Effect of moisture on corrugated sheet size

Steve asks,

I really appreciated the article on litho mounting…good stuff and thanks!

On a somewhat related topic…we are die cutting f flute for an application in which dimensional accuracy-and stability- are very critical. Particularly the body score dimensions, though all dimensions including depth are important. One of the issues we are concerned about is the effect of humidity on the growth of the finished product. We do not have environmental controls on our production floor and in summer, the humidity can get quite high, and change day to day. Conversely in winter we deal with very low levels.

We know this affects the sheet sizing…but to what degree? Is there a matrix for various board grades which may show the percent of growth in grain /cross grain direction?

What should we expect to see?   Any guidelines you can offer would be helpful!

I can address the issue of the magnitude of the changes in the finished product from changes in relative humidity. The literature reports that from zero to 80% relative humidity sheets can grow up to 1.6 percent parallel to the flutes and up to 0.7 percent perpendicular to the flutes. I believe there will be changes also in the Z direction (thickness of the sheet). There is a difference between kraft/semichemical containerboards and those with high amount of recycled content.

As the sheet takes on moisture it also becomes “soft” so depending on the die cut method, the sheets could be “squirming” around in the equipment. Do you index to one side of the sheet? Is there print image differentiation from one panel to the next?

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