1. First, consider the need to test the tensile force range of the material

The different pull force range determines the different sensors used, and also determines the structure of the pull test 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.

2. 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%, the stroke 1000 or 1200mm can be selected.

Third, the standard configuration problem.

There are 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 it is equipped with a computer, the manufacturer shall equip it with the corresponding software control system.

4. 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, with different clamps, it can be used for 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 reach less than three hundred thousandths).

Six, the main configuration of product machinery:

Transmission includes screw drive and rack drive. The former is expensive and is used for high precision and has high test repeatability; the latter is cheap and is used for low precision and has 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.

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.

7. 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 ordinary speed control systems, which have lower cost and roughness affects accuracy; the latter The use of servo system is expensive and high in accuracy. 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.

Accuracy issues include 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.

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 testing flexible packaging materials. Special attention indicators.

The tensile properties of plastics and rubbers are one of the most important and basic properties in their mechanical properties, and it largely determines 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 is 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 high-polymers are composed of long-chain molecules and have obvious relaxation characteristics in molecular motion. For example, high polymer materials have quite high elongation. Generally, the elongation at break of PE is 90%～950% (wherein the elongation 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 on 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 to study the mechanical strength of materials, and 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 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. Therefore, the tensile machine for testing high molecular polymers There is a certain difference from the usual tensile tester for material tensile properties, especially the effective stroke of the electronic tensile machine and the sample fixture.

3.1 Effective stroke

In the tensile test, although the size of the sample used is small, the elongation of the material is generally higher. Therefore, the tensile performance of the flexible packaging material needs to be equipped with a tensile machine with a larger stroke, otherwise the clamp 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", εt=[L-L0*100]/ L0 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 is drawn on the finished sample by printing or manually (the addition of the marking should not have any effect on the sample), and what is the distance between the markings? Different standards give this There is usually 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 film, the distance between the marking lines is usually between 25 and 50 mm.

In the tensile test, the deformation and elongation of the sample is not only within the marking line, but 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.

According to a random survey of the effective stroke of the electronic tensile machines currently on the market, it is learned that among the 72 tensile machines of several brands at home and abroad under investigation, the equipment whose stroke range is below 400mm, accounting for 13.9%; the stroke is between 400～ Equipment with 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 of the equipment with large strokes is used for the detection of flexible packaging materials, and some are used for tensile tests 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 sample 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 center line of the clamp.... Place the sample in the two clamps of the testing machine so that the longitudinal axis of the sample is aligned with the upper and lower The center 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 can not meet the requirements. Improper use will cause the sample to break at the clamp and cause the test to fail.