John asks:
Could you tell me the acceptable level of moister in 90# liners after they have been converted? We have an issue with 600# B/C converted boxes that crack at the scores. Our customer will not accept them and the corrugator that supplies us states that they have to apply enough heat to acquire the correct bond and that sometimes that allows the liner to crack. They say that they ran a moisture test and the moisture is 6.48%, but it does not affect the strength of the container. Is this correct?
Interesting that you bring this question forward now as I am creating a white paper on warp, bonding, and paper and board moistures.
However for a more immediate answer I would ask your supplier three questions. First , how they measure combined board moisture? Second, I would ask what their moisture targets are and if they experience any variations in the containerboards they purchase? Finally, I would ask what their pin adhesion values? Your sheet supplier, especially if it is an integrated, should be able to provide an internal expert on scoring to visit you operation.
Corrugators operate on a delicate balance between running an ideal seven percent combined board moisture and achieving the proper bond. Combiners with the proper process controls can control paper temperatures through the manufacturing phase, but cannot measure moisture with economically available controls.
Once you know what moisture contact you are working with, I would suggest that you contact one of the industry suppliers of creasing profiles to see if they can help you determine the best profiles given the moisture content of the combined board.
— Ralph
askralph 
March 3, 2015 at 11:09 pm
Seems to me that 6.48% moisture is a touch on the low side. Typically I would be looking for board moisture 6.5 to 8.5% at the converting operation. Moisture has to be added to the corrugating process to form a flute and make a bond. We need to remove excess water but not the water given to us by the mills in their paper. So, as a rule of thumb, add the moisture content from all 3 liners and 2 mediums and divide by 5. Assuming 7% for liners and 9% for medium that gives you 39 divided by 5 = 7.8% combined board moisture. It’s a crude calculation but it makes the point.
It’s winter and you are being supplied by a sheet feeder. His trucks are cold and he may use recycled paper (?) so it’s going to be more your challenge than his. If you find poor pin adhesion with gritty, dry glue lines… that points to corrugator over-heating and that’s his issue.
Curious about which liner is cracking: Check to see if you’re running the heaviest liner and the biggest flute on the inside of the box. Often sheet-feeders like to run their board “upside-down” because they’d rather run the heavy liner on the double-back side. It makes life easy for them but you have to flip the sheets.