What are corrugated flutes?

The principle is structural rather than material. A flat sheet of board bends easily; the same board formed into a series of arches and glued between two liners resists both crushing and bending far better, at almost no extra weight. It is the same reason a corrugated roof is stronger than a flat one.

Flute height sets the trade-off. Taller flutes (B) give more cushioning and stacking strength but a coarser surface, so print detail suffers and fine type can look ridged. Shorter flutes (E, and shorter still F) give a smoother surface that prints much better and a thinner, neater box, at the cost of some protection. Heavier double-wall constructions exist for transit cases where protection is everything and print is not.

For most branded retail and e-commerce mailers, E flute is the usual answer: thin enough to print well and fold neatly, strong enough for a courier network. B flute is the sensible step up for heavier contents or rougher handling, and F flute is used where print quality matters more than protection — small premium boxes, food packaging.

Flute direction matters as well as height. The board is far stronger along the flutes than across them, so how the box is laid out on the sheet affects how it survives being stacked. That is decided at the dieline stage.