The user, a medium-sized business, works in three shifts throughout the year, producing toboggans and child seats; of the toboggans, the company manufactures 70,000 to 80,000 units per season. After the blow molding process, pieces have to be cut out of the toboggans and the backrests of the child seats. Until now, five employees were required continuously 24 hours a day just for the deburring. The extreme stress that this work placed on the workers’ wrists was constantly resulting in disabling work-related injuries. An automated solution was therefore extremely desirable. The only kind of robot that came into consideration was one with six axes, since other designs, with their limited kinematics, are not suitable for this type of flexible motion sequence.
A totally new tool
Finding a suitable design concept for this situation was extremely difficult. To avoid having to provide a large amount of floor space for intermediate storage, the toboggans had to have the holes cut in them and be deburred immediately after leaving the blow molding machine. It would not have been economically justifiable to use either quick cooling, which would have caused structural changes in the plastic, or a cooling zone, which would have required extensive systems engineering work and additional handling. Therefore the machining had to take place while the plastic was cooling, and simultaneously shrinking by about two to three centimeters.
The use of burr spindles and milling cutters guided by a robot was ruled out, since at that point the toboggan was still at a temperature of about 100°C. Rotating tools would have generated poisonous fumes that would have had to be removed by means of an extraction system; another result would have been considerable dust and dirt.
A solution using non-rotating cutters was previously not possible, because robots could only carry out exactly the path motion they were programmed with, without being able to adjust for the variable shrinking process that was taking place at the same time. The automated process therefore required a completely new tool for deburring the blow-molded parts.
Roboter Technologie GmbH found an answer – a float-mounted tool unit, which can accommodate various cutter blades, for example straight, concave and convex cutters for various radii. To remove the plastic burrs, the blade must always maintain the correct cutting angle and a constant cutting pressure.
Higher quality
A KUKA KR 15 robot cuts the holes in the parts and deburrs them in a single operation. The cutter adapts to the contours of the workpiece. The advantage of this procedure over manual handling lies in the time savings, the consistent and significantly higher quality, and the minimal number of rejects.
During machining, the toboggans are gripped in a support fixture. The user gains an additional quality improvement due to the fact that in the fixture the toboggans retain their shape despite cooling.
Apart from the tool, the greatest demand placed on the robot was the need for high flexibility, so as to be able to process all products with a single system. Furthermore, the operator placed a high value on precise repeatablility and a controller that was easy to use. His expectations in this regard were met by the open PC technology and Windows interface offered by KUKA Roboter GmbH, based in Augsburg, Germany.
Roboter Technologie GmbH designed the tool in just six weeks, and then a test facility was set up in Mantel near Weiden, Oberpfalz, the company's manufacturing facility in Germany, from June to mid-August 1997. Development tests directed at determining the optimal positioning of the toboggan were carried out using the KUKA simulation system KR-SIM. Four weeks later, the soon-to-be operator visited the test facility and subsequently placed the order. In November 1997, the robotic cell with its periphery went into operation at the user’s plant. A team from the plastics manufacturer responsible for the system received preparatory training at KUKA College in Augsburg.
Simultaneous cutting and deburring
As soon as a blow-molded part leaves the plastic molding machine, a worker takes it, removes the flash and places the part in one of two form locations mounted on a rotary fixture. Then he operates a switch to clamp the toboggan. As soon as the robot located on the other side has finished its task, the fixture rotates. The new part thus enters the KR 15’s working area, and the completed part can be removed from the fixture, which opens automatically. The worker then packs the toboggan directly into a box that is standing nearby for that purpose. Cut-out pieces are removed from the robotic cell on a conveyor belt to be ground up and re-used.
After positioning, the robot begins immediately with simultaneous cutting and deburring. A zero is defined for the start of the operation. As soon as the side parts of the toboggan have been cut out, the fixture tilts it to a vertical position, so that the KR 15 can also easily reach the toboggan’s top side. The robot completes machining of the cooling toboggan within 40 to 50 seconds. Thus the cycle time of the robotic cell, including the manual handling for loading and unloading, is significantly shorter than that of the blow molding process, which takes 120 seconds.
During the processing of each toboggan, the robot exchanges tools once at a tool changer that is integrated into the system. In addition to a convex cutter for the cut-outs, a straight cutter is also used, with which the KR 15 gives the toboggan a smooth exterior contour.
The robot always begins with the interior contour. Time is saved by changing back to the curved cutter at the same time as the rotary fixture is turning. Reversing the sequence is ruled out because of the cooling process, the base data for which are stored in the program.
The toboggans are always produced continuously over a period of two to three months. The finished products are put in storage for the next season. When making a production changeover, the operator disassembles the robotic cell and moves it with forklift trucks to the other plastic molding machine. Because of these transfers, the size of the cell was an important factor; in its assembled form it measures 4.80 meters x 4.80 meters x 2 meters. No programming work is required after a changeover; the robot identifies each individual piece by means of the exchangeable coded form locations and the programs stored in the controller.
Paid for itself quickly
The robotic cell paid for itself within one season. Since it is practically impossible for the user to find new employees due to the health hazards posed by fumes emitted from the plastics, and since official regulations would require making additional investments in any event, the user is already thinking about using an additional robot for automatically feeding materials to the cell and removing the finished products.
Author: Jürgen Warmbold, freelance technical journalist, 27327 Martfeld