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Film thickness and non measurement methods

whether the film thickness is uniform is the basis for testing various properties of the film. Obviously, if the thickness of a batch of single-layer films is uneven, it will not only affect the tensile strength and barrier properties of the films, but also affect the subsequent processing of the films. For composite films, thickness uniformity is more important. Only when the overall thickness is uniform, the thickness of each layer of resin can be uniform. Therefore, whether the film thickness is uniform, whether it is consistent with the preset value, and whether the thickness deviation is within the specified range are the prerequisites for whether the film can have some characteristic indicators. Film thickness measurement is one of the basic testing items in film manufacturing industry

1. Rapid development of various and non thickness measurement technologies

thickness measurement of packaging materials non thickness measurement technology was first used for film thickness measurement. Later, with the continuous development of X-ray technology, the thickness measuring equipment installed with the film production line was gradually developed. Thickness measurement technology has been widely used since the 1960s, and now it is more able to detect the thickness of a coating. At the same time, non thickness measurement technology has also made great progress, and various non thickness measurement technologies have sprung up one after another. Thickness measurement technology is completely different from non-thickness measurement technology in terms of measurement principle. Thickness measurement technology generally adopts non-contact measurement methods such as ray technology, non-thickness measurement technology generally adopts mechanical measurement method or measurement method based on eddy current technology or electromagnetic induction principle, as well as optical thickness measurement technology and ultrasonic thickness measurement technology

2. Common thickness measurement techniques include β X-ray technology, X-ray technology and near infrared technology

2.1 β Ray technology

β X-ray technology is the first X-ray technology used in thickness measurement. It has been widely used in the thickness measurement of ultra-thin films since the 1960s. It has no requirements for measuring objects, but β The sensor is sensitive to the change of temperature and atmospheric pressure, as well as the fluctuation of the film. The equipment has high requirements for radiation protection devices, and the replacement cost of the signal source is expensive. The pm147 source can be used for years, and the kr85 source can be used for 10 years. The replacement cost is about 6000 dollars

2.2 X-ray technology

this technology is rarely used in plastic film production lines. The X-ray tube has short service life and expensive replacement cost. It is generally available for years, and the replacement cost is about 5000 dollars. It is not suitable for measuring polymers composed of multiple elements, and the signal source is highly radioactive. X-ray technology is often used to measure single elements such as steel plate

2.3 near infrared technology

low concentration of near-infrared industry. The application of these technologies in the field of thickness measurement has been affected by fringe interference, but now near-infrared technology has broken through the restriction of fringe interference on the measurement of ultra-thin film thickness, and can completely measure the total thickness of multi-layer films. Due to the characteristics of infrared technology, The thickness of each layer of material can also be given while measuring the total thickness of the composite film. Near infrared technology can be used for biaxially stretched film, cast film and multi-layer coextrusion film. The signal source is non radioactive and the equipment maintenance is relatively low

2.4 application of thickness measurement equipment

thickness measurement can obtain thickness test data at the fastest speed, and timely adjust the parameters of the production line through data analysis to shorten the start-up time. However, the thickness measuring equipment must be equipped with a scanning frame matching the production line, which limits the reuse of the thickness measuring equipment to a certain extent. And because the film production line often needs long-term continuous work, the corresponding thickness measuring equipment must also work continuously for a long time. In terms of equipment price, test equipment is generally much more expensive than non test equipment, and the operation and maintenance costs of the former are also relatively high

3. Non thickness measurement

non thickness measurement technology mainly includes contact measurement method and non-contact measurement method. The contact measurement method is mainly mechanical measurement method, and the non-contact measurement method includes optical measurement method, eddy current measurement method, ultrasonic measurement method, etc. Due to the low price and small volume of non thickness measuring equipment, it has a wide range of applications

3.1 eddy current thickness gauge and magnetic thickness gauge

eddy current thickness gauge and magnetic thickness gauge are generally small portable equipment, which use the principle of eddy current and electromagnetic induction respectively. It is specially used for measuring the thickness of various specific coatings. There may be errors when measuring the thickness of films and papers

3.2 ultrasonic thickness gauge

ultrasonic thickness gauge is also mostly small portable equipment, which can measure the thickness of metal, plastic, ceramic, glass and any other good ultrasonic conductor by using the ultrasonic reflection principle. It can work at high temperature, which is not available for many other types of thickness gauges, but it is selective for the types of test samples

3.3 optical thickness gauge

in terms of test principle, optical thickness gauge can achieve extremely high test accuracy, but this kind of thickness gauge has extremely high requirements in use and maintenance: it must be far away from the vibration source; Strictly prevent dust; Professional operation and maintenance, etc. The application range is narrow, and it is only applicable to composite films with few composite layers

3.4 mechanical thickness gauge

mechanical thickness gauge can be divided into point contact type and surface contact type. It is a contact thickness measurement method, which is essentially different from the non-contact thickness measurement method - it can apply a certain pressure (point contact force or surface contact force) to the measuring surface of the sample before thickness measurement, so as to avoid using the non-contact thickness gauge to measure those with certain compressibility When the surface is uneven, the data may fluctuate greatly. The mechanical thickness gauge adopts the most traditional thickness measurement method, with stable and reliable data and no selectivity for the sample. As the test accuracy of mechanical thickness gauge mainly depends on the accuracy of thickness measuring elements to prevent rusting, the test accuracy of mechanical thickness gauge in the market is uneven. In addition, the core components of mechanical thickness gauge - measuring head and measuring surface - are very sensitive to small vibration, so the measurement accuracy has no significance in the environment with vibration source. In order to avoid its own vibration and minimize the impact of external vibration, the equipment base is made of heavy and wide metal, which ensures the thickness measurement accuracy to a certain extent, but also brings great difficulties to the miniaturization and portability of the mechanical thickness gauge. Ambient temperature and wind speed can also affect the accuracy of the sensor, so it must be used in the laboratory environment. Many international standards on mechanical thickness measuring equipment have been formulated (this is rare in the field of packaging material thickness measurement, and other types of thickness measuring equipment are rarely supported by standards), ISO 534:1988, ISO 4593:1993, ASTM D, gb/t, etc

it should be pointed out that common mechanical thickness gages include point contact thickness gages and surface contact thickness gages. Because the contact area between the measuring head and the sample is different, the force application of the measuring head is different, and the force application speed is different. The same sample (assuming uniform thickness here) is likely to get different test results with these two types of thickness gages, This is mainly because the deformation rates of compressible specimens are often different under different conditions. Therefore, the test conditions and requirements of the referenced standards must be strictly implemented when selecting the mechanical thickness gauge for testing

3.5 the application of non thickness measuring equipment takes the installation of experimental machine as a major event.

the sales volume of non thickness measuring equipment is larger than that of thickness measuring equipment. On the one hand, its price is cheap; On the other hand, compared with thickness measuring equipment, non thickness measuring instruments can be easily handled and moved; In addition, the use of non thickness measuring equipment does not conflict with the use of thickness measuring equipment. The two can effectively cooperate to improve the product qualification rate. For some samples, different results may be obtained by using different thickness gages. On the one hand, the test principles of various thickness gages are quite different. Except for mechanical thickness gages, other types of thickness gages often have selectivity for sample materials and poor universality; On the other hand, most soft packaging materials are compressible. In 2004, Jinan Languang company successfully developed the first ultra-high precision mechanical thickness gauge chy-c1 in China, with a test resolution of 0.1 μ m. Such a high-precision mechanical thickness gauge is also very rare in the world, so the launch of LAN Guang chy-c1 thickness gauge has caused a great sensation in China's packaging industry

4. Many factors affect the thickness test results

characteristics of flexible packaging materials. Common flexible packaging materials are mainly made of polymers and paper products. Polymers are divided into three categories: rubber, fiber and plastic. Rubber is characterized by high elasticity at room temperature, that is, it can produce great deformation under the action of small external force; After the external force is removed, it can quickly return to the original state, and the elastic modulus is small, about 105 ~ 106n/m2. On the contrary, the elastic modulus of the fiber is large, about 109 ~ 1010n/m2, and the deformation is small under stress. The elastic modulus of the plastic is about 107 ~ 108n/m2, some of which are reversible and some are permanent. These three kinds of polymers are difficult to be strictly classified. For example, polyvinyl chloride is a typical plastic. 6. For plastic tensile testing machine 1, it is necessary to pay attention to adjusting the travel limit switch, but it can also be pumped into fiber. It can be made into soft products similar to rubber by adding an appropriate amount of plasticizer. Usually the paper is soft and has certain compressibility, and the surface is uneven. It can be seen that both polymer and paper products will deform after being stressed and are in a compressed state. Therefore, flexible packaging materials generally have a certain deformation rate. Therefore, whether there is pressure on the measuring surface of the sample during the thickness measurement of the sample will directly affect the test results

5. Summary

for film manufacturers, the thickness uniformity of products is one of the most important indicators. To effectively control the material thickness, thickness testing equipment is essential. However, the specific type of thickness testing equipment to be selected depends on the type of soft packaging materials, the manufacturer's requirements for thickness uniformity, and the testing range of the equipment. (end)

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