TL;DR
Folding angle variation produces boxes that do not close square. Check folding rail alignment, folding belt tension, and the squareness of the incoming blanks.
What you might see
- box corners not at 90 deg on finished box
- boxes racking or parallelogram under stacking
- fold angle varying from the machine start to end of run
- incoming blank scored lines not consistent
Likely causes
Folding rail misalignment creating a non-perpendicular fold path
Folding belt tension uneven between left and right sides, causing the blank to skew
Score line quality on incoming blanks inconsistent, leading to variable fold resistance
Box blank squareness varying from the upstream die cutter
Required tools
- Precision square and tape measure
- Belt tension gauge
- LOTO kit
Safety first
- Lock out the folder-gluer before reaching into the folding or delivery section.
- Running nips in the folding section are pinch-point hazards. Keep hands clear during production.
Procedure
- 1
Measure the diagonal dimensions of 10 finished boxes. Boxes out of square by more than 2 mm require investigation of the fold section.[1]
- 2
Check folding belt tension on both sides of the machine. Unequal tension skews the blank in the fold section. Adjust to equal tension per the machine specification.[1]
- 3
Verify folding rail alignment. Rails should be parallel to the machine centerline within the tolerance in the machine manual.
Warning: Lock out the folder-gluer before reaching inside the folding section to adjust rails or belts. - 4
Measure the squareness of the incoming blanks. Use a precision square on the corner of 10 blanks. Blanks out of square more than 1 mm compound into fold angle error.
- 5
Check the score line quality on the incoming blanks by hand-folding 5 blanks. Scores should break cleanly and uniformly across the full width.
- 6
If blanks are square and scores are good, perform a step-by-step fold rail alignment check from entry to exit of the folding section.
- 7
After adjustments, run 20 boxes and measure diagonal squareness before returning to full production.
Sources
Bobst Bobst Mastercut / Expertfold / Vision Fold Die Cutter / Folder-Gluer general technical documentation, Bobst
Folder-gluer folding section alignment and box squareness, general packaging machinery practice (general)
More guides for Bobst Bobst Mastercut / Expertfold / Vision Fold
How to fix die cut quality and register on a Bobst Mastercut Die Cutter
Die cut registration errors produce off-center cuts and wasted material. Check sheet feed timing, die lock-up, and impression pressure before adjusting register settings.
How to fix glue skips on a Bobst Expertfold Folder-Gluer
Glue skips cause box failures at the glue joint. Check glue pump pressure, nozzle cleanliness, and glue temperature before adjusting the application timing.
Stop fixing the same fault twice.
Dovient turns guides like this into your team's shared playbook, with AI that catches recurring issues before they break the line.