Design News for Mechanical and Design Engineers

Called the 900 mc, this fused deposition modeling (FDM) builds up parts from layers of selectively extruded thermoplastics — including variants of ABS, PC/ABS and PC. Designed with direct digital manufacturing in mind, this new machine is shaping up to be the company’s fastest, most accurate and most repeatable model to date. “As we move into manufacturing environment, we’re asking our machines to perform more like the CNC machines that manufacturing engineers are used to,” says Fred Fischer, product manager for FDM machines.

One obvious part of that move into manufacturing involves size, and the 900 mc doesn’t disappoint on that score with a build envelope of 3 x 2 x 3 ft. “In the past we measured our build envelopes in inches, not feet,”  Fischer says.

Becoming more CNC-like also triggered sweeping changes to 900 mc’s motion control system. From a mechanical standpoint, Stratasys switched to ball screws on all three axes of the gantry that moves the system’s extrusion head into place. Previous FDM machines used a belt-and-pulley actuation on the x-y axis. “But we found that the belts and pulleys didn’t provide the accuracy we wanted, especially as we scaled up to the larger platform,” says Jim Comb, a Stratasys systems engineer who helped design the new machine. He estimates that the ball screws offer about 0.002-inch positioning accuracy improvement given the size of the 900 mc’s large build platform.

Stratasys also changed its motion control system to allow its gantry and its extrusion pump to operate in parallel. In the past, FDM’s prototyping machines made mostly sequentially. Their z-axis moves and pump would operate in tandem but separately from the x-y moves. With the 900 mc, Stratasys moved to a new custom motion card and made software changes that coordinate three-axis gantry moves with the pump operation. 

“That’s an important change for the sake of accuracy and throughput,” says Comb. He explains that the new motion system allowed Stratasys to implement a proprietary method for smoothing gantry trajectories and eliminate some of the accuracy-reducing disturbances that would occur during gantry direction changes. As for throughput, the new motion system allows the system to move faster, particularly during moves that require no material deposition. Combs says these “glueless” moves have become about 1.5 times faster than on previous Stratasys systems. “That really helps improve throughput because glueless moves can make up a big part of the build cycle,” he says.

With the 900 mc so new, it’s too soon to tell exactly what the pay-off from the motion upgrade will be. Stratasys’ machine capability tests, which involve running hundreds of parts on multiple machines and measuring the results, take months and haven’t yet been completed for the 900 mc.

	Stratasys does more than talk about direct digital manufacturing. Its latest FDM system, the 900 mc, has not only been designed for manufacturing applications but also contains 32 production components that have been produced on other FDM machines.