Precautions for purchasing material testing machine

Selection of electronic tensile testing machine

Regarding the purchase of the electronic tensile testing machine in the overall construction plan of the flexible packaging laboratory, the performance and specifications should be considered comprehensively according to the needs of the film used, and comparisons should be made from the following aspects:

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

The different pulling force range determines the different sensors used, and also the structure of the pulling 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 material elongation exceeds 1000%, you can choose a stroke of 1000 or 1200mm.

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 of them are 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.

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.

7. Test speed.

The national standard stipulates that the test speed is 200mm/min. Some of the equipment on the market is 10~500mm/min, and some are 0.01~500mm/min. The former generally uses ordinary speed control systems, which are low in cost and roughness affects accuracy; the latter uses servo The system is expensive and high in accuracy. For flexible packaging companies, it is sufficient to use a servo system with a speed range of 1~500mm/min, which will not affect the accuracy and the price is within a reasonable range. {$nextpage$} 8. Measurement accuracy.

1. 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.

2. 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 markings are drawn on the finished sample by printing or manually (the addition of markings should not have any effect on the sample), and what is the distance between the markings? Most of the distances given by different standards have certain differences, and the same standard often gives different sample sizes for different materials, so the distance between the markings is also different, but this is conducive to the detection of extensions. Material with very large or very small growth rate 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 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 with the stroke range below 400mm accounted for 13.9%; the stroke was 400 ~700mm equipment, 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 electronic tensile machines, the equipment with large strokes is not all It is used for the detection of flexible packaging materials, and part of it is used in the tensile test of artificial wood panels, canvas, curtains, copper materials 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 clamp. The clamp 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.

3.3 GT-MS series intelligent electronic tensile testing machine

GT-MS LCD display intelligent electronic tensile testing machine, with a stroke of 600mm and a maximum of 1000mm. It can perform seven kinds of tests such as stretching, tearing, shearing, 180° peeling, and 90° peeling, and has been professionally designed. The test fixture dedicated to flexible packaging materials is a tensile testing machine tailored for the mechanical properties of flexible packaging materials