How to score and fold thick card without cracking

Paper is a mat of fibres. When you fold thin paper the fibres bend; when you fold thick card the outer fibres have to stretch further than they can and they tear, leaving a ragged white line along the fold. Printed card shows it worst because the ink or toner layer splits along with the fibres. Scoring pre-compresses the fibres along the fold line so they bend in a controlled channel instead of tearing. Anything above about 200 gsm needs it, and 160 to 200 gsm looks noticeably better with it.

Laser prints add a second problem. Toner sits on the surface as a thin plastic film rather than soaking in, and a hard crease through a solid area of toner cracks the film even on thin paper. Score first, and where you can, fold with the printed side inside so the toner is compressed rather than stretched.

A bone folder is the proper tool: a smooth blade of bone or plastic that scores and then burnishes the crease flat. If you do not have one, an empty ballpoint pen, a knitting needle, the back of a butter knife or the tip of a closed pair of scissors all work. The requirement is a rounded point that dents without cutting. You also need a steel or hard plastic ruler to run it against, and ideally a slightly soft surface underneath such as a magazine or cutting mat so the score has somewhere to go.

Lay the card flat on the soft base with the printed side up. Line the ruler up with the fold marks; Foldpic prints them as short grey ticks at the top and bottom edge of the sheet so they never cross a photo, and the instruction sheet says which folds are mountain and which are valley. Hold the ruler firmly and draw the scoring tool along it in one steady pass with firm, even pressure. You want a visible dent, not a cut. Do not go back and forth, which widens the score.

Which side to score is the part people argue about. The traditional rule is to score on the inside of the fold, so the crease folds toward the dent, because that gives the crispest edge. Many card makers prefer to score on the outside, so the card folds away from the dent, because it cracks less on printed surfaces. For an accordion the folds alternate, so score every line from the printed side and fold mountain lines away from you and valley lines toward you; test on a scrap of the same card and use whichever cracks less.

Fold along the score with your fingers, lining up the edges, then burnish the crease with the flat of the bone folder or the ruler edge. A sharp crease is what makes an accordion stand up straight. For a zig-zag, fold all the mountain folds first, then turn the strip over and fold the valleys, checking that the pattern alternates.

Grain matters more as card gets thicker. Card folds cleanly along the grain and roughly across it. A4 and Letter sheets sold for home printers are usually long-grain, so folds parallel to the long edge are the easy ones, and the vertical folds of an accordion strip run across the grain. Scoring compensates, but if you have a choice of stock, buy the one whose grain runs the way you will fold; a scrap test-folded in both directions tells you instantly.