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. This check is part of the file assessment for the related production service.

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.

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.

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.

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.