August 30, 2024
The original version of this article first appeared in this publication in November 2012. While we have questions on this property frequently, there has been an uptick in activity this year from members.
Often, I am asked to provide the calculation for determining the potential caliper for a given flute profile. Of course, we have to factor into the calculation the caliper variations of the liner and mediums used by the combiner and the degree of variation within any specific liner or medium. And this calculation is not what one may think should be the case, as medium is compressed in the corrugating roll labyrinth and rolls have varying amounts of wear. If you would like to know more about this formula, contact me directly. On the Ask Ralph blog (www.NOW.AICCbox.org/corrugated-2), you will find categories of dialogue that pertain to caliper. There are at least six additional resources on this matter.
Different mediums’ compression strength or resistance between the steel plates in a caliper gauge is dependent on the type of fiber and the papermaking process. We can often look to the differences between hard caliper and soft caliper to begin to evaluate the compressibility factor. These two procedures are highlighted in the Technical Association of the Pulp and Paper Industry (TAPPI) T-551 om-06 and TAPPI T-411 om-05, which describe the difference between the use of hard steel platens and soft synthetic rubber platens.
I am also often asked to comment on the loss of the edge crush test (ECT) because of the reduction of combined board caliper through converting equipment. There is not a solid answer for this question because the board can be “sev” crushed, yet the memory/rebound/recover rates of different mediums provide some spring-back of the crushed board—but ECT is lost forever. Also heavily impacted is the reduction of flexural rigidity or bending stiffness, the other key component of box compression.
The research study and report that AICC conducted in 2008 and independent studies that I have participated in through my private consulting company show that 25% of ECT can be lost through the crushing of the combined board. You can study this further in the Fibre Box Association and AICC jointly published fourth edition brochure, Understanding Box Performance. Order this from AICC at www.AICCbox.org/store.
Once again, we are talking about measurable caliper after the corrugator and the presses. While caliper is a very simple and convenient tool for smaller sheet plants to at least have as a gauge to measure crush, the measurement can be deceiving. While I would not be advocating for a $75,000–$100,000 TAPPI fully conditioned test lab with all of the test equipment to quantify various containerboard and combined board characteristics, it is important to have finished board tested by a facility whenever process changes occur in the manufacturing operation.
Relatively new to the United States—but not other countries and regions—is a rather inexpensive piece of test equipment that can measure the impact of board crush and its impact of medium degradation and loss of ECT. Please contact me to receive more information on this dynamic stiffness tester technology. While this way of testing combined board was brought forth by the U.S. Department of Agriculture’s Forest Products Lab in the 1950s, it was not until the mid-2000s that a commercial machine was developed elsewhere. In the U.S., we did the research and invented the idea, but it took another to design the commercial apparatus.
Above, we mentioned there are differences in mediums’ elasticity, resistance to crushing, and their resurgence in converted combined board. Semichemical neutral sulfite, medium from recovered fibers, and compression-engineered mediums all perform differently to converting nip pressures. Generally, mediums with high amounts of neutral sulfite semichemical pulp and lignin will exhibit lower caliper losses when confronted with the same operating conditions as combined board containing mediums with high amounts of recovered fiber. There I go again being technical.
If you want to see some of the accumulated data that supports this article, contact me. Many other pertinent resources are available through AICC and many outside sources. Be careful about caliper: Sustainability issues and newer ultralightweight containerboards will challenge us in this area.
Ralph Young is the principal of Alternative Paper Solutions and is AICC’s technical advisor. Contact Ralph directly about technical issues that impact our industry at askralph@AICCbox.org.
