Trending Content

Summer Reading List

By AICC Staff

September 13, 2018

I don’t know about you, but I hated hearing about my reading list during the last week of school. This article is only, at your leisure, to give you a preview of three major educational resources that will be available no later than the Fall 2018 AICC/TAPPI Corrugated Week meeting. They are an e-learning segment, Warp: Its Causes and Remedies, and then two white papers, Microflutes and Paperboard and Score Cracking: Its Causes and Remedies.

So, to tease you a bit regarding two of these new resources from your Association and to let you know there has been extensive time invested in these offerings, we are giving you a peek inside the offerings before their release.

Here are the questions that will be asked and answered from the section on warp.

Quiz #1

  1. Can containerboard moistures change over time?
  2. Am I all alone, or are their resources and people to help us? What is the maximum amount of moisture variation in liners for good bonding and minimum warp potential?
  3. Does linerboard expand and contract with moisture changes? What specific data should I seek from my suppliers?
  4. Can there be differences between virgin and recycled fibers?

Quiz #2

  1. Are corrugator temperatures critical to bonding and warp? Is moisture added or subtracted during the corrugating process?
  2. What does washboarding usually indicate?
  3. Yes or no? Older corrugators with more heat are better at flattening warp in the combining process. Name some corrugator conditions that can lead to warp.

Quiz #3

  1. What is the most common type of warp in your plant? What are the major causes?
  2. What are the major remedies? What elements of starch and its application affect warp?
  3. Where is parallelism most critical in the corrugating operation? Will you contact us if you have more questions or suggestions?

Here is a section from the white paper on score cracking.

Paper Physics

Containerboard, as we know it here, is a wood-based product made from cellulose fibers, just like cotton. Most of the other portion, hemicellulose, which acts as a natural paper strength additive, is lost in pulping. The first generation of newly pulped wood chips produces virgin long-length fiber with many bonding sites that allow the individual fibers to “stick together” both physically and chemically. Paper is a viscoelastic material with properties much like those of elastic binding or rubber bands. It is ductile, which means it is able to form around itself up to a point. It can be stretched and returns to its original shape and size unless it is broken or ruptured in the process. This is the stretch-to-break value. Virgin fibers are generally more pliable than recycled fibers, white-tops, and clay-coated boards. The formability of linerboard is also impacted by the applications of starch coatings and the low freeness of the pulp in the top ply.

To the extent that paper properties are responsible for score cracking, the key factor is stretch. Any factor that reduces paper stretch adversely affects score cracking. Stretch is a function of fiber length, width, degree of fiber entanglement, fiber-to-fiber bonding, and moisture content.

Score cracking can be caused by high-density papermaking and the use of recycled fibers. Highly pressed wet fiber mats of fibers, when dried, have less room to flex and bend.

We use the term “tension stress” to describe the ultimate strength that the containerboard sheet (strip) contains when it is pulled end-to-end, both MD and CD; its elongation (strain) is the percentage it will stretch until the point of failure. Both of these properties should be obtained from your linerboard suppliers, although sometimes not easily, but they are very good predictors of score cracking potential. Most mills have adjusted the MD/CD strength ratios over the years, but more MD than the CD, while the CD has about twice the elongation to failure as the MD.

The key factors related to paper cracking are stretch and moisture. Paper properties and corrugator temperatures are the most important elements to address to minimize score cracking.

Those 140 paper machines out there have an average chronological age of more than 40 years, but a technology age of less than 25 years. Once again, these are averages, and every printer and converter needs to ascertain as much about possible unique paper properties each machine imparts to its products.

Of course under TAPPI testing methodology, all labs operate at 72 degrees Fahrenheit and 50 percent RH. Both combined board moistures below 5.8 percent and RH below 50 percent create conditions for score cracking.

Papermaking processes include Fourdrinier vs. gap formers, single-ply dual or multiple-ply, recycled vs. virgin, and brown vs. white, to which, under scoring pressure, ply separation can occur. Traceability of cracking must be specific to a mill and then to an individual machine. There are 140 of them out there! Suppliers can also provide certificates of analysis comparing actual physical properties to agreed-upon specifications. A plant needs to know any changes that the doubleback liner supplier makes to its processes. There has to be open and constant communication. All participate in the end result.

What happens with increasing moisture is that papers swell, expand, and relax.

I am a large proponent of the use of heavyweight and stronger mediums to build cost-effective structural combined board. However, they can add to score cracking if they contribute too much rigidity to the board construction and score profiles have not been changed. Dual-arch constructions can be converted without fracturing, so it can be done.

Please get your hands around all the variables we have discussed in this paper. As we recycle more and more, additional types of fibers, both domestic and foreign, enter our “recovery” stream, making it more challenging to produce a uniform and consistent linerboard and, therefore, combined board sheet for laminating or direct converting. Score cracking will morph going forward.


PortraitRalph Young is the principal of Alternative Paper Solutions and is AICC’s technical advisor. Contact Ralph directly about technical that impact our industry at askralph@aiccbox.org.