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Board Strength
The bursting strength of corrugated board is measured in the number pounds per square inch required to break through the board. This is often called the Mullen Test of the board.
Attention is turning away from this traditional measurement of bursting strength to a measurement called Edge Crush Test (ECT) which is the edgewise compressive strength, parallel to the flutes of a short column of corrugated board. Test results are reported as the pound-force per inch required to cause compressive failure.
A board's strength is largely determined by the different weights of linerboard facings and mediums used in its construction. These are identified by their basis weight (e.g. 26#, 38#, 42#, 69#). The basis weight is the weight, in pounds, of 1000 square feet of the linerboard or medium. Various combinations of linerboard facings and mediums are used to achieve the different strengths of corrugated board. For example:
200# sw Test (ECT 32) is comprised of two 42# facings (one on each side of the medium).
275# sw Test (ECT 44) is comprised of two 69# facings (one on each side of the medium).
250# swTest (ECT 40) is comprised of one 42# facing and one 69# facing (one on each side of the medium).
The minimum combined weight of facings is stated in the box maker's certificate along with the bursting strength and/or the ECT. See the Conversion Chart.