I need to print cutting lines onto black card and the ink is invisible — what actually works?

If you do want marks directly on the card, the mechanism matters and it explains why your inkjet fails.

Inkjet ink soaks in. On black paper the ink is absorbed and sits below a black surface, so nothing shows. There is no ink density that fixes this.

Laser toner sits on top. It is melted plastic fused to the surface rather than absorbed, so a light toner colour on black card is often faintly but usably visible. If you have access to a laser printer, printing your lines in yellow or light grey is worth one test sheet.

White toner printers exist and are genuinely the right tool, but they are specialist equipment and a print shop is the realistic route if you need it often.

For one-off marking by hand, a white or silver gel pen, a chalk pencil or a white coloured pencil all show clearly on black card and rub off or hide on the inside of a fold.

Since the marks do not need to survive, the simplest answer is far better than any printing trick: print on ordinary white paper and use it as a template.

Two ways:

• Temporary adhesive. Spray adhesive or a repositionable glue stick, stick the printed sheet to the black card, cut through both, then peel the paper off. Cut lines, fold lines and numbers are all visible while you work and gone when you finish.
• Print and pierce. Lay the printed sheet over the card and prick through the corners and key points with a needle. Remove the paper and join the dots with a bone folder or a light score.

The first is faster for complex shapes, the second leaves absolutely no residue. Both avoid the entire problem of getting a mark onto black card.

A cutting machine, if you have access to one, solves it a third way — it cuts from the file directly and never needs to mark anything.

One caution on spray adhesive with black card: test the peel on a scrap first.

Some black craft card has a coated surface that lifts when tape or adhesive is removed, leaving a pale scar that is far more visible than any pencil line. Repositionable adhesives are much safer here than anything described as permanent, even for a temporary application.

A different angle that people often miss: do the folding and cutting from the back.

Most black card is black on both sides, but a lot of craft card is coloured on one face only. If yours has a lighter reverse, print or trace on that side, cut from there, and score so the fold goes the right way. The finished model shows only the black face.

Even with card that is black throughout, working on the back means any stray pencil mark ends up inside the model. It is worth checking which side you actually need to be clean before deciding how hard the marking problem is.