Glass container industry : Cold end

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.

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