The Hardest “Cardboard”:
The Various Factors that Affect Corrugated Strength, From Fluting to Stitching

 
Previously, in our Cardboard vs Corrugated blog, we discussed the different ways of making corrugated fiberboard. Now we’re going to get further into that discussion, bringing up different liners, medium, and construction techniques, which combine to produce a variety of corrugated strengths. It turns out that measuring the hardness and strength of corrugated fiberboard is more complex than it might seem.

These are among the many factors that affect the strength of corrugated fiberboard: the number of walls, fluting style, liners, medium, glue and construction of the box. We’ll discuss each one and show you how it effects overall construction. And then we’ll talk about how all of this is measured.

The Basics

As we have already explained, corrugated comes in single face, single wall, double wall and triple wall. Generally speaking, the more layers you have, the stronger the corrugated is. But within that cardinal rule, there is a lot of variation. For instance, not all double wall corrugated is equal, nor all triple wall. Even single wall corrugated can vary in strength based on the medium and the liner.
 

Double Wall Variations: Liner, Medium, Stitching and Glue

Take two double wall corrugated boxes and inspect them. Let’s say the first is AC fluting and the other is BC fluting. The AC double-wall corrugated is about 1mm thicker. This thickness, for most people, gives a sense that the corrugated is stronger. But looks can be deceiving.

With corrugated, looks can be deceiving…

These two types of double wall construction are very common. But let’s look at the various ways that these corrugated constructions can be drastically different. Namely, we’ll need to look not just at the thickness of corrugated, but also at variables like liners, glue, medium and stitches in order to understand strength.
 

Liner Variation

Liners (the flat, non-corrugated layers) play a huge role in the strength of the corrugated construction. Not only do they provide different rigidity and weight to the process, they also have the important role of managing the transfer and retention (or deflection) of moisture. One big factor in liner strength concerns how much recycled material they utilize. Liners with more recycled material tend to be less strong because they have shorter fibers and therefore lower tear strengths. Fiber length also affects moisture retention. On the other hand, recycled fibers tend to be less expensive.

Recycled material has really changed the equation in corrugated construction. Learn more about that in this blog.
 

Liner Glue

The glue that adheres the liner to the medium also affects the strength of the corrugated construction. Glue can be more or less water resistant, which can make the corrugated fiberboard more or less susceptible to the adverse effects of humidity and moisture. In Mil-Spec corrugated, for instance, there are very strict requirements about glue and its ability to resist and withstand moisture. More about Mil-Spec corrugated here.
 

Medium variation

Corrugated medium (the wavy part that makes up the wall) comes in different weights and a heavier medium will reinforce the corrugated construction drastically. As mentioned above, the glue that bonds the medium to the liner can also affect strength.
 

Stitching vs. Gluing

We haven’t yet talked about the construction of the box itself. Generally speaking, a box is cut so that a flap can be either glued or “stitched” (using large staples) to be secured. Gluing tends to be the cheaper option but it is less strong. Stitching creates the stronger box. Think of it this way: glue only secures the box liner to liner. The glue on one liner adheres to the other liner. On the other hand, a stitch goes all the way through the corrugated (all the walls and media), bonding all the layers of the construction. This makes the bond stronger.

In Mil-Spec applications, corrugated must be stitched, not glued. This is because stitching is also more secure. It is harder to open a stitched box and then reseal it than it is to open a glued box and then re-glue it.

At Reid, we stitch all of our corrugated boxes, for added strength. One remaining question we ask our clients: should the flap be stitched on the inside or the outside of the box? Outside is stronger, but inside gives a cleaner look. This is the kind of specificity we bring to our custom crates and boxes.
 

Using the BMC (Box Manufacturer’s Certification) to Determine Corrugated Strength

Luckily, if you’re looking for a way to reliably determine the strength of a corrugated box, you can do so with a stamp that is found on every box. It’s called the BMC and it’s on the bottom of most boxes. The BMC is a necessary interface between the person packing the box and the person transporting the box, so that they can both effectively protect what’s inside.

A BMC will give you the important information about the max gross weight and the size of the box. But it will also give you certain test specifications that are two different versions of “strength.” These are either listed as “bursting test” or as “edge crush test (ECT).” We get into the difference between these two tests in our corrugated testing blog (coming soon). But suffice to say, a higher bursting test reading or a higher ECT reading means that the box is stronger.

Curious how we test custom corrugated boxes? Here’s a video of our practical corrugated box crush test that we use to verify strength.

Custom corrugated tested and built specially for your needs

So rather than simply eyeing it based on the relative thickness of the walls or some other factor, you can see precisely what the tested strength of the box is. Check out our next blog (coming soon) for more on how corrugated fiberboard is tested and what the tests mean.

As always, get in touch with Reid Packaging for a custom corrugated box perfectly suited to your transport packaging needs.