Archive for the ‘Containerboard’ Category

How Much Washboarding is Acceptable

April 24, 2018

Paul asks,

I would like to know about washboarding in corrugated – how much variation in surface flatness (in microns) is typically seen for the types of corrugated board used in applications like shelf ready packaging?

I don’t know that there is a published standard on how smooth the surface of a corrugated sheet should be for the type of printing used for display or shelf ready packaging, but I think we are safe to say pretty darned smooth.

One of the key elements of printing the type of quality high-graphics usually found in display work and shelf ready packaging is minimizing the impression (pressure) between the plate and the substrate. The greater the impression pressure the more the printing plates distort and that distortion results in growth and distortion of the dots that create your image. Typically in high-graphics printing we are looking for a kiss impression, just enough that the printing plate just touches or “kisses” the surface of the board and transfers the ink. For the best print results we want .0015” to .003” (~38 to 76 microns) impression when printing. Oh course the lightest impression we can get away with.

Now, if the high and low points of the board surface exceed this kiss impression depth then additional pressure will be necessary in order to obtain coverage in the low points caused by fluting. Then, as stated above, as we add more impression our printing starts to suffer. With large block print you might think you can get away with over impression and perhaps you can a tad more than you can with process or fine lines and text, but excess impression on solid coverage will result in color variation through striping of the print.

We should also note that typically washboarding is less prevalent on small flute (such as a E, F or N) because there are more support points and they are closer together than a C or B flute.

Digital printing may be a little more forgiving than offset, but we still have to remember that the smoothness of the surface contributes to the overall aesthetics of the packaging and not just the print quality.

—Ralph

Global Percent of Medium

March 6, 2018

Mike asks,

I have a question on what the % medium should be globally. I’m trying to get a handle on it algebraically so to speak.

Classic corrugated sheet, in my mind, is one 26# medium sheet and two 42# liner sheets.  The medium has a wiggle factor of ~1.3 so,

Medium % = 26*1.3/(26*1.3+42*2) = ~ 29%.

Does this sound reasonable?  As BWs tend downward, my gut tells me there is more % medium overall but I am thinking still less than a third.

Twenty five years ago, here, it was indeed 42/26/42 with C flute having a take up factor of 1.43.  Then the sheet feeders began to request 23# medium and corrugating rolls started to have lower profiles which reduced the take-up factor.  Then we moved to 35# and today that probably is 33/23/33.  Globally the World Containerboard Organization http://www.wco-containerboard.org/ tracks total containerboard production.  Additional information may be available through them.

— Ralph

Bake a cake in a box

January 31, 2018

Stan asks,

We have a customer who wants to bake a cake in our corrugated box. We use recycled paper from XXXXXXXXXX Mill in XXXXXXX. We also have a letter of ‘No Objection’ from Health Canada. Do you foresee any problem or restriction on this method of cake preparation?

Assuming you have some type of suitable and accepted barrier coating the only other concern would be heat. I assume that they are planning to bake in a conventional or convection oven. Since cakes typically bake at 350 degrees F you shouldn’t have to worry about the 451 degree combustion point of the paper.

If a microwave oven is to be used then there may be a few concerns about the paper and what comes into direct contact with it. A mass resting against paper in a microwave can cause temperatures to reach a point significant enough to cause scorching and even combustion of the paper. (I’ve ruined enough bags of pop corn to prove this point)

I’m not sure how the AIB may play into this if the products are to ship into the United States.

Let’s toss this one out to the readers and see what knowledge they have to share.

– Ralph

Checking Moisture of Corrugated Sheets

January 31, 2018

Tim asks,

Recently we encountered corrugated sheets with dry liners and medium that created problems converting. Is there a way for us to test sheets for moisture content at our facility? If so, is there a listing of the ranges?

Moisture meters, or moisture analyzers, can be used to test the moisture content of corrugate sheets. There are a number of portable, hand-held models available. (Most look surprisingly like a taser) They are rather inexpensive (ranging from $50 to $600) and a modest range of testers with varying capabilities and features are available in this range. These units provide instant readings of paper moisture content and should meet your needs. If you purchase a meter/analyzer make sure it is suitable for measuring paper. Some of the least expensive models are only for wood or hard materials. Also check the accuracy rating of the device. If it has a .01% or .02% for paper it should do fine. The same devices may have a 1% – 3% accuracy for wood, but that won’t affect testing paper.

6 to 8 percent moisture is the sweet spot for corrugated paperboard. Below 6% and the risk of cracking scores will increase, perhaps significantly depending on the paper. If the moisture content goes above 8% you will start to see a decrease in compression strength. Reports indicate compression strength can decrease as much as 6.5% per every 1% of moisture over 8%.

Now, in your case (simple checking for machineability) moisture meters/analyzers should work fine. However, they are not the most accurate method for checking paperboard moisture. Should you ever need to certify board for food or pharmaceuticals you may want to find a testing facility that can provide results via the oven method.

– Ralph