Archive for the ‘Testing and Specifications’ Category

Calculating Theoretical Box Compression

April 9, 2019

Steve asks:

How can you calculate a theoretical box compression value if you know the ECT value of the combined board, you have the dimensions of the box and can figure out the Square root of the perimeter? What is the formula we should use?

You have everything listed above except one important factor, the caliper. You need the caliper and then you’re all set. So the formula you would want to use is 5.87 x ECT x (the square root of (caliper x box perimeter)).

(Steve has provided caliper and other the carton specs as)

  • multiplier = 5.87
  • ECT value = 154.7
  • Perimeter = 148”
  • Caliper = .4375” (AA DW)

So our equation would look like this,

Predicted BCT = 5.87 x 154.7 x (√ (.4375 x 148)) = 7307.11 lbs.

Or… 5.87 x 154.7 x 8.0467 = 7307.11 lbs

Just a note, rounding can have a significant effect on the result. It may be better to round down than to round up for the sake of safety.

— Ralph

Tuck Top vs Tuck with Locking Tab, or Friction Tuck

March 1, 2019

Rich asks,

I have a customer who orders a lock bottom box with a tuck top from us. It ends up at distribution in Walmart and Walmart is having issues with the top flap coming open. It does not get taped closed. They are asking if we have packaging engineers that can provide documented test results or studies on performance changing it to a tuck top with a locking tab, or friction lock tuck top tab, or extending the tuck on the top tuck. Have you seen anything like that or are you aware of any hard documentation that says how much it changes the function of the top tuck? Thanks for your help with this.

I would start by checking with your sheet supplier and/or their containerboard provider to see if they have any information regarding the information you are looking for. Often integrated companies do these types of studies. Some will be willing to share and others are not as open to sharing their results. I have an associate in Canada that may want to weigh in on this issue. I’ll touch base with him and then update this post with any input he may have. I also searched through my George Maltenforts’ books, but found no discussions or reported research in this area.

However I do believe that a friction fit is the best way to go short of some type of press applied coating.

— Ralph

Calculating Partition Strength

February 28, 2019

Rodrigo asks;

Calculating Partition StrengthI’m trying to figure out the stacking strength of the partitions shown in the picture (How much weight can be held on top). A corrugated board bed is set on top covering all of it. Products are located inside the 4 cells of the partition. I would like to know what is the process or logic of calculating the maximum weight it can hold before it collapses. Resistance used is 26 ECT.

If I see correctly the pieces are set at an angle so we cannot determine compression strength because the elements are not fully vertical. (left-most sections in the image)  What is more significant here is the torsion or flexing of the components. Because of the distance between connecting points, failure is more likely to come from the twisting action of the components than an edge-wise crush. It’s just like a vehicle traveling over a rough road and all four wheels and suspension act independently of each other.  If the pallet of the load is uneven or starts to shift, then angular forces can be applied potentially causing the long spans to flex and collapse.

We can’t tell exactly how large your partition assembly is, but do you have access to a compression tester with a footprint large enough to test at least one square?

— Ralph


Tom adds,

Consider pointing Rodrigo to the book Corrugated Shipping Containers An Engineering Approach by George G. Maltenfort. Chapter 7 of the book discusses compression strength estimation for boxes with inserts and shows 25 different styles. These are not the partition shown in your question but MIGHT give him a place to start your predictive work or at least highlight attributes he needs to consider.

Thanks Tom!

How Can the Cobb Rating Affect Flexo Printing?

March 26, 2018

Kim asks,

What effect does the Cobb rating of paper have on printing with flexographic inks?

Cobb determines the paper’s ability to absorb water. The higher the Cobb reading, the more water is absorbed into the paper.

Since flexographic inks are predominately water, the Cobb reading has a great affect on how the ink interacts with the paper. Color, drying and coverage are all affected by the Cobb rating of the paper.

A lower Cobb rating will absorb less liquid into the paper and therefore more ink will stay on the surface of the sheet. If the ink is formulated to match the paper the results can be superior coverage, deep bold colors and remarkable surface effects. If the ink is formulated for a higher Cobb rating it may dry slower, offset from one print station to the next down and color and coverage may suffer.

A higher Cobb rating will draw more liquid into the paper, usually faster, leaving less ink on the surface of the sheet. This has its advantages and disadvantages too. The ink may dry faster minimizing offsetting, but coverage and richness of color may be sacrificed as the inks and solids are drawn into the paper and less stays on the surface.

So, it is very important that the paper, inks and printing plates are closely matched for each job. If you were to run a job on a high Cobb rated kraft and then simply switched to a low Cobb rated high hold-out paper without switching inks and plates, the results would most likely be far from favorable.

Now there is a little leeway and ink viscosity and pH can be adjusted to a certain extent to control drying, transfer and coverage. But it’s always best if the ink is formulated to match the desired results to the characteristics of the paper.

This is a very simple explanation of how Cobb can affect flexographic printing, but it could be a complete seminar on its own. If you have a question about a specific job your ink supplier is a great resource.

— Ralph