On/Offline Measuring of Film Thickness

Offline thickness measuring refers to the process in which finished film is detected by measuring instruments (mechanical or optic) in lab. This measuring adopts random sampling and the result can not be obtained before production comes to the end. Thus, offline measure appears to be behind the times for the increasingly efficient automatic production nowadays.

On the other hand, online measuring enables users or control system to obtain real-time results so that they would make timely response to the abnormal. Plus, online measuring is able to conduct 24-hour detection of film thickness along with production.

Commonly-used online measuring technologies of film thickness include β-ray, X-ray and near infrared.

• β-ray

β-ray is the first technology to be applied to online film thickness. Β thickness gauge is a radioisotope instrument, which is used to measure film thickness based on the principle that part of B ray emitting from Kr85 or Pm weakens due to absorption after passing through the film. B-ray has been widely used for online measuring of ultrathin film as early as in 1960s. The ray has no requirement for measured object but B sensor is sensitive to changes of temperature and atmospheric pressure and film up and down vibration. Also β-ray is under the strict application mainly because it is a radioactive substance, which poses a harmful impact on humans and environment. The device has a high requirement for radiation protection and cost of replacing signal source is also high. Pm-147 source would be under service for 5~6 years and Kr85 for 10 years, costs for replacement amounts to 6,000 USD.

• X-ray

Barely applied to the plastic film production, X-ray light tube, which can be under service for 2~3 years, has a short lifespan and a costly replacement expense up to 5,000USD. X-ray is unsuitable for measuring polymer and its radioactive level is very high. This technology is commonly applied to measurement of sole element such as steel plate.

• Near Infrared

The principle is based on the absorption characteristics of various materials on near infrared ray (1.0~3.0μm). Some of infrared ray would be selectively absorbed by film during passing through and the selective absorption has something to do with wave length. The rest of ray would be free of absorption, which is called general absorption. And it has nothing to do with wav length. By comparison, we choose 3.4μm absorption wave length and 3.1μm reference wave length to measure absorptions. Then according to Bell’s law of absorption, by comparing signal changes of wave lengths above can obtain film thickness values.

Based on the principle cited above, near infrared measuring features high accuracy and high resolution, stable measurement and immune to ambient temperature; the accuracy of film thickness can be ensured even film vibrates in the gap of sensor; also measuring each film thickness of multilayer is feasible as different materials absorb varied wave lengths.

Shini-Syncro Near Infrared Online Measurement System

Based on extrusion film production, there are two types of Shini-Syncro measurement systems.

• Optymex Blown Film Measurement

OptYmex provides a continuous measuring of the thickness during process. It is installed on an oscillating ring is able to refresh the profile every 2-3 minutes since not influenced by the haul off speed. Measurement can be displayed via touch screen and be transmitted to IBC control system. It is capable of measuring transparent, semi-transparent or opaque films.

• Optyscan Extrusion Film Measurement

Optyscan operates on reciprocating orbit and goes around film in a speed of 10m/min to fulfill horizontal measurement. Measurement can be displayed via touch screen and be transmitted to IBC control system.

Compared with X-ray and B-ray, near infrared ray can spray a much smaller measuring point and maximum accuracy up to 1mm. Emitter and receptor can be combined together due to scattering near infrared light. Therefore, the device has a more compact size.