Impact of inner liner on RDC stacking strength

Don asks:

Can you explain (technically) how bumping up the inside liner on an RSC box increases stacking strength has a big impact?

In most distribution environments corrugated boxes in compression situations fail in a two part mode. First the corners of the box take the load of the boxes above them and this is best predicted by ECT. Then the side walls of the box begin to bulge and we predict this by testing for four point bending stiffness or flexural stiffness.

This latter physical property is dependent on the caliper and the stiffness of the board. Since side wall deflection begins with the deformation of the inside liner this is the component that one should begin with to add basis weight, caliper and stiffness.

This is the scientific side of the construction, but medium is the most cost effective way to build more ECT. There is a balancing act. Of course this all depends on the corrugator’s ability to maintain the strength of the containerboards and preventing or minimizing crush to the sheets during converting.

— Ralph

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One Response to “Impact of inner liner on RDC stacking strength”

  1. Brett Kendall Says:

    I’d just like to add that we’re finding unbalanced constructions to be a key tool in our efforts to optimize board constructions and reduce excess / unproductive fibre. As Ralph states, ECT is simply vertical compression so the specific location of the fibre really doesn’t matter. For bending resistance, it’s the inside liner that offers the greatest compression resistance in a bulging box. However, two points…

    a) We don’t really see the benefit of a heavier inside liner when testing boxes in a lab. First of all, you’ve really got to trend your data due to the enormous +/- in BCT but… empty boxes on a compression tester tend to buckle IN while a product-filled box in a warehouse bulges OUT. It’s possible that a heavier inside liner has a negative impact on lab-tested BCTs.

    b) Not realizing the importance, many corrugator supervisors will move the heavier liner to the double-backer in order to pick up their run speeds and/or reduce concerns of added SFer waste. In doing so, your added BCT efforts have just been thwarted.

    Corrugator crews don’t like unbalanced liners over fears of warp and post-warp but these concerns are often exaggerated unless we’re talking about very large imbalances. We also see some corrugators running double-wall with the larger flute profiles on the outside. This has a similar negative effect on bending resistance and BCT but zero impact on ECT.

    From a structural engineering view-point, it would be interesting to know the ideal SF versus DB weight ratio to maximize the compression/tension relationship for the best flexural stiffness response….

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