What can be remarkable at times is how so many materials that are characterised as being weak can have remarkable levels of strength relative to their weight and purpose.

Glass, for example, whilst seen as weak enough to be shattered with a high enough singing voice, is toughened and often laminated to the point that it can become a structurally integral part of a car or building.

On a similar note, cardboard is capable of remarkable compressive strength, allowing it to store goods that are orders of magnitude heavier than the box itself.

This is particularly true with custom box designs, which can improve stability or provide a softer cushion to keep fragile items such as glasses safe.

What gives cardboard boxes that strength? What can affect their strength? And are there ways to make cardboard even stronger?

Why Is Cardboard Stronger Than It Looks?

When we discuss cardboard, particularly in terms of its remarkable strength, we are typically talking about corrugated cardboard.

Unlike paperboard, which typically consists of a single layer of thick paper suited for small items, corrugated cardboard has multiple layers designed in such a way that the whole is greater than the sum of its parts.

Whilst there are various configurations and structural elements to cardboard which can affect their strength, corrugated cardboard consists of three main elements:

• Outer Layer - The liner board keeping the whole structure in place

• Inner Layer - The liner board keeping the internal structure supported

• Fluting - The corrugated middle, which provides the strength and structural rigidity

In various combinations and with a range of designs, these three layers provide strength in three major ways:

Tensile Strength

Also known as pulling strength, tensile strength is how well a material can resist being stretched before breaking entirely. Thanks to the liner boards, cardboard has remarkable tensile strength, which is why cardboard boxes can be dented or even bent without breaking.

Compression Strength

Also known as pushing strength, compression strength is the forces placed on top of a box before it collapses on itself.

Because cardboard boxes are often stacked in stock-keeping units and pallets, it is vital that a box does not collapse.

This is where the fluted layer comes in, as much like an I-beam or a shipping container, a corrugated cardboard box can hold remarkable amounts of weight without collapsing.

Bursting Resistance

Also known as the Mullen test, a bursting test or puncture test evaluates how resilient a box is to puncturing, tearing and ripping.

The combination of carefully designed flutes and strong liner boards helps cardboard to be remarkably resilient.

What Affects The Strength Of A Cardboard Box?

Not all boxes are equivalent, of course, and whilst some boxes are so strong that stunt performers rely on them to land safely, others are more prone to tearing, breaking and falling apart.

Different cardboard boxes have different purposes, and this usually affects how they are designed, and there are no right or wrong answers in this regard.

Some weaker boxes look beautiful, are thinner, lighter, more workable or have the scope to have amazing designs printed on them. Meanwhile, stronger cardboard boxes tend to be more restrictive in their decoration or require stickers or wraps to be more aesthetically pleasing.

With that in mind, here are some of the factors which affect a box’s strength the most, and when you most have to make compromises in the name of utility.

Flute Size

The biggest determining factor in a cardboard box’s strength is not typically visible when you use it.

The fluting contributes considerably to the compressive and bursting strength of a box, but changing the size and design of the flute can transform how much it can withstand:

• Smaller flutes are lighter and weaker but are much better suited to light items and luxury items where detailed printing and vivid colours are necessary.

• Larger flutes provide better cushioning, which not only makes them stronger but also provides greater cushioning from impacts.

Moisture

Cardboard needs to be kept within a relatively confined moisture tolerance once it has been produced, as exposure to water or high humidity weakens the fibres, reducing its strength. 

This can be helped with the help of treatments to improve moisture resistance, but it can prove a problem when delivery boxes are left outdoors.

Reinforcements

There are various ways to reinforce a box, with the most common of these being a double-walled construction. This involves the use of multiple layers of fluting in order to increase the compressive strength.

Similarly, glueing the joints together or using particularly robust packing tape can help keep boxes together.