Wednesday, 7 June 2017

Effects of Annealing at the Top End of the Range

It is possible to begin your annealing at any point in the annealing range.

The annealing point is the temperature at which the glass most quickly relieves the stress within.  This occurs at the glass transition point

The  annealing range is between the softening point and the strain point of the glass.  No annealing can be achieved above the softening point, nor below the strain point.  This range, for practical purposes can be taken to be 55°C above and below the published annealing point.  For thick slabs, Bullseye has chosen to start the anneal 34°C below the published annealing point of 516°C.

High Annealing Point

They could have chosen to use a higher point, even up to 571°C, the approximate strain point of the glass.  The effect of this is an extended anneal cool.  The reasons are as follows.  

The anneal soak does not need to be extended, as the purpose is to get all the glass at the same temperature in preparation for the annealing cool. 

The cooling rate must be slower (approximately one third the rate) than an anneal soak at a lower temperature, as the glass must be maintained at the same temperature throughout the long cool.  

Also, the initial rate of cool needs to be maintained down to the strain point, which is 110°C below the softening point.  Of course, after that initial cool, the speed of cooling can be increased.

Low Annealing Point

Starting the anneal cool closer to the strain point requires a longer soak to ensure the glass is all at the same temperature (+/- 5°C) before the anneal cool begins.  Typically, this initial soak would be for an hour before the initial cool begins (for a 6mm to 9mm thick piece).


Effect of the Differences in Approach

The advantages and disadvantages centre around these needs to 

  • soak long enough to get all the glass to the same temperature and secondly, to 
  • cool slowly enough to maintain the even temperature distribution throughout the glass.


Example

If you think of an example of a piece of Bullseye glass 12mm thick, it will show the differences in approach.

High temperature soak
A soak of 30 minutes at 571°C (the highest possible start for an annealing soak) is required to even the temperature.  To ensure the temperature differentials in the glass do not deviate from the +/-5°C, the cool needs to be at 18°C per hour down to 461°C.  It is possible then to increase the speed to 36°C down to 370°C.  This gives you a total annealing cool of just over 5 hours.

Low temperature soak
Starting the anneal at 482°C requires an hour soak followed by a decrease in temperature of 55°C per hour to 427°C, and an increased rate of 110°C to 370°C.  This gives an anneal cool time of 3 hours and 30 minutes.


The example shows how, although the annealing result may be the same, there is considerable time saved (for thicker pieces) in using the lower part of the annealing range to begin the annealing.  It also will save some electricity.

However, an anneal of 30 minutes at 516°C with a cool of 80°C per hour to 370°C will still give a perfectly adequate anneal for 6mm thick pieces.