Linerboard Coefficient of Friction (COF) Target and Max

February 3, 2021

Cassie asks,

I’m doing some investigation to determine what is the minimum, target and maximum COF (coefficient of friction) for linerboard. Can you point me to any information on this?

Instead of answering your question directly from a mill process point of view, which we can do, here are a couple links to several specification sheets from domestic mills. (Domestic High Performance Linerboard, COF Spec Sheets or Slide Angle)

The Europeans do not seem to deliniate slide angle as a physical property characteristic.  Since they have been making linerboard from recovered fibre much longer than we have in the Americas, it may be just universally understood there.

Let me know more if you have more specific questions. I would also recommend membership in TAPPI’s Corrugated Board Technical Committee where you will have access to the best minds in this business.

—Ralph

Stitching Tolerance

December 30, 2020

Dave asks,

 We have an old foot pedal stitcher we manually stitch boxes with. Is there a standard spacing between staples that we must maintain?

Dave, this was a new one for me.  Took a bit of research using a number of resources, but this should address your question.

Singlewall: Starting and ending staples shall be 1 to 1½ inches from the flap score lines.  Intermediate staples shall be spaced no more than 2½ inches apart.

Doublewall: Starting and ending staples shall be double stitched no more than 1 to 1½ inches from the flap score lines.  Intermediate staples shall be spaced no more than 2½ inches apart.  For products weighting 100 lbs. or more spacing of intermediate staples shall be reduced to 1 to 1½ inches apart.

Triplewall: Starting and ending staples shall be double stitched no more than 1 to 1½ inches from the flap score lines.  Intermediate staples shall be double stitched and spaced no more than 1 to 1½ inches apart.

— Ralph

Hazmat Linerboard Weight Tolerance

December 30, 2020

Dave asks,

What does the AICC hazmat guide say about tolerances on basis weight? Customer says they must run: “We must run the 31-31-35 hazmat board combo” but the corrugator no longer runs that combo.

The DOT has relaxed the “49 CFR 178.516 – Standards for fiberboard boxes” from +/- 5% to now allow +/- 10 percent basis weight tolerance in the linerboards.

Per the Legal Information Institute “UN4G combination packaging with outer fiberboard boxes and with inner fiberboard components that have individual containerboard or paper wall basis weights that vary by not more than plus or minus 10% from the nominal basis weight reported in the initial design qualification test report.” Click on this link to learn more at the Legal Information Institute.

How did your customer arrive at the 31-31-35 combination? Is it from a previous requirement? As long as they the meet the strength requirements for the content it should be acceptable to use another combination. The +/-10% relates to the base weight of the liner used and may not mean that you could directly substitute a 28lb for a 31lb. If a 28lb liner is used then the tolerance would be +/-10% of the 28.

So I think it would be important to first see if you can provide a combination that meets their strength requirements WHILE remaining while remaining within the +/-10% tolerance.

— Ralph

Glue vs Staples

September 9, 2020

Andrew asks,

Glue joints vs. stitch joints. What are the benefits of each type of manufacturing joint and which is stronger? Often I am told glue joints are stronger because glue penetrates and bonds the liners. Are there any test results that support this?

The intended use and environment that the container is going to be used in are key factors in selecting glue or staples for your manufacturer’s joint. For the majority of uses, glue is the preferred method because it provides a higher strength in most cases. Glues also enhances productivity because, in the case of a flexo folder-gluer, you can print, fold, glue and stack or bundle in a single operation while stapling would require a second operation. In applications where gluing is a second operation, it can typically be accomplished at production speeds far greater than stapling.

So why use staples? Well, they still have their purpose. Staples are often preferable for containers that are subject to high levels of moisture or exposure to outdoor elements and temperature change. These types of containers are typically coated with materials that repel water such as wax and other weather resistant coatings. Glue usually does a poor job of bonding these types of materials.

Both methods have their disadvantages too. As mentioned above, glued joints can be more susceptible temperature and moisture changes and may not adhere well to some printed or coated surfaces. Glued joints take a little longer to set, but since they are going through a stacking to compression device that’s usually a non-issue.

Staples may have a few more disadvantages. They are typically not desirable for use with food products and can complicate the recycling process. Staples can also scratch or damage products inside of the container, though this is more likely in manual than automatic operations. They can also create a bit more of a safety risk especially if they are not properly installed.

Here are some documents that provide additional information.

— Ralph