This is more important for the laminated films. Only the whole thickness is even can confirm the thickness of every layer of colophony is even. Hence, for film, the preconditions of some special technical index are that: thickness of film is even, the thickness of film is as same as the preset value, and the error is in the range of tolerance. Thickness test is the basis test item in film industry.

1.Packaging materials thickness test
Non-online testing technology is the earliest one in the film thickness test. Then, with the development of radial technology, online thickness test equipment, which can cooperate with the production line, was developed. During the 60fs of last century, online test technology was applied widely, now it can test the thickness of a certain coating layer. Meanwhile, non-online technology has developed a lot, various non-online technologies appeared.

There are obvious differences between online test technology and non-online test technology. On-line technology generally introduces non-touching method. Non-online technology generally introduces mechanical method, electrical vortex method or electromagnetism method; some testers introduce optical or ultrasonic technology.

2.Online thickness test
Familiar online thickness test technology isƒŔradial technology, X radial technology and infrared technology.

a)ƒŔradial technology
ƒŔradial technology is the earliest radial technology which is used to online thickness test. It requests nothing for test specimens, but it is easily affected by the temperature, atmosphere pressure, and movement of film. And it requests best radiation protect equipment, the price of the signal resource is very expensive, the lifetime of Pm147 is 5 or 6 years, Kr85 is 10 years, the price is about USD6000.00.

b)X radial technology
This technology is rarely used for film production. Lifetime of X -ray tube is 2 or 3 years, its price is about USD5000.00. And it can not be used for measurement of polymer which is made up of various elements. The Radioactivity of signal resource is high. It is usually used in steel board test, which is made up of single element.

c)near-infrared technology
This technology has ever been affected by stripe-effect phenomena. But modern near-technology has broken though the effect. It can be used for total thickness test and certain layerfs thickness test. Modern infrared technology can be used for double direction elongation film, cast film, and multiplayer copolymer. No radioactivity, easy to maintain.

d)On-line tester
On-line tester can get the data rapidly. By data analysis, producer can adjust the production parameter. But on-line tester must be configured by suitable scan frame; this limits the repeat test in a certain extent. Because the production line must keep working for a long time, the on-line thickness tester must work for a long time too. On-lone tester is more expensive, and the running fee and maintain fee are more expensive too.

3.Non-online thickness test
Generally speaking, there are two kinds of non-online thickness test method: touching method and non-touching method. Touching method is mechanical. Non-touching methods include optical method, electrical vortex method, ultrasonic method and so on. Because non- online testers are cheaper, small and other properties, they are used widely.

a)Vortex thickness tester and magnetism thickness tester
Vortex thickness tester and magnetism thickness tester are small portable testers. They are specially designed for coating test. There may have error when these kinds of testers are used for paper and film test.

b)Ultrasonic thickness tester
Ultrasonic thickness testers usually are small portable testers too. They can be used for metal, plastic, ceramic, glass and other good conductors of ultrasonic. It can work under high temperature, but it is not that all specimens can be tested by this kind of tester.

c)Optical thickness tester
As for the principle, optical thickness tester can get high accuracy, but this kind of testers is strict with the use and maintain: far from shaking, dusty and professional operation and maintain. Specimenfs range is narrow, just for laminated film which layers are not too much.

d)Mechanical thickness tester
Mechanical thickness tester includes point touching tester and plane touching tester. It is a touching method, is different from the non-touching method: it can press the specimen before test. This can avoid the data fluctuation caused by the materialsf properties, for example the materialsf surface are uneven and can be compressed. Mechanical thickness testers employ the traditional test method, data is steady, and mechanical thickness testers are not of materials selective.

Because the precision of mechanical thickness tester is decided by the precision of measurement elements, the levels of testersf precision are different in the market. Furthermore, the core elements of measurement elements, test head and test surface, are sensitive to weeny shaking, so precision is unmeaning if it is under the shaking situation.

To avoid own shaking and decrease the effect of conditions, mechanical thickness testers are made of heavy and wide steel base. This confirms the precision but it is difficult to be portable. The ambient temperature and wind speed can affect the precision of sensor too, so this tester must be used in the lab. There are many standards for mechanical thickness tester (This is rarely in the field of thickness testers), other thickness test methods have no standard almost), ISO 534F1988CISO 4593F1993CASTM D 645-97CGB/T 6672-2001 and so on.

What to indicate is that: because of the difference between the area of test heads, force of test, and speed of test, there will be different results for same specimen tested with different thickness testers. The reason is that the distortions of compressible specimen under different condition are different. So users must strictly implement the relative standard when they use mechanical tester.

e)Non-online testers
Sales volume of non-online tester is larger than online testers, one reason is their price is lower, the second reason is non-online testers are easy to move, the third one is the two kinds of testers are compatible. Use the two kind of testers together, users can get more efficiency. For some materials, the data put out by different testers may be different. One hand, the principles of different testers are very different. Except of mechanical thickness tester, other kinds of testers are materials selective, can not be used widely. On the other hand, flexible packaging materials are compressible.

Labthink Testing Instruments has developed our own mechanical thickness tester(Type CHY-C1) whose resolution is 0.1ƒĘm. Itfs very uncommon even in the world.

4.Characteristics of flexible packaging materials
Common packaging materials are made of polymer and paper. Rubber, fibrin and plastics are polymer. Characteristics of rubber is high elasticity under room temperature, in another words, little force can produce great distortion, and it can come back after the force is moved, its elastic of modulus is little, about 105`106N/m2. Contrary, fibrinfs elastic modulus is larger, it is about 109`1010N/m2, distortion is little. Plasticfs elastic modulus is about 107`108N/m2Cparts of distortion is reversible, parts of distortion can not. It is difficult to divide these three kinds of polymer strictly. For example, polyvinyl chloride is typical plastic, but it can be drawn into fibrin, and after adding suitable elasticizer it can be made into flexible materials, which is familiar with rubber. Generally, paper is flexible and compressible. Its surface is rugged.
So, polymer and paper can be distorted by force. So flexible packing materials generally have a degree of deformation. The forces on the surface affect the test result directly.

5.Conclusion
For manufacturer of film, the evenness of thickness of products is the most important index. Thickness tester is absolutely necessary to control the thickness. But they must know the packaging materialsf category, requirement of thickness evenness, test range of tester and other factors, then they can choose the tester.