How Glass is Made – From the Batch House to the Lehr
Glass has become one of the most popular building materials used today because it offers virtually unlimited aesthetic options, combined with outstanding performance. What ends up as large, sweeping glass panels in a high-rise office building, healthcare facility, school, or other construction project starts as a simple combination of sand, soda ash, limestone, dolomite and some other minor ingredients.
The glass making process commences in the batch house, which is where all of the incoming raw materials are offloaded on to a conveyer and sent to their respective storage silos. The actual batching begins when the raw materials are moved, weighed and mixed, and sent via a conveyor belt to the charging end of the melting furnace. The melting furnace is similar to an old fashioned brick baking oven, but much, much bigger.
The batch materials are combined with cullet (which is crushed scrap glass) and melted to form liquid glass. Sand is the major ingredient in glass, and sand on its own typically doesn’t melt until it reaches a temperature of about 3,000 degrees Fahrenheit. However, when sand is combined with other raw matierals and cullet, it melts at a temperature well below 3,000 degrees Fahrenheit.
As the combination of batch materials enters the furnace, pre-heated air is pumped into the chamber by fans. The pre-heated air is then combined with jet streams of natural gas, which, in turn, produce torch-like flames that spew across the batch and cause it to react and melt in a matter of minutes.
The fining process is next. During this step, the bubbles that are formed during the melting process rise to the surface and escape into the chamber atmosphere. The glass then moves from the chamber through a canal into what is known as the float bath. The liquid glass floats as it hardens on a bath of liquid tin.
The glass moves under toothed wheels in the hot end of the float bath, which are known as stretch machines, to alter the thickness and width of the glass. In addition, heating elements above the stretch machines also control the glass thickness as it moves towards the exit.
The next step is cooling, which occurs with a series of water coolers in the cold end of the float bath. The glass is then carefully lifted out of the liquid tin and on to conveyer rolls at about 1100 degrees and goes into the annealing lehr. The job of the lehr is to further cool the glass at a controlled rate in order to ensure proper stresses are put into the glass so it can be cut easily and accurately.
The glass comes out of the lehr at about 350 degrees Farenheit and is then further cooled towards room temperature by open air fans. The glass is then inspected for any flaws prior to cutting. Before cutting, the glass must also first pass under a machine that drops a special powder on the surface in order to provide separation between each piece of glass and stain prevention.
From Cutting to Low-e Coatings
we covered how glass starts as a simple combination of sand, soda ash, limestone and dolomite, moves to the batch house, enters the furnace, and then goes through the melting, fining, forming, annealing, and cooling processes.
The next parts of the glass making process are focused on cutting and shipping. Once the glass has been cooled and prepped, it is cut by first scoring it with carbide cutting wheels. The process starts by snapping it over a roller that acts as a fulcrum. The main line scoring equipment consists of slit cutters, which score the glass in the direction of the flow and the cross cutters that score the glass across the flow. After scoring, the glass remains as a continuous ribbon that enters the main line where the cross cutter scores are opened.
The glass, now in plate form, proceeds from the main line to the packing lines. The glass is transferred to the packing lines by a corner table, which is a belt and roll conveyer combination. After leaving the corner table, the glass enters the snapping conveyers where slit scores are opened. After the snapping conveyors, the glass is transferred one section at a time on to narrow conveyers. Pieces of glass that do not pass inspection are recycled through the cullet system.
Next, the glass moves on the conveyor to an automatic vacuum transfer module (VTM) system. Working like a vacuum cleaner, the glass is sucked off of the main conveyer and placed on to a finished stack. As the stack gets higher, an elevator table indexes downward in order to maintain a consistent drop in the glass. Once complete, the elevator table is lowered and the completed rack is moved aside and replaced with an empty one.
The final step in the process involves placing the glass in the Magnetron Sputter Vacuum Deposition (MSVD) chamber where a low-e coating is applied to the glass. In this step, the glass is rolled into the vacuum chamber where microscopic minerals, mainly silver, are bonded on to the glass surface. A final quality inspection is performed and the glass is then stacked and wrapped, moved to shipping and sent out the door.