Moisture Test Comparison MRA vs. non-MRA Box

May 24, 2017

Steve asks,

Would you have or could you please direct me towards any testing data/statistics on a box with MRA vs. a non-MRA box?  I would like to see what kind of improvement it shows in a moisture-related test.

The dimensions, flutes and composition range (very small to large boxes; B/C, E/B, and B flutes; 32ECT to 71ECT), and I don’t have much info on supply chain.  A customer just looking for any testing data that he can get on MRA to demonstrate that it does improve performance in moisture.

For assistance with answering this question I reached out to Clayton Clancy at Kruger. He has provided this this detailed study on stacking performance when using water-resistant adhesives, complied by the Institute of Paper Chemistry, which high-lights paperboard performance improvements when using WRA additives. He cautions that there are varying terminologies used when discussing water resistance such as (MRA, WRA and WPA), with that in mind I hope this will be helpful.

— Ralph

Cracking Litho-lam, Does ‘Sides’ Matter

May 8, 2017

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

Checking Starch Viscosity

May 4, 2017

Ralph asks,

Lately we have been having a debate about the perfect temperature for the starch viscosity testing. We use the stein-hall cup for our tests. Is there any specific temperature where it is best to test at?

Temperature certainly plays a major role in the viscosity of starch. Starch viscosity changes about 10% for every 2 degrees Fahrenheit. We discussed your question briefly with Herb Kohler of Kohler Coatings. Herb says that most corrugating plants that he is familiar with try to maintain temperature of starch in their storage tanks between 100 and 105°Fahrenheit (37.8-40.6°C). This helps maintain a steady viscosity and optimizes the running speed of the corrugator. Above 105°F (40.6°C) and you’ll start to experience viscosity growth. Maintaining a set point of 102°F (38.9°C) is commonly recommended by industry experts. His experience has been that is far better and more accurate to check the starch viscosity at approximately the same temperature each time than to rely on temperature/viscosity correction charts.

Additional information about regarding corrugator starch viscosity can be found in this Ask Ralph article from Roman Skuratowicz Ideal Starch Viscosity for Corrugated.

— Ralph

White Linerboard Cracking at Score

March 24, 2017

Dana asks,

Regarding cracking on white liners, is there one specific liner type that will fare better than another?  Considering Kemi, Bleached White and Mottled White liners which is likely to provide the best performance and quality for rotary die-cutting with a perforation pattern of 3/8″ x 3/8″ perf-score or 1/2″ x 1/2″ perf-score.

In my experience, we have compared Mullen grades to ECT grades and understand the longer fibers to fare better than shorter fibers, but I have never before been asked this question in regards to the specific liner board color. I am curious to know if you have any data or experience related to this.

Kemi is extremely uniform and well-formed of virgin Northern European short hardwood fibres. Bleached white from the US South will be different than Canadian bleached. There is no more mottled white.  White tops can be composed of virgin or recycled fibres or a combination of both.  These are also made in the US South and Canada.

I would invest in a small microscope that you can connect to a laptop and look at the cross sections of both good and bad scores to see what is happening to the medium in both situations.  You might also consider sending samples back to your sheet feeder for flat crush testing. You may need a different medium.

Do you have the right rule and rubbering for the dies?  What does the supplier say?  If they do not know I will recommend an expert to you. Also, are your dies in good condition with no damaged rule or rubber? And don’t forget your anvil covers/blankets. Are they in good condition and do they have an even surface? It’s important that they are rotated, ground or trimmed frequently to maintain the proper anvil surface.

Cracking during the winter is often a matter of low moisture.  What combined board moisture do you have at the time of converting.  Do you know the individual containerboard temperatures at the time of combining on the corrugator?  It’s possible your supplier is ‘cooking’ the sheet on the corrugator causing it to become more brittle than desired.  Consider a Denver Moisture Analyzer somewhere in your system?  You need at least 7% moisture to have any success at scoring properly.

How is the maintenance on your presses?  Do you experience cracking more on one diecutter than another? Are there differentials between shifts?

Let me know how your investigation proceeds. I would be very interested in what you discover.

— Ralph