What Should We Do When There Is White Haze White Line Happend at The Middle of Tempered Glass Made by Glass Tempering Machine?

First checks which we made was made on hot furnace:

1. Blowers of convection, does it work right hZ, does it switch on/off on right time of the cycle. Everything works fine.

2. Heaters. Resistance and load of each heater. Everything is ok.

3. SSR which are switching ON/OFF heaters. It works fine.

4. Temperature controllers which control all SSRs. It works perfect. If the temperature goes down, controller immediately send signal to SSR and according to power setting on PC to switch it ON maximum power or less power, depends from the settings. We even tried to run and temper glasses with 0% power. Then all SSRs remain OFF during all heating time.  

5. Thermocouples was tested with special professional tester. The temperatures on the tester for most thermocouples was same as Northglass monitor show us. Some heaters were 2°C colder, some 2°C hotter. It shouldn’t be big issue in this case.

During all these tests we were tempering glasses and tried to play with a recipe to make the glass perfect flat on the rollers. Some glasses on the same loading had no white haze, some of it had small white haze line in the middle of the glass (too hot from the bottom part), and some of it had white haze just on the front and rear part of the glass (too much convection from the top).

After these tests and finding nothing wrong we tried to use SO2 gas but it didn‘t help as well and in few days, we decided to cool down furnace and check everything inside the furnace.

We checked:

1. Glass particles on bottom thermocouples. Condition is perfect, furnace is empty from the glasses.

2. We checked top convection pressure and found small differences in some areas. After inspecting convection pipes condition, we found some gaps which caused us loss of convection. After fixing it, the pressure is really similar in all parts of the furnace. Same test we did for top and bottom furnace parts.

3. We found that isolation material leaves some dusts on the hand if you touch the surface of the material. I don’t know is it the reason, but on Glaston FC500 we had some insulated materials falling down from too high vibrations and it caused us just burned in particles on all glass surface, but not white haze. Of course, the materials are little bit different on Northglass and Glaston but anyway it should give negative effect for whole surface of the glass. We cleaned all the furnace with vacuum cleaner.

4. We changed new bearing of the rollers and green drive belts for each roller. Checked speed of the rollers and motor of main drive.

Technician did detect any problems who might have influence for this white haze. That’s why final decision was made to change new fused silica ceramic rollers.