Robot machining cell for high-quality plastic products - Highly flexible milling
Otto Graf GmbH, with headquarters in Teningen near Freiburg, is a pioneer when it comes to rainwater harvesting. Over the past five decades the plastics processor, founded in 1962, has grown into a prospering enterprise with about 450 employees worldwide and annual sales of about 95 million euros.
Today Graf markets a complete range of plastic products for water management. Its product portfolio extends from rainwater barrels, infiltration systems and retention cisterns to separators and firefighting water tanks. The 130-page product catalogue contains solutions for every conceivable use of rainwater for private and municipal customers.
It is this diversity of products that calls for a high degree of flexibility in the manufacturing process. There is a demand for new technologies that make the processing of injection- and blow-moulded products not only more flexible, but also faster and more cost-effective, while adhering to the highest quality standards.
Jürgen Keßler, plant manager at Graf company headquarters in Teningen, puts in a nutshell what that means for the milling of individual product groups: “Milling operations in our production lines are becoming increasingly complex. We not only have strongly fluctuating batch sizes, but different plastic qualities including fibreglass-reinforced materials. In these circumstances and given the existing quality requirements, the ideal solution appeared to be a six-axis robot.”
A robot cell for every task
A suitable system integrator for implementing the project robot milling cell was quickly found in CNC-Automation Würfel, based in Singen on Lake Constance. The team led by company owner Oliver Würfel can look back on a large number of successfully implemented automation solutions. “What’s so special about this robot cell is its flexible configuration without knowing details of the tasks ahead. Of course, there are two different products for which the system was specifically designed. But given the innovative power of Graf it is only a matter of time before further versions or new products are added,” says Würfel.
A look at the system shows how CNC-Automation implemented the existing requirements. The large Motoman MS80W, equipped with a water-cooled IMT milling spindle, functions as a guarantee of flexibility per se. With its remarkable range of 2.236 millimetres, the six-axis robot covers an extremely wide area of operation. Limitations in processing due to protruding components are not anticipated.
Another plus in terms of autonomy: the cell has an automatic tool changing fixture with a milling station at which the Yaskawa robot itself collects the required tool. In addition, it features a combination of automation and manual operation: “We automated the procedures that are tedious or time-consuming when performed manually, but relied on manual work for simple operations such as loading of the cell. This hybrid design ensures the highest degree of efficiency and flexibility,” confides Keßler.
From individual processing up to batch sizes of 1,000 and over
The two products that have been manufactured since the start of operations in the fourth quarter of 2016 could not have been more different. The first is the plastic Barrica rainwater barrel, made by extrusion blow-moulding. The bayonet lock for the lockable lid is milled by the Yaskawa robot. Task number two is the cutting of accesses for pipes on shaft components made of fibreglass-reinforced polypropylene for an underground infiltration system.
The products milled on the system are as different as their batch sizes. Keßler explains: “In the production of the blow-moulded Barrica rainwater barrel, 5,000 to 7,000 items are manufactured in ten to twelve working days. In this case the robot spends several weeks milling only the bayonet locks for the barrel lid. That is quite different with infiltration systems, which are customized. Here we are closer to batch size 1 for milling. In practice this means that retooling procedures vary from zero times per week to several times a day.”
To make retooling procedures and system programming as simple as possible, CNC-Automation relied on the all-round competence of the Yaskawa Group. That started with the programming of the system. The tool paths can be prepared on the PC in user-friendly offline mode with the CAD-CAM function of Yaskawa MotoSim VRC software and transferred to the system control with a mere mouse-click. The milling cell can then be operated in production even without in-depth expertise. A touch panel made by Yaskawa subsidiary Vipa displays all information in plain text. Thanks to this visualization, after a short briefing any employee can assume the operation of the cell.
Tool wear becomes a challenge
A further aspect that proved to be a real challenge in test operation was tool wear, which – in contrast to the working of metallic materials in plastic processing – is often underestimated. It is an oversight that often causes headaches, although that was not the case with our experienced project participants: “In particular the milling of fibreglass-reinforced plastics proved to be a ‘tool killer’. We finally found a reliable solution in a complex series of tests with the tools of various manufacturers and trials with all conceivable spindle rotation and cutting speeds,” says Würfel.
Because Graf places great value on the quality of all its products, differentiating them from low-price products, first-class milling is a must. In particular, the milled bayonet lock of the high-quality rainwater barrels is in full view and perceived as a quality feature. “Customers who have opted for our aesthetically pleasing rainwater barrel in wood finish have a right to place high demands on the quality. Thanks to its excellent trajectory control and exemplary repeat accuracy of 0.07 millimetres, Yaskawa’s six-axis robot can satisfy the existing quality requirements with ease. The milling is so precise that no post-machining on the bayonet lock is necessary,” says Keßler.
In all other respects, the innovative robot milling cell achieves or exceeds the expectations pinned on it. It runs reliably with maximum availability, if needed in three-shift operation around the clock. But it is not only the cost-effectiveness of the robot solution that convinced Kessler of the solidity of this investment: “Robot milling enables three-dimensional movement patterns within an extremely large working range virtually without limitation. That is an advantage gives us greater design freedom in our product development and promotes the development of new, innovative Graf products,” remarks Keßler.
Text & Pictures: Ralf Högel
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