Lamp and Light Bulb Manufacture
There are a range of processes used to produce the wide array of different types of bulbs, lamps and tubes used across the world, these include:
This page also provides information regarding:
The type of glass most commonly used in the lamp industry is soda-lime-silicate glass, since with only minor variations in batch composition it is used as the envelope material for general incandescent, fluorescent and low wattage discharge lamps.
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 1,000 bulbs a minute can be achieved. The ribbon process is illustrated and explained in further detail below.
Relatively cheap raw materials are fed continuously into one end of a large tank furnace at a rate which balances that at which the molten glass is delivered to forming machines at the other end. The properties of the glass are designed to provide the optimum viscosity-temperature characteristics to enable components to be made on high speed machines at maximum efficiency.
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.
In the high-speed manufacture of incandescent bulbs on the Corning ribbon machine (Tooley, 1971), glass flows from the furnace forehearth between two rotating, water-cooled, rollers and onto the Ribbon machine as a horizontal ribbon of glass. This ribbon travels between two belts - the top belt consisting of a series of blowing heads and the bottom belt consisting of a corresponding series of moulds.
As the belts and the glass ribbon pass through the belts, the glass is blown by the blowheads on the top belt into the moulds on the lower belt which have been sprayed with water and are rotating. As the glass is blown into the mould a 'blister' forms into bulb envelope. The steam cushion formed between the glass and the mould leaves the bulb with a polished surface whilst the rotation eliminates mould seams.
The shaped bulb is release from its mould, cooled by air jets and then finally tapped off (or cracked) from the ribbon and dropped onto an annealing conveyor. 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.
This process can produce over 60,000 bulbs per hour per machine, depending on bulb size. If pearl bulbs are required, the interior of the bulb is treated, subsequent to the annealing process, with an etching fluid, comprising mainly hydrofluoric acid, and then washed and dried. Powder coatings are also applied to clear bulbs during lamp making to obscure the filament or for decorative effects; this obviates the environmental problems associated with etching.
Tubing for fluorescent lamp envelopes is continuously drawn from the same type of furnace using either the Danner or the Vello process (Doyle – Glassmaking Today 1979). In the Danner process, glass flows from the furnace at a controlled rate onto the top of an inclined, rotating, hollow refractory mandrel. Air is blown down the centre of the mandrel as the tubing is drawn off the bottom by a drawing machine, which may be 50m distant. The glass tube, as it solidifies, is supported between the mandrel and the drawing machine by a series of shaped carbon rollers placed at regular intervals. The size of the tubing drawn depends on the diameter of the mandrel, the draw speed, the amount of blowing air, the glass temperature and the cooling rate.
In the Vello process the glass in the furnace flows in to a refractory bowl which has an orifice plate in its base (the “ring”). A vertical mandrel, the bottom end of which is flared (the “bell”), is suspended through the ring. Around the mandrel is a rotating sleeve which provides stirring for the glass in the bowl. Glass is drawn between ring and bell, initially vertically downwards, but then, as it cools, it is pulled through almost 90 o by the drawing machine on to the carbon support rollers. Dimensional stability is achieved by methods similar to those used in the Danner process, but the size range is limited and a change of tube size may require both ring and bell to be changed. Consequently, the Vello process is best used for long runs of a standard size, while the Danner process provides greater flexibility for shorter production runs over a wider range of sizes.
The internal glass components of fluorescent and general incandescent lamps, and also many types of small incandescent bulbs, particularly for wedge base type lamps, are made from lead-alkali silicate glass. Lead glass, as it is termed, is preferred to soda-lime glass because of its higher electrical resistivity, which prevents electrolysis occurring in the pinch seal. The glass seals readily to soda-lime envelopes and has a somewhat lower softening point and longer working temperature range than does soda-lime glass, all factors which assist in lamp making. The standard lead glass traditionally used in lamp making contains about 30% by weight of lead oxide. Since the early 1980s, however, because of the relatively high cost and the introduction of increasingly stringent health and safety regulations concerning lead compounds, glasses containing 20-22% lead oxide have almost entirely replaced the old standard lead glasses. These new glasses have similar working characteristics and, although they have a lower electrical resistivity, they are adequate for the majority of lamp applications.
The drive to reduce levels of environmental lead has, however, continued and has been primarily responsible for the development of lead-free glasses, which are increasingly being used in those lamp making applications traditionally reserved for lead glasses. These new formulations rely upon the use of barium oxide as the major replacement for lead oxide, in addition to which oxides of strontium, calcium, magnesium and boron may be added. The amounts of the other oxides are also adjusted to produce glasses that have similar electrical properties to the current lead glass but with a shorter working temperature range. This faster setting can be accommodated to a large extent by the progressive increase in the speed of lamp making machinery. It is likely that, in the longer term, glasses free from both lead and barium will eventually become the industry standard.
For lamps in which the operating temperature is too high for soda-lime glass, such as envelopes for high-wattage discharge lamps, borosilicate glass is used. In addition to its ability to withstand higher operating temperatures, it also has a much lower thermal expansion coefficient and thus withstands greater changes in temperature. This leads to its use in sealed beam and other specialised types of lamps which may be subjected to sudden temperature variations. Lead and borosilicate glasses are usually melted and formed in a similar way to soda-lime glasses, although often on a smaller scale.
Where even higher service temperatures are required, aluminosilicate glass is used. This is the most refractory conventional glass used in the lamp industry. Its thermal shock resistance, however, is inferior to that of most borosilicate glasses. It is produced in relatively small quantities and is used for the envelops of lw-wattage, single-ended tungsten halogen lamps, including some automobile headlamps. Glasses for this application are essentially free from alkalis and are usually of the alkaline-earth aluminosilicate type.
A glass having a small and very specialized application in the lamp industry is sodium resistant glass. The powerful reducing properties of hot alkaline vapours produce rapid blackening in normal silicate glasses by reduction, and this can be eliminated by using glasses which contain little or no silica or other readily reducible oxides.
The primary process for manufacturing tubing for lamps uses quartz sand as its major raw material. These are a by-product of the mining of feldspars. The purified sands are fed into an induction-heated furnace fitted with a central mandrel which passes through an orifice in the base. Melting takes place in a refractory metal crucible in an atmosphere of helium and hydrogen (Antezak, 1973). The feed rate is controlled to maintain a constant level of the melt as tubing is drawn through the base. Since both helium and hydrogen are readily soluble in fused silica, there are few airlines and the optical quality is good.
Translucent, or satin vitreous silica, which is used in the manufacture of some linear heater lamps, is produced by externally reheating a fused slug of sand and drawing it down the tubing. The striated, entrapped bubbles give the tubing its characteristic sheen. The main advantages of vitreous silica re its transparency, resistance to thermal shock and high operating temperatures.
There are several different types of lamps and lighting on the market. The most efficient light source today is the low-pressure sodium lamp. It emits monochromatic orange light, but has no colour rendering properties. In contrast, incandescent and tungsten halogen lamps have excellent colour rendering properties but relatively low luminous efficacies*. Listed here are some of the more common lamps that illuminate our daily environment – pause for a moment and imagine where we would be without these unassuming glass articles in our daily activities? Listed here is a brief description of the kinds of lighting we use in our homes, our streets, and in industry:
- Incandescent Lamps;
These are available in a range of voltages, from just a few volts for battery operation up to mains voltage. They are a low cost item, need no auxiliary circuitry and are available “off the shelf” from supermarkets and general stores. Their principal application is still domestic lighting, and they are used in many places where compact low voltage lamps are needed, e.g. torches, panel lighting etc. Only about 10% of the input power is radiated as light, with a typical life expectancy from a few tens to a few thousand hours.
- Tungsten Halogen Incandescent Lamps;
These occupy a much smaller volume for the same wattage than non-halogen lamps of the same rating, permitting high gas filling pressures with heavier (and more expensive) gases. These changes result in enhanced life, or efficacy. Again, they can be run directly from a power source and do not require any control circuit. They are extensively used for motor vehicle lighting, in projection systems, for spot lights, low cost flood lighting, stage and studio lighting and any other applications where compactness, convenience and enhanced performance over “non-halogen” lamps is advantageous.
- Low-Pressure Sodium Lamps;
As mentioned above, these are the most efficient source of artificial illumination, but because they emit monochromatic yellow light, it is impossible to discriminate colours under them. Their principal use is for road lighting, security lighting and similar outdoor applications. They have an efficacy of some 200 1mW -1 (twice that of a fluorescent lamp and 10 times more than a tungsten halogen lamp) and, like the fluorescent lamp, they are in long, tubular form – usually bent into a “U” shape – contained in a vacuum outer jacket coated with an infrared reflecting film to conserve energy and afford maximum efficacy.
- High-intensity Discharge (HID);
These lamps are high-pressure discharge lamps, characterised by short, high brightness arc tubes, usually contained in some form of glass (or quartz) outer jacket, which may be clear, diffused or phosphor-coated to enhance the red radiation. High-pressure mercury vapour (HPMV) are the simplest form of HID lamps, where the discharge takes place through mercury vapour, within a quartz tube – usually mounted inside a phosphor-coated outer jacket. They have only a very moderate efficacy and colour rendering, and for this reason are mainly used for outdoor lighting and some industrial interior applications. High-pressure sodium (HPS) lamps require a ceramic arc tube to contain the corrosive sodium vapour at temperatures in excess of 1000 o C. The arc tube is mounted inside a glass bulb or quartz tube to isolate it from the outer atmosphere. HPS lamps have the highest efficacy of all HID lamps and have very long lives (around 24,000 hours), which makes them a favoured source for city centre lighting, car parking lots, industrial premises etc., where their moderate colour rendering is usually more than adequate. Improved colour rendering and “white light” versions are also available, but at the cost of lower efficiency. Metal halide (MH) lamps are the most complex form of HID lamps; in these, light is generated by the excitation of metal atoms, often involving several metallic elements, usually introduced as halides, to give a white light with good colour rendition. The arc tubes may be made from quartz or ceramic and like the HPS lamps, are mounted inside a glass bulb or tubular quartz outer jacket. Applications are numerous – in fact they can be used anywhere requiring high quality white light, at a high efficacy. Typical applications include up-lighters and down-lighters, flood lighting and spot lighting, and the compact forms are particularly useful where accurate beam control is required.
- Induction Lamps;
These are also known as electrodeless lamps and are a relatively new technology. In these, the power is coupled to the discharge by a high-frequency field, where the discharge effectively forms the secondary winding of a transformer. In the present format they are an alternative to compact fluorescent lamps, but high-pressure versions are also possible. They are not constrained to a tubular form, and they give instant light. They operate at a frequency of several MHz and require a special electronic circuit to drive and control them.
- Electroluminescent Lighting;
This embraces light-emitting panels of various sorts and light emitting diodes (LEDs). To date- their main application has been for signs and indicators, but high brightness LEDs are now being used for vehicle rear lights and “flashing” rear lamps on bicycles. Their low current drain makes them attractive for this type of application.
* efficacy – (ŋ) is the luminous flux divided by the power consumed (1mW-1)
Further Reading – “Lamps & Lighting”, Coaton and Marsden, ISBN: 0470235896 (Wiley)
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