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How Glass Bottles Are Made

Although traditional glass-blowing and blow-molding methods are still used by artists and for custom applications, most bottle manufacturing is an automated process.  The development of glass bottle machining peaked with the advent of feed and flow machines, which enabled manufacturers to generate larger production runs than was previously possible. Glass production is broken down into two general categories: container production and sheet production. Bottle machining is part of glass container production.

Hot End Processes

Bottle manufacturing takes place at a glass container factory in multiple steps. The first stage of glass-container making begins with the hot end processes, which typically employ high amounts of heat to produce and shape a glass container. A furnace is first used to mold molten glass, which fed to the furnace as glass feed stock. Soda-lime glass stock accounts for the majority (around 90 percent) of glass products, and is typically largely comprised of silica, with about 10 percent each of calcium oxide and lime. Small amounts of aluminum oxide, ferric oxide, barium oxide, sulfur trioxide, and magnesia also account for about 5 percent of soda-lime glass. Before melting, cullet (recycled glass) is added to the stock, accounting for anywhere between 15 and 50 percent of the final glass composition.

Once the stock has been fed into the furnace, temperatures inside can be as high as 1675 degrees Fahrenheit. Next, one of two method forming methods is applied: press-and-blow or blow-and-blow.

  • Press-and-Blow

Press-and-blow formation takes place in an individual section (IS) machine and is the more commonly used method in glass-container production. IS machines have between five and 20 sections, all identical, which can each carry out the glass-container forming process simultaneously and completely. The result is that five to 20 containers can be produced with one machine at the same time.

When the molten glass reaches between 1050 and 1200 degrees Celsius it is said to be in its plastic stage, and it is during this phase that press-and-blow formation begins. A shearing blade is used to cut and shape the glass into a cylindrical shape, called a gob. The cut gob falls, and using gravitational force, rolls through the appropriate passage to reach the moulds. A metal plunger presses the gob into the blank mold, where it assumes the mould’s shape and is then termed a parison. Next, the parison is moved into a final mold, where it is blown into the mould to assume its final dimensions. This process is typically used for wide-mouthed glass containers, but can also be used to manufacture thin-necked bottles.

  • Blow-and-Blow

Like press-and-blow formation, blow-and-blow takes place in an IS machine, where a gob is released during the plastic stage and moved along to the moulds. However, in blow-and-blow formation, the gob is forced into the blank mould using compressed air to push the gob into place. The gob, now a parison, is then flipped into a corresponding final mould where it is blown again, to form the interior side of the glass container. Glass bottles of varying neck thickness can be made using blow-and-blow formation.

After formation, bottles often undergo internal treatment, a process which makes the inside of the bottle more chemically-resistant, an important factor if the bottles are intended to hold alcohol or other degrading substances. Internal treatment can take place during formation or directly after, and typically involves treating the bottles with a gas mixture of fluorocarbon. Glass containers can also be treated externally, to strengthen the surface or reduce surface friction.

Annealing

Once formation is complete, some bottles may suffer from stress as a result of unequal cooling rates. An annealing oven can be used to reheat and cool glass containers to rectify stress and make the bottle stronger.

Cold End Processes

At this stage in glass production, the bottles or glass containers are inspected and packaged. Inspection is often done by a combination of automated and mechanical inspection, to ensure the integrity of the final product. Common faults include checks (cracks in the glass) and stones (pieces of the furnace that melt off and are subsequently worked into the final container), which are important to catch because they can compromise the component. Packaging methods will vary from factory to factory depending on the specific type of bottle and the size of the production run.

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