In order to achieve product uniformity, noncontact temperature measuring methods are increasingly being used and are an accepted and important technique for process control and quality. This article looks at some of the benefits and existing applications.
Benefits of noncontact measurement
Accurate temperature measurement and control are critical in many glass manufacturing processes. In addition to point thermometers, which are able to measure the surface of a single spot, more and more infrared line-scanners are being used.
Line-scanners are capable of profiling temperatures. The benefits of noncontact measurement are:
* No contact with the material, no influence on the target.
* No damage to the object and no effect on the surface.
* Durability.
* Fast response time, better quality control of product.
* Temperature measurement profiles can be achieved by IR scanners.
* Measuring the temperature of moving targets is simple.
Horizontal strengthening
Glass sheets have to be treated on their surface after the cutting process in order to make them stronger.
This is done by transferring the glass panes into an electrically heated oven where the glass surface is heated as evenly as possible to a temperature of around 600°C.
After approximately 4 min, the glass is transferred to a section of the plant where it is cooled down as evenly and quickly as possible to ambient temperature by forced air.
This process can be problematic because often the glass is heated unevenly or at too low a temperature, which then causes internal deformation and breakage.
When partial overheating occurs, problems with glass flatness arise.
The glass panes leave the hardening furnace at a speed of 1 m per second. In addition, the position of the 24 heaters in a 3 x 8 layout, and the average temperature of the glass under each heater is monitored. With the help of area identification, it is possible to adjust the single electric heaters to work in a very efficient manner. The yield can be increased considerably, especially when changing the types of glass or cross-sections frequently for short runs.
Pre-stressing of car windows and vertical bending
To deform the glass in a press it is necessary to heat it to the required bending temperature.
For the measurement of temperature and for the detection of irregularities in the temperature distribution, line-scanners are located between heating and forming sections in order to quickly take temperature profiles during the rapid transport of glass from the furnace into the press.
With the knowledge of the shape of the glass sheet, an area-related analysis of individual sections can be obtained.
By use of the data, the heating process can be optimised.
Thermal image displays can be employed for optimisation of the heating section temperature. The average temperature of the glass segments, which are heated by the respective heater sections, are displayed together with the position of the heating sections.
The pre-stressing of the glass panes is done by forced air cooling. A certain temperature and time cooling regime has to be followed. Here, line-scanners are also successfully used.
The scan of the temperature profile is carried out before and after the cooling section.
By an accurate referencing, the required temperature difference can be checked point by point. In realtime it can be decided if the pre-defined quality parameters are maintained or if the current piece of material is to be rejected as scrap.
Rapid return on investment
The above applications are just some of the many. Automotive windscreens are similarly monitored when gravity-sag formed, and many other glass products, for example containers and flat sheets, are subjected to infrared monitoring and control in various applications.
Raytek infrared sensors and line-scanners have been introduced into the glass industry over a number of years, meeting different customer requirements across the globe.
For all the abovementioned applications, PREI Instrumentation offers complete solutions including an advanced Windows-based software.
The use of noncontact temperature measurement instruments provides a rapid return on investment through improved product quality control, reduced scrap rate, lower energy costs and production optimisation.
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