Performance, runability and convertability of corrugated packaging are to a large extent defined by paper properties. Paper properties are broken into three elements, optical, surface and physical properties.
Optical properties are primarily related to the visual element of paper. Surface properties are important for corrugated board manufacturers, specifically the bonding process. While physical properties are of most interest to board performance.
Important physical properties of paper include tensile stiffness and strength, Short Span Compression Test (SCT), Corrugated Medium Test (CMT) and bursting strength. For the purposes of looking at the primary contributors to how cases perform under compression, SCT and Tensile stiffness are commented upon.
Raw material properties significantly contribute to the properties of a finished case; this is simply illustrated by considering one of the key facets of case performance, the Box Compression Test (BCT).
BCT is dependent on the corrugated board stiffness and the compressive strength along the flutes, this is measured as Edge Compression Test (ECT). It is therefore possible to predict BCT based on ECT using the commonly used McKee formula. The ECT value is generated through a combination of the SCT values for each of the constituent liners and fluting within board. SCT taken from the cross direction of the paper machine, is therefore representing compression strength when in a vertical position within a case.
The SCT is measured through the maximum force applied to paper before destruction is caused. A paper strip 15mm wide is firmly clamped in its plane by two clamps with a gap leaving 0.7mm free paper between them. One of the clamps is pressed hydraulically towards the other along the plane of the paper, tending to compress the narrow strip of paper. The pressure at the point of collapse is measured.
Tensile stiffness also has an important connection to the performance of a case, particularly the relationship to bending stiffness. This has a significant effect upon the bulge that occurs with ECT and BCT tests. A paper’s ability to withstand the effect of bulge can contribute to the case strength.
Paper properties are a fundamental element of box performance and have a significant effect upon the box performance. Although other test methods and therefore properties of paper exist, SCT provides a link between paper compressive strength and the box compressive strength. However, it should be considered against other paper properties for a thorough understanding of board and box performance.
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