Coating thickness variation measurement

Purpose

The measurement of coating thickness variation will reveal if this is the cause for mottle on the printed samples or if the print results is bad because of other paper or paper board properties like surface roughness or white-top mottle.

The measurement can be used to answer:

• Measure if the mottle (e.g. coating thickness variation) level is acceptable and stable over time
• Check if coating thickness variations is the cause for mottle
• Monitor how new settings in the productions affect the coating thickness variation
• Check if new products are better or not when it comes to coating thickness variation

Method

First the samples are treated with a burnout technique where the fibres become black, but the coating remains white. Next step is to scan the samples in a flatbed scanner and the gray level image is calibrated to reflectance using a calibration set.
Variations in the range 1 to 8 mm are summed since these have been found to be the most disturbing range in visual assessments.

Deliveries

The mottle value, which is the reflectance variations as Coefficient of Variation in size interval (wavelengths) 1-8 mm proved to correlate to the perceived mottle at a normal reading distance of 400 mm. Also, detailed mottle values divided in spatial octave bands, ex. 0.25-0.5 mm, 0.5-1 mm, 1-2 mm, 2-4 mm, 4-8 mm and 8-16 mm revealing the coating thickness variations in fine scale, medium scale and large scale. The results are saved in an Excel file.
Optional: To also get the orientation of the coating thickness variations.