|Less uniform sheet More uniform sheet|
Nonuniformity of paper within a length scale of 2-20 mm is most frequently associated with a tendency of fibers to form flocs. It is important to keep in mind that a certain degree of fiber flocculation can be expected, regardless of chemical conditions in a papermaking furnish. The flocculation occurs because a typical papermaking fibers have length-to-thickness ratios between about 50 and 100. That means that the fibers tend to collide with each other and become somewhat entangled. At the same time, hydrodynamic shear also tends to break down the fiber flocs, and the degree of flocculation can be understood as a dynamic equilibrium between these two tendencies.
Papermakers employ the following kinds of strategies to try to minimize the level of fiber flocculation in the paper: (a) adjustments of the papermaking equipment, (b) selection of fibers or manipulation of mechanical aspects of the furnish, and (c) adjustment of the chemical environment.
There is an interrelationship among roughness, porosity, and optical formation measurements. Those regions that are calendered the heaviest will be smoother, denser, and will have lower opacity than the adjacent regions that are calendered lightly. Local variations in opacity will show up as having poor formation on optical formation testers. A rough surface will absorb more ink, and that same rough surface will be more porous, as it received less calendering action. The more porous region will also absorb more ink into its interstices. When printers correlate poor printing with poor formation, perhaps it is the roughness and porosity variations that are the culprits, and the formation tester is one additional piece of test equipment that verifies the root of the problem. These formation problems occur on a small scale, smaller than what a basis weight process control system will discriminate. Formation problems are perhaps quite detectable by analyzing the standard deviation of porosity and roughness measurements within a small region. The roughness and porosity variations caused by fiber flocs are much smaller than the test area of either the roughness or porosity measuring heads. There can be other reasons for high standard deviations, such as non-uniform sizing, which may also be caused by the localized absorbency properties in the region of fiber flocs.
- Scott, William & Trosset, Stanley, "Properties of Paper: An Introduction", 1989.
- Hubbe, Martin, "Formation Uniformity", http://www4.ncsu.edu/~hubbe/TShoot/G_Formn.htm