How to detect a missing label on a multi-lane web
A missing label on a multi-lane web is a gap in the pitch of a lane, not a failed artwork match. A gap wider than half a label pitch stops the press, and the lane number is written down. A camera that only compares artwork treats the empty cell as designed white space.
A missing label on a multi-lane web is found by looking for a gap in the pitch of each lane, not by comparing the artwork alone. If the gap is wider than half a label pitch, stop, and write the lane number. A camera that only matches artwork will treat the gap as designed white space.
Why it is judged this way
When the die cut takes one label away and the liner is still there, that cell is empty. The same place on the template may also be a light colour, so “close enough to the master” will let it pass. The space between the two neighbours becomes about two pitches. That is the missing label.
What to compare
| What you see | Call it | Do not |
|---|---|---|
| One lane’s spacing is about two pitches | One label missing. Stop | Treat it as designed white space |
| The artwork is all there, only register is a little off | Register, not a missing label | Record it as a missing label |
| A whole lane is empty | A web break or labels not coming out. Count it apart | Use the same code as one missing label |
Check at a job change or a new roll
- At the job change, measure one pitch. Set the gap threshold at half a pitch.
- Take one label off and run the web. Confirm that lane alarms and the neighbouring lanes do not.
- The alarm record must carry the lane number. Do not write only that there is a defect.
