Clint asks,
I have a question that I hope you can answer. When using the same liner board on both the inside and outside of a singelwall sheet, why do you see more of the “flute lines” on one side versus the other? What causes that and does it have more effect on fracturing the liners when you are creasing one side versus the other?
This came up when one of my customers was laminating a litho label on SInglewall. They normally laminate it to the good side (less visible fluted lines). Because some of the sheets were slightly warped, they laminated some on the bad side (more lines). After converting there was some fracturing of the litho (not liners). We saw less fracturing of the litho on the labels that were laminated to the bad side. From a logical standpoint, if you put the label on the bad side and it is a weaker paper, I think the label may stretch easier versus being glued to a heavier liner. Also, the inside of the box would look better (less visible flute lines) so I would assume this should be the norm unless you are just spot labeling something. Keep in mind, the liners on the inner and outer were supposedly the same paper.
What are your thoughts on this?
Thank you for an excellent question. Yes, I can add some insight.
Tensile strength and stretch of the liners are impacted differently in the singleface and doubleback operations. Also temperatures and starch application are different in the two operations. Fracturing can occur in one liner and not the other. Learning flutes can have an impact. Microphotographs of the cross section of the combined sheet can begin to reveal some issues.
Of course starch application and amount can signal issues as you described. World class consumption would be about 1.4 # per msf.
– Ralph
askralph 
May 9, 2017 at 12:28 pm
Visible flute lines on the single-face side of the sheet likely point to the fact that your supplier is using a pressure-roll single-facer versus pressure-belt. This is completely normal. The cracking when you laminate to the opposite side is another issue. Based on what you say, I gather your warp was up-warp. So… laminating to this “good side” means you are laminating to a liner that has shrunk due to excess moisture, and… you are asking this sheet to stretch even more than it normally would have to but the tensile strength is not enough. That’s how you get increased cracking along score lines. Questions: Are you die-cutting from the label side? Are you using the proper profile of CITO matrix?
May 9, 2017 at 12:34 pm
Sorry, I meant to say “down-warp” above. This would indicate the corrugator had a moisture profile imbalance with the higher moisture profile on the bottom (the “good”) liner. When this side dries it also shrinks and when it shrinks it loses the ability to stretch. As a result, it cracks when you try to bend it.