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Glass formulations (melts) for cathode ray tubes (CRTs) produce some of the highest quality glassware available. This glassware equals or exceeds the finest crystal in lead content and comes very close to being defect free in both contaminant-induced artifacts and optical properties. A glass plant is a 24-hour-a-day, 7-day-a-week operation. The raw materials are relatively inexpensive, but the energy required to sustain tons of molten glass accounts for most of the operating cost. Glass tanks, which are a substantial capital investment, are rated by the tons of glass poured in 24 hours. A small tank would be rated at 10–20 tons and is best suited for smaller-sized glassware to maintain a balance between parts/hour and optimum flow rate.
Molten glass is drawn from the center third of the vertical axis to feed the molds. The top third contains contaminants being burned off, while the bottom third contains heavier particles of raw material that will not melt plus particles from the wear and tear of the firebricks. A continuous pour must be maintained at the tank’s rated capacity to maintain the middle third at the proper level within the tank. Once a specific melt formula is established, a usable product can be produced. Glass poured before stability on a new tank is achieved is crushed (collet) and cycled back into the tank at up to 20% of the total raw material. Once a tank is pouring glass, it is preferable to run it without interruption on high-volume parts. But since CRTs come in many sizes, there is a need to change molds. To avoid downtime, glass tanks have multiple taps going to forming stations (turrets with four to six mold sets are typical), so that glass can be poured on two or more stations while the molds are being changed on another station. The mold sizes must be balanced for rate of pour and tonnage to keep the tank flowing within its capacity.
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