Tempering soft coat glass.

Guest User
Mon, 01/02/2010 - 15:47

I am new to tempering glass and i am currently trying to set up my furnace to toughen soft coat(elite) glass. What differences sould i expect to tempering float glass?

I currenly have set heating time much longer with more top convection and slightly adjusted tempeture, glass is fully tempered but i keep getting a wave or blip in the middle of the glass.

Any advice on resolving this problem?

Guest User
Tue, 02/02/2010 - 03:40

The distortion you are seeing is called lensing and is due to the glass bowing up off the rolls during the early stages of heating. This happens because the top coated surface is reflecting the IR radiation and not heating as fast as the lower surface.
You need some form of addition heat input to the top surface of the glass, and normally for a soft coated glass this would be achieved with air convection (hot gas fans or heated compressed air).

Guest User
Tue, 02/02/2010 - 05:15

Thanks for your repy.

Yes, I have top convection fans. In fact, iv turned up the fans to the max for when the glass first goes into the furnace. I currently have top temp at 710 and bottom at 700. Maybe i should decrese the tempetures, especially the bottom, would this resolve the problem do you think?

Guest User
Tue, 02/02/2010 - 18:41

For a stable operation when the glass enters the furnace the Upper and Lower idle power levels should be similar. You can try reducing the lower set point temperatures until the lower element power levels are very low at idle, furnace empty. If you reduce the set points below this level then the roll bed is being heated solely by the upper zones and you have no control of the roll bed temperature. When the glass enters the furnace it effectively separates the heating top to bottom and the roll bed temperature will then reduce to a point where power from the lower elements is required, but by this time the damage to the glass will already have happened. At the end of the cycle the furnace should have recovered to the idle condition with the unbalanced power levels and when the next glass enters the process will repeat. The condition is unstable and the process will be very susceptible to any glass load size changes.
I personally think your furnace temperatures are far to high and would suggest you lower them to around 680 over 660 with a longer cycle time and see what happens.

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