Beams Shine, Chips Fly

Lasers have a real flair for metalworking. They can cut, weld, heat-treat, and perform various surface modifications. Yet injection molds and similar types of complex tooling mostly consist of components that require traditional machining operations such as milling, turning, EDM, and grinding. These two approaches to metalworking have now joined forces, thanks to engineers at the Fraunhofer Institute's Center for Coatings and Laser Applications.

They have developed new manufacturing systems that use lasers to enhance, rather than replace, traditional tool building techniques. One such system adds laser welding heads to existing machining centers. The other system employs lasers in the construction of more efficient injection molds and die casting tools.

More than milling

To create machines that can perform both laser processing and milling operations, Fraunhofer engineers start with an existing three- or five-axis machining center. According to Project Engineer Thomas Himmer, the mechanical aspect of integrating the laser equipment is straightforward. Fraunhofer's engineers have created compact welding heads consisting of the laser optics and a dispensing nozzle for metal powder materials. The head installs inside the machine's work envelope, riding on a vertical track attached to the spindle frame. This arrangement allows the head to cover the entire work piece as it moves along with the spindle in the X and Y axes before lowering itself into "firing position" on the track. Himmer notes that this modular unit can accommodate Nd:YAG, CO2, and diode lasers up to 6 kW.

Once installed on the machine, the laser head can deposit additional material on a work piece. In this cladding process, the laser energy creates a melt pool on the surface of the work piece, while the nozzle simultaneously sprays metal or metal-and-ceramic powders into that pool. Typically, the head would lay down a 1 to 2 mm wide swath of material, moving back and forth with the spindle to cover larger areas. As the melt pool solidifies, it leaves a built-up area that has a metallurgical bond to the original work piece.

According to Himmer, cladding can be used for repairs-adding material to worn or miscut sections of existing tools. It can also be used to build up new geometry that would be difficult to create through machining. "There's a good potential to create functionally graded materials," Himmer says.

Following the cladding application, which typically has an accuracy of +0.3 to +0.5 mm, a part would be finish machined to bring it to its final dimensions. And the beauty of this system is that the finish machining takes place in the same machine, usually with a single setup. Fraunhofer engineers integrated the laser head and the machining center controls. "So the same CNC program is used for the cladding and the finish machining," he says.

Aside from cladding, the hybrid laser and machine tool can be used for other purposes, too. Himmer reports that Fraunhofer has successfully performed heat treating, cutting, and brazing within a machine tool environment.

Cool tools

Fraunhofer engineers have also been working to use a laser-based system to create cores and cavities for injection molds and die-casting tools. The system builds tools from laminated stacks of steel that have been precision-cut by an Nd:YAG laser. Called MELATO-for "metal laminated tooling"-the system works with just about any type of steel available in sheet form. "The application requirements would determine the type of steel and layer thickness," Himmer says.

MELATO actually works a bit like a rapid prototyping system. It begins with a solid CAD model of the tool insert. Software then "slices" the model into layers, creating an STL file. This file then drives the laser cutting system, which cuts out the individual cross sections. Next, these cross sections are stacked and joined using a high-temperature (more than 1,000C) brazing process. Himmer says the system typically employs a nickel-based alloy for the brazing, an alloy chosen because it can withstand the thermal loads and stresses of the injection-molding process. "The strength of the connecting material reaches that of original steel," he says, noting that the brazing process has been optimized to create sealed, non-porous structure. Finally, the tool is then finished using conventional milling and polishing techniques-mostly to smooth the layered surfaces and bring the tool to typical machined tolerances.

The whole process typically takes a couple of days, which is not necessarily the fastest way to make a mold. Some high-speed machine tools could arguably cut core and cavity inserts just as quickly. But as Himmer explains, speed isn't the only goal. MELATO's main advantage comes down to making more efficient molds. The process readily produces cooling lines that conform to the contours of the molded part. Compared to the more linear cooling passages produced by machining, these conformal lines can reduce molding cycle times dramatically, which ultimately lowers the cost of the molded parts. Himmer says that Fraunhofer's tests have shown that conformal cooling has reduced cycle times by 35 percent or more.

Still, MELATO won't suit all applications. Even after the tools have been finished, their laminations may replicate on the molded part. "This could be a problem for some cosmetic parts," Himmer says. But Fraunhofer engineers have worked out a couple of ways around this problem. For one, MELATO can be used to create cooling inserts that fit inside a machined core. "This way, there is no layer structure on the tool surface," Himmer says. The other way relies on the institute's integrated cladding and machining system. The cladding could cover the laminations and then the same machine would begin the finish work, Himmer explains.

Fraunhofer has already produced a handful of MELATO systems for use in the automotive industry in Europe. The institute has also built two of its integrated machining and cladding systems in Germany. Himmer says both technologies will now be available in the United States.