is "greener," more environmentally correct. Machining not only results in the finished part, but also such things as metal chips, used cutting fluids and dull tools, all of which must be handled. DMD results in parts. Period. Any metal powder that isn't fused can be reused. There's no waste.
The Big Three of Manufacturing
DMD for the processing of molds, dies and prototype parts, Morgan maintains, provides what can be thought of as the Big Three of Manufacturing:
• Speed—as in faster type to market. A study performed by the National Center of Manufacturing Science indicates that die production time can be reduced by 40% with DMD.
• Economy—lower tooling costs due to factors including the reduction of labor (this is an automatic process) and capital equipment costs (there is one machine that does the lion's share of the work; a conventional metal-cutting machine or EDM is typically used for part clean-up.)
• Quality—the parts produced are generally 0.0010-in. oversized, so after a quick clean-up (see above) they are ready to go.
|The working elements of the Direct Metal Deposition system.|
The Real Deal
Morgan points out that there are other processes that employ a laser and powdered material. But he goes on to say that the DMD process can make parts with materials such as H13 tool steel while the other processes may use an alloy of, say, bronze and steel, which gets you to 80% of the properties of tool steel. "We're focused on not `near' anything, but on tool steel." As well as other metals, including aluminum.
The molds and dies produced with DMD are meant to be used in production, just as those produced by machining are.
There is no "standard" system for DMD. Morgan explains that the size and configuration of the equipment depends on the application. But there is a machine in the shop area of POM that, from the standpoint of overall appearance, resembles a large machining center (it has travel of 24 in. in the X, Y and Z axes), albeit a machining center with a mezzanine on top, where the laser is located. The laser used on this system is a 2.4-kW CO2 unit from trumpf. The controller, located just where it tends to be on conventional machine tools—to the right of the doors (the housing is a Class One container), is a GE Fanuc Series 180i-M CNC.
Inside in the work area, the clamps and fixtures characteristic of machining are absent: they aren't necessary for this additive process. The laser head is positioned where a vertical spindle might be; it is flanked by a CCD camera-based optical feedback device; it is this feedback device that distinguishes the DMD process from all others. Morgan explains that this is a critical advantage, because it assures that the layering occurs as it is supposed to. Given that hundreds of layers are involved in building up an object, feedback is critical for quality assurance. Adjacent to the sensor is the tube that expels the powdered metal into the work zone.
Reversal of Programming
The programming of the system can be performed with the native CAD file for the part, or an STL file can be generated. One interesting aspect of the programming is that although tool paths are created, before generating the G-code for the controller it is necessary to shuffle the tool paths from their conventional order: remember that this is the additive process starting on an empty work surface, so instead of cutting downward, it is all about building up the object.
A Big Benefit: It's Cool
In molding processes, there are generally five steps:
1. Close mold
2. Fill mold
3. Pack and hold
4. Cool part
5. Eject part
According to Morgan, the single biggest time-consumer in that sequence is part cooling, which accounts for 44% of the cycle time. Mold and die makers are thoroughly familiar with the cooling challenges. For years they've been machining (i.e., drilling) cooling holes into injection and die casting molds. More recently, software programs have been brought into play to create thermal models. These models permit designers and engineers to locate the hot spots, which are then addressed through the generation of cooling channels.
One big limitation: the holes in question are straight while the parts are generally contoured. Consequently, the cooling is somewhat limited.
One of the opportunities that DMD provides is to create cooling channels that wrap around the part, which is called "conformal" cooling. POM engineers have been developing what they're calling "Cool Mold" technology. They've determined that while conformal cooling can reduce the amount of time required for a molded part to cool, the additive nature of DMD facilitates the means by which cooling time can be significantly cut. This is accomplished by adding copper heat sinks (the copper can be deposited and melted during the DMD routine, which permits the combination of metal powders during operation—first spray the tool steel powder, then switch the material feed line over to copper, then back to tool steel) or inserted as solid pieces.
The results of one test that Morgan shares have conventional cooling of a molded part requiring 5.2 seconds, conformal cooling in 4.8 seconds, and conformal and copper in 4.5 seconds, or a cooling time reduction of 13.5% compared to the conventional. He claims that a time savings of as much as 50% can be attained with conformal cooling with copper (it is part dependent). Additionally, conformal cooling plus copper helps maintain a better temperature relationship between the surface of the mold and part, thereby reducing the possibility of part warpage.
Reduced molding cycle time and fewer warped parts can mean a reduction in the number of molding machines required for production, which is potentially a huge savings. Morgan submits that even if it is possible to machine a mold faster than it can be created via DMD, the ability to provide better cooling is a key advantage.
Beyond the generation of conformal cooling lines and the implementation of copper heat sinks, DMD provides the opportunity to tailor molds and dies still further.
Thermocouples and other sensors can be embedded in the mold for process control. (The small heat-affected zone produced by the laser keeps the sensor from being melted.)
Because various materials can be used to create a given object, it is possible to vary the surface characteristics of the mold to provide high heat- or wear-resistance.
Fix It Fast
When stamping dies crack, the typical fix involves welding. Materials like D2 steel need to be pre-heated at 1,000°F for several hours (with any components on the die that can't take the heat disassembled) before welding, then post-heated after the crack is repaired. The heating is required to avoid cracking and thermal distortion.
The DMD system can effect the repair without requiring the pre- and post-heating because of the small heat-affected zone that's characteristic of the process.