So, when glass is annealed with a typical annealing lehr, the first process objectiveis to equalise the glass product to the annealing point (1000°F/537.8°C).This assures all the molecules are in a similar state of expansion, but not so excited as to cause the glass to begin transitioning back to a liquid state and slump or deform (viscous flow). The second objective is to control the cooling rate of the glass to avoid creating the large variations in temperature
through the thickness of the glass as described above.As the glass cools, molecule realignment also occurs at a diminishing rate. Eventually the glass reaches a point where the significant contraction due to molecule realignment (viscous flow) is complete. This, as noted above, is
described as the lower annealing point or strain point. Once the cooling rate to
this temperature is controlled the actual annealing process is complete.Permanent annealing strain cannot be introduced into the glass. The third and final process aim is to continue cooling the glass to the required lehr exit temperature to allow for lubricity
coating, handling, inspection and packaging, approximately 300°F – 148.9°C. What is not widely recognised is that after the glass is below the strain point it can be cooled right up to the
rate of thermal shock without changing the quality of the annealing.Thermal shock is defined as temporary strain as opposed to permanent annealing strain described above. Thermal shock results in violent and immediate excitement and expansion of the glass molecules (through rapid temperature change), which results in immediate failure or not. Thermal shock has no impact on the level of permanent annealing strain. Note that the forming quality of the glass can have an impact on how much thermal shock is required to result in fracture/failure. As would be expected, successful execution of the process objectives above are dependent on creating the
necessary glass time and temperature relationship with the annealing lehr. An understanding of the specific temperature goals is clear. The time requirements relate mainly to glass weight, thickness and heat transfer. Although weight does play a factor it is the glass products maximum thickness that has the greatest impact. As noted above, glass is a natural insulator. The thicker the glass the more time required for conduction or heat transfer to take place. This is necessary