SUND Knowledge: Until it tears - stress tests in the laboratory

11 Oktober 2021

The series "SUND Knowledge" takes you through the laboratory: Quality assurance is a fundamental part of the SUND Group. For this purpose, various machines are used with which the products can be accurately tested. Lab manager Gabi Krügel explains the test equipment. Part 1: the traction test.

"We test the load capacity of the garbage bags in several steps, such as the tear strength in the longitudinal and transverse direction. You don't mix that because the technical properties are different. Since, among other things, the elongation in the transverse direction is also significantly higher than in the longitudinal direction," explains Krügel before she tests a cut-to-size strip of a garbage bag to test for its thickness. 

To determine the thickness of the sample, a small measuring device is used that mechanically measures the thickness of the film. Each sample is measured five times to determine an average. "Since we want to determine not only the force when the film breaks, but also newtons per square millimeter, or megapascal value, using a formula, the software also calculates this value for us. For this, the thickness of the sample is determined."

The specimen strip must be clamped straight. The difference between longitudinal and transverse direction is crucial, because in the longitudinal direction the force is constant. Fixed at two points, the arm of the tensile machine slowly moves upwards until the test strip tears. The force required for this can be read directly in software. "I can call up two test specifications via the software, where different starting positions are specified. This is particularly important for thicker films, so that we also have enough space in the transverse direction to determine the value," continues Krügel. A graph visually shows how much force was needed to cause the sample strip to tear. In this case, it took 10.09 newtons for tearing to occur.

In the transverse direction, there is an initial maximum force; a waist forms as the strip is pulled. There, where the strip has already deformed, is the first load value that is measured. But it then takes time for the strip to tear. The elongation in the transverse direction is significantly higher due to the polymer chains.

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