July 3, 2025
During the 15 years we have responded to questions to Ask Ralph or the 10 years of articles for BoxScore, the issue of glue tab strength continues to emerge.
While the specifics of the dimensions of the glue tab and the adhesive application or placement of the staples are given in Rule 41 and Item 222 in the Fibre Box Handbook, there is no mention of the strength of the bond or the mechanical force necessary to cause failure. Designers often ask if there is an advantage to using a wider glue tab or placing the tab inside the body. Here is the original question we received from an AICC member:
“We have been reviewing glue lap size and glue lap strength, looking to standardize our offering to customers [and glue head sizes]. In doing a modest amount of research, the only definitive information I found for the size of a glue joint is in the Fibre Box Handbook. Under the voluntary standards in Item 222, single-wall (SW), double-wall (DW), and triple-wall (TW) minimum glue joint sizes are listed: 1¼” for SW and DW and 2″ for TW. There does not appear to be any information that looks for a correlation between the size (width) of the glue lap to the strength of the glue lap.”
There are no great quantitative glue joint tests that we could easily use to say, “This joint is twice as strong as that one.” There are a few general challenge tests, but while they give the same directional trends, they give different kinds of quantitative results.
There isn’t much that would directly answer the question about the inches of the glue tab versus the strength of the joint. Whether it is linear or even matters past a certain point, we need the joint to be only as strong as it needs to be. What that point is will depend on how the joint is being stressed; bulge or vertical load or panel separation apply to different loads. It will also depend on how the glue is applied; the adhesive should get as close to the scores as possible.
The same width of adhesive coverage that starts far from the score will enable nucleation of failure to begin more easily. And once the glue bond begins to rip the paper on one liner or another, the subsequent propagation of failure occurs much quicker and at lower loads.
These joints also play a large role in the case erecting process by determining how and which case erectors work most cohesively with your existing production line. Converting case blanks into fully erected, bottom-sealed cases may not sound like a difficult process. However, if you’ve struggled to open a package or construct a perfectly square case, you know the joints that support cases and the machines that assemble them are no small part of any case.
No one seems to know the effect of going from 1 3/8″ to 2″ to 3″ glue laps. We suspect an increase in strength. However, a determination of how to measure it has not been made, and the rate of the increase has not been calculated.
If someone has done the work, it has not been made public. When failure is present, it tends to be 75% containerboard-related, then equipment, adhesives, design, internal and external dynamics, and shipping and handling.
While it is relatively easy to determine the Z-directional force per ply bond for strength through the thickness of the bond, shear strength remains elusive. There is no Technical Association of the Pulp and Paper Industry test method.
Research has been reported on this topic in the FBA Handbook and in the works by George Maltenfort. We cannot find any published work on this structural issue from Clemson University in South Carolina, which has done wonderful work on printing; or any other packaging schools, except for California Polytechnic State University (Cal Poly) in California, which published a 1969 presentation on adhesive coverage.
We connected with Baumer hhs; H.B. Fuller; Henkel; Advanced Design Technology; Applied Paper Technology Inc.; and Jay Singh at Cal Poly, who wrote the last definitive research paper on glue tab testing, for their input.
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.
