askralph

Often we are asked through the blog, directly in emails, or over the phone about how to determine the maximum measurable combined board caliper off the corrugator. This is not always an easy answer. As we shared in the recent webinar series, “What Convertors Need to Know About Board Crush,” the medium at the singleface can be compressed at the flute tip by as much as 35 percent of its thickness and its spring back recovery is not always complete. See the formula at the end of this article and see how this typically 3 thousands shrinkage loss is included in the caliper formula.
During the webinar at the end of January we shared how caliper is lost at various stations on the corrugator. Corrugated board can have a remarkable spring back or recovery from significant crush or deformation during the combining operation. The Institute of Paper Science and Technology has published studies that show 50 thousands actual board crush may only result in 8 thousands loss of recovered caliper.
Determining actual caliper potential at the end of the double backer is not as straight forward as one would believe. Following is the only formula I have discovered for estimating caliper potential. It must be remembered that containerboard calipers vary by grade, both liners and medium. In an example of 35/36# performance liners the calipers would range from 8.5 to 13.6 thousands, and 23# medium even though it does not play in the equation has caliper ranges from 6.6 to 9.8 thousands.
Here is a singlewall example:

Thickness of the two liners ____
+ Flute height ____
– Flute wear allowance ____
– Crush Allowance 0.003
= Potential Combined Board Caliper

You have to establish your own expectations based on the paper machines that supply you. Consider purchasing the second edition of “Understanding the Key Characteristics of Linerboard and Medium and Their Impact on Combined Corrugated Board” for more insights on various physical properties.

You may have heard me state that caliper is a very poor predictor of in-use combined board performance. Look for new and better ways to measure the actual loss in board strength as a result of crushing.

— Ralph

This entry was posted on February 10, 2014 at 2:36 pm and is filed under Corrugating. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.