Plastic Flexibility

Plastics sheet extrusion requires precise gap control between rollers to control sheet thickness. But traditional mechanical gap setting and control methods can be difficult and time-consuming, requiring manual calibration and creating a safety hazard for operators. Davis-Standard in Pawcatuck, Conn., wanted to provide not only a safer environment with its plastics production machinery, but precise gap settings that could be changed in a matter of seconds. We were able to achieve that by switching from mechanical gap control to an electrohydraulic hands-free system.

Davis-Standard builds equipment for the plastics industry, producing machines for foam, flexible and rigid sheet plastics extrusion. The company's fully integrated extrusion solutions are at work in packaging, automotive, building trade, medical and many other industries.

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In plastics production, a rollstack machine draws raw polymer material between heavy rollers, which cools the material and gives the plastic sheet its desired surface finish and thickness. Thicknesses range from 0.008 in. (0.2 mm) to more than 1 in. (25 mm), depending on the application, which can include pharmaceutical containers, consumer products packaging, arts and crafts products, film and much more. Producing plastics for these applications requires precise control of the distance between rollers to control thickness.

Traditional gap setting and control methods are based on mechanical devices such as lever arms and pancake cylinders, which are nudged to the desired position according to the product on the production line. Operators must also enter the nip area (where heavy nip rolls pull material out of the machine), and perform manual calibration using feeler gauges. Not only is this a potential safety hazard, it can be more difficult or time-consuming to change gap or force requirements for different products. Once set, the gap also can be difficult to monitor using traditional feedback methods because machine frames can flex under heavy loads, causing slight discrepancies in positioning. In addition, the use of numerous mechanical components can cause maintenance and inventory issues.

Hands-Free Flexibility

To help customers become more flexible in their manufacturing, we addressed these challenges on our XP Express roll stand system, using industrial hydraulic components to provide the intelligent control that allows customers hands-free flexibility for quick and smooth gap or force changes. The key to the hands-free system is electronic control of the hydraulic cylinders that actuate the chill rollers, including digital position feedback to monitor cylinder positioning at any time. The chill rollers measure 36 in. in diameter by 80 in. wide, weigh about 5,000 lb each, and can process more than 7,000 lb of material per hour. The electrohydraulic components chosen allow positioning to be maintained at specifications down to 0.0001 in.

"The gap setting is the key to producing the finish customers want for their plastics products," says Larry Mellow, project manager at Davis-Standard. "We recognized that we could design a machine that would let electronic signals and transducers, not mechanical parts, control the gap settings and force. Now we have a way to adjust those settings almost instantly for more flexibility in product runs."

Davis-Standard worked with a local distributor, Airline Hydraulics, which recommended Bosch Rexroth's intelligent system — specifically, four HACD digital controllers, each commanding a CST4 hydraulic cylinder with a 3.25 in. bore and a stroke of 5-12 in. There are two hydraulic cylinders per roller (one at each end), with each cylinder controlled independently. That allows operators to change settings or "float" either end of the roll separately if necessary.

The HACD controllers communicate digitally to a third-party PLC via DeviceNet or Profibus, and receive signals from both digital SSI linear position feedback and analog pressure transducers. The digital position feedback enables 1 µm resolution. The pressure transducers allow force to be limited if desired. The force can be monitored and used for feedback for the overall process. The controllers also supply command signals to servo solenoid proportional valves, which ensure smooth control of the up and down roll motion. Rexroth servo valves and pressure transducers come fully assembled on the hydraulic cylinders and require no assembly time. The hydraulic pressure in the system generally ranges from 2,000 to 2,500 psi, but can go as high as 3,000 psi for larger sheet products.

Airline Hydraulics also specified Rexroth mechanical components in the form of linear guide rails for carrying the bearing blocks associated with the rollers. The rails are 35-45 mm wide and 396-575 mm long. The final element of the hands-free gap control system was a custom Airline Hydraulics manifold containing the components needed for emergency retraction of the chill rolls if necessary.

"The hands-free system allows the customer to set the exact gap or force that's needed to make a sheet product to any specification," Mellow says. "What's more, the system offers operators the choice of position control or force control."

In position control, which can be changed by the operator during the process, the chill roll gap is keyed into the system along with the production recipe (including melt temperature, chill temperature, nip pressure, line speed, etc.), Mellow says. With linear transducers handling the position sensing, the roll gap is held constant. Gap changes can be made through the system while in operation, usually in seconds. If force control is chosen, hydraulic force expressed as nip roll pounds per linear inch is keyed in and force is maintained by the computer's digital feedback system. Force changes also can be made on the fly.

Many products such as embossed plastic or those with a high-gloss finish run best by running a primary nip in position (gap) control mode with a secondary nip in force control mode. That allows the operator to use the chill roll "float" capability to impart a final polish to the sheet. When operating in either force or position modes, the alternate mode is displayed as a reference. All information is digitally stored and recorded.

The electrohydraulic system also makes it easy to set force limits for extra protection measures. "Operators can input a low force setting for the second chill roll, which they couldn't do before," Mellow explains. "If the nip detects a force change from foreign matter passing through, it can be set to open automatically. This helps reduce the risk of damage to the roll or plastic sheet."

Flexing Some Muscle

Airline Hydraulics and Rexroth engineers also worked to solve the tricky problem of frame flexing. Recognizing that some flexing was inevitable if the machine was going to have high-load capabilities, they changed the position feedback system so that key feedback components were not on the hydraulic cylinders. Instead, the feedback components are placed on the bearing blocks at the end of the rolls, where a magnetic sensing system allows excellent position monitoring.

With the XP Express system, plastics manufacturers can have the versatility of specialized finishes down to thousandths of an inch. Changeovers for different production requirements can be made in seconds without shutting down an entire line. The digital controller reduces the need for operators to interpret feeler gauge readings and use trial-and-error methods to determine the right settings. That, in turn, minimizes wasted material caused by improper settings, or settings that become invalid because of flexing or vibration. Once the recipe is keyed in, the intelligent hydraulics immediately begin controlling material output as needed.

"The more changeovers a customer has, the greater the gain in productivity from using a hands-free system," Mellow says. "It also maximizes operator safety, virtually eliminating the need to enter the nip area to make adjustments and changes. This can help users to produce a wider variety of plastics products and increase return on investment even more."