How Are Glass Bottles Made?

How are glass bottles made? Although traditional glass-blowing and blow-molding glass methods are still used by artists and for custom applications, most glass 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.

In this guide, we’ll be breaking down the steps on how to make a glass bottle, from hot end to cold end processes, with additional details on their applications. If you'd like to learn more about plastic bottles, you can also check out our guide on the plastic bottle manufacturing process.

Glass Bottle Manufacturing Process

The glass bottle manufacturing process takes place at a glass container factory in multiple steps. First glass is taken through hot end processes to shape it into a bottle, including melting and blowing, then the new bottle may get an internal treatment. From there the bottles may be annealed. Finally, they are inspected and packaged in cold end processes.

Hot End Processes

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.

Additional elements to color the glass are also added before the glass is melted. Amber glass bottles get their color from nickel, sulfur, and carbon, although natural impurities in sand can also cause this color. Other colors can be achieved through adding various materials:

• Black: iron, although it can also include magnesia, copper, and carbon.

• Purples and reds: manganese or nickel oxide.

• Blue: copper and cobalt oxide.

• Aquamarine (which can vary between blues and greens): either naturally present iron or added iron. You can also get blue-greens from mixing cobalt and chromium.

• Green: iron, chromium, and copper. Yellowish green can be achieved with chromium oxide.

• Opaque white glass: tin, zinc oxide, calcium, flourides, or phosphates.

Once the stock has been fed into the furnace, temperatures inside can be as high as 1675 degrees Fahrenheit. The molten glass is then fed through a refiner, which allows air bubbles to escape the liquid. 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

Once formation is complete, some bottles may suffer from stress as a result of unequal cooling rates. An annealing oven, also known as a lehr, 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.

Applications of Glass Bottles

Glass is chemically inert and more impermeable than plastic, making it better able to withstand processes like pasteurization while containing more corrosive contents. Glass bottles are especially crucial in the food and beverage industry, since they don’t affect flavors, or enable flavor and carbonation to be lost as quickly as plastic does. While many are familiar with glass bottles used to hold beer and wine, there are also glass milk bottles, baby bottles, soda bottles, and water bottles. Outside of beverages glass bottles are used to hold food ingredients, sauces and jams. Additionally, glass is used to package products like medication, cosmetics, and essential oils; it can also be used in laboratory or spray bottles. For more on these containers, you can check out our guide on types of glass bottles.

Amber glass bottles are especially popular for medication, beer, and other contents that are sensitive to light. The amber color of the bottle filters out UV and blue light, allowing it to prevent the breakdown of these products, called photooxidation. Without this filtering of higher-energy light waves, beer will break down over time and ruin its flavor, while essential oils may shift in scent.

Conclusion

Now that we’ve covered how glass bottles are made, including hot and cold end processes, as well as their applications, we hope this information will better enable your sourcing.