The strength of a corrugated box starts with its material.
A corrugated sheet consists of two major components – linerboard and medium. Linerboard is the flat paper that covers both sides of the sheet and the medium is the “fluted” or arched paper found between both liners. The flute, when anchored to the linerboards with a starched-based adhesive, resists bending and pressure from all directions. When placed vertically on its ends, the flutes form vertical columns, capable of supporting considerable amounts of weight.
A way to be sure that the material of the box that you’re purchasing meets industry standards is to look for the Manufacturer’s Certification Stamp, usually printed on one of the bottom flaps of the box. The stamp identifies the material as “singlewall,” “doublewall” or “triplewall.”
|Minimum Bursting Test, Singlewall, Doublewall (lbs. per sq. in)||Minimum Edge Crush Test (ECT) (lbs. per in. width)||Minimum Combined Weight of Facings, Including Center Facing(s) of Doublewall||Maximum Weight of Box and Contents (lbs.)||Maximum Outside Dimensions, Length, Width, and Depth Added (inches)|
It also certifies the Mullen Bursting Test ( most common is 200 lbs. per square inch) or the Edge Crush Test (ECT 32 lbs. per inch). The Mullen Test measures the bursting strength of the corrugated linerboard while the Edge Crush Test measures linerboard stacking strength. The 200 lbs. Mullen box and the 32 ECT box are comparable in stacking strength. But that is where the similarity ends. The Mullen Test box is better suited for the protection of heavier contents while the Edge Crush Test box provides lighter weight cartons with good stacking characteristics. When in doubt as to which box to use, always select the Mullen Test box because the construction of the linerboard cannot be altered. For example, 200# Mullen Test linerboard must be made of 42# per square inch basis weight paper and the medium or flute paper cannot be less than 23# per square inch basis weight.
Flutes come in five basic heights and shapes – the most common are “B-Flute”(used for die-cut boxes) and “C-Flute (used for RSCs). B-flute is compressed and appears thinner, but don’t be fooled. It is made with more paper to provide stronger side wall protection from blows and punctures. C-flute is taller, with more air space, but offers enhanced stacking strength. For excellent graphic reproduction, consider E-flute.
The amount of virgin pulp fibers and the length of those fibers in a corrugated sheet substantially contribute to box strength. For example, the difference between a 200# test box and a 275# test box is that the latter has more pulp fibers in its corrugated linerboard. The 200# test box is rated to hold up to 65 lbs. of box and contents while the 275# box can hold up to 95 lbs. A 350# test box is rated to hold up to 120 lbs. of box and contents. The following chart shows Bursting Test and Edge Crush Test performance standards of corrugated box liners based on the combined weight of a box and its contents.