Selecting a channel from grammage alone assumes that boards of the same weight have the same caliper and fibre behaviour. A safer starting point measures the real board and the crease rule together. The same measurement is part of the file assessment for a creasing counterplate order.

Measure real caliper, not nominal grammage

Board thickness can vary across a sheet and between batches. Measure at least three sound points with a micrometer, avoiding crushed edges and coating bubbles. Record the range as well as the average in the job note.

Measuring points are chosen from the middle and both edges of the sheet; where batches differ, the thickest and thinnest values are written together, because the channel has to pass the thickest sheet without cracking and the thinnest without wandering. Without a micrometer, the caliper on the supplier’s data sheet is enough to start.

  • Measurements are taken at the middle and the edges, at least three points.
  • The thickest and thinnest values are written together in the job note.
  • Without a micrometer, the caliper on the supplier’s data sheet is used.

Depth and width have different jobs

Depth influences how far board is displaced into the channel. Width controls the room available for fibres around the crease rule. A starting calculation relates depth to material caliper and width to both board and rule thickness.

If depth is insufficient the board does not enter the channel fully and the fold line stays shallow; if width is insufficient the fibres are pinched and cracking starts on the outer face. The two are set in turn, not together: depth from caliper first, then width from rule thickness; one variable per step.

  • Depth is set from board caliper, width from board plus rule thickness.
  • A shallow fold points to depth, cracking on the outer face to width.
  • One variable is changed per trial.

Grain and finish change the result

A fold across the grain can show higher resistance and cracking risk. Film, varnish and heavy ink coverage change surface extension. Calculated dimensions are therefore a proof starting point; the final decision belongs to the real printed substrate.

The proof is made on the real printed board, on two lines, one along and one across the grain; a line that folds cleanly on raw board can crack on a varnished surface. On heavily printed fold lines the channel is chosen a little more generously and the result is checked on the folded sample under light.

  • The proof is made on real board that is printed and finished.
  • Lines along and across the grain are tried separately.
  • On heavily printed lines the channel is more generous; the result is checked under light.

Use the defect pattern to adjust

A channel that is too tight or shallow can crack and spring open. An excessively wide or deep channel can create a soft shoulder and visible marking. More pressure is not a universal correction; verify the channel-material match first.

The defect pattern gives the diagnosis: a box that cracks and springs open means a narrow or shallow channel; a soft shoulder and visible mark mean a wide or deep one. More pressure makes both patterns worse; the channel-to-material match is corrected first, and pressure is the last and least-touched setting.

  • Cracking and spring-back mean a narrow/shallow channel, a soft shoulder a wide/deep one.
  • Raising the pressure enlarges both faults.
  • The channel-to-material match is corrected first, pressure last.

Application note

Send the board sample, measured caliper and crease-rule information together.

  • Use the real production substrate and final file revision.
  • Correct one variable at a time.
  • Keep the approved sample with the job record.