Burr-free punching with drive technology from Bosch Rexroth

Burrs and flash (also referred to as angel hairs) occur when thermoplastic plastics are punched. Stanova Stanztechnik GmbH has developed a procedure thanks to which common plastics such as PVC and PC-ABS can be punched without burrs.

With the new punching process, there is no need for the usual processing steps which involved removing burrs on plastic parts chemically, thermally or mechanically. Products such as wiring ducts were bombarded with tiny balls for example in order to remove burrs. However, classic procedures such as vibratory finishing or milling were also used.  With the new technology, there is no need for this deburring process. This saves time, space and costs when producing plastic parts, and has a huge influence over the total cost of manufacturing.

The company faced a number of challenges along the way. “It took a lot of trial and error loops until we could achieve substantial improvements,” said Katrin Lechler, Managing Director at Stanova. Using PVC as a material, good results can be achieved with high punching speeds in combination with a small cutting gap. Stanova achieves the high punching speeds of 900 strokes per minute by using eccentric punches which are driven by dynamic synchronous servo motors.

The demands on the servodrive technology were therefore very high from the start. Numerous test series and benchmarks with various drive concepts from different manufacturers were carried out. Bosch Rexroth convinced Stanova with the functionality of its drive product range together with its expert knowledge and its wealth of application know-how. “Bosch Rexroth’s drive experts offered professional support on site and encouraged us to achieve our ambitious goal, namely a perfect punching result, on a step-by-step basis,” said Manuel Schulz, an automation technician at Stanova.

It was necessary not only to achieve 900 punching strokes at a speed of 1500 mm/s but also to handle the dynamic control requirements for the this multi-axis system. This was only possible by using the high level of functionality offered by the Rexroth drive controller in conjunction with Rexroth’s high-performance MS2N synchronous servo motors. Compared to the previous generation, these have a higher power density with a very low torque ripple. With an overload capacity of up to five times, acceleration processes in machines can be carried out even more dynamically. High short-time torques are therefore possible even at high speeds as the motors are specially designed for operation with field weakening. Without field weakening, the maximum possible speed is limited by the output voltage of the control device. For operation with field weakening, an additional current component is implanted into the stator. This weakens the magnetic flow and reduces the induced counter-voltage. As a result, the useful torque-speed range can be increased considerably. Higher maximum speeds can thus be reached at the same voltage.

A high production speed is particularly useful where an endless material is punched (inline) or a feed system speeds up the workpieces to the speed of the punching unit (offline). The offline procedure is simple and complex at the same time: the tool no longer travels with the workpiece – it is stationary while the material is fed in. This is challenging because the feed movement and the punching have to be synchronized exactly. That is possible thanks to an additional encoder for the feed drive which is read in as a second encoder on the Rexroth drive controller and thus allows quick control without a time delay. This synchronization concerns all drives, the two eccentrics which are responsible for punching and the two drives which feed in the material. When punching wiring ducts, a third eccentric press must also be synchronized. It punches the fixing holes in the base which have different spacings from the side punches. This is a highly complex interaction and everything takes place at almost 16 strokes per second.

For Stanova, it was not just about developing the innovative punching process and making it available on the market. The company also thought about an integrated machine concept, durable and freely combinable with an innovative punching process for the plastics industry. The solutions are structured according to the four company principles – modularity, usability, flexibility and sustainability. They can be integrated easily and flexibly into existing systems and, if necessary, can also be used at different locations.

Sustainability plays an important role at Stanova. Thanks to the consistent use of efficient servodrive technologies, the Stanova eccentric punching machine has a power consumption of 7-10 kW at the maximum stroke rate (1000 strokes/min). By way of comparison, a hydraulically driven punching machine requires more energy (approx. 30-40 kW) and achieves fewer strokes per minute. The Rexroth servodrive concept offers further energy-saving potential. Thanks to DC-bus coupling, energy can be shared between the drives. Unlike previous systems, when a drive’s braking energy was converted into heat, it can ideally be used to speed up another drive. Depending on the load profile, up to 30% of kinetic energy can be saved. In order to exploit this further, capacitor modules are used so that the DC-bus energy is available optimally as a buffer. The use of regenerative supply units with the new modular ctrlX DRIVE range can further increase efficiency. The “Smart Energy Mode”, the intelligent energy management system developed by Bosch Rexroth, comes into play here. The control system ensures a DC-bus voltage which is independent of the grid voltage and at the same time uses capacitors as energy storage components. This avoid peak loads on the grid side and reduces losses in the grid connection line. The result: reduced energy consumption, improved grid compatibility and the use of smaller components to achieve the same machine performance.