Buckle and Knife: Two Ways a Folder Makes a Fold

A folded brochure and a folded book signature look alike on the table. The creases may have been made by two different machines, and the choice between them is settled by paper weight, fold direction and the number of folds needed in one pass.

Buckle folding

The common folding machine works with buckle plates. The sheet runs between two rollers, hits a stop plate, and the paper behind it keeps moving. The sheet has nowhere to go, so it buckles upward into the gap between a second pair of rollers. Those rollers catch the buckle and pull it through, pressing a fold into the paper at the point where it bent.

The position of the stop plate sets the fold length. Move the stop a few millimetres and the panel changes size, which is how a 100 mm roll fold and a 210 mm gate fold come off the same machine.

A buckle folder makes one fold per plate. Four plates in a row give four folds, all parallel to each other, all in one pass, at speeds that reach tens of thousands of sheets an hour. That is why parallel folds are cheap.

Knife folding

A knife folder does the opposite. The sheet is held flat on a table, and a blade comes down along the fold line, pushing the sheet through a gap between two rollers below. The rollers take the fold and pull the sheet through, so the crease forms where the knife pressed.

A knife fold is slower and takes more floor space, but it handles stock a buckle plate cannot. Heavy cover board, thick signatures, and paper above roughly 170 gsm tend to crack or refuse to buckle evenly. The knife pushes the fold into place instead of relying on the paper's own willingness to bend.

Knife folders also make right angle folds easily. A second fold at ninety degrees to the first cannot be made in a straight buckle run, because the sheet is now twice as thick at the spine and the panels are no longer aligned with the plate.

Where each machine stops

Buckle plates work best on uncoated and lightly coated stocks from 60 to 150 gsm. A coated 170 gsm sheet will fold in a buckle plate, but the coating can crack along the crease, and the crack shows as a white line on a dark solid. Above that weight the paper resists buckling, the fold lands off the line, and the sheet jams.

The usual solution for heavy stock is a combination folder: buckle plates for the light parallel folds and a knife unit for the cross fold, or a creasing unit ahead of the fold. A crease compresses the sheet along the line so the coating stretches instead of tearing when the fold is made.

Grain and the direction of the last fold

Paper folds cleanly along the grain and resists across it. A fold across the grain puts the fibres under tension, and a coated surface can crack at the fold before the sheet is even pressed flat.

On a multi-fold job the last fold is the one a reader sees, so it should run parallel to the grain. The imposition decides the rest. If the page layout forces a cross-grain fold at the end, the printer needs to know before the paper is ordered, because a different sheet size or a different signature can sometimes move that fold onto the grain.

What the crease does to the panel

Every fold takes up space. The paper has thickness, and at the spine the fold needs room to turn. A 16-page signature on 100 gsm stock folds into a spine about 1.6 mm thick, and the panel at the fold pulls inward by roughly that amount.

If the design places a heavy solid across the fold, the fold line becomes a light line, because the paper is stretched at the crease and the ink film breaks. Printers call it a fold crack. Scoring reduces it, and so does a paper with a softer coating. On a perfect bound cover, the same effect appears at the spine, where the cover wraps around the block.

Register between folds

Each fold after the first works on a sheet that may be a fraction out of square. A buckle fold references the sheet edge, so a sheet trimmed 0.3 mm out of square gives a fold 0.3 mm off at the far end. On a right angle fold, that error doubles and shows as panels of unequal width.

Folders have a register guide and a movable stop for the second fold, and the operator sets them against the printed line rather than the sheet edge on critical work. That is why a brochure with a wide colour band across the fold costs more to fold than a plain text page.

What to check in a dummy

Ask for a folded dummy on the actual stock before the run. Open it flat and look at the crease: a cracked coating shows as a pale line and will get worse with handling. Check that the panels line up at the head and foot, and that the innermost panel does not push past the outer one.

Then fold the dummy again along the same crease, as a reader would. Paper that has been folded once takes a second fold badly if the first was scored too deep. The dummy shows that in a minute, and the press run cannot show it until the sheets are printed.