3001 Heverlee, Belgium
Tel: (016) 298363; Fax: (016) 298319
Date Visited: 23 October 1995
JTEC/WTEC Attendees: B. Kramer (report author)
Professor Jean-Pierre Kruth, Chairman, PMA
Dr. Jan Detand, Senior Research Associate, PMA
Dr. Bart van der Schueren, Manager of Prototyping, Materialise
The Catholic University of Leuven's Division of Machine Design, Production, and Automation (PMA) is the largest of four divisions in the university's Department of Mechanical Engineering. PMA has 110 full time employees, including 7 faculty, 7 research managers, 72 research engineers, 10 technicians, and 5 administrative staff. Approximately 150 fourth- and fifth-year students conduct design projects and thesis work in the PMA. The laboratory is well-equipped, with a broad range of conventional and NC machine tools, many robots of various types, an assembly cell, and a filament winding machine. The available equipment for rapid prototyping includes a 3D Systems SLA unit, a university-built SLA machine, and an experimental unit for direct selective laser sintering of metallic powders.
PMA is conducting a broad range of manufacturing-related research. The division is distinguished by a desire to conduct research of practical importance in the context of a strong fundamental perspective on manufacturing issues. In keeping with this philosophy, rapid prototyping research is being conducted in tandem with a major effort to integrate design, process planning, and 5-axis milling to develop systems to produce fast-turnaround tooling by the milling process. A recently completed project sponsored by industry exploited PMA strengths in reverse engineering, SLA, EDM, 5-axis machining, and concurrent engineering: the division developed production prototypes of a line of 7 different die-cast lighting fixtures, with delivery of cast parts in two months from the time of delivery of a wooden model of the designer's concept. Materialise N.V. is a 5-year-old spin-off company, founded to commercialize the rapid prototyping research of PMA. The company generates cash flow as a service bureau, employs 20 people, and is located at a technology incubator in a separate facility within easy walking distance of the PMA laboratories. Materialise has a 3D Systems SLA machine, a developmental SLA system of PMA design, and extensive facilities for replication of SLA models by vacuum molding polyurethanes in silicone rubber molds, spin molding of zinc die casting alloys in vulcanized rubber molds, and investment casting of high-temperature alloys in plaster molds. Materialise also has an extensive program to develop software tools for SLA: its MAGICS software for visualization, manipulation, support generation, and quality control of models in the STL format, and its CONTOUR TOOLS software for the production of SLA models directly from 3D CAD files. These are apparently receiving broad acceptance among European rapid prototyping users.
Materialise is forming a new division in the United States in partnership with Laserform, a service bureau in the Detroit area that has represented its software in the United States. Similarly, Materialise has obtained the right to represent Sanders' plotter-based rapid prototyping machines in Europe.
Research and development activities relevant to rapid prototyping include projects related to stereolithography, selective laser sintering of metals, and rapid production of polymer injection molds and metal casting dies by 5-axis NC machining.
PMA is a resin test site for SLA resins developed by DuPont and Zeneca (a division of ICI). The greatest perceived need relative to resin is for resins with higher strengths comparable, for example, to thermoplastic ABS resins. Epoxy resins are clearly superior in this regard to acrylates, with Allied Signal and DuPont in strong competition to develop improved epoxy resins for SLA. Development is rapid, with frequent lead changes. The best available epoxies have processing speeds and build accuracies comparable to acrylates, and have low shrinkage. Further improvements are needed (and anticipated) in all of these properties, as are improved heat resistance (currently limited to 150°C), flexibility, and toughness. Patented software to produce complex, lattice-like support structures for SLA models has been developed by PMA and commercialized by Materialise N.V. Their support geometry requires only 10% of the resin volume per unit support volume of conventional structures and therefore requires only 10% of the build time to produce. Significant efforts are underway to model and characterize critical aspects of the SLA process to reduce variability. Inadvertent or unavoidable exposure to ambient light causes changes in cure characteristics as the resin ages, making the accurate setting of laser power and scan speed difficult. Therefore, a simple, 2-5 minute test pattern has been developed to characterize the aging of resin systems over time. It has also been found that the thickness of liquid resin spread over each point in a model depends sensitively on the detailed local topography of the already-built model, and that the thickness variation can be reduced by careful adjustment of the resin-spreading speed. The PMA/Materialise SLA machine attempts to increase laser on-time by providing two separate resin vats in the build zone. The pattern can be written in one vat while the resin is being spread in the other, providing a 50-100% increase in laser duty cycle.
Work is also underway to incorporate reinforcing fibers in SLA parts. Research was initiated four years ago with woven glass mats, which were transparent to the laser energy and could be laid into the uncured resin during the build. The project was dormant for some time but has recently been reactivated. Surprisingly, the samples shown did not seem to display any ill effects from resin wicking by the mats. However, they were of very simple geometries.
A particular strength of Materialise N.V. is the development of techniques for producing SLA models from CAT and NMR scans for medical applications. The work is partially funded by the BRITE-EuRAM PHIDIAS project in cooperation with Zeneca, Siemens Medical Systems, and PMA. Materialise researchers have developed excellent software, Mimics, for interpolating data from medical scanners. They showed the JTEC/WTEC team a medical model that employed a special resin that could be colored by varying the curing conditions in selective volumes to highlight, for example, diseased tissues.
PMA has built a prototype SLS machine for the direct sintering of metal powders. The unit uses a 500 W Nd:YAG laser with a fiberoptic head, an XY stage for beam scanning, and a 5 mbar vacuum environment in the powder chamber. The machine had been operational for only two months at the time of the team's visit, with the research effort concentrated on steel/copper powders. Early samples showed good bonding within the layers but some problems with delamination between layers. It was suggested that significant improvements could be obtained through adjustment of the sintering parameters.
The NC milling efforts are extensive and tightly integrated with the overall program to develop software for concurrent design and manufacturing. The research effort is partially supported by the ESPRIT COMPLAN and BRITE-EuRAM KERNAL II projects and spans the spectrum from reverse engineering to computer-aided process planning, NC code generation, and hard milling of die steel (up to R 54). A particular feature is a post-processor that develops true 5-axis NC programs for specially designed toroidal milling cutters. The resulting tool path minimizes scalloping in the mold surface, greatly reducing the need for mold polishing compared with similar surfaces produced by ball milling. Sample parts that the team examined showed impressive results, but they involved modest curvatures. No full molds were observed.
The research group at PMA is large, and is directed at critical problems of manufacturing productivity; rapid prototyping is viewed as one technology that may provide needed solutions. The work in rapid prototyping is well conceived, and there is excellent cooperation between the PMA researchers and Materialise N.V., with Materialise providing the opportunity to test new research results and identify new problems for researchers to address.
Katholieke Universiteit Leuven. 1994. Annual Report of the Mechanical Engineering Department.
Katholieke Universiteit Leuven. 1995. KU Leuven Newsletter (June).
Materialise N.V. n.d. Rapid Prototyping Software Tools.
_____. Medical Models.