3 common Tempering mistakes that are easily fixed on “Heavy Glass” = White line (SKUNK TAIL), Blown Corners/ edge pitting (HEAT SHOCK), Crack from Hinge Cut-Out (SHARP EDGES).

First up is the dreaded “SKUNK TAIL”. Usually makes its appearance at the change of seasons when dynamics in the heating chamber change. This ugly white line down the middle of the glass is caused by too much heat in the bottom of the Heat Chamber causing glass to bend upwards - think smiley face. The resulting weight shift to a minimal area picks up scuffs from the hot ceramic rollers causing the offending white line. The fix is simple … reduce the heat on the bottom by 10 degrees (to start) which will allow the glass to ride flat on the rollers by distributing the heat evenly, It may be nescssary to decrease heat more and add a few seconds to heating time to compensate but this simple change will fix the issue.

Second is “BLOWN CORNERS” or ”HEAT SHOCK”. Another heat related issue caused by too high a temperature at the glass edge “popping off” the corner of the glass. The broken corners leave a trail of glass residue to be deposited on the ceramic rollers and transferred to upcoming product. Even if the corners do not blow off excessive heat will cause dimpling along the edges of the glass lites. Not good. What to do? Reduce the heat in the chamber adding time if needed, slower heating = better quality and when combined with “HEAT PROFILE” settings - more on that in a few - will eliminate this particular issue.

Third prevalent cause of breakage is found in the cooling section. Hinge cut outs crack from the holes causing the entire lite to blow during cooling. This is mostly caused by inferior edge work on cut outs, hinges and holes during fabrication. All edges should be sanded smooth after drilling/notching, not left “as is”. This is very easy to fix on CNC Drills = simply program in countersunk holes and “touched” corners. The other way is to use a countersink bit in a hand held drill to touch both sides of a hole and a 1 1/8″ sander to hit the remaining sharp edges and corners. By adding these few steps to the fabricating process a dramatic reduction in breakage will be immediately realized. I also prefer to keep the temperature the same for 1/2″and 3/8″ glass changing the time only. This eliminates heating up/ cooling down issues and the operator can flip back and forth between thicknesses as production dictates. Extremely useful when one or two lites are needed in a hurry or to finish an order. I find the increased flexibility with less damage during thickness changes more than compensates for the additional heating time on 1/2″ glass.

Another quick addition that reduces Hinge cut-out related breakage is to increase thediameter of the holes drilled by 1/8″ on half inch glass and 1/16″ on 3/8″ glass. In all the years I have done this I can count the number of times any customer noticed the differance on one hand. Also cutting a “relief” from a hole that is close to edge distance tolerances pays big dividends especially in 1/2″ thick Glass. It is always good practice to consult with the customer first on relief cuts and hole dimension changes even if they probably would never notice.

Now we have these 3 issues put to sleep lets look inside the heating chamber. Good Tempering operators can visualize the process and make adjustments as needed. Heat Profiles are the way to go here. Picture a Heat Profile as a tent being put above the glass with the peak being adjustable from side to side and high to low. Positioning the peak of the heat profile to one side or the other allows uneven heat distribution in the heat chamber resulting in even heating throughout the lites, especially useful on 1/2″ glass with 3 or more hinge cut-outs on one side, The Heat profile can be set to concentrate the heat away from the notched edge – which picks up excess heat due to having more exposed glass edge – offsetting overheating and pitting. Multiple “Heat Profiles” can be set up to cover most scenario’s. It doesn’t take much to set up before hand and/or change in-process. Lite glass is all about “aspirators” inside the heat chamber, especially with Low-E or coated glasses. Aspirators blow super heated air on the top of the glass counteracting the radiated heat that is reflected away by the glass coating thus keeping the heating process even over the entire glass surface. Think of these as similar to Heat Profiles on Heavy Glass and set up accordingly.

One last thing to examine is during (scheduled) maintenance down time. When the heat chamber is cool enough to work in, take a pick and assure each and every aspirator hole is clear. It is a monotonous job, however, not doing this as part of routine scheduled maintenance will result in aspirator holes getting blocked and uneven air flow over the top of the glass. Same goes in the cooling chamber. Clear each and every air hole especially on the bottom where broken glass accumulates.

Well thanks again for checking in. I could only describe the very basics in this article due to space constraints, so…. if anyone has questions please e-mail or message me and I will be happy to expand on the processes. Still to come…To So2 or not to So2…that is the question and more on the cooling process.