I have serious problems with HST, because all my glass plates break during the process (every 100 plates 80 are broken). The plates are 1500 x 2500 mm, glass tickness is 12 mm, the holes are D=12mm and are 24mm distant from the long sides. There are total 8 holes. At the corners the holes are 160mm and 30mm distant (from the edges). During the tempering process all is OK. In the HST process during the cooling phase the glass break.
I don't know if the problem is the tempering process or maybe the position and the diameter of the holes.What do you suggest me?
Thanks in advance
Robert from Italy.
Thamk you very much for your reply.
We have made same test. Next week we will buy a stress mesaurement system (Strainoptic GASP), and we will analize the tensions near the holes. 80% the problems are the holes, 20 % the problem is the tempering furnace. We have to chek if the tempering process is homogeneus.
By the way we tried to put in the HST chamber only few pieces, and the result was "only" 30% breakages.
I hope we will find the problem.
Kind regards.
Robert
As long as you can toughen it properly witout problem. It is O. K. You can think about improve the edge quality, especiaaly arround the holes circumferential adges. You can use longer heating time to fix it during the tempering. But in the heat soak, you can do very little bit. Because during the heat soak the glass surfaces experince expansion and contraction, the tiny cracks along the hole edges can initiate a crack easily. Even though you do not make any heat soak process it also will crack some time, may be during delivery or installation or in customer's site.
80% breakage continuously is very abnormally, there should not be so much Nickel Sulphide stone to initiate the breakage.
Try to improve the hole edges quality and extend the cooling phase time ----- the time take from 290C ( contant temperature value ) to room temperature or the process end temperature. Because sometimes too short cooling time ( in some heat soak furnace ) can cause abnormal breakage or bend a lot after the process for very long glass.
Thomas.