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Glass distribution
Glass bottles for cucumber slices
A Soviet mayonnaise jar
A modern "French Kilner" jar

Glass production comprehends two types of glass: (i) sheet glass, made by the float glass process, and (ii) glass-container glass.

Contents

Glass container production

Glass container factories

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 furnaces, annealing ovens, and forming machines; and the cold end handles the product-inspection and -packaging equipment.

Hot end

The following table lists common viscosity fixpoints, applicable to large-scale glass production and experimental glass melting in the laboratory:[1]

log10(η, Pa·s) log10(η, P) Description
1 2 Melting Point (glass melt homogenization and fining)
3 4 Working Point (pressing, blowing, gob forming)
4 5 Flow Point
6.6 7.6 Littleton Softening Point (Glass deforms visibly under its own weight. Standard procedures ASTM C338, ISO 7884-3)
8-10 9-11 Dilatometric Softing Point, Td, depending on load[2]
10.5 11.5 Deformation Point (Glass deforms under its own weight on the μm-scale within a few hours.)
11-12.3 12-13.3 Glass Transition Temperature, Tg
12 13 Annealing Point (Stress is relieved within several minutes.)
13.5 14.5 Strain Point (Stress is relieved within several hours.)

Furnace

Batch feed (doghouse) of a glass furnace

The hot end of a glassworks is where the molten glass is formed into Glass Products; beginning when the batch is fed into the furnace at a slow, controlled rate. The furnaces are natural gas- or fuel oil-fired, and operate at temperatures up to 1575°C. [3] The temperature is limited only by the quality of the furnace’s superstructure material and by the glass composition.

Forming process

Glass container forming

There are, currently, two primary methods of making a glass container — the blow and blow method and the press and blow method. In both cases a stream of molten glass, at its plastic temperature (1050°C-1200°C), is cut with a shearing blade to form a cylinder of glass, called a gob. Both processes start with the gob falling, by gravity, and guided, through troughs and chutes, into the blank moulds. In the blow and blow process, the glass is first blown from below, into the blank moulds, to create a parison, or pre-container. The parison is then flipped over into a final mould, where a final blow blows the glass out, in to the mould, to make the final container shape. In the case of press and blow process, the parison is formed with a metal plunger, which pushes the glass out, into the blank mould. The process then continues as before, with the parison being transferred to the mould, and the glass being blown out into the mould.

Forming machines

IS machine during bottle production[4]

The forming machines hold and move the parts that form the container. Generally powered by compressed air, the mechanisms are timed to coordinate the movement of all these parts so that containers are made.

The most widely used forming machine arrangement is the individual section machine (or IS machine). This machine has a bank of 5-20 identical sections, each of which contains one complete set of mechanisms to make containers. The sections are in a row, and the gobs feed into each section via a moving chute, called the gob distributor. Sections make either one, two, three or four containers simultaneously. (Referred to as single, double, triple and quad gob). In the case of multiple gobs, the shears cut the gobs simultaneously, and they fall into the blank moulds in parallel.

Internal treatment

After the forming process, some containers—particularly those intended for alcoholic spirits—undergo a treatment to improve the chemical resistance of the inside, called internal treatment or dealkalization. This is usually accomplished through the injection of a sulfur- or fluorine-containing gas mixture into bottles at high temperatures. The gas is typically delivered to the container either in the air used in the forming process (that is, during the final blow of the container), or through a nozzle directing a stream of the gas into the mouth of the bottle after forming. The treatment renders the container more resistant to alkali extraction, which can cause increases in product pH, and in some cases container degradation.

Annealing

As glass cools it shrinks and solidifies. Uneven cooling causes weak glass due to stress. Evenpo cooling is achieved by annealing. An annealing oven (known in the industry as a Lehr) heats the container to about 580°C then cools it, depending on the glass thickness, over a 20 – 6000 minute period.

Cold end

The role of the cold end is to inspect the containers for defects, package the containers for shipment and label the containers.

Inspection equipment

Glass containers are 100% inspected; automatic machines, or sometimes persons, inspect every container for a variety of faults. Typical faults include small cracks in the glass called checks and foreign inclusions called stones which are pieces of the refractory brick lining of the melting furnace that break off and fall into the pool of molten glass which subsequently are included in the final product. These are especially important to select out due to the fact that they can impart a destructive element to the final glass product. For example, since these materials can withstand large amounts of themal energy, they can cause the glass product to sustain thermal shock resulting in explosive destruction when heated. Other defects include bubbles in the glass called blisters and excessively thin walls. Another defect common in glass manufacturing is referred to as a tear. In the press and blow forming, if a plunger and mould are out of alignment, or heated to an incorrect temperature, the glass will stick to either item and become torn. In addition to rejecting faulty containers, inspection equipment gathers statistical information and relays it to the forming machine operators in the hot end. Computer systems collect fault information to the mould that produced the container. This is done by reading the mould number on the container, which is encoded (as a numeral, or a binary code of dots) on the container by the mould that made it. Operators carry out a range of checks manually on samples of containers, usually visual and dimensional checks.

Secondary processing

Sometimes container factories will offer services such as labelling. Several labelling technologies are available. Unique to glass is the Applied Ceramic Labelling process (ACL). This is screen-printing of the decoration onto the container with a vitreous enamel paint, which is then baked on. An example of this is the original Coca-Cola bottle. The Absolut Bottles have various added services such as: Etching ( Absolut Citron/) Coating (Absolut Raspberry/Ruby Red)and Applied Ceramic Labelling ( Absolut Blue/Pears/Red/Black)

Packaging

Glass containers are packaged in various ways. Popular in Europe are bulk pallets with between 1000 and 4000 containers each. This is carried out by automatic machines (palletisers) which arrange and stack containers separated by layer sheets. Other possibilities include boxes and even hand sewn sacks. Once packed the new "stock units" are labelled and warehoused.

Coatings

Glass containers typically receive two surface coatings, one at the hot end, just before annealing and one at the cold end just after annealing. At the hot end a very thin layer of tin oxide is applied either using a safe organic compound or inorganic stannic chloride. Tin based systems are not the only ones used, although the most popular. Titanium tetrachloride or organo titanates can also be used. In all cases the coating renders the surface of the glass more adhesive to the cold end coating. At the cold end a layer of typically, polyethylene wax, is applied via a water based emulsion. This makes the glass slippery, protecting it from scratching and stopping containers from sticking together when they are moved on a conveyor. The resultant invisible combined coating gives a virtually unscratchable surface to the glass. Due to reduction of in-service surface damage the coatings often are described as strengtheners, however a more correct definition might be strength retaining coatings.

Ancillary processes – compressors & cooling

Forming machines are largely powered by compressed air and a typical glass works will have several large compressors (totaling 30k-60k cfm) to provide the needed compressed air. Furnaces, compressors and forming machine generate quantities of waste heat which is generally cooled by water. Hot glass which is not used in the forming machine is diverted and this diverted glass (called cullet) is generally cooled by water, and sometime even processed and crushed in a water bath arrangement. Often cooling requirements are shared over banks of cooling towers arranged to allow for backup during maintenance.

Marketing

Glass container manufacture in the developed world is a mature market business. Annual growth in total industry sales generally follows population growth. Glass container manufacture is also a geographical business; the product is heavy and large in volume, and the major raw materials (sand, soda ash and limestone) are generally readily available, therefore production facilities need to be located close to their markets. A typical glass furnace holds hundreds of tonnes of molten glass, and so it is simply not practical to shut it down every night, or in fact in any period short of a month. Factories therefore run 24 hours a day 7 days a week. This means that there is little opportunity to either increase or decrease production rates by more than a few percent. New furnaces and forming machines cost tens of millions of dollars and require at least 18 months of planning. Given this fact, and the fact that there are usually more products than machine lines means that products are sold from stock. The marketing/production challenge is therefore to be able to predict demand both in the short 4-12 week term and over the 24-48 month long term. Factories are generally sized to service the requirements of a city; in developed countries there is usually a factory per 1-2 million people. A typical factory will produce 1-3 million containers a day. Despite its positioning as a mature market product, glass does enjoy a high level of consumer acceptance and is perceived as a “premium” quality packaging format.

Lifecycle impact

Glass containers are wholly recyclable and the industry in many countries retains a policy (or is forced to by Government) of maintaining a high price on cullet to ensure high return rates. Return rates of 95% are not uncommon in the Nordic countries (Sweden, Norway, Denmark and Finland). Return rates of less than 50% are usual in other countries. Of course glass containers can also be reused, and in developing countries this is common, however the environmental impact of washing the container as against remelting them is uncertain. Factors to consider here are the chemicals and fresh water used in the washing, and the fact that a single use container can be made much lighter, using less than half the glass (and therefore energy content) of a multiuse container. Also, a significant factor in the developed world's consideration of reuse are producer concerns over the risk and consequential product liability of using a component (the reused container) of unknown and unqualified safety. How glass containers compare to other packaging types (plastic, cardboard, aluminium) is hard to say, conclusive lifecycle studies are yet to be produced.

Float glass process

Environmental impacts

Local environmental impacts

As with all highly concentrated industries, glassworks suffer from moderately high local environmental impacts. Compounding this is that because they are mature market businesses they often have been located on the same site for a long time and this has resulted in residential encroachment. The main impacts on residential housing and cities are noise, fresh water use, water pollution, NOx and SOx air pollution, and dust.

Noise is created by the forming machines. Operated by compressed air, they can produce noise levels of up to 106dBA. How this noise is carried into the local neighbourhood depends heavily on the layout of the factory. Another factor in noise production is truck movements. A typical factory will process 600T of material a day. This means that some 600T of raw material has to come onto the site and the same off the site again as finished product.

Water is used to cool the furnace, compressor and unused molten glass. Water use in factories varies widely, it can be as little as one tonne water used per melted tonne of glass. Of the one tonne roughly half is evaporated to provide cooling, the rest forms a wastewater stream.

Most factories use water containing an emulsified oil to cool and lubricate the gob cutting shear blades. This oil laden water mixes with the water outflow stream thus polluting it. Factories usually have some kind water processing equipment that removes this emulsified oil to various degrees of effectiveness.

The oxides of nitrogen are a natural product of the burning of gas in air and are produced in large quantities by gas fired furnaces. Some factories in cities with particular air pollution problems will mitigate this by using liquid oxygen, however the logic of this given the cost in carbon of (1) not using regenerators and (2) having to liquefy and transport oxygen is highly questionable. The oxides of sulphur are produced as a result of the glass melting process. Manipulating the batch formula can effect some limited mitigation of this; alternatively exhaust plume scrubbing can be used.

The raw materials for glass making are all dusty material and are delivered either as a powder or as a fine-grained material. Systems for controlling dusty materials tend to be difficult to maintain, and given the large amounts of material moved each day, only a small amount has to escape for there to be a dust problem. Cullet is also moved about in a glass factory and tends to produce fine glass particles when shovelled or broken.

Global environmental impact

The main global impact factor is the production of CO2 due to the burning of fossil fuels in the heating of the furnace and production of electricity to supply the compressors. Typically a tonne of glass packed will liberate between 500 and 900 kg of CO2, assuming a gas fired furnace and coal fired electricity usage. In areas with predominantly renewable or nuclear energy, the CO2 released comes only from the conversion of carbonates to oxides in the ingredients of the glass itself.

See also

References

  1. ^ Werner Vogel: "Glass Chemistry"; Springer-Verlag Berlin and Heidelberg GmbH & Co. K; 2nd revised edition (November 1994), ISBN 3-540-57572-3
  2. ^ The dilatometric softening point is not identical with the deformation point as sometimes presumed. For reference see experimental data for Td and viscosity in: High temperature glass melt property database for process modeling; Eds.: Thomas P. Seward III and Terese Vascott; The American Ceramic Society, Westerville, Ohio, 2005, ISBN 1-57498-225-7
  3. ^ B. H. W. S. de Jong, "Glass"; in "Ullmann's Encyclopedia of Industrial Chemistry"; 5th edition, vol. A12, VCH Publishers, Weinheim, Germany, 1989, ISBN 3-527-20112-5, p 365-432.
  4. ^ Copy from German Wikipedia: Glasmaschine, IS-Maschinen

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