Is it true that if the glass is cooked longer (sacrify the optical quality) and reduce quench pressure then glass breakage will go down?
For 12mm glass we set oven temp at 685, heating time is 610 seconds. We do not have thermo meter but use the equation Te=(Tfurn-Tamb)*(1-e-k)+Tamb given by Jonathan Barr in the the "Tempering by numbers" article. Exit temp is found to be 649.4 C. The glass comes out with many circular marks on it (just like acnes on human skin), so my guess is the glass is well heated. Of cource glass still breaks at a few percents. I told my operator to reduce heating time, and he said that would increase glass breakage.
hi Jonathan... i m operating furnace last since five years...i can explain u why glass broken in the quench...i m agree with u r point but i have tempered lots of critical glass and there is no brake in the furnace and also in the chiller.....if u have any critical glass then see furnace temperature its should be equal than if u have any 12mm glass which having center hole or cut out than try to increased center temperature with respective heating time whatever u give..because there r only one reason....is that the radios of hole and cutout..its also the reason of brake....if u set the quenching pressure like 21kpa or 25mmaq...its should be less than given value.....but most imp think is that furnace center temperature should more then entrance area and also exit area......also the hole an cut out should be well finished...and also need to known the lehre in the glass....because there so many glass manufacturer who annealing glass very fast for increasing production ...and glass having stress inside the glass....this is the major problem that glass broken in the furnace..if u help or support plz email me @ harish_panthi@yahoo.in
Generally speaking, increasing the exit temperature of the glass and reducing the quench pressures will improve the yield. However, 12mm glass exiting at 640 degrees plus will definitely suffer from marking on the bottom surface.
There are several reasons why glass will break in the quench, even when processed very hot, and these include:
1. Poor cutting and edgework or holes too close to the edge of the glass.
2. Damage to the corners or edges of the glass during heating.
3. Non uniform heating or quenching across the glass area.
4. Quench pressures too high.
5. Sudden (step) change from Quench pressure to Cooling pressure especially if it is too early.
You could try dropping the furnace temperature to 660 with the heating time at 560 seconds. Glass will exit around 610 (which is still hot for 12mm glass) but should look much better than before.
Watch the glass carefully during the heating phase and ensure that it does not go into a water shedding condition inside the furnace as this will damage the corners of the glass against the rolls.
Email me and let me know how you get on with the suggestion.
Regards,
Jonathan Barr