Section 3.4: Forming Process

Viscosity

Viscosity, a measure of liquid fluidity, varies inversely with temperature. As the temperature decreases, viscosity increases. As glass cools, it gets "stiffer" or more viscous.

Viscosity is measured in poise at a specific temperature. Molten glass in the forehearth at 2,200 degrees F has a viscosity of about 1,000 poise. As a point of comparison, consider the following examples:

• Water at 70 degrees F has 1/100 poise
• Light oil at 70 degrees F has 1 poise
• Heavy oil at 70 degrees F has 7 poise
• Honey or syrup at room temperature is 100 poise
• Molasses in January (32 degrees F) is 10,000 poise
• Asphalt is from 1,000,000 to 1,000,000,000 poise depending on the temperature

Gob Formation and Shapes

• A gob is a specific amount of molten glass, which is eventually formed into a glass container.
• The shape of the gob is important because it affects the way it enters into the Individual Section Machine.
• There is an optimum gob shape for each glass container produced.
• Gob length and diameter are dependent on the shape and weight of the container.

• Consider these facts about the size, weight and heat content of a gob during formation:

  Gob weights vary from ½ oz. to 48 oz.

  Gob lengths typically range from ½ inch to 6 inches.

  Gob diameter can be from 3/8 of an inch to 4 inches.

  A 7 oz. gob is typical for a 12 oz. beer bottle.

  At a temperature of 2,100 degrees F, the viscosity of a gob has the consistency of thick honey and it is uniform throughout.

  A typical 7 oz. gob contains 275 BTUs; 40 gobs contain the same amount of heat as a small kerosene heater puts out in 1 hour.

  An I.S. Machine can produce up to 700 bottles per minute.

Feeder and Delivery

Molten glass flows with the help of gravity from the refiner through the forehearth. From there it is carefully cooled to a uniform temperature and viscosity prior to reaching the feeder. Using the pull of gravity, the hot glass flows through the orifice at the bottom of the feeder. Glass flow is controlled by the height of a ceramic tube in the feeder; a raised tube creates a heavy flow while a lowered tube results in a reduced flow.

The glass flow undergoes a “mixing action” created by the rotation of the ceramic tube. This helps to make the temperature consistent while the downward motion of the plunger accelerates the glass flow. This pumping action is timed with the shearing of the glass flow as it falls beneath the feeder to shape the falling gobs. After the gob has been sheared from the feeder it falls into a series of chutes where it is delivered to the blank mold on the I.S. machine.

The I.S. Machine

The I.S. Machine or "Individual Section II Machine" is designed to ensure efficient production so that operators can take one or more sections out of production for repairs without shutting down production in other sections. Gobs enter the I.S. Machine and are formed into containers through a process of controlled shaping and cooling of the glass.

The machines have anywhere from 6 to 20 sections and each section can produce one to four bottles simultaneously.
• The total time need to produce a container varies, but beer and soda bottles take approximately 10 seconds.
• Depending on the container’s size and shape, the machine’s production speed may be as fast as 700 containers per minute.

Parison

• A parison is a hollow and partially formed container that will be blown up like a balloon in the blow mold to form a bottle.
• It has a cooler skin or enamel outer surface and a temperature of 1700 degrees F on its outer skin.
• Parisons are formed on the blank side of an I.S. Machine from gobs and greatly differ in shape for each type of container design.

Container Formation

Manufacturers use three different types of forming processes to make glass containers, depending on the type of container to be produced:

• Blow and Blow
• Wide Mouth Press and Blow
• Narrow Neck Press and Blow

Blow and Blow Process

• In the Blow and Blow process, compressed air blows a cavity into the molten gob in the blank mold of the forming machine thereby creating a preform shape known as a parison.
• From there the parison is transferred to the blow mold where compressed air is used to blow the bottle into its final shape.

Wide Mouth Press and Blow Process

• In the Wide Mouth Press and Blow process, a metal plunger is used to press the cavity into the gob to create the parison in the blank mold.
• The parison is then inverted and compressed air blows the container into its final shape. This process is used to manufacture containers with wide finish diameters (38mm and larger).

Narrow Neck Press and Blow Process

• The Narrow Neck Press and Blow process is similar to the wide mouth press and blow except the metal plunger in the blank mold is much smaller in diameter. This process is used to manufacture containers with narrow finish diameters (38mm and smaller).
• The introduction of this process has enabled glass manufacturers to increase overall productivity and reduce weight and variations in the thickness distribution of beer and beverage bottles.

continue to Section 3.5: Glass Manufacturing Quality