700 perfectly synchronized drives

Initial situation

High-quality plastic films are found e.g. in accumulator batteries, capacitors or flat screens. For these applications, plastic films have to have special characteristics that require a complex production processes. In a film stretching line, the blank or pre-film is "stretched" into a film that is a few micrometers thick. Individual grippers are responsible for the stretching process. The grippers grip the pre-film by the edges and stretch it. The distance of the grippers from each other is gradually increased, giving the plastic film its shape and characteristics.

In conventional film stretching lines, the stretching could only be done in one direction at a time, either horizontally or vertically. For this reason, the stretching actions had to be carried out successively.

Task

The customer of ARADEX developed a new method that combines the two processes of horizontal and vertical stretching in one step. Unlike conventional methods, the new one offers absolute flexibility in setting the stretching conditions. In this way, film characteristics can be achieved that are not possible with conventional stretching. As a development partner, ARADEX was responsible for the drive technology of the film stretching line.

Implementation

In the new film stretching line, the grippers are moved and individually controlled by linear motors. Over 1400 grippers are used. Each pair of grippers is driven by a VECTODRIVE® inverter. A total of 726 ARADEX drives are used.

During the stretching process, the gripper distances are continually increased perpendicular to the running direction, as in a reversed bottleneck. In this way, the approx. one-meter-wide pre-film becomes an up to nine-meter-wide film. In the end, films are produced that are only a few micrometers thick.

The position of each individual gripper is determined and controlled without sensors by ARADEX drives. A VECTONUM® VD Control centrally processes and logs the characteristics of all 726 axes. Position and speed are directly and therefore quickly controlled on the axes.

After development on a prototype with 60 drives, the initial installation took place directly at the end customer's. The first attempt had to function directly, and it did.

Results

The new film stretching line processes over five tons of plastic per hour. The precise, sensorless synchronization of the 726 drives guarantees maximum quality for the plastic film. The simultaneous method achieves a speed of 400 m/min, up to four times that of conventional stretching methods.

The installation runs continuously: 24 hours a day, 7 days per week. Over 8000 operating hours per year. It thereby fulfills the highest requirements for reliability and availability.

ARADEX technology also considerably reduces the energy requirements of the linear motors. In order to have sufficient energy reserves for all operational conditions, the motors have previously been operated with the maximum possible current. This resulted in considerable losses in the motor. As a result of the new method, the current losses have decreased dramatically, in this case by 66 %, which means annual savings of 1.6 million kWh or currently around 200,000 €.