Box Crush Test and the Chalmers DST

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Summary

BCT can be predicted from paper properties and box dimensions but only for undamaged boxes.

Crushing corrugated board lowers the boards MD Torsional Stiffness (MDTS) which is the most sensitive structural property of corrugated board. Failure of MDTS leads to bending stiffness failure which leads to box bulging which leads to compression failure which leads to box collapse.

With enough test data, the BCT for any one box type can be predicted from MDTS with the uncrushed highest MDTS box having the highest BCT. Because the BCT of a box is reliant on box dimensions etc, MDTS cannot be used alone to predict BCT and data from different boxes should not be mixed without careful consideration.

The Chalmers DST is the fastest and most accurate way to measure MDTS and can be used at any point of the manufacturing process for speedy QC.

Did you ever wonder why your hand-made box samples were always better for BCT than the production result?

What makes BCT?

Where does a box get it’s BCT from?

It has been shown by McKee et al that the BCT of a box can be largely predicted from the components used to make the corrugated board, the flute type and the dimensions of the box. The Ring Crush test (RCT or SCT) of the components can give the ECT result which can be used (McKee) to estimate the BCT, see below.

ECT and BCT can be calculated from paper components using the sum of the RCTs of the components and the McKee Equation. This should be the best result obtainable from the components used.

• ECT = k(RCT L1 + RCT Med xTUF + RCT L2) TUF = take up factor
• BCT = 5.87 x ECT x Caliper x Box Perimeter kN (McKee)

Neither of these formulas incorporate a factor for how well the board is made or treated.

The ‘Structure’ is ignored and McKee warned that the equation may not apply if boxes had fabrication defects such as crush, low flat crush, leaning flutes or poor adhesion. Also the height of the box is not considered. The caliper in this case is largely the measure of the flute type rather than the crushed board thickness. One of McKee’s original equations used the square root of the multiple of the md and cd bending stiffnesses which would be affected by crush but these stiffness are very difficult to measure and caliper was substituted to make easy use of the McKee equation possible though accuracy was lost.

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