What is a glue flap?
A glue flap is the narrow strip left on the edge of a dieline so a folding carton can be closed. Glue is applied to the strip and the flap is bonded to the opposite panel. No graphics or text go on it; the strip is what turns flat board into a three-dimensional box.
Look at a flat box layout and you will see a narrow tab next to one of the panels: that is the glue flap. When the box is closed, the flap sits against the inside of the opposite panel and is fixed with glue. Most folding cartons have a single glue flap; its position and width depend on the structure of the die.
On a dieline the glue area is usually shown as a hatched zone. This is where the glue sits, and no artwork goes there. The edge of the flap is often slightly angled, which makes assembly easier and prevents snagging while the box is folded.
Two points matter for design. First, do not place text or a logo on the flap: once folded it stays inside and cannot be seen. Second, the flap must hold glue well: coated and glossy surfaces grip glue poorly, so printers often leave this area free of lamination or varnish. Discussing this with your printer in advance keeps the workflow smooth.
A glue flap is not limited to outer boxes; some structures, such as sleeves, can have one too. Because it is not the same in every die, check the drawing of your own die when you prepare the file.
How you do this in Grapxo
In Grapxo, cut, fold and glue lines come on three separate layers as editable vector; the position and width of the flap depend on the box structure. The exact die is confirmed with your printer.
For more, see Reading a tuck-end dieline.
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