2. 1 X-ray technology
X-ray technology was first used in online thickness measurement technology, and was widely used in online thickness measurement of ultra-thin films in 1960s. It has no requirements for the measured object, but the sensor is sensitive to the changes of temperature and atmospheric pressure and the fluctuation of the film. Equipment requires high radiation protection devices, and the replacement of signal sources is expensive. Pm 147 source can be used for 5-6 years, Kr85 source can be used for 10 years, and the replacement cost is about 6000 dollars.
2.2 x-ray technology
This technology is rarely used in plastic film production line. X-ray tube has short life and high replacement cost. Generally, it can be used for 2-3 years, and the replacement cost is about 5,000 dollars. Moreover, it is not suitable for measuring polymers composed of various elements, and the signal source has strong radioactivity. X-ray technology is usually used to measure single elements such as steel plates.
2.3 Near-infrared technology
The application of near infrared technology in the field of online thickness measurement was once influenced by fringe interference, but now it has broken through the limitation of fringe interference for ultra-thin film thickness measurement, and can completely measure the total thickness of multi-layer films. Moreover, due to the characteristics of infrared technology itself, the thickness of each layer of materials can be given while measuring the total thickness of composite films. Near-infrared technology can be used for biaxially stretched film, cast film and multilayer extruded film. The signal source is non-radioactive and the equipment maintenance is relatively difficult.
2.4 Optical coating technology
For light-transmitting materials, there is a one-to-one correspondence between transmittance and measured thickness under certain conditions, so it is a very mature scheme to measure the thickness of materials by measuring light transmittance (optical density). In the winding coating industry, such as aluminum plating and various packaging films, it is a very mature scheme to monitor the production quality online by monitoring the transmittance of the film. For example, LS 152 vacuum coating on-line thickness gauge of Shenzhen Shanglin Technology uses the principle of optical transmission to realize non-contact on-line thickness measurement. The instrument supports RS485 communication interface and MODBUS communication protocol, and can communicate with PLC on the coating machine to realize closed-loop control.