How to Use Repeat Length to Find Which Roller a Defect Is On
Measure the distance between the centers of two defects of the same type; that is the repeat length. Compare it with the circumference of each roller: circumference equals diameter multiplied by 3.1416. The roller whose circumference is within a few millimeters of it is the suspect. If none match, check the incoming material; don't waste time on your own rollers.
Measure the distance between the centers of two defects of the same type; this is the repeat length. Compare it with the circumference of each roller: circumference equals diameter multiplied by 3.1416. The roller whose difference is within a few millimeters is the suspect. If none match, check the incoming material, and don't spin your wheels on your own rollers.
How to measure without getting it wrong
Only measure defects of the same type. An ink crust spot and a later pinhole are not a pair. In the paper travel direction, find the next spot of similar shape and size, and use a ruler or the meter reading of the inspection system, reading center to center. When the web is stretched, measure on the section just printed, not after rewinding has stretched it further.
For the plate cylinder, include the plate and double-sided tape in the diameter. The anilox roller, chill roller, and guide roller each have their own diameter, all listed on the equipment nameplate or in the last measurement record. A difference of 2 to 3 millimeters can be considered a match; a difference of one tooth or ten-plus millimeters means it is a different roller.
After it matches
| Repeat length close to | Check first | Don't touch first |
|---|---|---|
| Plate cylinder circumference | Plate, double-sided tape, plate cylinder surface | Ink formulation |
| Anilox roller circumference | Anilox roller damage, dried ink | Plate curve |
| Close to none of them | The periodicity of incoming material | Take apart all rollers |
Do this first this round
- Confirm the two spots before and after are the same type of defect, then read the center distance.
- Multiply diameters by 3.1416 to list plate rollers, anilox rollers, impression rollers, and chill rollers.
- Only take apart the one that matches. If none match, cut a section of unprinted material and see whether the raw material has the same periodicity.
