How to choose a practical tensile testing machine-Jiangdu Kaiyuan Testing Machinery Factory
At present, there are many tensile testing machines for testing the tensile properties of materials on the market, but not all testing machines are suitable for the tensile testing of flexible packaging materials. This paper combined with China's material testing standards, analyzed the selection of tensile testing machines for flexible packaging materials. Especially concerned indicators. Polymers, stretching, tensile machines, strokes, the tensile properties of plastic and rubber fixtures are one of the most important and basic properties of their mechanical properties, which largely determines the type of plastic and rubber Use occasion. The quality of the tensile properties can be checked by a tensile test.
1. Tensile properties of high molecular polymers When used as materials, high molecular polymers are required to have necessary mechanical properties. It can be said that for most applications of high molecular polymers, mechanical properties are more important than other physical properties. High molecular polymers have the widest range of mechanical properties among all known materials. This is due to the fact that polymers are composed of long-chain molecules, and the molecular motion has obvious relaxation properties. If the polymer material has a relatively high elongation, the break elongation of PE is generally 90% to 950% (including the linear elongation of low-density polyethylene LLDPE is higher), through special manufacturing process, some materials The elongation rate can be above 1000%, and the elongation at break of ordinary polymer materials is mostly between 50% and 100%. Generally, the heat-shrinkable film and stretched film are required to have higher tensile properties.
2. Tensile test Tensile test (stress-strain test) generally clamps the two ends of the material sample on two fixtures at a certain distance. The two fixtures separate and stretch the sample at a certain speed to determine the sample The stress on the surface changes until the specimen fails. Tensile test is one of the most widely used methods to study the mechanical strength of materials. It requires the use of a constant-speed tensile testing machine. According to the different load measurement methods, the tensile testing machine can be roughly divided into two types of pendulum tensile testing machine and electronic tensile testing machine. At present, the electronic tensile testing machine is mostly used.
3. Selection index of electronic tensile testing machine Since flexible packaging materials are mainly high molecular polymers or its related materials, as mentioned above, the elongation of high polymer materials is far superior to metals, fibers, wood, plates and other materials, so There is a certain difference between the tensile machine for detecting high molecular polymers and the conventional tensile machine for material tensile properties. In particular, it is necessary to pay attention to the effective stroke of the electronic tensile machine and the sample fixture. 3.1 Effective Stroke During the tensile test, although the size of the sample used is small, the elongation of the material is generally relatively high, so the tensile performance of the flexible packaging material needs to be equipped with a tensile machine with a large stroke, otherwise the fixture will run It may exceed the service limit of the stroke and cause damage to the equipment.
It should be noted that in the calculation of elongation, we only collect the elongation between the two markings on the sample. The marking lines are drawn on the finished sample by printing or manually (the addition of the marking line should have no effect on the sample), and what is the distance between the marking lines? Most of the distances given by different standards are different, and in the same standard, different sample sizes are often given for different materials, so the distance between the marking lines is also different, but this is conducive to the detection of extension Very long or very small materials and get accurate test results. For plastic films, the distance between the marking lines is usually between 25-50mm.
Because the deformation and elongation of the sample in the tensile test are not only within the marking line, all samples between the two fixtures will get different degrees of tensile deformation. In the standard, the initial distance between the clamps corresponding to the distance of the marking line is within 80 to 115 mm. If the sample between the two clamps can maintain the same elongation and assume 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 tension machine should be at least 880mm to ensure the normal test.
A random survey was conducted on the effective strokes of electronic tension machines currently on the market. Among the 72 tension machines of several domestic and foreign brands under investigation, the distribution of stroke ranges is shown in Figure 1.
Figure 1. Analysis of the stroke range of the tension machine
In the figure, illustration 1 represents equipment with a stroke of less than 500mm, accounting for 13.9%; illustration 2 is equipment with a stroke of 500-900mm, accounting for 56.9%; illustration 3 is equipment with a stroke of 901-1100mm, accounting for 20.8%; Figure 4 is a device with a stroke of more than 1100mm, accounting for only 8.4%. However, in this statistical electronic tension machine, not all of the large-stroke equipment is used for the detection of flexible packaging materials, and part of it is used in the tensile test of artificial wood, canvas, curtains, copper and other materials.
3.2 Sample fixture
The description of the fixture in GB13022-91 "Test Method for Tensile Properties of Plastic Film" is: "The test machine should be equipped with an appropriate fixture. The fixture should not cause the specimen to break at the fixture. When any load is applied, the fixture on the test machine It should be able to be immediately aligned into a line so that the long axis of the sample coincides with the direction of stretching through the center line of the fixture .... Place the sample in the two fixtures of the testing machine so that the longitudinal axis of the sample is up and down The connection of the center of the fixture coincides, and it should be tight and suitable to prevent the sample from slipping and breaking in the fixture. The fixture should be lined with elastic materials such as rubber. "Due to the special mechanical properties of polymer materials and the special use of flexible packaging materials The method makes the thickness of the sample detected by the soft package material very thin, and the general fixture cannot meet the requirements. Improper use will cause the sample to break at the fixture and cause the test to fail.
3.3 KY8OOOB micro-control electronic tensile testing machine
Jiangdu Kaiyuan Testing Machinery Factory is an enterprise specializing in the production of flexible packaging material testing equipment. In particular, KY8000B micro-control tensile testing machine (see Figure 2) is made for the characteristics of large elongation of flexible packaging materials. Seven kinds of tests such as peeling, tearing and heat sealing, and professionally designed various types of test fixtures dedicated to flexible packaging materials. It is a mechanical property tensile testing machine tailored for flexible packaging materials. See the picture for details, please go to the company website
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