How to inspect both sides of a web in one pass
To see both sides in one pass, use two cameras, one on the face and one on the back, or a turn bar so one face passes one camera twice. Both sides must share one metre zero, or defects will not land on the same metre. Light from one side must not enter the other picture.
To see both sides in one pass, use two cameras, one on the face and one on the back, or a turn bar so the same face passes one camera twice. The two sides must share one metre zero, or a defect on the face and a defect on the back will not land on the same metre. Light from one side must not enter the picture of the other.
Zero and light leak
If the two cameras each keep their own frame count, at rewind you do not know whether 100 metres on the face is 100 metres on the back. At start, zero both counts from the same splice or the same roll start. A turn-bar setup has only one camera, so the metre count is the same by nature, but after the turn you must confirm the web is not flipped left-right so far that it no longer matches the operator side.
If face light passes through a thin web, it becomes backlight on the reverse. Dirt on the reverse is washed out, or bright spots appear that are not there. Hood the lamps, or stagger the two sides. Do not aim them at each other.
What to check
| Item | Pass | Fail |
|---|---|---|
| Metres | Both sides share one zero | Each counts its own frames |
| Light | Neither enters the other picture | A thin web is lit through |
| Across the web | Both write distance from the operator side | After a turn, left and right are reversed and not declared |
Check at a job change
- Put one mark on the face and one on the back at the same metre.
- See whether the two reports give the same metre number.
- Switch off the lamps on one side. Brightness on the other side should not follow.
