Products that are stacked in the course of production, storage, transport or display must be sufficiently robust within desired or industry-standard stacking heights. Top-load or column-crush testing defines methods for ensuring that products consistently meet these quality requirements for axial load.
Plastic bottles and containers, cans, glass jars, or cardboard cartons, will all behave differently according to contents, materials and structural design. Cost and environmental pressures for lighter packaging using less raw materials, also affect performance during filling and capping, as containers become more susceptible to crushing, or deforming in ways that must be designed out.
A common example of a stacked container is the PET bottle, used globally for beverages, cooking, cleaning and other liquids. It has design features that affect axial load strength, including closure, handles, grip areas, and shoulder and base design. Some designs are made for unit-to-unit stacking to further minimise batch packaging and increase stack stability. Top-load testing is therefore as integral a part of the design process, as it is of production line quality testing.
A top-load test essentially involves applying a downwards compression to measure resistance to crushing of a product, usually a container. Test methods define the speed of compression and extent of deformation, and peak force measurement determines the product sample strength. An appropriate universal tester will also be able to measure accurately the initial and recovered height of the sample, for conformance to specification.
In the case of multi-wall cardboard materials, standardised samples of the material itself are assessed for rigidity by edge crush testing, since this is predictive of final construction strength. Contents, head space and weight, as well as humidity and storage conditions greatly affect the load-bearing of a cardboard container. The strength and suitability of a complete cardboard box may therefore also involve compressive burst testing under various conditions.
Compression fixtures account for the behaviour of the sample, so a plate for crush testing a bottle may be vented, or have a cone centre that prevents a bottle slipping sideways. A plate for crushing a box may be self-levelling to follow the pattern of failure. Edge crush methods may require special fixtures, for example to retain a circular ring of cardboard. If a filled container such as a beverage can is to be tested, a suitable enclosure and containment is required. If glass top-load is to be done, additional safety enclosures are essential.