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Flat vacuum-calibrator plates, typically used to extrude hollow profile sheet, are showing up in new forms and new uses. A new version invented in Italy is said to boost output and improve surface finish. A flat calibrator system developed in the U.S. is a new alternative to calender-roll stacks for wood-plastic composite (WPC) sheet. Again, surface finish is a major benefit.
Omipa SpA in Italy, well known for dies and extrusion systems to produce hollow profile sheet used in glazing panels and packaging, has patented a calibrator that creates an air cushion on which the profile floats. The calibrator uses two parallel cooled vacuum plates. In addition to cross-wise slits that create a vacuum in the gap between the profile and plates, this calibrator also has slits that introduce pressurized air into the gap. Each calibrator half has a separate vacuum circuit and pressurized-air circuit.
Pressurized air introduced by one set of slits is continuously removed by the vacuum slits, increasing cooling capacity and raising output as much as 15% to 18% with PP profiles, Omipa says. A less dramatic increase in output has been observed with hollow PC-acrylic profiles.
The air cushion also reduces friction between the profile and plates, which improves surface smoothness. With polypropylene, this gives a better printing surface; with PC-acrylic it improves optical properties, according to Omipa.
The first prototype was built five years ago. It was first offered commercially two years ago after the patent was issued (U.S. 6634880). Omipa’s patent reportedly covers use of the controlled air cushion for any hollow profile, including pipe.
Sheet of WPC and other natural fibers is typically calibrated with a stack of three chill rolls. But ESI-Extrusion Services Inc. has come up with a novel alternative using a series of flat, 48-in.-wide vacuum calibrators in a vacuum calibration tank 5 ft wide x 20 ft long. Smoother surface is said to result.
The first commercial system is being tested now at a customer’s plant. This is believed to be the first time solid sheet has been produced with flat profile calibrators.