Glass production involves two main methods – the float glass process that produces sheet glass, and glassblowing that produces bottles and other containers. It has been done in a variety of ways during the history of glass.
Glass container production Broadly, modern glass container factories are three-part operations: the "batch house", the "hot end", and the "cold end". The batch house handles the raw materials; the hot end handles the manufacture proper—the forehearth, forming machines, and annealing ovens; and the cold end handles the product-inspection and packaging equipment.
Batch processing system (batch house) Batch processing is one of the initial steps of the glass-making process. The batch house simply houses the raw materials in large silos (fed by truck or railcar), and holds anywhere from 1–5 days of material. Some batch systems include material processing such as raw material screening/sieve, drying, or pre-heating (i.e. cullet). Whether automated or manual, the batch house measures, assembles, mixes, and delivers the glass raw material recipe (batch) via an array of chutes, conveyors, and scales to the furnace. The batch enters the furnace at the "dog house" or "batch charger". Different glass types, colours, desired quality, raw material purity/availability, and furnace design will affect the batch recipe.
Hot end 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 following table lists common viscosity fixpoints, applicable to large-scale glass production and experimental glass melting in the laboratory:
Furnace
The batch is fed into the furnace at a slow, controlled rate by the batch processing system. The furnaces are natural gas- or fuel oil-fired, and operate at temperatures up to 1,575 °C (2,867 °F). The temperature is limited only by the quality of the furnace’s superstructure material and by the glass composition. Types of furnaces used in container glass making include "end-port" (end-fired), "side-port", and "oxy-fuel". Typically, furnace size is classified by metric tons per day (MTPD) production capability. Modern furnaces use electric heating methods that improve energy efficiency compared to traditional fossil fuel systems, contributing to reduced pollution and emissions. Electrodes made from molybdenum, graphite, or alloys are used in glass furnaces to conduct electricity and generate energy.
Forming process
There are currently two primary methods of making glass containers: the "blow and blow" method for narrow-neck containers only, and the "press and blow" method used for jars and tapered narrow-neck containers. In both methods, a stream of molten glass at its plastic temperature (1,050–1,200 °C [1,920–2,190 °F]) is cut with a shearing blade to form a solid cylinder of glass, called a "gob". The gob is of predetermined weight just sufficient to make a bottle. Both processes start with the gob falling, by gravity, and guided, through troughs and chutes, into the blank moulds, two halves of which are clamped shut and then sealed by the "baffle" from above.
In the "blow and blow" process, the glass is first blown through a valve in the baffle, forcing it down into the three-piece "ring mould" which is held in the "neckring arm" below the blanks, to form the "finish". The term "finish" describes the details (such as cap sealing surface, screw threads, retaining rib for a tamper-proof cap, etc.) at the open end of the container. Then compressed air is blown through the glass, which results in a hollow and partly formed container. Compressed air is then blown again at the second stage to give final shape. Containers are made in two major stages. The first stage moulds all the details ("finish") around the opening, but the body of the container is initially made much smaller than its final size. These partly manufactured containers are called "parisons", and quite quickly, they are blow-molded into final shape. The "rings" are sealed from below by a short plunger. After the "settleblow" finishes, the plunger retracts slightly, to allow the skin that's formed to soften. "Counterblow" air then comes up through the plunger, to create the parison. The baffle rises and the blanks open. The parison is inverted in an arc to the "mould side" by the "neckring arm", which holds the parison by the "finish". As the neckring arm reaches the end of its arc, two mould halves close around the parison. The neckring arm opens slightly to release its grip on the "finish", then reverts to the blank side. "Final blow", applied through the "blowhead", blows the glass out, expanding into the mould, to make the final container shape.
In the press and blow process, the parison is formed by a long metal plunger which rises up and presses the glass out, in order to fill the ring and blank moulds. The process then continues as before, with the parison being transferred to the final-shape mould, and the glass being blown out into the mould. The container is then picked up from the mould by the "take-out" mechanism, and held over the "deadplate", where air cooling helps cool down the still-soft glass. Finally, the bottles are swept onto a conveyor by the "push out paddles" that have air pockets to keep the bottles standing after landing on the "deadplate"; they're now ready for annealing.
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