Box Crush Test and the Chalmers DST

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BCT versus Board Crush

It has been shown many times that boxes made from crushed corrugated board do not perform as well as boxes made from uncrushed board. In the Oct 1989 Paperboard Packaging Journal, Joseph Bick presented a paper called “The Cost of Crushing Flutes is High”. Figure 1 shows Bick’s data where BCT compression loss and the increase in paper costs to allow for this BCT loss are plotted against percent board crush.

Figure 1: Bick’s data of BCT and paper costs versus percent corrugated board crush.

Bick also made the comment in his paper that “Crush is difficult to measure and commonly understated”.

Chalmers (Appita Journal Sept 2007) also showed that board crush adversely affects BCT but also has a far more dramatic effect on performance in the service environment as simulated by constant compression under a cyclic humidity environment. Figure 2 shows BCTs and cyclic humidity performance of three corrugated boxes made from uncrushed board and two levels of crushed board. The degree of crush is measured by Chalmers DST bpi levels (MDTS). On the left hand side of Figure 2 are the cyclic humidity performance curves where for example if the box has to sustain a 80 kg weight in the service environment the uncrushed board (bpi = 24) would last for 40 cycles, the board crushed to 21 bpi would last 15 cycles and the box made from the board crushed to 14 bpi would only last 8 cycles.

Figure 2: BCT and cyclic humidity longevity results for 3 levels of board crush. Uncrushed board (24 bpi) had a BCT result of 4.65kN while the most severely crushed board (14 bpi) had a result of 4.28 kN.

What has definitely been shown over the years is that uncrushed board gives the highest BCT level that the components and manufacturing equipment is capable of at the time of manufacture. There are many ways to improve the quality of the manufactured boxes and that is usually achieved in two stages by eliminating crush during conversion first then optimising the processes on the corrugator to make stronger board off the corrugator.

BCT testing unfortunately has many problems in its implementation. The major problem is the large standard deviation of the results and the time and resources required to try and minimise this figure. Boxes have to be pre-conditioned at about 30% RH then conditioned to 50% RH. This takes a lot of time and space and can only be done in conditioned laboratories. This is not suitable for online QC as the boxes are usually long gone before the results are obtained. But even on well conditioned samples the standard deviation is still high and many boxes are needed to be 90% sure of the final mean result.

DST on the other hand is very quick and reliable and you can guarantee that for any box design that if the board is made well and not crushed as measured by the DST then the BCT of that box will be maximised. Once you start crushing as measured by the DST your BCT will be heading down.

Why is BCT not as sensitive to crush as cyclic humidity lifetime performance? All cellulose fibre products are subject to creep when under load (compression or tension). This is a time dependant property that is made worse in corrugated boxes by cycling humidity. Damage to the structure of the board by crush significantly speeds up the creep process. BCT on the other hand is a short time test that has very little creep component. The faster you do your BCT test, the stronger the box will appear to be. Long term stacking tests using deadweights in a controlled atmosphere will show significantly reduced results compared to the BCT because of the creep effect only.

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