By Neal Gigliotti
Plastic Marketing Applications Engineer
Bosch Rexroth Corp.,
Welex Corp., Bluebell, Pa., recently developed an improved system for controlling the roll gap in the sheet take-off system of its extrusion equipment to produce higher quality plastic sheets. The higher precision control is needed where the polymer passes through extrusion dies that give the sheet its basic shape and then passes to
the take-off rolls.
Sheet thicknesses range from 0.006 in. (150 11m) to 1.5 in. (38mm). The rolls cool the output, give the sheet the specified thickness and surface finish, and accumulate the sheets in uniform rolls or flat stacks. The sheet take-off rolls are computer designed to achieve maximum heat transfer with minimal roll deflection under pressure. The rolls have a double-shell construction, with multiple spiral baffles to ensure high-velocity cooling fluid for uniform roll temperatures. A heavy rigid inner shell supports the shrunk-fit outer shell. To prevent indentation from high sheet pressure, rolls are constructed with a thick, hard stainless steel overlay. Proper cooling requires the appropriate roll dia-meter. If the rolls are too small, the line capacity will be limited. If the rolls are too large, the sheet may not lie flat after cooling or there can be excessive sheet sagging between the die and the roll nip.
The gap between the rolls is also critical. The system uses three rolls. The middle roll is fixed, and the upper and lower rolls move to set the proper gap. The top roll must maintain a fixed gap between it and the center roll to achieve the proper sheet thickness. The bottom roll, which provides the final polish and surface finish, is also set to a gap, but is allowed to float based on a force setting. The float is necessary to prevent ripples from forming on the sheet.
The existing system, in use since 1991, is an analog system that requires a custom electrical control to interface it to the overall machine controls. The company decided it was time to design a new and improved system for automatically controlling the roll gaps. Its first attempt at designing a new system had a few drawbacks. First, it included a custom conversion box, but numerous potentiometers needed adjustment during startup. In addition, the servo valves were sensitive to contamination of the hydraulic oil, creating support problems. However, the biggest drawback, according to Mike Mitchell, We1ex’ s director of engineering, was that the system could only control the position of the rolls. “We wanted to sense both the position of the roll and the force being exerted on the sheets. While the position of the roll is a good indicator of the gap, being able to also monitor the force and make on-the-fly adjustments gives us even better results in sheet quality.” Forces range to 40,000 lb at lines speeds of 300 fpm.
The Welex takeoff machine controls the roll-gap using a hydraulic system from Bosch Rexroth.t.
The real new design
Welex decided to work with Gary Jermyn of Airline Hydraulics, Bensalem, Pa., a Rexroth distributor, and Neal Gigliotti of Bosch Rexroth to design a new control system based on its VT-HACD digital hydraulic axis controller, which would be used to monitor and control the movement of the two outer rolls. Bosch Rexroth CST4 hydraulic servo cylinders were selected to move the rolls, which provides precise control based on both position and force.
The system uses two interacting control loops. The closed-loop position system sets the basic gap required. The closed-loop force controller monitors the hydraulic force and limits the amount of force that is exerted on the roll. This force limiting function allows the roll to float on the surface of the plastic sheet and protects the roll from damage due to obstructions. Because each roll is a high-value, precision-manufactured component, damaging one not only halts production, but also represents a costly repair involving considerable downtime. The closed loop hydraulic force control system will automatically sense an obstruction by the added force it measures. It will open the rolls to allow the obstruction to pass through and then close them again to the proper gap. Since the operator does not have to clear the obstruction manually, this approach has obvious safety benefits.
The VT-HACD controller and CST4 cylinder together can control the gap with 0.0001-in. accuracy over a 5-in. range of travel. As Mitchell explained, “Manual gap control is a tedious trial and error process. The new system gives us control of the gap with greater precision than can be obtained with feeler gauges and hand wheel adjustments.”
The VT-HACD system has four servo axes to control the movement of the rolls. Each end of the two movable rolls forms one axis. Coordination at each end is required to ensure that the rolls remain parallel with a consistent gap across the span. Any skew can turn production into scrap. The VT-HACD monitors the four axes and provides on-the-fly adjustment in ms. Set points and data are transferred between the axes and machine programmable logic controller (PLC) over a PROFIBUS fieldbus, with the PLC able to command multiple control loops and sequence steps.
The VT-HACD gives the performance required to maintain high-quality production. Even better, it is fast and easy to commission and setup. Gigliotti explained, “The HACD is user configurable through an intuitive Windows-based graphical interface using pull-down menus, check boxes, and value fields. There is no programming language to master. A system designer can configure the controller in only a few hours.” The first system was up and running in less than a day.
The sophisticated VT-HACD digital hydraulic axis controller comes in
a single Eurocard format.
The CST4 hydraulic servo cylinders play a critical role in achieving the precision movements required. Airline’s Jermyn noted that the cylinders allow the valves to be mounted directly on the cylinder. “The closer you can bring the valve to the cylinder, the quicker and more accurate the response you can achieve. In addition, the digital position feedback transducer is inside the cylinder, where it is rigidly mounted and protected from damage.” The integration of feedback device, valve, and cylinder into one unit offers packaging advantages in terms of faster assembly and minimum space requirements.
Also critical to high accuracy positioning are the low-friction seals in the CST4 cylinders. Standard piston seals create a condition called stick-slip. Because the friction of the piston seal is higher when static than while moving, when the cylinder piston first moves the seal will flex instead of slide. As the piston moves further, the static friction between seal and cylinder is overcome and the seal slides, snapping back to the original shape. If, as is the case with standard lip-seal piston rings, the static friction is very high the piston will overshoot the intended position. “Stick-slip presents no problems if you’re talking about small errors within the allowable tolerances of an application, but for accuracies of ten thousandths of an inch you need to absolutely minimize stick-slip by using lowfriction seals,” explained Gigliotti.
For more information, see www.boschrexroth-us.com.
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Filed Under: Factory automation, Motion control • motor controls