Creating glass containers can be accomplished by one of two different processes – the Blow and Blow, or the Press and Blow process. Each process is chosen based on the kind of glass bottle being made. All glass bottles start out as raw materials. Silica (sand), soda ash, limestone, and cullet (furnace-ready, recycled glass) are combined into a specific mixture based on the desired properties of the bottle. The mixture is then melted at high temperatures in the furnace until it becomes a molten material, ready for formation. The type of glass this mixture will produce is known as soda-lime glass, the most popular glass for food and beverages.
Glass Forming Methods
Molten glass gobs are cut by a perfectly-timed blade to ensure each gob is of equal weight before it goes into the forming machine. The weight of a gob is important to the formation process for each glass container being made. The molded glass is created by gravity feeding gobs of molten glass into a forming machine, where pressure forms the neck and basic shape of the bottle. Once the neck finish and the general glass bottle shape has been achieved, the form is known as a parison. To achieve the final container shape, one of two processes are used.
Press and Blow Process
The Press and Blow process is the most commonly used method in glass bottle manufacturing. It uses an individual section (IS) machine, which is separated into varying sections to produce several containers of the same size simultaneously. The molten glass is cut with a shearing blade into a specific gob size. The gob falls into the machine by force of gravity. A metal plunger is used to push the gob down into the mold, where it starts to take shape and become a parison. The parison is then transferred into the blow mold and reheated so that the parison is soft enough to finish off the dimensions of the glass.
Once the parison is reheated to blowing temperature, air is injected to blow the container into shape. Press and blow methods are typically used for manufacturing wide-mouth bottles and jars as their size allows the plunger into the parison.
Blow and Blow Process
The Blow and Blow process is used to create narrow containers. It also requires an IS machine, where gobs of molten glass are gravity fed into the mold. The parison is created by using compressed air to form the neck finish and basic bottle shape. The parison is then flipped 180 degrees and reheated before air is again injected to blow the container into its final shape. Compressed air is once again used to blow the bottle into its desired shape. Blow and Blow methods are best used for glass bottle manufacturing requiring different neck thicknesses.
Finishing the Process
Regardless of the process used, once the bottle has been completely formed, it is removed from the mold and transferred to the annealing lehr. The lehr reheats the bottes to a temperature of about 1,050 degrees Fahrenheit then gradually cools them to about 390F. This process allows the glass to cool at an even rate - eliminating internal stresses in the glass that could lead to cracking or shattering. Bottles are then subjected to careful inspections to ensure they meet quality control guidelines. Any bottles showing imperfections, including bubbles, cracks, or misshapen areas, are removed from the line and used as cullet. All remaining bottles are sorted according to size and type. The bottles are then packaged on pallets and prepared for shipping.
For an infographic on the glass manufacturing process, click here: Infographic - From Grit to Glass, How Are Glass Bottles Made.
View the video: From Grit to Glass: How Glass Bottles are Made