Abstract: There are many tensile testing machines on the market for testing the tensile properties of materials. However, not all testing machines are suitable for tensile testing of flexible packaging materials. This paper analyzes the tensile testing machine for selecting soft packaging materials in combination with China's material testing standards. Indicators that should be especially concerned with.
Polymer, stretching, tensile machine, stroke, fixture
The tensile properties of plastics and rubbers are one of the important properties of Zui in the mechanical properties, which largely determines the use of plastics and rubber. The tensile properties can be tested by a tensile test.
1. Tensile properties of high molecular polymers
When used as a material, it is required that the polymer has the necessary mechanical properties. It can be said that for most applications of high molecular polymers, mechanical properties are more important than other physical properties.
The high molecular polymer has a wide range of mechanical properties in the variability range of all known materials because the high polymer consists of long chain molecules and the molecular motion has significant relaxation characteristics. For example, the high polymer material has a relatively high elongation. Generally, the elongation at break of PE is between 90 % and 950 % (where the elongation of the linear low-density polyethylene LLDPE is high), through special manufacturing processes, some materials. The elongation can be above 1000 %, while the elongation at break of ordinary polymer materials is also between 50 % and 100 %. Generally, a heat shrinkable film, a stretched film, and the like are required to have high tensile properties of the material.
2 , tensile test
Tensile test (stress-strain test) generally involves clamping the two ends of the material sample on two fixtures separated by a certain distance. The two clamps separate and stretch the specimen at a certain speed to measure the stress change on the specimen. Until the sample is destroyed.
The tensile test is one of the widely used methods for studying the mechanical strength of materials, and it is necessary to use a tensile tester with constant speed. According to the different methods of load measurement, the tensile testing machine can be roughly divided into two types: pendulum type tensile testing machine and electronic tensile testing machine. Currently, the electronic tensile testing machine is used more.
3 , electronic tensile testing machine selection index
Since the flexible packaging material is mainly a high molecular polymer or its related materials, as described above, the elongation of the high polymer material is far superior to that of metals, fibers, wood, plates, etc., so the tensile machine for detecting the high molecular polymer There is a certain difference between the tensile testing machine and the tensile testing machine of the usual materials. In particular, it is necessary to pay attention to the effective stroke of the electronic tensile machine and the sample fixture.
3.1 Effective itinerary
When the tensile test is carried out, the size of the sample used is small, but the elongation of the material is generally high. Therefore, it is necessary to equip the tensile force of the flexible packaging material to provide a tensile machine with a large stroke, otherwise the operation of the clamp may exceed The limit of use of the stroke, resulting in damage to the equipment.
It should be noted that in the calculation of the elongation, we only collect the elongation between the two lines on the sample. The marking is drawn on the prepared sample by printing or by hand (the addition of the marking does not have any effect on the sample), and what is the distance between the markings? Most of the distances given by different standards are different. In the same standard, different sample sizes are often given for different materials, so the distance between the lines is different, but this is beneficial for detecting the extension. Very long or very small materials and accurate test results. For plastic films, the distance between the lines is usually between 25 and 50 mm .
Since the deformation elongation of the sample in the tensile test is not only within the marking line, the sample between the two clamps will have different degrees of tensile deformation. In the standard, the initial distance between the clamps corresponding to the distance of the marking is within 80 to 115 mm . If the specimen between the two clamps can maintain the same elongation and is assumed to be 500 %, the effective stroke of the tensile machine needs to be 480. ~ 690mm , if it is a sample with an elongation of 1000 %, the effective stroke of the tensile machine is at least
The author conducted a random survey on the effective travel of the electronic rally machines currently on the market. In the 72 rally machines of several brands at home and abroad , the travel range distribution is shown in Figure 1 .
Tensile machine stroke range analysis chart
Itinerary
Sample fixture
GB13022-91 "Test method for tensile properties of plastic film" describes the fixture as follows: "The test machine should be equipped with a suitable fixture, which should not cause the specimen to break at the fixture. When any load is applied, the fixture on the test machine It should be immediately aligned into a line so that the long axis of the specimen coincides with the direction of stretching through the centerline of the fixture. ... Place the specimen in the two fixtures of the test machine so that the longitudinal axis of the specimen is up and down The center line of the fixture is coincident and elastic, so as to prevent the sample from slipping and breaking in the fixture. The fixture should be lined with elastic material such as rubber. " Because of the special properties of the polymer material and the special use of flexible packaging materials. The method makes the thickness of the sample tested by the soft packaging material very thin, and the general fixture cannot meet the requirements. Improper use may cause the specimen to break at the fixture and cause the test to fail. Â
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