What is stiffness? How to measure?

Stiffness is a measure of the strength of paper and board against bending, and also indicates its soft or stiff nature. Stiffness has a great relationship with the thickness of paper and cardboard . In theory, the stiffness is proportional to the cube of the thickness. For example, when the tightness is kept constant, the increase of the stiffness is proportional to the cube of the thickness; When the thickness is constant , the stiffness is directly proportional to the tightness. Paper made from pulp with a high degree of beating also has a greater stiffness. Therefore, the stiffness of paper and cardboard mixed with a certain amount of straw pulp is better. The stiffness of the carton reflects the ability of the carton to resist deformation, which mainly depends on the stiffness of the cardboard . If the carton is subjected to a compressive load or has insufficient bending resistance after being filled with items, it is easily deformed or even broken.

Commonly used stiffness Testers include Clark Stiffness Tester, Taber Stiffness Tester, Gurley Stiffness Tester, Swedish Horizontal Stiffness Tester and French Resonance Stiffness Tester. The Clark Stiffness Tester expresses the stiffness by the bending degree of the sample under the action of its own weight, and is mainly used to measure the stiffness of paper. The Gurley Stiffness Tester is the maximum bending force required to bend the sample to a certain degree to indicate the stiffness. It is mainly used for the determination of paper, and can also be used for thin cardboard. The Taber stiffness Tester expresses , and is mainly used to measure the stiffness of thick paper and thin cardboard. The horizontal stiffness Tester of Sweden L&W Company expresses the stiffness by the force when bending a 38mm wide sample to an angle of 15° or other angles. It can be used for the determination of both paper and cardboard stiffness. However, its bending distance is different when measuring paper and cardboard. The bending distance when measuring paper is 10mm, and the bending distance when measuring cardboard is 50mm. The resonance stiffness Tester in France obtains the stiffness value by calculating the length of the sample when the resonance produces the maximum amplitude, which can be used to measure the stiffness of paper and cardboard. The horizontal stiffness Tester and the resonance stiffness Tester are convenient to test and the data are reliable.