Paper and cardboard testing with a Texture Analyser – Part 2
3 minute read
Reliable testing for packaging and print
Pin adhesion test
Example standard: TAPPI T821
Another factor that controls the quality of corrugated board is the strength of adhesion between the corrugating medium and its facings. This adhesion can be measured directly using a ‘pin adhesion test’, where pin adhesion is the force to separate the two components. In this test, pins are inserted between the facing and fluted medium, then attached to rigs connected to the loading arm and base. The rigs are pulled apart at constant speed until the facing is separated from the flute tips. Separation of all bond lines is not necessary. This test assesses the quality of the bond formed when the board was combined, and can be used to detect some manufacturing defects such as poor adhesive penetration and spotty adhesive application.
Pin adhesion test diagram
Pin adhesive strength has a small effect on the cardboard’s column strength, and so is not widely used. Additionally, there is a drawback to this test – when a flexible liner is used, it bends around the pins and causes an artificially low reading. However, the test is very useful for boxes destined for high humidity environments, where the adhesive may not perform as necessary. In this case, a water-resistant adhesive can be used. To test for these conditions, the test is carried out on moistened board samples.
Stable Micro Systems are able to develop bespoke testing solutions based on customer sample dimensions and testing requirements with the ability to analyse data in a number of standard formats or the development of a desired customer analysis technique.
Flexural stiffness test
Example standard: ISO 5628
As mentioned above, corrugated board boxes are designed to be stacked on top of each other during storage and transport. They sustain a large top load, particularly if they are at the bottom of the pile. When a box is loaded in this way, its walls deflect and bend either in or out. This causes a stress concentration at the box corners, which means the corners have a large part to play in the strength of the box. However, the deflection of the walls that causes this stress on the corners can be controlled to some extent by ensuring the cardboard has adequate flexural stiffness: a higher flexural stiffness gives a more rigid container and a higher box compression strength.
Flexural stiffness can be measured in a bend test, such as three-point bending. In a test such as this, a cuboid sample is placed over two curved supports (termed ‘rollers’) and a third roller is used to apply a load to the centre of the sample, halfway between the two support rollers.
Flexural stiffness test
Flexural stiffness, strength and deformation to failure are all measured from a bend test graph as long as the sample’s dimensions have been measured accurately.
When using flexural stiffness in formulae used to predict box strength, the geometric mean is taken of the stiffness values of the box in both directions (perpendicular and parallel to the flutes) as the sides are stressed in both directions.
Puncture test
Some box makers carry out puncture testing, measuring the force or energy required to pierce through cardboard samples. This test ultimately predicts the penetration resistance of a complete box. Some older standards require the use of a pendulum to measure burst energy, and these tests are still used today, but a Texture Analyser is ideal for measuring the force to puncture at a specific speed, with a full graph of the indenter’s movement through the sample (rather than a single data point result).
Texture Analysers give a more controlled application of force than a pendulum impact tester, and will consequently give more repeatable results.
Puncture test diagram
During a puncture test, a pyramid or conical shaped tip is pushed into the surface of a cardboard or paper sample suspended over an aperture. Puncture resistance may be reported in terms of a force or a stress. A variant, known as ‘Mullen testing’ or ‘burst testing’, uses hydraulic pressure to burst through the sample.
