VIII.  Prevent Glass from Overheating 

When heating temperature exceeds the softening point of glass, glass deformation could occur, severely affecting the surface quality of the tempered glass.

As the temperature of the heating element increases, the maximum radiation intensity will shift more and more towards the shorter wavelength part of the spectrum. The increased heat is wasted since the radiation will penetrates through the glass without absorption.  

IX.  Prevent Glass from Under-heating 

If the heating temperature is too low, the rate of release of internal stress will decrease as the temperature gradient is small.  

In the meantime, the inner layer of the glass cannot be fully heated and residual stress is not completely released. It will affect the redistribution of stress after cooling and tend to cause glass breakage in quenching process.  

When the heating temperature is too low, the mechanical properties of the tempered glass are usually poor and result in larger particles of shattered glass.  

X.  Key Factors for Even Heating

The heating chamber must maintain the crosswise and longitudinal load balance. In other words, uniformity should be maintained for each batch in terms of the number of glass panes placed, positions of the placement, and spaces between the pieces. 

The heating area in the chamber may be divided into many small heating sections controlled by microcomputer system separately. While there is always glass absorbing heat in the central section of the chamber, the power or temperature settings of the heating elements in this area is usually higher.  

XI.  Main Factors Affecting Heating Time 

Heating temperature, the chemical contents of the glass (the higher the iron content, the faster the rates of heating), thickness of the glass, number of the glass panes placed and the spaces between them, the color of the glass and ambient temperature, etc.