1. The tensile range of the test material should be considered first.
The difference in the tension range determines the difference in the sensors used, which also determines the structure of the tension machine, but this item has little effect on the price (except for the door type). For general flexible packaging manufacturers, a pull range of 200 Newtons is sufficient. Therefore, it was decided to use the single-arm type.
The structure corresponding to the single-arm type is the portal structure, which is suitable for relatively large pulling forces, such as one ton or more. Therefore, flexible packaging manufacturers basically do not need it.
Second, the problem of test schedule.
According to the performance and requirements of the flexible packaging film, the stroke can be 500-600mm. If the material elongation exceeds 1000%, the stroke of 1000 or 1200mm can be used.
3. The standard configuration problem.
Three basic intelligent configurations: host computer, microcomputer, and printer. If the microcomputer has strong functions, you can print directly. It can also be equipped with ordinary computers. With a computer, you can perform complex data analysis, such as data editing, local zoom, adjustable report format, and statistical analysis in groups. If equipped with a computer, the manufacturer should be equipped with the corresponding software control system.
4. Output results.
The test result output can be arbitrarily set: maximum force value, elongation, tensile strength, constant force elongation, constant elongation force value, yield strength, elastic modulus, maximum test force 8 items. This can be said to be the most comprehensive result output when the microcomputer is operated. The products of some foreign manufacturers can generally output these 8 items. Some domestic manufacturers can output 5-6 items, and some manufacturers can only output the maximum force value, average value, and minimum value.
Fifth, on experimental projects that can be done.
Flexible packaging requires a multi-purpose tensile machine, that is, on the basis of different fixtures, it can be used for tensile, compression, bending, tearing, shearing, 180 degree peeling, and 90 degree peeling tests.
In addition to the above items, there are some high-end tension machines on the market. Due to their high sensor accuracy (some of them reach less than three hundred thousandths), a friction coefficient tester has been developed to test the friction coefficient.
6. Main configuration of product machinery:
Transmission, there are screw drive and rack drive, the former is expensive, used for high precision, high test repeatability; the latter is cheap, used for low precision, low test repeatability.
The lead screw plays a decisive role in the measurement of tension accuracy. The general ones are ball screw, trapezoidal screw and general screw. Among them, the accuracy of the ball screw is the highest, but its performance can only be achieved by the operation of the computer servo system, and the whole set is relatively expensive. The precision required for flexible packaging can be achieved by using general screws and trapezoidal screws, that is, 0.5-1% accuracy.
Transmission, there are gear transmission and chain transmission, the former is expensive and used for high precision; the latter is cheap and used for low precision.
The main cost of the sensor lies in the life span. Photoelectric sensing is one of the more advanced technologies. It is generally available for more than 100,000 times. It can be achieved by imported and domestic joint venture manufacturers.
7. Test speed.
The national standard stipulates that the test speed is 200mm / min, some of the market equipment is 10 ~ 500 mm / min, and some is 0.01 ~ 500 mm / min. The former generally uses a common speed regulation system, which has a lower cost and roughness affects accuracy; the latter The use of a servo system is expensive and has high precision. For flexible packaging companies, the servo system is used, and the speed range of 1 ~ 500mm / min is enough, so that the accuracy is not affected, and the price is within a reasonable range.
8. Measurement accuracy.
Accuracy issues include force measurement accuracy, speed accuracy, deformation accuracy, and displacement accuracy. These precision values ​​can be up to plus or minus 0.5. But for general manufacturers, 1% accuracy is sufficient. In addition, the resolution of the force value can almost reach 1 / 100,000.
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.
The tensile properties of plastics and rubbers are one of the most important and basic properties of their mechanical properties, which largely determine the use of such plastics and rubbers. The quality of the tensile properties can be checked by a tensile test.
1. Tensile properties of high molecular polymers
When used as a material, high molecular polymers are required to have 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.
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
The tensile test (stress-strain test) generally clamps the two ends of the material sample on two fixtures at a certain distance, and the two fixtures separate and stretch the sample at a certain speed to measure the stress change on the sample. Until the sample is destroyed.
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 the flexible packaging material is mainly a polymer or its related materials, as mentioned above, the elongation rate of the polymer material is far superior to that of metals, fibers, wood, plates, etc. There is a certain difference from the usual tensile testing machine for tensile properties of 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
In 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 operation of the fixture may exceed The service limit of the stroke causes damage to the equipment.
The calculation formula for elongation at break or yield elongation (εt, unit is%) given in GB13022-91 "Test Method for Tensile Properties of Plastic Films", εt = [L-L0 * 100] / L0 where: εt is the elongation at break or yield elongation; L is the distance between the marking lines when the specimen breaks or yielding; L0 is the distance between the marking lines
. 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 can guarantee the normal test.
According to a random survey of the effective strokes of electronic tension machines sold on the market, among the 72 tension machines of several brands at home and abroad surveyed, the equipment with a stroke range of less than 400mm accounted for 13.9%; the stroke was 400 ～ 700mm equipment accounts for 71.9%; equipment with a stroke of 701 to 1000mm accounts for 10.8%; equipment with a stroke of more than 1000mm accounts for only 3.4%. 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.
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