High nipping loads across the entire width of the chrome roll face can cause the roll to deflect, resulting in a thick band at the center of the sheet, according to PTi. "This new invention gives processors the ability to produce thin-gauge sheet that is uniform in thickness and enables them to reach thicknesses that were previously unattainable," said Sushant Jain, Senior Scientist for PTi. For reference, if the sheet gauge target is 0.010 in. (250 microns), even a 0.002 in. (50 micron) roll deflection will show up as a 20% variation in sheet thickness. The target variation is typically ±3%. Roll skewing compensates for the deflection and enables the operator to obtain the ±3% variation target at the 0.010 in. (250 micron) sheet thickness.
PTi's roll skewing mechanism employs a cross-axis positioning of the roll such that the roll deflection is wrapped about the mating roll, thus creating a uniform gap over a wide operating range. The roll skewing system is highly versatile and recommended for processing thin-gauge materials such as PP because of its adjustability over a wide range of deflection patterns and sheet gauge targets.
The U.S. patent (9,182,067) was granted to PTi president Dana Hanson and colleague Mitchell Gritzner. Within the PTi device, a bearing assembly mounted to the chrome roll journals is used within, and keyed to, an outer housing. The machine direction position of this bearing assembly within the outer housing is controlled by a threaded push/pull device mounted on the front cap of the outer housing. Using this device to adjust one bearing assembly forward and the other backward by the same amount causes the center of the top roll to remain centered over the center of the middle roll while the gap of the top roll will "wrap" around the middle roll. The device enables processors to adjust each side of the top roll by ±0.75 in. (19 mm). PTi noted that PP has been successfully run down to 0.010 in. (0.25 mm) using its roll skewing device.
PTi's roll skewing mechanism is being used by several processors for the production of thin-gauge PP sheet. The company offers the device as an option for all of its roll stands.