Toughened or tempered glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the inner surfaces into tension. Such stresses cause the glass, when broken; to crumble into small granular chunks instead of splintering into jagged shards as plate glass (i.e. annealed glass) creates. The granular chunks are less likely to cause injury.
Properties: Toughened glass is physically and thermally stronger than regular glass. The greater contraction of the inner layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass.
In such glasses, the surface compressive stress exceed 100 megapascals (15,000 psi), for as greater the surface stress, the smaller the glass particles will be when broken.
It is this compressive stress that gives the toughened glass increased strength. This is because any surface flaws tend to be pressed closed by the retained compressive forces, while the core layer remains relatively free of the defects which could cause a crack to begin.
As per the nature of the glass, any cutting or grinding must be done prior to tempering because Cutting, grinding, and sharp impacts after tempering will cause the glass to fracture.
Now here we will be discussing the issues related to optical defect:
The surface of tempered glass does exhibit distortion or surface waves caused by contact with flattening rollers. This Optical issue of waviness is a significant problem in manufacturing high-end façade projects.
The strain pattern resulting from tempering can be observed with polarized light or by using a pair of polarizing sun glasses.
There are three main reasons causing the roller wave effect:
High exit temperature of glass from furnace:
While tempering the glass, heat treatment requires uniform heating of the glass to 621 +/- 3 deg C, while holding the glass in a flat state.
The glass softens and is prone to cause internal bending as it approaches the critical temperature closer to 650 ˚C. So hotter the glass that exits the furnace, the worse is the quality.
The colder the glass, the better is the quality. However, the risk of breakage increases. Sometimes, overheating is needed to compensate for bad edge work or hole finishing.
Corrections: Although the furnace is designed to control thermodynamic conditions, variations are difficult to eliminate.
So for perfection precisely enough heat is added to relieve stress without making the glass too soft. But in reality, such as glass type, glass thickness, coatings, furnace temperature, atmospheric temperature, ambient humidity etc. define the final outcome.
Key rule is:
“Decrease the furnace temperature and increase the heating time.”
Any roll eccentricity imparts deformation to the glass.
The leading and trailing edges of each lite form a cantilever as the glass leaves a roll. The glass sags under this load. Overheating causes sag between the rollers. Uneven heating and inconsistent loading exacerbate the problems due to hot spots in the furnace.
Large roller pitch:
There are differences in roller settings in tempering furnaces. When the distance between consecutive rollers is longer, the glass travels a longer distance unsupported. Longer roller pitch results in a larger peak-to-valley roller wave value, thus creating a visually more noticeable distortion.
Roller shape accuracy along the whole roller length also plays an important role in roller wave quality.
This problem is mechanical, related to the roller pitch of a furnace and should be taken care while opting for new tempering line. Later it is extremely challenging to change it.
For better quality results the total indicated run out (TIR) of rollers is checked and measured and high-quality rollers should be used in the process. They have a direct impact on the quality.
Better accuracy with high-quality rollers, improves the overall flatness of the roller bed and thus prevents roller “vibrations” and level differences, which can greatly affect the overall roller wave values.
The oscillation speed is too low.
Lower oscillation movement in the end of the heating cycle increases the time that the glass is without roller support, and thus causes optical distortion. A faster oscillation speed has the better result to improve roller wave effects.
Since glass as a material changes during the heating, it’s good to have dynamic control of the oscillation speed. New control systems can control the conveying process more accurately and at different speeds. Such system makes it possible to set different oscillation speeds for the various heating cycle stages to improve roller wave defect.
: Pay attention to pre-processing quality :
Accurate cutting, grinding and drilling processes will allow colder glasses to exit the furnace and improve the roller wave quality. It is to be assured that pre-processing is not a bottleneck for the tempered glass quality and yield.
The need for higher quality, distortion-free, heat-treated glass is creating both challenges and opportunities for the glass industry. While furnace technologies are advancing to meet more stringent requirements, improved measurement of roll distortion is required for process control of the furnace.
A new machine-vision technology has been developed and is now available to accurately measure roll distortion. The LiteSentry Roll Wave Distortion Measurement system, measures the peak-to-valley optical distortion on-line as glass exits the furnace. This system provides the tool necessary to improve and maintain the quality of heat-treated glass, thereby opening new markets for tempered and laminated glass.
(The float glass process can be used to provide low-distortion sheets with very flat and parallel surfaces.)