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.
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