Rigid packaging made of corrugated board

One of the most important functions of packaging materials is, on the one hand, to protect the contents against physical influences from outside and thus against possible damage or other impairments. On the other hand, the packaging must be able to safely hold the weight of the goods packed in it.

For corrugated board packaging, among other things, the so-called bending stiffness therefore becomes a relevant parameter. In simple terms, this term refers to the ability of a material to withstand an applied force without (permanent) bending and plastic deformation, which could possibly damage the contents.

Corrugated board packaging is particularly suitable for this purpose due to the basic structure of corrugated board and the materials used for its manufacture. As a result, they generate both a high level of protection for the goods and products packaged in them and resistance to heavy weights - as well as other factors. In the following article, we will show you everything you need to know about this special form of packaging.

What does rigid mean?

To explain the concept of bending stiffness, one cannot avoid delving into the world of physics. Specifically, into the complex of topics "forces", their direction of action and effects.

The so-called bending force (or correctly: the bending moment) refers to a moment acting on a component that triggers a curvature about a bending axis due to its direction of occurrence and action.

By a moment, the expert understands the effect of an influencing factor that is applied to a certain point. Most often, this influencing factor is one or more forces.

As a simple example, one can imagine a road bridge. This rests on girders at both ends. Gravity and the load from vehicles push or pull the bridge downward. Those parts that are particularly far away from the girders are subjected to the greatest load by the downward force. Viewed from a distance, an insufficient bending stiffness of the bridge (exaggeratedly formulated) would result in a straight line bent downwards.

Different forces act in this straight line or in the bridge itself:

The bridge surface (or the carriageway) is subjected to horizontal compression by the bending moment, i.e. it is compressed.
The underside of the bridge, on the other hand, is subjected to tensile stress, i.e. it is stretched there.

This is where bending stiffness comes into play: It generally refers to the ability of a body to resist such a bending moment through constructive and material stiffness. The body can thus retain its original shape regardless of an acting moment - no matter what this original shape looks like.

In terms of rigid corrugated board packaging, this means that the mostly straight surfaces of this type of packaging remain

a) straight, can
b) react flexibly to a bending moment by deformation with subsequent recovery and retain
c) their original shape even in the face of large forces.

Of course, all this has its limits. Just as the aforementioned bridge could permanently bend and ultimately even break under excessive load, the same applies to corrugated board packaging. This means that the bending stiffness always relates only to the respective design of the packaging material.

The type (and number) of the respective flute of the corrugated board plays a role here, as does the possible tensile and compressive load of the cover papers used, their own bending stiffness, and the cohesive and adhesive forces of the adhesive that binds the layers together. In addition, the size of the individual corrugated board surfaces also has an important part to play. A small shipping carton would therefore be more resistant to bending overall than a significantly larger carton, all other things being equal in terms of corrugated board and construction.

What is the difference between rigid and flexible packaging materials?

The calculation of whether a packaging material is flexurally stiff or flexurally soft is mathematically relatively complex, since various variables must be known and used in formulas.

However, as far as the practical application of corrugated board packaging is concerned, it is much simpler: as soon as a packaging material can be deformed by simply touching it without the use of great force, it is referred to as flexible packaging material.

Let's take as an example our shipping cartons and Stand-up pouch. Of course, the former could also be permanently deformed. However, a great deal of force would be required. About a fist blow to the center of the hinged lid.

The Stand-up pouch , on the other hand, would not offer any significant resistance to the touching hand and would immediately deform.

However, this distinction does not mean that rigid packaging is always "better" and flexible packaging "worse". In practice, it always depends on exactly what a packaging material is to be used for. And last but not least, if we look at the subject of cushioning materials, then flexural softness can even be a significant factor. For it is only when such a material is flexurally soft that it can cushion forces, transform them through deformation and thus protect the surrounding products.

How is the bending stiffness of corrugated board packaging created?

In principle, bending stiffness can be produced in various ways. These include materials that are inherently more difficult to bend (stiffer) as well as a design that provides greater resistance to the bending moment.

Corrugated board packaging uses both approaches. Due to its multilayer structure with at least two layers of liner paper and one flute the following happens here:

The face sheets facing away from the bending moment absorb the tensile forces generated during bending. Therefore, it is irrelevant for a piece of corrugated board whether the bending moment acts from inside or outside.
The flute ensures that the otherwise only horizontally aligned product is given a vertical plane in the form of the ascending and descending wave flanks. Quite similar to what is the case, for example, with a steel T-beam.

The flexural rigidity of corrugated board packaging is further increased because the flute is comprehensively bonded to both cover layers.

Is corrugated board automatically always rigid?

In principle, yes, but the degree of bending stiffness differs depending on how a piece of corrugated board is oriented. The reason for this fact lies in the internal shaping of the corrugated board, specifically the flute itself:

Longitudinally to the course of the flute , the bending stiffness is basically the highest. Exactly how high it is depends on the respective type of flute . Therefore, for example, the B-flute is stronger than the E-flute, but both are topped by the (doubled) EB-flute.
Transverse to the course of the flute , on the other hand, the bending stiffness is lower. This is because the corrugations are not added as a stiffening element here. In this direction, the bending stiffness therefore depends mainly on the tensile strength of the cover papers and their own bending stiffness.

Again, here is a simple example: Corrugated metal sheets. Lengthwise to the corrugation, these are significantly more resistant to bending than a flat piece of sheet metal. Transversely, on the other hand, they can even be rolled.

However, since corrugated board, as mentioned, always has at least one outer and one inner cover, its bending stiffness transverse to the corrugation is always greater than that of a cardboard product consisting of corrugated board alone - or a product made of solid board. Therefore, yes: corrugated board is automatically always stiff in bending, just not equally stiff in all directions.

To improve this factor, there are also packaging materials made of corrugated board, which consist of two or three corrugated layers lying on top of each other. This packaging has a particularly high load-bearing capacity.

Rigid corrugated board packaging is always suitable when the expected external loads exceed the load-bearing capacity of the goods to be packaged. For this reason, shipping cartons are made of corrugated board practically across the board.

What are the rigid corrugated packaging materials?

There are many different packaging materials made of corrugated board - including shipping cartons and folding cartons.

So let's briefly state: in principle, all corrugated board packaging is stiff to a certain degree, because at least the flute automatically causes this product property.

Based on this, there are many packaging materials made of corrugated board, which can bring this property into the field. For example:

Shipping boxes
Folding cartons
Moving boxes
Shipping sleeves / shipping tubes
Displays for the point of sale
Advertising displays and similar marketing products
Transport boxes as a substitute for shopping bags
Exhibition trolleys
(some) shipping bags
Slipcases and similar office solutions

The enormous flexural strength of corrugated board is demonstrated not least by a recent trend: furniture made exclusively of corrugated board, but which, due to its great flexural rigidity, is hardly less resilient than furnishings made of more "traditional" materials.

What are the advantages and disadvantages of rigid corrugated board?

In fact, rigid corrugated board packaging has various advantages. However, depending on the application, these may well turn out to be the opposite:


Advantage	Disadvantage
High mechanical protection of the goods	Permanent deformation and damage if the design forces are exceeded
Large payloads possible	Possible transfer of forces to the inside of the packaging. Therefore, padding is sometimes required for very sensitive goods
Long service life with proper handling, thus possibly second and third uses possible	Higher material costs than solid board due to more complex manufacture
Easier to recycle than other rigid materials	Requires special folding for space saving storage
Due to the air layers created by the flute , the corrugated board has another possibility to cushion forces

Conclusion - what are the benefits of rigid corrugated board packaging?

Bending stiffness always refers to the ability of a material to absorb comparatively large bending forces without causing any noticeable, permanent (plastic) deformation.

From this point of view, corrugated board has one of the highest bending stiffnesses of all packaging materials based on paper or related materials - without, however, having an unbearably high weight per unit area.

This makes packaging made of rigid corrugated board "light and strong" in the best sense of the word and perfectly suitable for protecting all products that can be damaged by external stresses. Furthermore, these packaging materials are ideal when it comes to packing items that have a high weight - which could overtax other packaging materials made of less rigid materials.

FAQ

Does corrugated board retain its flexural stiffness when wet?
Among other things, wetness negatively affects the tensile strength of all paper-based packaging materials. Both wet corrugated board and that which has subsequently been dried again is therefore no longer as flexurally stiff as it was in its original state.
How can bending stiffness be named and calculated mathematically?
In principle, bending stiffness is a product of a) the cross-section and b) the modulus of elasticity. The formula _Sb =_Mb / k =_Mb p is usually used for this purpose.
Is there also corrugated board that is equally rigid on all sides?
These are either staggered double flutes or various honeycomb constructions. However, such corrugated board packaging materials are only actually necessary for a few special applications.