Site: Institute for Polymer Testing and Polymer Science (IKP)
University of Stuttgart
Pfaffenwaldring 32
D-70569 Stuttgart (Vaihingen), Germany

and

Nebenstelle
Boblinger Str. 78 E
D-70199 Stuttgart, Germany

Date Visited: 26 October 1995

JTEC/WTEC Attendees: A. Lightman (report author), F. Prinz, L. Weiss

Hosts:

Dipl.-Ing. Hans-Christian Ludwig, Head, Engineering Dept.
Dipl.-Ing. Bernd Keller
Dipl.-Ing. Bernd Wiedemann

BACKGROUND

The Institute for Polymer Testing and Polymer Science (IKP) at the University of Stuttgart was founded in 1963 by Prof. Dr.-Ing. S. Wintergerst. There is a staff of 62, organized into 5 departments: engineering; life-cycle engineering; polymer and environmental analysis; polymer physics and plastography; and nondestructive testing. There are ~200 students. The annual budget is DM 6.5 million (~$4.7 million), of which DM 1.0-1.5 million is provided by the state of Baden-Wurttemberg. IKP information is available on the World Wide Web (http://www.ikp.uni-stuttgart.de/).

Prof. Dr. -Ing. P. Eyerer, Director of IKP since 1979, has since 1994 also been director of the Fraunhofer Institute of Chemical Technology (ICT) located in Pfinztal (near Karlsruhe). Besides other joint projects related to polymer engineering, there is a strong link between the two institutes in special areas such as rapid prototyping, tooling, and life cycle assessment. The two institutes complement each other's work, as IKP performs fundamental research and ICT performs applied research.

IKP started its RP activities in 1991 with high-power laser (Nd, CO2) metal melting. The process is similar to that at Los Alamos National Laboratory (circa 1970s). The IKP process is focused on laser cladding - laser surface treatment. IKP researchers used it for building material from single lines but found the process too restrictive. Under the circumstances, a 5-axis system is needed for complex geometries, and the system becomes quite expensive. The Fraunhofer IPT in Aachen is implementing such a system, but IKP management questioned whether this could ever become commercial, and IKP went on to laser sintering. (Sulzer Innotech of Switzerland has been doing laser welding buildup since the early 1970s using a 3-axis system and cobalt- or nickel-based material for blades.)

IKP has considerable strength in polymer characterization and testing, with extensive laboratory work focused on microscopic analysis (both surface and subsurface) and chemical and physical analysis. This strength has naturally led the institute to materials research for stereolithography (SL) and to plastics for sintering. IKP has both types of systems in operation in research programs. The stereolithography machine is an EOS Desktop model, and the sinterstation is a noncommercial unit.

Before beginning its program, IKP surveyed industry to determine the desires for material properties, long-term stability of the materials, high productivity versus high accuracy material (i.e., faster build with lower accuracy versus slower build time with higher accuracy), and applications focus. The JTEC/WTEC team's hosts at IKP would not discuss the results of the survey other than to say that the results are guiding the institute's efforts and that industry currently considers tooling to be the most important application.

RAPID PROTOTYPING RESEARCH

The IKP programs have focused on materials development and characterization. IKP researchers helped EOS to develop a polystyrene composition having suitable flow properties (lubrication without agglomeration) and material conversion properties upon exposure by the laser. They have also performed extensive characterization of the photopolymers used in SL. IKP has developed an extensive RP material properties bibliography that is currently proprietary. This is part of a software package called "Rapid Prototyping Selector" developed at the Bremer Institut fr Betriebstechnik und angewandte Arbeitswissenschaft (BIBA) within a common European project. The operator specifies the desired material properties (material type, formation process, etc.), and the program shows how to achieve these properties either in a single step (if there is a suitable RP process available) or as a series of processing steps of an RP chain (e.g., create RP mold master, cast tool, inject). This acts as an expert system, relieving the operator of having to possess an intimate knowledge of all the optional processes.

At the time of the team's visit, IKP's SL system was conducting shrinkage and strain measurements in standard photopolymers using a balance beam rigged with both a displacement transducer and a strain sensor (similar to the apparatus at the University of Dayton). The researchers drew a 10 mm strand from a fixed post to the microbalance. Exposure was performed either slowly to get the required single-strand exposure with the appropriate cure depth, or quickly by making fast, multiple passes to build up the same exposure. They were studying SL5170, SomosTM 6100 epoxy materials, and newly developed acrylate resins. Results indicated the time delay until shrinkage began, the development of shrinkage, and finally the eventual relaxation as the material swelled (the exposure only yields partial polymerization). They are performing tests on a variety of materials. Eschl presented papers at the Dayton RP conference in 1995 and at the Nottingham conference in 1996. Further investigations on photopolymers are being carried out on the final properties of these materials with respect to process parameters, suitability for investment casting, and morphology of photopolymer parts. Both IKP and ICT have significant funding from the EC for rapid tooling and other state and industry collaborations. There are 10 projects with average funding of DM 0.5 million. These include: BRITE EuRAM Automobil; BRITE EuRAM Laser Sintering; BMFT Material Development -- Photopolymers; state-supported tooling application software; a special RP research project; and several industrial collaboration projects. ICT purchased its EOSINT P system and an SLA 500 as an entrée into collaborative research on sintering materials. IKP representatives stated they would not purchase equipment if IKP could not get an associated project to help pay for some of the cost.

IKP and Mercedes-Benz (MB) made the SL mold faces for an injection mold used by MB. The faces were held in a standard steel frame and reinforced with Al shot and epoxy. The mold was used for ~120 shots using several materials, including ABS and 30% glass-fiber-reinforced nylon. The mold is showing deterioration. Mold flash issues were not discussed.

IKP has a program to evaluate the capability of each RP technology for every tool production process and the resulting part production system. One goal of this program is to develop a software package in which the user specifies the material, surface finish, strength, etc., of the end part, and the software will guide the user by selecting the processing sequence needed to produce the part and the RP process required to fabricate tools that meet the production specifications. IKP is also working on a proprietary post-curing system for SL and on new hatching strategies for sintering and SL.

OTHER

IKP representatives mentioned the University of Munich's development of a process similar to one developed at MIT. In this process, an ink jet head disperses photopolymer into a powder bed. The photopolymer acts as a binder. A UV lamp polymerizes the material, thereby defining the geometry. The powder form can then be burnt out and sintered. IKP participates in a special research project involving 13 institutes, including BWI, Daimler Benz, IMA, FhG-IAO, FhG-IPA, and the Institute for Laser Technology (IFSW) in Stuttgart, to address how development and testing of innovative products will be manifested in practice. The participants intend to demonstrate that RP technologies can be integrated into the innovative structure of a development team for improved cooperation and communication. Later, they intend to broaden the scope to geographically dispersed firms and eventually to place the project within the "Global Engineering" concept under development. Prof. Dr.-Ing. H.J. Bullinger of FhG-IAO chairs this project.

Used at the IFSW are a nozzle and high-power lasers in a lathe system. Researchers use an additive process for nickel- and cobalt-based materials. They use a 2 kW Nd:YAG laser for both laser cladding and laser ablation to form shapes with both additive and subtractive processes, useful for cylindrical patterns. Objects of interest include valves for high-performance engines. IKP has also done ceramic (monolithic material) ablation using an excimer laser to form a trapezoidal screw (a time-consuming fabrication process).


Published: September 1996; WTEC Hyper-Librarian