Smooth ride; how to heat glass uniformly in a tempering oven

Tempering requires that the glass gets heated to about 630 degrees Centigrade, or about 1,165 Fahrenheit. It is important to have all areas of the glass at the same temperature as it enters the tempering quench. It also is important to minimize the glass temperature gradients during the time the glass is heating.

From the time the glass enters the oven,  keep it riding flat on the ceramic rollers, thereby controlling the glass temperature gradients. As the glass is heating, if the top surface gets hotter than the bottom surface, the top will get longer than the bottom and the glass will bow into a water-shedding shape. A cold top surface and a warmer bottom surface make the glass take a water-holding shape. Area wise, if the edges get hotter than the center, the edges wrinkle, making the glass saddle-back shaped. Colder edges and warmer center cause the central area to pop upward or downward in a bi-stable shape.

Each of the above distorted conditions is undesirable and results in surface scratches, sometimes called skunk stripes, and corner chips that may make the glass unusable. The problem of non-uniform heating is related to the various ways glass is heated. Heat transfer takes place by radiation, conduction and convection. The glass will ride flat through the furnace when all three heat-transfer methods are under control.

Heating the two surfaces by the ceramic rollers is complicated.  The rollers are good insulators and act as a heat sink. The heat transfer to various colored glasses and coated glass further complicates keeping the glass flat.

Convection is built into some ovens using blowers or aspirators. Ovens with neither are plagued with natural convection, usually called drafts. For uniform glass heating, the drafts must be controlled.

Conduction is minimal except from the rollers. True conduction is when things are in contact, and heat flows from the hotter to the colder object. The rolls are round and therefore touch the glass only on a theoretically zero-width line, which should give zero heat transfer. Operators of tempering ovens know that the rollers play a significant role in how the glass heats. Conduction to the glass from the roll actually takes place through the air space between the glass and the rolls. This air gap is so small that a lot of heat transfers to the glass from the rolls. This phenomenon changes when the rolls are grooved, introducing a convection component around the roll. Grooved rolls often help to reduce the fast heating of the glass bottom surface that occurs when the glass comes into a hot, idle oven.

Top to bottom temperature set-point changes try to obtain correctly balanced heating, but often this does not work. The rollers get heated from the bottom elements, but also by top heaters when there is no glass on the rollers. How do operators solve this problem and keep losses from scratches and chips to a minimum? My choice is to substantially reduce the set points in the front end of the oven. Try using a 500-degree Centigrade set point in the first section of a shake-and-bake oven, or in the first three zones of a continuous heater. The lower top and bottom set points will probably still result in area-to-area and top-to-bottom temperature gradients, but they will be much smaller gradients. Smaller gradients will mean reduced thermal-bending forces and a flatter ride for the glass. Production rates might drop, but not as considerably. More than half the heating of the glass happens in less than half the residence time in the furnace, so doing less heating in the first half may not show a serious production loss. If you are making scrap, obviously getting some good ones out is better than getting nothing salable out. For the best ride of the glass, and incidentally also for the best optics and temper break pattern, maintain the best thermal uniformity possible.

In summary, if the glass is not flat when it comes out of your oven, try adjusting set points, providing more heat on top for a water-holding shape and less for water-shedding shapes. If you still have a problem, heat slower and minimize the delta Ts—or differences in temperature— and keep the temperature differences small over the entire piece of glass.