Compression: The Test That Shows Whether Packaging Can Withstand Real-World Conditions

A corrugated box rarely has to support only the weight of the product it carries. From the factory to its final destination, it must also withstand the weight of boxes stacked above it, the stresses of transportation, extended periods in storage and changes in ambient humidity. The ability to withstand these vertical loads without deforming or collapsing has a technical name: compression strength.

What Compression Strength Measures

In corrugated packaging, compression strength is the ability of a box to withstand vertical loads. In practice, it helps determine how many boxes can be safely stacked on a pallet without the boxes at the bottom giving way under the weight above them.

This property is directly tied to the engineering of the board: the flute profile used, the basis weight (grammage) of the papers, the number of walls (single, double or triple wall) and the structural design of the box. A package with insufficient compression strength can crush while still in storage, damage the product inside, and cause losses across the logistics chain, often without the root cause ever being clearly identified.

How Compression Strength Is Measured

In Brazil, compression testing for corrugated boxes is standardized by ABNT NBR 6739, equivalent to the international ISO 12048 method and TAPPI T804. The test involves placing the box, assembled and sealed as it would be in real use, between the plates of a compression tester, applying controlled vertical load up to the point of structural collapse. The result is expressed as the maximum force sustained, typically in kgf or newtons.

This test is usually paired with another one, edge crush strength of the board (ABNT NBR 6737, also known as ECT, edge crush test). ECT measures the edge crush strength of the board itself, and it’s one of the parameters used in the McKee formula, developed back in the 1960s by the Institute of Paper Chemistry, to estimate a box’s compression strength from three variables: the board’s edge crush strength, its thickness, and the box’s perimeter. In practice, the formula works as an initial estimate during package development, with final validation usually coming from a direct test on the assembled box.

One more factor enters this equation: relative humidity. Paper is a hygroscopic material, and its mechanical strength drops as humidity rises. That’s why compression tests are usually run under controlled temperature and humidity conditions (the standard conditioning and testing atmosphere of 23°C and 50% relative humidity, defined by ABNT NBR NM-ISO 187), and the final design of a package for a given logistics route accounts for expected changes in humidity along the route and during storage.

Why This Matters in Practice

Insufficient compression strength has both direct and indirect costs. Direct costs include damaged packaging, product damage and returns. Indirect costs, often greater, include emergency redesigns, loss of usable storage space due to stacking limitations and, when problems recur, erosion of trust between supplier and client.

Theoretical calculations, like the McKee formula, are a valuable tool early in package development, helping guide the first design decisions. When significant changes are made to the packaging structure, product or logistics route, simulating real stacking, load and storage-time conditions through physical testing adds an extra layer of confidence, confirming whether the designed strength matches what the operation actually demands.

Technical References

• ABNT NBR 6739: Corrugated fiberboard packaging, compression test using a compression tester
• ABNT NBR 6737: Corrugated fiberboard, determination of edge crush resistance (ECT)
• ABNT NBR NM-ISO 187: Paper, board and pulps, standard atmosphere for conditioning and testing
• ISO 12048 and TAPPI T804: (international equivalents to the packaging compression test)
• McKee, R. C.; Gander, J. W.; Wachuta, J. R. “Compression Strength Formula for Corrugated Boxes.” Paperboard Packaging, August 1963, Institute of Paper Chemistry