Lightweighted boards can be used without compromising carton quality or performance. This was confirmed by box compression tests on the performance of cartonboard packaging in demanding real-life situations such as stacking and palletising.
Since in packaging substance equals weight and heavier weight equals higher cost, this is significant news. Desirable carton stiffness can indeed be achieved in tandem with cost savings. What’s more, there are sustainability benefits as less raw material is required to manufacture lightweight boards.
The studies, undertaken by an external research institute for Metsä Board Consumer Packaging, performed box compression tests on dummy cartons. Box compression test (BCT) values were measured from the point where the cartons had been compressed 4 mm in vertical direction. After this, deformation is considered irreversable and cartons no longer saleable.
The tests showed that bending stiffness provides the best correlation with the stacking strength of cartons. Boards such as Metsä Board’s Simcote, with high bulk and optimal layer structures to achieve very good stiffness even in low grammages, are better able to resist the bulging of side panels under loading conditions. To achieve the same load-carrying ability as with an Metsä Board grade, the cartons would need higher substance depending on the board type.
Fibre direction and humidity affect carton strength
Further box compression tests evaluated how elements such as carton dimension, fibre direction and climate conditions affect carton strength. The results showed that changing the carton size − the height and width of the side panels − does not affect the BCT value, as the corners bear most of the load.
Rotating fibre direction can, however, make a dramatic difference. Most cartons use horizontal fibre direction to improve vertical creasing and provide a more rigid feel when handled. But when fibre direction is changed to run parallel to compression, stacking strength can be increased by 20%. This benefit, however, must be weighed against the more commonly-preferred advantages of horizontal fibre direction.
Finally, tests found that increasing the relative humidity of air lowers the strength of a board. This is because it is made from a natural raw material – wood fibres. Both board and carton strength decreased when relative humidity rose from 50 to 80%. Bending stiffness decreased by 5–10% and compression strength by 30–40%, depending on the board grade. These are important results to consider when cartons are kept in cold storage or transported in humid regions.