In my previous post on Glass Tempering or Toughening Process, I had mentioned how the process is executed and the physics involved (Refer Back). Also a brief mention how the toughened glass quality is assessed after it breaks, this is very important because safety is the reason for we spend on tempering. There are also other issues in terms of quality when glass is tempered. These are mainly optical distortion, roller marks, waviness and bend, edge strength, coating burns, fragmenting, burns, spontaneous breakage, etc.
Quality of tempered glass mainly depends on the quality of equipments used and the quality control procedures adopted. Optical Distortion , is mainly a blurred appearance in images when seen through the glass, as well as on the reflection on the glass. This quality issue in tempered glass is common to all types of glasses. Even though minor levels of optical distortion is present in most of the tempered glasses, but it gets magnified when the quality is that poor and the glass is applied on high rise building facades. The minor level optical distortion is inherent on tempered glass, considering the fact that glass nearly reaches it’s softening point as it is heated up to a temperature of 726 degrees, and also the fact that this glass moves in rollers, therefore it is also called roller wave distortions. Such distortions could be easily identified in reflective and low-e coated glasses. Roller wave distortions could be easily controlled by adopting suitable technology and quality control procedures (use of forced convection furnaces instead of radiation furnaces).
Reflective glasses, as it’s name suggests, reflects heat incident on it. While tempering such glasses, the uncoated side of
the glass is in the bottom and the reflective side faces upwards, while inside the furnace. Since the coated side keeps on reflecting heat, the bottom side gets heated faster and wait for the top side to reach the same temperature. At this point, the bottom side is in contact with the rollers. This is the major factor for the cause of roller wave distortion in reflective glasses. Another cause for optical distortion is the difference in temperature across the furnace.
Optical distortion could be minimized to a great extend by following simple standards in terms of cleaning and maintaining the rollers and furnace, constant monitoring of temperature gradients inside the furnace and by immediately taking corrective action as and when such defects are observed.