Riesselmann & Sohn GmbH & Co., founded in 1925 in the town of Lohne in Lower Saxony, started out as a manufacturer of corks. The company branched out into plastics processing in the late 1940s when some corks were produced with plastic tops. The plastics business enjoyed a remarkable upswing in the 1970s thanks to intensive cooperation with the automotive industry. Since the beginning of the last decade, the company has concentrated exclusively on these customers and the production of technical plastic components and subassemblies.
The company, which has QS 9000, DIN EN ISO 9001 and VDA 6.1 certification, specializes in technically demanding, custom solutions for plastics applications in car engine compartments and interiors. The expertise of this global player encompasses the entire process chain, from initial design drafts through to series production. 70 percent of Riesselmann’s DM 130 million annual turnover comes directly from car manufacturers, the other 30 percent being attributable to system suppliers. The company has a staff of 560 and a reference list which reads like the ‘Who’s Who’ of the international automotive industry.
Riesselmann’s production facilities are highly automated and include more than 90 modern injection molding machines with clamping forces of 25 to 1,800 tonnes. However, continually increasing demands, in terms of design, quality and cost-effectiveness, mean that all production processes must also be subjected to continuous optimization; at the end of the day, this is of vital significance for the future of the plant, the employees and the company as a whole.
Complex geometry
“Because of the complex geometry of the cable ducts, these were previously fitted with clips manually; we saw that we could make production more efficient using robots,” explains Dipl.-Ing. Markus Mechelhoff, Production Manager at Riesselmann. “Our expectations have been fulfilled. Reduced personnel costs have meant sustainable reductions in manufacturing costs. Furthermore, we have been able to shorten cycle times, achieve more consistent quality and spread the load more evenly over the production installations. In addition to this, we have seen a massive drop in customer complaints. As far as the defect quota, measured in ‘parts per million’, is concerned, we are well on the way to hitting zero. This is a decisive factor, because a defect on our part would entail significant costs, both for the customer and for us.”
Cable ducts contribute to a more attractive layout of the engine compartment, but their primary function is to protect the wiring harness. They are used, for example, to provide lasting protection for heat-sensitive wiring harnesses, shielding them permanently against high temperatures in the engine compartment by routing them through the vehicle in a precise manner.
The only economically viable way of automating the handling of the cable ducts is to use a six-axis jointed-arm robot. The prices for this type of robot have fallen drastically in the last few years, sparking greater interest even among medium-sized manufacturing companies.
Riesselmann invested in two robotic cells, each based around a KUKA KR 15 robot. Each of the robots, which were commissioned in June 2000, ‘cooperates’ with an injection molding machine. These machines, with clamping forces of 125 and 200 tons, produce a total of six different cable ducts. Annual output is about 1,500,000 components.
This figure is dependent on the short cycle times which the KR 15 robots are able to deliver, thanks to their remarkable acceleration capacity. High levels of repeatability and flexibility are also vital. On top of all this, the user needed robots with a low floor space requirement, but at the same time a long reach. In a three-shift day, the two KR 15 robots attach about 14,000 clips to cable ducts. In peak demand periods, they are in operation three shifts a day, six days a week.
Exact synchronization
Before deciding in favor of KUKA Roboter GmbH, Augsburg, Riesselmann tested alternative systems offered by the competition. Markus Mechelhoff had the following to say about the selection process: “The determining factors were KUKA’s experience and global presence, information gathered at trade fairs and the attractive price/performance ratio. These were backed up by numerous positive references from the automotive industry, including plastics handling applications. We were also impressed with the PC-based KUKA controller, particularly the control panel with its familiar Windows-based man-machine interface. Furthermore, KUKA came up with the best solution for removing the cable ducts from the injection molding machine: an exactly synchronized parallel motion of the robot and the machine ejector.”
The robotic cells were supplied by KUKA systems partner DAT Automatisierungstechnik GmbH, based in Pappenheim, Bavaria. DAT took responsibility for the complete layout of the manufacturing cell, including the integration of the robots, the interfaces and the design of the clip mounting stations and the grippers. There are separate grippers for each article. The user changes these about once a week along with the injection molding tool.
As far as the design of the grippers was concerned, the twisted shape of the cable ducts and the varying positions of the mounting points presented a particular challenge. The robot must hold the cable ducts in a number of different positions when attaching the clips. The cable duct may on no account be bent, or it will not be possible to attach the clip. The development engineers were faced with special requirements for two cable ducts in which so-called film hinges had to be pre-bent. This procedure, which obviates the need for cable straps, prevents the lid of the cable duct from breaking. Before installing the cells, DAT carried out an in-house test run, leaving the systems partner in a position to guarantee perfect functioning from the handover date onwards.
Controlled process
The robot removes the cable duct from the injection molding machine and places it in the mounting station where it presses a clip into the form-fit snap connection in the plastic component. To ensure that this procedure goes smoothly, the clips are sorted in a vibrator and positioned ready for the KR 15. Depending on the product, three to five clips are required per cable duct. The controller uses a sensor system to check whether the station is holding a clip ready and again to see whether it has been mounted. This results in 100% checking and thus high process reliability.
The KUKA robot controller KR C1 communicates with the injection molding machine via an interface and synchronizes the processes in this way. The conveyor that transports the completed cable ducts away from the cell also receives information about the feed rate and runtime from the robot controller. The KR C1 also exchanges data with the cell gates and the photo-electric barriers fitted as a safety measure.
A separate program is stored in the controller for each type of cable duct; the required program can be called up easily by the operator, using the Windows interface of the KUKA control panel. Riesselmann made use of the courses offered by KUKA College in Augsburg when preparing its workers for their new tasks.
Economic perspectives
“Since commissioning, the robotic cells have demonstrated high availability. As far as the payback period is concerned, we are looking at about a year,” says Markus Mechelhoff. “In light of the excellent economic potential, we are already planning on acquiring further robots.”
Prime examples of this are applications involving smaller machines, for which shelf-mounted robots would be ideal. Furthermore, the current prices of six-axis robots put them in a strong position in terms of competition with three-axis devices. Also under consideration is the possibility of using KUKA robots to weld and assemble engine components – activities the user has hitherto carried out manually.
Riesselmann is redeploying workers thus freed from their previous tasks as operators for the robotic cells, and in the manufacture of new products for which the pros and cons of automation cannot yet be fully assessed.
Author: Jürgen Warmbold, freelance technical journalist, 27327 Martfeld, Germany