Making Glass

GLASS FORMING PROCESSES

Like treacle and pitch, glass is fluid at high temperature and its fluidity decreases at the temperature is reduced. In other words its viscosity decreases as the temperature increases. Unlike water, which turns from liquid to a solid at a specific temperature, glass has no specific melting or freezing point but is gradually changed from a stiff solid to a liquid mass as the temperature is increased. It is this property of variable viscosity which is utilised in forming a mass of glass into articles of beauty or utility.

Glass Blowing

For nearly 2,000 years mouth blowing was the main method of forming glass articles. The last few years of the 19th century saw the beginnings of blowing glass by compressed air and the 20th century brought in the revolution of mechanisation.

For mouth blowing, a hollow blowing-iron or pipe is dipped into a pot containing molten glass and the glass is gathered at the end of the pipe by rotating it, similarly to gathering treacle onto a spoon. The collected glass, known as the gather, cools to about 1000^oC and is marvered (rolled on an iron slab) to form a parison. The parison is then manipulated by allowing it to elongate, re-heating it and blowing air into it to bring it into a shape which resembles the final article to be formed. It is then placed in an iron or wooden mould, which is kept wet by water, and the glass is blown to the final shape of the interior of the mould. There is no contact between the glass and the mould, due to the water being present, a cushion of steam forms a barrier preventing this. During the blowing the pipe is rotated continuously, thus preventing mould joints or other mould imperfections appearing in the glass.

Semi Automatic Bottle Making

Until the second half of the 19th century bottles were made by hand gathering, mouth blowing and finishing the neck, which was to receive a closure, by manual manipulation with simple tools. The mouth of the bottle, being made last, was known as the "finish".

No way was discovered of imitating this process by semi-mechanical or mechanical means until it was realised that the only way was to make the "finish" first.

The glass is thus gathered on a solid rod and is allowed to flow into a parison mould until sufficient glass is judged to have entered the mould. It is then cut off by means of hand shears. At the bottom of the parison mould is the "finish" with a plunger, which forms the opening into which compressed air is blown. A puff of compressed air blows the glass upwards against the sides of the parison mould and a plate at its top. Thus a parison, which is a thick walled bottle vaguely resembling the final product, is formed. The parison is removed from the mould and by absence of contact with the iron, the heat from the outer surface is no longer conducted away and the parison surface is re-heated. It is then transferred to the final mould and blown again and the parison surface is re-heated. It is then transferred to the final mould and blown again by compressed air to its final shape. The mould is opened, the bottle is removed and placed in a re-heating tunnel, called a lehr, for annealing.

Semi-automatic bottle making has practically disappeared in the developed countries and has been replaced by fully automatic production.

Making Glass Containers by Semi-Automatic Process

The gob is gathered by hand on an iron, and the correct amount of glass is dropped into a preliminary mould. Compressed air is introduced to form the neck of the article. The embryo shape (parison) is then transferred to the finishing mould in which the final shape is blown.

Automatic Container Production

The principle of automatic production is exactly the same as that previously described, except that instead of gathering the glass on an iron rod and allowing it to flow into the parison mould, gobs of glass of pre-determined shape and weight are formed above the parison mould and are allowed to drop into it.

Making Glass Containers by Automatic Process

The Press and Blow Process

The Blow and Blow Process

The machine which has almost replaced all others is the IS machine. It is not certain whether "IS" denotes its inventors, Ingle and Smith, or its main characteristic which is independent synchronised units with a synchronised gob distribution system each section.

It can consist of several sections and 10 section machines are by no means uncommon. The machine can operate on blow and blow or press and blow principle and double gob production, i.e. delivery of two gobs of glass at the same time is quite common. Triple gob machines are also in existence. The machine is capable of producing more than 200 containers per minute.

Flat Glass

The main flat glass products are float glass for high quality glazing in homes, offices, hotels, shops, transport and public buildings glass for horticulture: wired glasses for fire resistance; patterned glass for privacy and decoration; and a wide range of glasses for environmental control and energy conservation.

Other flat glass products include toughened glass doors, suspended window assemblies, cladding glasses for the exterior of buildings, mirrors and diffuse reflection glass for reducing reflection on glazed pictures and instrument dials.

The two manufacturing processes for producing flat glass in the UK are the float glass process and the rolled process.

The Float Glass Process

The float glass process, invented by Pilkington Brothers PLC and introduced in 1959, is now the principal method of producing flat glass throughout the world.

The glass is held in a chemically controlled atmosphere at a high enough temperature (1000 ēC) for a long enough time for irregularities to melt out and for the surfaces to become flat and parallel. Because the surface of the molten tin is flat the glass becomes flat and the thickness of the ribbon, in the range 2.5 to 25mm, is controlled at this stage. The ribbon is cooled down while still advancing along the molten tin until the surfaces are hard enough (600 ēC) for it to be lifted onto the conveyor rollers without marking the bottom surface. The ribbon passes through the annealing lehr to the automatic warehouse where computers govern the cutting of the ribbon to match customer’s orders. A large modern float glass plant will produce 5000 tonnes of glass per week. It operates continuously 24 hours a day, 365 days a year for several years. The glass produced has a uniform thickness and bright fire-polished surfaces without the need for grinding and polishing.

The Rolled Glass Process

The rolling process is used for the manufacture of patterned flat glass and wired glass. A continuous stream of molten glass is poured between water cooled rollers.

Patterned glass is made in a single pass process in which glass flows to the rollers at a temperature of about 1050 ēC. The bottom cast iron or stainless steel roller is engraved with the negative of the pattern; the top roller is smooth. Thickness is controlled by adjustment of the gap between the rollers. The ribbon leaves the rollers at about 850 ēC and is supported over a series of water cooled steel rollers to the annealing lehr. After annealing the glass is cut to size.

Wired glass is made in a double pass process. The process uses two independently driven pairs of water cooled forming rollers each fed with a separate flow of molten glass from a common melting furnace. The first pair of rollers produces a continuous ribbon of glass, half the thickness of the end product. This is overlaid with a wire mesh. A second feed of glass, to give a ribbon the same thickness as the first, is then added and, with the wire mesh "sandwiched", the ribbon passes through the second pair of rollers which form the final ribbon of wired glass. After annealing, the ribbon is cut by special cutting and snapping arrangements.

Glass Fibres

Glass in the form of fibres has found wide and varied applications in all kinds of industry. Its composition depends on the intended use.

For building insulation and glass wool the type of glass used is normally soda-lime. For textiles, an alumino-borosilicate glass with very low sodium oxide content (E glass) is preferred because of its good chemical durability and high softening point. This is also the type of composition employed for the fibres used in the reinforcement of plastics, familiar for their application in protective helmets, boats, piping, car chassis and many other articles.

In recent years, great progress has been made in making optical fibres which can guide light and thus transmit images round corners. These fibres are applicable to endoscopes for examination of internal human organs, changeable traffic message signs now in common use on motorways for speed restriction warnings and communications technology for transmitting telephone conversations much more efficiently than copper cable.

There are two broad groups of glass fibre products: continuous glass fibre which is used for the reinforcement of plastics, rubber and cement; and glass wool, which is used for thermal insulation and which is produced by the Crown process.

Glass Fibre Manufacture

Continuous glass fibre is a continuous strand, made up of a large number of individual filaments of glass.

Molten glass is fed from the furnace or "tank" through a channel or "forehearth" to a series of bushings which contain over one thousand six hundred accurately dimensioned holes or "forming tips" in its base.

A constant head of glass is maintained in the tank and forehearth and the temperature of the glass in the bushings is controlled to very fine limits. Fine filaments of glass are drawn mechanically downwards from the bushing tips at a speed of several thousand metres per minute, giving a filament diameter which may be as small as nine microns, or one tenth the diameter of a human hair. From the bushing the filaments run to a common collecting point where size is applied and they are subsequently brought together as bundles, or "strands", on a high speed winder.

Glass fibre is produced in a range of filament diameters and strand dimensions to tight tolerances for different end uses. It is used to strengthen and stiffen thermosetting plastics, thermoplastics, nylon and polypropylene as well as inorganic matrices, such as gypsum.

Glass Wool Manufacture

Glass wool is made in the Crown process. From the forehearth of the "tank" a thick stream of glass flows by gravity from the bushing into a rapidly rotating alloy steel dish "Crown" which has several hundred fine holes round its periphery.

The molten glass is thrown out through the holes by centrifugal force to form filaments which are further extended into fine fibres by a high velocity blast of hot gas. After being sprayed with a suitable bonding agent, the fibres are drawn by suction onto a horizontally moving conveyor positioned below the rotating dish.

The mat of tangled fibres formed on the conveyor is carried through an oven which cures the bonding agent, then to trimmers and guillotines which cut the product to size. The mat may be further processed into rigid sections for pipe insulation. The mats are made into many products for heat and sound insulation in buildings, transport vehicles and domestic appliances.

Optical Fibre Manufacture

Communications are increasingly based on eletro-optic systems in which telephones, television and computers are linked by fibre optic cables which carry information by laser light.

Making glass optical fibres is a highly specialised aspect of glass manufacture. Optical fibres consist of two distinct glasses, core of highly refracting glass surrounded by a sheath of glass with lower refractive index between the two glasses, it is guided by total reflection at the core-sheath interface to the other end of the fibre. In theory, a wide range of glasses can be used as long as the difference in refractive index is appropriate but the higher the refractive index of the core relative to that of the sheath glass, the greater the carrying capacity of the fibre. A typical system available commercially comprises a germanium doped silica core and a borosilicate cladding.

The aim in manufacture is to produce a fibre of glass which is so pure and free form defects that light inserted at one end will emerge at the other end a distance of 1 kilometre or more away. There are many manufacturing processes being used to produce cored fibre; two of these will illustrate the principles. All the processes require ultra-pure starting materials.

Chemical vapour deposition - high silica glass fibres are prepared by chemical vapour deposition in which layers of SiO, are deposited to make a preform, either on the outside of a mould or on the inside of a fused silica tube. The layers are doped during the deposition to control the refractive index. The preform is then drawn to a rod and subsequently to a fibre of 100-125mm diameter. The surface is protected from damage by a plastic coating.

The double crucible method - The double crucible uses purified glasses in separate crucibles in a controlled atmosphere furnace. Fibre drawn from the tip consists of a uniform core drawn from the central crucible and a cladding drawn from the outer crucible.

Tubing

Manufacture of Tubing Danner Process

The Danner Process was developed for the continuous production of glass tubing and rod. Subject to equipment design the process can make tubing of 1.6mm to 66.5mm diameter and rods of 2.0mm to 20mm diameter at drawing rates of up to 400m a minute for the smaller sizes.

Glass flows from a furnace forehearth in the form of a ribbon which falls on to the upper end of an inclined refractory sleeve carried on a rotating hollow shaft or blowpipe. The ribbon is wrapped around the sleeve to form a smooth layer of glass which flows down the sleeve and over the tip of the shaft. Tubing is formed by blowing air through a blowpipe with a hollow tip and rods are made by using a solid tip on the shaft.

The tubing is then drawn over a line of support rollers by a drawing machine situated up to 120m away. The dimensions of the tubing are determined as the glass cools through its setting point at the catenary or unsupported section between the blowpipe and the first line roller. A given range of size is based on the diameter of the refractory sleeve, and variations within the range are obtained by adjusting the temperature of the glass, the rate of flow, the pressure of the blowing air and the speed of the drawing machine.

Manufacture of Tubing Vello Process

The Vello process was a later development with a production capacity greater than that of the Danner process but based on a different principle.

Glass flows from a furnace forehearth into a bowl in which a hollow vertical mandrel is mounted or a bell surrounded by an orifice ring. The glass flows through the annular space between the bell and the ring and travels over a line of rollers to a drawing machine up to 120m away. Tubing is made by blowing air through a bell with a hollow tip and rod is produced by using a bell with a solid tip. The dimensions of the tubing are controlled by the glass temperature, the rate of draw, the pressure of the blowing air and the relative dimensions of the bell and ring.

Automatic Domestic Glassware Production

The Westlake machine was developed for blowing bulbs for domestic lamps and radio valves at production rates of up to 75,000 a day (gross). It has since been adapted for making drinking glasses, including stemmed ware, at up to 55,000 a day (gross).

The machine copies the action of a handblower in gathering glass from the furnace, forming a parison and blowing the article in a cast iron mould. Twelve pairs of spindles or blowpipes, together with their blowing air valves and past moulds, travel around a central column. The gathering equipment is carried on top of the column and sets of cams are fitted around the column to control the sequence of operations.

Glass is gathered by vacuum into a pair of blank moulds and the pairs of blanks are transferred in turn to each pair of spindles. The spindles are rotated and swung down, and air is introduced to form each blank into a parison, controlling the profile and distribution of the glass before blowing the required shape in the wetted mould.

The mould opens and the spindle jaws release the article which is then transferred to the stemming machine. Here the neck formed in the mould is reheated and stretched to the required length. The article then passes to the burn-off machine where oxygen-gas flames remove the "moil" or waste glass which was originally formed at the gathering position, and the finished piece is conveyed to the lehr for annealing.

Electric Light Bulb Envelope Production

The ribbon machine was developed for the high speed manufacture of bulbs for domestic lamps, auto lamps, vacuum flasks, etc. Its main feature is that glass travels through it in a straight line rather than on a rotary path as with the Westlake machines. Production rates in excess of 1000 a minute can be achieved.

From the furnace forehearth molten glass flows down between two rotating water cooled rollers and on to the Ribbon machine. On leaving the rollers the ribbon of glass is carried through the machine on a series of orifice plates, forming a continuous belt pierced with holes.

As the ribbon moves forward, a continuous chain of blowheads does the glassblower’s job for him. It blows the glass through the hole and the "blister" forms into a bulb inside a rotating mould which meets and closes around it from below. Still moving forward on the ribbon, the shaped bulb is released form its mould, cooled by air jets and then tapped off the ribbon to fall onto the scoops of a rotary turntable which tips it on to a conveyor belt. This carries it through an annealing lehr and air cooling to inspection and packing. The unused part of the ribbon passes direct to a cullet system for re-melting.

Electric Light Bulb Envelope Manufacture

Molten glass flows continuously between water cooled rollers and the ribbon so formed on orifice plates. Blowheads from above blow the glass through the holes in the plates. Moulds form below meet and close round these "blisters". The mould fall away revealing the formed bulbs which are cooled by air jets and tapperd off the ribbon. They fall into scoops on the rotary turntable which tip them onto the conveyor belt to the annealing lehr. More than 1,000 bulbs per minute can be produced on such a machine.

Pressed Glassware

Pressing is used for objects with a simple basic shape where the opening is wider than the base, this does not restrict surface decoration which may be complicated. A plunger is used to form the inner surface of the article by pushing the glass against the outer mould. Pressing can be hand-operated or fully automatic.