Glass container industry

Glass containers are a common part of everyday life - we enjoy beverages such as water, soft drink, juice, beer, wine, spirit from bottles - jams and spreads from jars. The glass container's manufacture often involves a far greater level of complexity, automation and involvement than the products they contain. This article deals with the mass production of the common glass bottle or jar. Vials, hand made or hand blown items, glass tubes and tableware are excluded here.

Glass container factories

Modern glass container factories are broadly divided into three parts: the batch house, the hot end and the cold end. The batch house is concerned with raw materials. In the hot end we find the furnaces, machines that make the containers (forming machines) and annealing ovens. In the cold end there are the inspection and packaging equipment.

Batch house

The batch house holds the raw materials for creating glass, primarily sand, soda ash and limestone (as well as numerous others). These materials are received (typically by truck or rail transport) and elevated into storage silos. From the silos they are accurately weighed out into a batch of several tonnes. The batch is mixed and sent to silos over the furnace.

Hot end

Furnace

The hot end of a glassworks is where the molten glass is formed into containers. This begins when the batch is fed at a controlled rate into the furnace. The furnaces are natural gas or fuel oil fired and operate at just below 1600ºC (this is the temperature at which the refractory roof bricks begin to melt). Glass furnaces typically operate an energy recovery scheme known as regeneration. The hot exhaust gas flow back over one of two piles of loosely packed bricks, called regenerators. These bricks become hot and every 20-30 minutes the flow of the combustion system is changed over so that the combustion air, which is mixed with the gas, is drawn through the heated bricks, and the combustion exhaust flows through the other pile of bricks. The batch melts inside the furnace which is maintained as a great pool of molten glass, perhaps 1200mm deep by 200m². The molten glass flows from a subducted channel known as the furnace throat into the refiner and forehearth channels. These channels, 1200mm wide and 400-150mm deep transport the glass to the glass bottle forming machines. The role of these channels is to cool the glass very precisely so that the glass at the forming machine is of a uniform and exact temperature.

Forming process

These days, there are two main methods of making a glass container - the blow and blow method and the press and blow method. In all cases a stream of molten glass at its plastic temperature (1050ºC-1200ºC) is cut by a shearing blade to form a cylinder of glass called a gob. Both of the processes start with this gob falling by gravity and guided by troughs and chutes into the blank moulds. In the blow and blow process, the glass first is blown from below into the blank moulds to create a parison or pre-container. This 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, the parison is formed by 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.

In both cases the moulds are in two halves, and open to allow the parison or final container to be transferred out. The break between the two halves is responsible for the characteristic lines you will see running lengthwise on a container. So containers are not, as some may think, made in two halves and stuck together.

Forming machines

While the huge furnaces are a glasswork's fiery heart, the forming machines undoubtedly make the brain. These complex and highly engineered machines contain the mechanisms that 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), invented in 1903 by Michael J Owens in Illinois, U.S.. This type of machine is arranged as a bank of 8-12 identical sections, each of which contains one complete set of mechanisms to be able to make containers. The section are lined up in a row, and the gobs feed into each section via a moving chute, called the gob distributor. Sections are engineered to make either one, two, three or four containers at once. (Referred to as single, double, triple and quad gob respectively). In the case of multiple gobs, the shears cut the gobs simultaneously, and they fall into the blank moulds in parallel.

Annealing

As glass cools, it simultaneously shrinks and solidifies (by the way, it does not flow below the liquidus temperature - see Glass). If the cooling is uneven then stress caused by thermal shrinkage will set inside the glass, and this will cause the glass to be weak. The forming process, occurring over a short period of time (6-20 seconds), leaves the container with internal stresses which then need to be relieved through annealing (See Annealing (glass)). (The stress is compression on the outside and tension on the inside. A typical material properties demonstration involves an unannealed container being used to drive a nail into a piece of wood - the container is then scratched on the inside and it breaks easily or just falls to pieces. Also see Prince Rupert's Drops). An annealing oven (known in the industry as a Lehr presumably after the eponymous German manufacturer of the same) first heats the container up to 580ºC then cools it, depending on the glass thickness, over a 20 – 60 minute period. Annealing ovens have a huge steel mesh conveyor a bit like a pizza oven on steroids.

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 - that is to say every container is inspected. Automatic machines are set up to inspect for a variety of bottle faults. Typical faults include small cracks in the glass called checks, foreign inclusions called stones, bubbles in the glass, called blisters and thin glass. As well as rejecting faulty containers inspection equipment also gathers and correlates statistical information and relays it to the forming machine operators in the hot end. Computer systems collect and correlate fault information to the mould that produced the container. This is done by reading the mould number off the container, which is encoded (as a numeral, or a binary code of dots) on the glass container by the mould that made it. As well as automatic on line inspection, operators will carry out a range of checks manually on small samples of containers, usually visual and dimensional checks.

Secondary processing

Sometimes container factories will offer their customers value added services such as labelling. A number of labelling technologies are available; unique to glass however 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. A well known example of this application is the old Coca-Cola bottle.

Packaging

Around the world, 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 kept in a warehouse, ready for shipping.

Coatings

Glass containers typically receive two surface coatings, one in the hot end, just before annealing and the one in the cold end just after annealing. In the hot end a very thin layer of tin is applied. This keeps the surface of the glass in compression (thus making it more resistant to breakage) and makes the glass more adhesive to the cold end coating. In the cold end a layer of either wax or polyethylene is applied. This makes the glass slippery, protecting it from scratching and stopping containers from sticking together when they are moved on a conveyor.

Ancillary processes – compressors & cooling

Forming machines are largely powered by compressed air and a typical glass works will have several large compressors (totalling 30k-60k cfm) to generate the required 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 it’s positioning as a mature market product, glass does enjoy a high level of consumer acceptance and is perceived as a “premium” quality packaging format.

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 & 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.

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 rate of 99% are not uncommon in Western Europe. Return rates of less than 50% are unusual 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.

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 900kg of CO₂, assuming a gas fired furnace and coal fired electricity usage.

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