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Glass Bottle Manufacturing Process
The glass bottle manufacturing process takes place at a glass product factory in multiple steps.
Hot End Processes
The hot end of a glassworks is where the molten glass is manufactured into glass products. The batch enters the furnace, then passes to the forming process, internal treatment, and annealing.
The first stage of the glass bottle production process 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 feedstock. Soda-lime glass stock accounts for the majority (around 90 percent) of the types 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 Molding Glass
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 molds. A metal plunger presses the gob into the blank mold, where it assumes the mold’s shape and is then termed a parison. Next, the parison is moved into a final mold, where it is blown into the mold 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 molds. However, in blow-and-blow formation, the gob is forced into the blank mold using compressed air to push the gob into place. The gob, now a parison, is then flipped into a corresponding final mold 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 Glass Bottles
As glass cools, it shrinks and solidifies. Uneven cooling causes weak glass due to stress. Even cooling is achieved by annealing. An annealing oven (known in the industry as a lehr) heats the container to about 580 °C (1,076 °F), then cools it, depending on the glass thickness, over a 20 – 60 minute period.
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.
The role of the cold end of glass container production is to complete the final tasks in the manufacturing process: spray on a polyethylene coating for abrasion resistance and increased lubricity, inspect the containers for defects, label the containers, and package the containers for shipment.
The formula for making a glass bottle
The formula for making glass is as simple as silica sand, soda ash, limestone, and a whole lot of heat. The raw materials for making modern glass are silica sand and soda ash (familiar so far?), limestone (added to lower the temperature at which the mix melts), and frequently magnesium oxide and aluminium oxide (to provide for better chemical durability).
Another basic and highly-essential ingredient is cullet. Cullet is recycled glass that helps reduce the amount of raw materials and energy needed to make new glass. A glass batch may consist of 25 to 60 percent cullet by volume. Cullet is crushed glass, generally of the same composition as the mineral mixture, and comes from regrind from previous production runs and recycling sources. It is included because its characteristic of melting in the furnace before the other minerals helps accelerate the batch’s reaction into molten glass. It is an important energy-savings ingredient that both lowers the amount of energy needed for the melting process and in turn helps reduce greenhouse gas emissions. The resulting glass that is produced typically contains about 70 to 74 percent silica by weight.
Every ingredient is carefully measured — by weight — and combined into a batch mixture. It is at this stage that additional ingredients may be added to create various colors of glass.
The color of a glass bottle
QUICK FACT! The purer the silica sand is, the lower its iron content will be, which in turn means more control over the ultimate color.
Glass that has virtually no color is called colorless glass. Colorless is the preferred term instead of the word clear. Clear refers to a different value: the transparency of the glass and not its color. The proper use of the word clear would be in the phrase “clear green bottle.”
Aquamarine colored glass is a natural result of the both the naturally occurring iron found in most sands, or through the addition of iron to the mix. By reducing or increasing the amount of oxygen in the flame used to melt the sand, manufacturers can produce a more bluish-green color or a greener color.
Opaque white glass is commonly called milk glass and sometimes called Opal or white glass. It can be produced by the addition of tin, zinc oxide, fluorides, phosphates or calcium.
Green glass can be made through the addition of iron, chromium, and copper. Chromium oxide will produce yellowish green to emerald green. Combinations of cobalt, (blue) mixed with chromium (green) will produce a blue green glass.
Amber glass is produced from the natural impurities in sand, such as iron and manganese. Additives that make Amber include nickel, sulfur, and carbon.
Blue glass is colored with ingredients like cobalt oxide and copper.
Purple, amethyst and red are glass colors that are usually from the use of nickel or manganese oxides.
Black glass is usually made from high iron concentrations, but can include other substances such as carbon, copper with iron and magnesia.
Whether the batch is destined to be clear or colored glass, the combined ingredients are known as the batch mixture and is transported to a furnace and heated to a temperature of about 1565°C or 2850°F. Once melted and combined, the molten glass passes through a refiner, where trapped air bubbles are allowed to escape and then it is cooled to a uniform yet still formable temperature. A feeder then pushes the liquid glass at a constant rate through precisely-sized openings in a heat-resistant die. Shear blades cut the emerging molten glass at the precise moment to create elongated cylinders called gobs. These gobs are individual pieces, ready for forming. They enter a forming machine where, using compressed air to expand them to fill a die of the desired final shape, are made into containers.
Newly formed containers are H-O-T and naturally start cooling rapidly. Left unchecked, this rapid cooling will occur unevenly — the outside cools more rapidly than the inside — and result in stress within the glass that can lead to both immediate failure or failure long after they have been formed. That’s why these newly-made containers pass through a special type of kiln called a lehr. The lehr precisely reheats the containers to a point just below the solidification point of glass, then very gradually and evenly cools them to room temperature, relieving stresses. This process is called annealing. Anneal cooling rates are generally tens of degrees Celsius per hour for a typical glass container. Annealing is a key reason why glass is such an incredibly versatile and strong packaging material perfectly suited for so many applications.
Once cool, the glass containers undergo rigorous final inspections to ensure the highest quality. Scanning equipment and technicians review the containers and those that do not meet stringent design specifications are remelted and the process begins again.
In Conclusion
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