Selection of electronic tensile testing machine and mechanical tensile testing machine
1. First, consider the need to test the tensile force range of the material.
The difference of the tensile force range determines the different sensors used, which also determines the structure of the material testing machine, but this item has little effect on the price (except for the door type). For general flexible packaging manufacturers, a pulling force range of 200 Newtons is sufficient. Therefore, it was decided to use a single-arm type.
The structure corresponding to the single-arm type is the door type 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 question of the test stroke.
According to the performance and requirements of the flexible packaging film, the stroke is 500-600mm. If the elongation of the material exceeds 1000%, you can choose a stroke of 1000 or 1200mm.
Three, the standard configuration problem.
Three basic configurations of intelligence: host, microcomputer, and printer. If the microcomputer is powerful, you can print directly. It can also be equipped with an ordinary computer. With a computer, you can perform complex data analysis, such as data editing, partial enlargement, adjustable report format, and group style statistical analysis.
If a computer is equipped, the manufacturer shall equip it with the corresponding software control system.
Fourth, output the result.
The test result output result can be arbitrarily set: maximum force value, elongation rate, tensile strength, constant 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 during microcomputer operation. The products of some foreign manufacturers can generally export these 8 items. Some domestic manufacturers can output 5-6 items, and some manufacturers can only output the maximum force value, the average value, and the minimum value.
5. In terms of experimental items.
Flexible packaging requires a multi-purpose tensile machine, that is, on the basis of equipped with different clamps, it can do tensile, compression, bending, tearing, shearing, 180-degree peeling, and 90-degree peeling tests.
In addition to the above items, some high-end tensile machines on the market have developed a friction coefficient tester that can test the coefficient of friction due to their high sensor accuracy (some of them are less than three hundred thousandths). Such as Japan Shimadzu, Beijing Landmaker and other companies.
Six, the main configuration of product machinery:
transmission, there are screw transmission and rack transmission, 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 tensile force accuracy. Generally, there are ball screws, trapezoidal screws, and general screws. Among them, the ball screw has the highest accuracy, but its performance depends on the operation of the computer servo system, and the whole set is more expensive. The precision required by flexible packaging, namely 0.5-1% precision, can be achieved by using general lead screws and trapezoidal lead screws.
transmission includes gear transmission and chain transmission. The former is expensive and is used for high precision; the latter is cheap and is used for low precision.
The main cost of sensors lies in their lifetime. Photoelectric sensing is one of the more advanced technologies, which can generally be used more than 100,000 times, which can be achieved by imported and domestic joint ventures.
Seven, test speed.
The national standard stipulates that the test speed is 200mm/min. Some of the equipment on the market is 10~500 mm/min, and some are 0.01~500 mm/min. The former generally uses an ordinary speed control system, which has lower cost and roughness affects accuracy; the latter uses The servo system is expensive and has high precision. For flexible packaging companies, it is sufficient to choose a servo system with a speed range of 1~500mm/min. This will not affect the accuracy and the price is within a reasonable range.
8. Measurement accuracy.
Precision issues, including force measurement accuracy, speed accuracy, deformation accuracy, and displacement accuracy. These accuracy values can reach up to plus or minus 0.5. But for general manufacturers, 1% accuracy is sufficient. In addition, the force resolution can almost reach one hundred thousandths.
The basic configuration above is about 30,000 yuan.
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 article combines the material testing standards of our country and analyzes the application of the tensile testing machine for flexible packaging materials. Special attention 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 occasions of such plastics and rubbers. The tensile performance can be tested by tensile test.
1. The tensile properties of polymer
When used as a material, the polymer must 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 variability among all known materials. This is due to the fact that high polymers are composed of long chain molecules and have obvious relaxation characteristics in molecular motion. For example, high polymer materials have a relatively high elongation rate. Generally, the elongation at break of PE is 90% to 950% (where the elongation rate of linear low-density polyethylene LLDPE is higher). Through a special manufacturing process, some materials The elongation can be above 1000%, and the elongation at break of ordinary polymer materials is mostly between 50% and 100%. Generally, heat shrinkable films and stretched films have higher requirements for the tensile properties of materials.
2, tensile test
Tensile test (stress-strain test) is generally to clamp the two ends of the material sample on two clamps separated by a certain distance. The two clamps separate and stretch the sample at a certain speed to measure the stress change on the sample. Until the sample is broken.
Tensile test is one of the most widely used methods for studying the mechanical strength of materials. It requires the use of a tensile testing machine that moves at a constant speed. According to different load determination methods, tensile testing machines can be roughly divided into pendulum tensile testing machines and electronic tensile testing machines. At present, electronic tensile testing machines are more commonly used.
3, selection index of electronic tensile testing machine
Since the flexible packaging materials are mainly high molecular polymers or its related materials, as mentioned above, the elongation rate of high polymer materials is much better than that of metals, fibers, wood, plates and other materials, so the tensile machine for testing high molecular polymers There is a certain difference from the normal tensile testing machine for material tensile properties, especially the effective stroke of the electronic tensile machine and the specimen clamp.
3.1 Effective stroke
In the tensile test, although the size of the sample used is small, the elongation of the material is generally high. Therefore, the tensile performance of the flexible packaging material needs to be equipped with a tensile machine with a larger stroke, otherwise the fixture operation may exceed The use limit of the itinerary causes damage to the equipment.
The calculation formula for elongation at break or elongation at yield (εt, unit is %) given in GB13022-91 “Test Methods for Tensile Properties of Plastic Films”,
Where: εt is the elongation at break or the elongation at yield;
L is the distance between the marking lines when the sample breaks or yields;
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 line is drawn on the finished sample by printing or manually (the addition of the marking line should not have any effect on the sample), and what is the distance between the marking lines? Different standards give this There is a certain difference in the distance, and the same standard often gives different sample sizes for different materials, so the distance between the marking lines is also different, but this is conducive to the detection of very large or very small elongation Materials and get accurate test results. For plastic films, the distance between the marking lines is usually between 25 and 50 mm.
Because the sample deforms and elongates in the tensile test not only within the marking line, all the samples between the two clamps will get different degrees of tensile deformation. The initial distance between the clamps corresponding to the marking distance in the standard is within 80～115mm. If the specimens between the two clamps can maintain the same elongation and assume it is 500%, the effective stroke of the tensile machine must be 480 ~690mm can ensure the normal progress of the test.
The author conducted a random investigation on the effective stroke of the electronic tensile machines currently on the market. Among the 72 tensile machines of several domestic and foreign brands investigated, the equipment whose stroke range is below 400mm, accounting for 13.9%; the stroke is 400 Equipment with a distance of ~700mm, accounting for 71.9%; equipment with a stroke of 701-1000mm, accounting for 10.8%; equipment with a stroke of 1000mm or more, accounting for only 3.4%. However, in the statistics of the electronic tensile machine this time, not all the equipment with large strokes is used for the detection of flexible packaging materials, and some are used for the tensile test of artificial wood panels, canvas, curtains, copper and other materials.
3.2 Specimen fixture
The description of the fixture in GB13022-91 “Plastic Film Tensile Performance Test Method” is: “The testing 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 testing machine It should be able to align to a line immediately, so that the long axis of the sample coincides with the tensile direction passing through the centerline of the fixture….Place the sample in the two fixtures of the testing machine so that the longitudinal axis of the sample is aligned with the upper and lower The central lines of the clamps overlap and should be tight enough to prevent the specimen from slipping and breaking in the clamps. The clamps 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 In this way, the thickness of the sample tested by the soft package material is very thin, and the general clamp cannot meet the requirements. Improper use will cause the sample to break at the clamp and cause the test to fail.