Once an extruder starts to produce film, there is one golden rule—no shutdowns. Just like in steelworks, paper plants or the semiconductor industry, every break in production is extremely costly as restarting the equipment is an elaborate, time-consuming process.
A critical process station for film extruders is the downstream winder that places the film onto rolls. Once a roll is complete, the film needs to be cut, redirected and placed precisely onto the next roll without glue in a continuous process. During this time, a material accumulator buffers the film that continues to be produced.
These in-line winders and web storage units are the specialty of Sander Maschinen und Anlagen in the Westphalian town of Bad Oeynhausen in Germany.
Storage volumes and times must be precisely aligned with automatic roll changes and the insertion of film. “We would have a big problem on our hands if even a single cylinder or valve in this system were to fail. Reliable, durable components are an absolute must,” explains Managing Director Fred Sander.
“Since our company’s founding in 2002, we have had good experiences with the proportional pneumatic valves from the Aventics ED family. And, due to the specific features of their PRA cylinders, we have branched out to include a wider spectrum of Aventics pneumatic components including proportional valves, AV03 valve systems and SM6 position sensors.”
Individual-function pneumatic circuits govern insertion and cutting, including the control of the dancer, a freely moving pulley (Figure 1). Pneumatic cylinders move the pulley up and down to ensure that the tension setting remains constant.
The ED02 electro-pneumatic pressure control valve regulates this tension setting (Figure 2). This highly dynamic regulator uses closed-loop control to compare actual and target values and respond automatically to volume changes in the dancer cylinder.
“The difficulty lies in switching between an idle state and a fast startup. The dancer cannot jerk,” Sander explains. “Previously, we used expensive, hard-to-install custom cylinders to prevent a stick-slip effect.” This occurs when static friction is much higher than dynamic friction, which is a common occurrence due to seals on the piston rod and the piston in pneumatic cylinders.
“The smooth-running variant of the Aventics PRA cylinder series provides an optimal solution to this longstanding issue,” says Sander. “Lubrication and sealing material are designed to minimize the stick-slip effect. We haven’t experienced any interruptions in production sequences with the use of these cylinders (Figure 3).”