Archive for the ‘Paper’ Category

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

Effects of Transitioning to 100% Recycled

December 19, 2016

Andrew asks,

I was speaking with a customer yesterday, and they asked about 100% recycled content boxes, and how it might affect their process if they moved to that type of box. They currently do food manufacturing and stack the boxes roughly 80” high (1 pallet).  The boxes also withstand quite a bit of moisture, as the product is refrigerated during/after delivery.

I remembered some of the information from our Corrugated 101 class, and spoke to the fibre degradation between virgin and recycled, and how a 100% recycled box may not be able to withstand their current way of operating. I told the customer I could provide an article, or something with more info on it, to help educate him, if he were to get more questions from his managers.

I found this article on Cracking Scores on Recycled Paper on “Ask Ralph” but didn’t know if you have more info or a different article that would be helpful to share with the customer.  Thanks in advance!

It must be remembered that any corrugated structure should be designed and engineered for a fit-for-use environment. Both virgin and recycled fibred boxes parallel each other in most end use applications including food and agricultural.

Each suppliers system has its own unique value and sustainability proposition. High density and low density repulping process appear to deliver “acceptable” fractionation which separates short length fibres from long length fibres especially in the case where both OCC and recovered municipal and office papers are used. These are then recombined to make linerboard with targeted physical properties.  So the containerboard making process is critical to the quality of the containerboard and corrugated boxes and not all mill processes are the same.

It is important to know your supplier. Initial box compression is the best method and it is important that you have established combined board testing protocols to establish your own quality levels. The AICC has several resources to assist you in these areas.

— Ralph

Cracking Score on White Liner – update

June 27, 2016

Kaleb asks,

We are facing cracking issue in our white test liner when it is used in a particular material combination. Pictures are attached for your reference.

Kindly provide your expert opinion.

 

 Cracked Score 3 Cracked Score 2 Cracked Score 1

Allow me to make a few observations and ask a few questions that may help us find an answer to your question. Since this is double wall corrugated I assume this was converted on a rotary diecutter and not on a flat bed diecutter.

When was the last time you changed out the slotting knives and scoring heads?

Do you change scoring heads and profiles between single wall and double wall?

What was the relative humidity and board moisture at the time of converting?

It appears that the white top did not have enough tensile strength and elongation potential to survive the folding. What does the mill have as far as retained physical property tests and samples for the top sheet that you combined and converted?

Can you supply the scoring head profile so that I can review the design with some of my domestic associates?

–Ralph


Here’s an update on Kaleb’s cracking issue. After some back and forth discussion we’re still looking at Kaleb’s problem.

He has provided the following additional information on his issue.

The box is being die-cut on a rotary diecutter.

  • They change the slotting knives and scoring heads on a weekly basis, but they do not change between singlewall and doublewall orders.
  • Relative humidity of conversion area was about 45-55% and moisture of board was 7-8%.
  • Average Tensile strength and elongation values of top liner are as following:
    • Tensile Strength MD: 7.6KN/m
    • Tensile Strength CD: 5.0KN/m
    • %age Elongation MD: 2.5%
    • %age Elongation CD: 5.5%

Rick Putch of National Steel Rule provided this info…

Pre-crushing before slotting can help reduce score cracking. Also, closing the slotter head nip a little tighter than normal may create additional crush. This can reduce stretch of the outer liner when folding at 180 degrees. I would suggest asking him to run at significantly different speeds to see if that has any influences on the cracking. Also ask Kaleb to put a dial indicator on score heads to see if they are concentric. If they are running out they may not be consistently crushing the score. Have him try these things and report his findings back to us.

How does his tensile and elongation numbers compare to US kraft liner?

These elongation values should be good for any liner.

Tensile values depend on basis weights which he not provided. For example 7.6 KN/m MD tensile stiffness would be typical for DS Smith’s 150g Eurotest, their 130g Testliner 3, less than 100g SCA kraftliner, and their 100g Eurokraft.

Please feel free to share your knowledge and experience. We look forward to hearing from our readers!

—Ralph

Litho-label Score Cracking

December 2, 2015

Gary asks,

Could you tell me the key variables in the Litho Label and the Corrugate Board that will affect Label Fracturing. We are seeing fracturing variability in the litho label after laminating and scoring.

Moisture, paper grade and condition, glue/adhesive, scoring profiles, scoring rule, impression and the manufacturing process are some of the variables that can cause score cracking (fracturing) in both litho-label and corrugated. When a label is glued over a liner you’ve added another ply and basically reinforced that surface. Now, when that surface is scored and folded the label is stretched around the corrugated liner causing stress to the label causing it to tear.

Moisture may play one of the most key roles in score cracking. If the moisture level is too low, the paper loses its elasticity, the fibers become brittle and fail resulting in a cracked score. However, if the moisture level is too high, or there is too great a difference between the moisture levels of the corrugated and the label then you can risk warp and shrinkage issues.

In an earlier post on this topic Brian Tankersly from Lewisburg Printing offered insight through his experience with the affects of moisture on litho labels. Click here to read that post.

We tend to take great care of our litho-labels. We handle them so that are not damaged. We keep them in controlled environments to maintain moisture levels, cleanliness, etc, but do we take the same care to control and monitor the sheets to which the labels are being applied?

If the corrugated liner under the label tears, the result will most likely carry through to the label. So the condition and “conditioning” of both the corrugated and litho-label are critical to the quality of the score.

Then of course there is the paper itself. Every paper machine has its own unique profile and engineers each one of its products with its own DNA characteristics. As in life, there are tradeoffs to make: improving one property may diminish another. In the case of high graphic substrates as those typically used for labels, the formulas, combinations and engineering of the paper can be proprietary information. An example can be the hardwood to softwood fibre mix. The hardwood to softwood ratio is used to improve the coating application. However, enhancing the “coatability” of the paper can increase the incidence for score cracking. Also some types of coating can be more prone to cracking than others.

 

I hope this provides some help and variable to consider. If you have any further questions, or there is anyone out there that has information to contribute, please don’t hesitate to comment or contact us.

— Ralph