There are several manufacturing processes that shape a continuous web and wind the product on a roll. When metallic materials are plastically deformed through a series of roll nips, the end product is quite uniform, due to the malleable properties of the materials. The modulus of elasticity is quite high for metallic materials, as compared to that of paper. Metallic materials leave a roll nip substantially the same thickness as the roll gap, with a surface finish somewhat equal to the roll finish.
By comparison, paper is extremely compressible. There are numerous voids in paper, including the presence of air within the fibers, which resemble small capillary tubes. In the calendering process, the nips are loaded to a certain nip pressure, “pli”, or pounds per linear inch. The resulting distance between the mating roll surfaces is primarily a function of the nip loading, the compressibility of the paper and the deformation of the roll surfaces. The mechanical action in the nip imparts a smoother surface on the paper, and there is a decrease in the thickness of the web after it is calendered. Further, there is a difference in the stacking height of such calendered papers, due to both the thickness reduction, and the way in which rough surfaces stack together. The properties that affect stacking height are surface roughness, compressibility, and stack
When building a reel, the highest priority is to wind a uniform roll, as a roll with ridges and valleys will give a perceived value of poor quality, and also there can be runnability issues with such rolls.
The process control systems in use today have a strong history of development around basis weight, moisture, and caliper (thickness) control. Basis weight and thickness variations can cause the calendering action to be different across the web. When a web has reasonably uniform basis weight, but has caliper uniformity problems, a process control system can make very small, but effective adjustments to the calender stack to build a level roll. Such adjustments may affect the smoothness profile, but generally there are no on-line sensors that provide such feedback. When a CD strip from a reel is run through a profiling smoothness tester, there are generally regions of high and low smoothness values that show up at the same places, reel strip after reel strip. When the smoothness values fall within the accepted limits, there is generally no concern in fixing the problem. The astute production manager will observe such trends, and take action before the measurements reach upper or lower control limits.
Related posts include information on the relationship between paper roughness (smoothness) and the following items:
- Papermaking Process
- Printing processes
- Parker Print Surf Test
- Sheffield Test