Hybrid machine tools for moldmakers

The advantages of combining additive manufacturing with finish-machining processes in a single setup are plentiful, from shorter cycle times to the ability to create complex, unlikely geometries. The most commonly available hybrid machines include machining centers with laser-metal-deposition (LMD) nozzles for 3D printing.

Because of the specific needs of their industry, moldmakers might feel left out. However, a few machine builders are filling this niche-within-a-niche.

‘I Am Your Density’

“When it comes to choosing a hybrid machine for moldmaking, there are only two systems I’m aware of that can do the job,” said William Gillcrist, national product manager – machining/additive divisions for MC Machinery Systems Inc., “and that’s Lumex and [the OPM250L system by] Sodick.” The Wood Dale, Ill.-based Mitsubishi subsidiary is the U.S. distributor for Lumex, a line of hybrid machine tools born of a collaboration between Japanese machine tool builders Mitsubishi Corp. and Matsuura Machinery Corp.

Sodick’s OPM250L hybrid machine blends CNC milling with powder-bed fusion
for moldmaking applications. Image courtesy of Sodick.

The reason for the low level of competition has a lot to do with technological challenges inherent to the process, according to Evan Syverson, marketing manager for Sodick Inc., Schaumburg, Ill.

“In order to print something that is adequate for moldmaking, you need to use direct metal laser sintering,” he explained, adding that DMLS provides a suitable density for molds.

Also known as powder-bed fusion, the process involves firing a fiber laser into a bed of powder metal (PM), which melts or welds the material together to create the part structure. While LMD is by far the most common method in hybrid machines, that process—in which PM is essentially sprayed and then melted onto the work surface—creates structures that are too porous for molds, Syverson noted.

“Those types of machines have considerable utility in repair and fabrication work,” Syverson said, “but with a powder bed, the material density endemic to the process lends itself more closely to moldmaking.”

Density is critical, he explained, because even small deviations can dramatically reduce the lifespan of a mold. For example, a 0.1 percent difference in density can yield a 50 percent reduction in tool life. Sodick’s OPM250L offers a reliable part density of 99.99 percent, while other 3D printers may be limited to 99.7 percent density or lower, according to Syverson.

The combination of additive and subtractive manufacturing can yield complex, intricate mold geometries
with extremely fine surface finishes, without involving an EDM. Image courtes of MC Machinery Systems.

However, milling a workpiece surrounded by PM in the bed poses unique challenges, which may explain why the technology is not more widespread. For example, the presence of PM risks reducing tool life by increasing friction and adding to the amount of material the tool must work through. Plus, conventional oil-air mist lubrication cannot be applied because of safety issues related to the exposure of a flammable material to laser radiation.

MC Machinery’s Gillcrist has seen demonstrations of several 5-axis hybrid machines with spray-deposition capability, but, he said, when switching between additive and conventional machining, the coolant must be drained from the work area so it does not interfere with the powder-deposition process. This drawback is not present with DMLS systems.

“Lumex is a dry system,” he said, “so, without coolant to worry about, the part can move back and forth without difficulty or interruption.”

Printing in Process

DMLS hybrids offer a number of distinct advantages, from shortening lead times to enhancing design flexibility to—well, achieving the previously impossible. Sodick’s Syverson explained that there is not yet a 3D printer on the market that can achieve the necessary surface resolution without finish machining. Using a hybrid machine rather than moving to another machine for finishing eliminates the need for multiple setups, and allowing the spindle to finish a part as it is being printed cuts cycle time.