Microwave Oven

Microwave Oven
Background :
Microwaves are actually a segment of the electromagnetic wave spectrum, which comprises forms of energy that move through space, generated by the interaction of electric and magnetic fields. The spectrum is commonly broken into subgroups determined by the different wavelengths (or frequencies) and emission, transmission, and absorption behaviors of various types of waves. From longest to shortest wavelengths, the spectrum includes electric and radio waves, microwaves, infrared (heat) radiation, visible light, ultraviolet radiation, X-rays, gamma rays, and electromagnetic cosmic rays. Microwaves have frequencies between approximately .11 and 1.2 inches (0.3 and 30 centimeters).

Microwaves themselves are used in many different applications such as telecommunication products, radar detectors, wood curing and drying, and medical treatment of certain diseases. However, certain of their properties render them ideal for cooking, by far the most common use of microwave energy. Microwaves can pass through plastic, glass, and paper materials; metal surfaces reflect them, and foods (especially liquids) absorb them. A meal placed in a conventional oven is heated from the outside in, as it slowly absorbs the surrounding air that the oven has warmed. Microwaves, on the other hand, heat food much more quickly because they penetrate all layers simultaneously. Inside a piece of food or a container filled with liquid, the microwaves agitate molecules, thereby heating the substance.

The ability of microwave energy to cook food was discovered in the 1940s by Dr. Percy Spencer, who had conducted research on radar vacuum tubes for the military during World War II. Spencer's experiments revealed that, when confined to a metal enclosure, high-frequency radio waves penetrate and excite certain type of molecules, such as those found in food. Just powerful enough to cook the food, the microwaves are not strong enough to alter its molecular or genetic structure or to make it radioactive.

Raytheon, the company for which Dr. Spencer was conducting this research, patented the technology and soon developed microwave ovens capable of cooking large quantities of food. Because manufacturing costs rendered them too expensive for most consumers, these early ovens were used primarily by hospitals and hotels that could more easily afford the $3,000 investment they represented. By the late 1970s, however, many companies had developed microwave ovens for home use, and the cost had begun to come down. Today, microwaves are a standard household appliance, available in a broad range of designs and with a host of convenient features: rotating plates for more consistent cooking; digital timers; autoprogramming capabilities; and adjustable levels of cooking power that enable defrosting, browning, and warming, among other functions.

Design
The basic design of a microwave oven is simple, and most operate in essentially the same manner. The oven's various electronic motors, relays, and control circuits are located on the exterior casing, to which the oven cavity is bolted. A front panel allows the user to program the microwave, and the


The oven cavity and door are made using metal-forming techniques and then painted using electro-deposition, in which electric current is used to apply the paint.
The magnetron tube subassembly includes several important parts. A powerful magnet is placed around the anode to provide the magnetic field in which the microwaves will be generated, while a thermal protector is mounted directly on the magnetron to prevent damage to the tube from overheating. An antenna enclosed in a glass tube is mounted on top of the anode, and the air within the tube is pumped out to create a vacuum. Also, a blower motor used to cool the metal fins of the magnetron is attached directly to the tube.
door frame has a small window to enable the cook to view the food while it is cooking.

Near the top of the steel oven cavity is a magnetron—an electronic tube that produces high-frequency microwave oscillations—which generates the microwaves. The microwaves are funneled through a metal waveguide and into a stirrer fan, also positioned near the top of the cavity. The fan distributes the microwaves evenly within the oven. Manufacturers vary the means by which they disburse microwaves to achieve uniform cooking patterns: some use dual stirrer fans located on opposite walls to direct microwaves to the cavity, while others use entry ports at the bottom of the cavity, allowing microwaves to enter from both the top and bottom. In addition, many ovens rotate food on a turntable.

Raw Materials
The cover or outer case of the microwave oven is usually a one-piece, wrap-around metal enclosure. The oven's inside panels and doors are made of galvanized or stainless steel and are given a coating of acrylic enamel, usually light in color to offer good visibility. The cooking surface is generally made of ceramic or glass. Inside the oven, electromechanical components and controls consist of timer motors, switches, and relays. Also inside the oven are the magnetron tube, the waveguide, and the stirrer fan, all made of metal. The hardware that links the various components consists of a variety of metal and plastic parts such as gears, pulleys, belts, nuts, screws, washers, and cables.

The Manufacturing
Process
Oven cavity and door manufacture
•1 The process of manufacturing a microwave oven starts with the cavity and the door. First, the frame is formed using automatic metal-forming presses that make about 12 to 15 parts per minute. The frame is then rinsed in alkaline cleaner to get rid of any dirt or oil and further rinsed with water to get rid of the alkaline solution.
•2 Next, each part is treated with zinc phosphate, which prepares it for electro-deposition. Electro-deposition consists of immersing the parts in a paint tank at 200 volts for 2.5 minutes. The resulting coating is about 1.5 mils thick. The parts are then moved through a paint bake operation where the paint is cured at 300 degrees Fahrenheit (149 degrees Celsius) for 20 minutes.

The chassis or frame is mounted in a pallet for the main assembly operation. A pallet is a vise-like device used in conjunction with other tools.
•3 After the door has been painted, a perforated metal plate is attached to its window aperture. The plate reflects microwaves but allows light to enter the cavity (the door will not be attached to the cavity until later, when the chassis is assembled).
The magnetron tube subassembly
•4 The magnetron tube assembly consists of a cathode cylinder, a filament heater, a metal anode, and an antenna. The filament is attached to the cathode, and the cathode is enclosed in the anode cylinder; this cell will provide the electricity that will help to generate the microwaves. Metal cooling fins are welded to the anode cylinder, and a powerful magnet is placed around the anode to provide the magnetic field in which the microwaves will be generated. A metal strap holds the complete assembly together. A thermal protector is mounted directly on the magnetron to prevent damage to the tube from overheating.
•5 An antenna enclosed in a glass tube is mounted on top of the anode, and the air within the tube is pumped out to create a vacuum. The waveguide is connected to the magnetron on top of the protruding antenna, while a blower motor used to cool the metal fins of the magnetron is attached directly to the tube. Finally, a plastic fan is attached to the motor, where it will draw air from outside the oven and direct it towards the vanes. This completes the magnetron subassembly.
Main chassis assembly
•6 The chassis assembly work is performed on a pallet—a work-holding device used in conjunction with other tools—located at the station. First, the main chassis is placed on the pallet, and the cavity is screwed on to the chassis. Next, the door is attached to the cavity and chassis by means of hinges. The magnetron tube is then bolted to the side of the cavity and the main chassis.

In a completed microwave oven, the magnetron tube creates the microwaves, and the waveguide directs them to the stirrer fan. In turn, this fan points the waves into the oven cavity where they heat the food inside.
•7 The circuit that produces the voltage required to operate the magnetron tube consists of a large transformer, an oil-based capacitor, and a high voltage rectifier. All of these components are mounted directly on the chassis, close to the magnetron tube.
Stirrer fan
•8 The stirrer fan used to circulate the microwaves is mounted on top of the cavity. Some manufacturers use a pulley to drive the fan from the magnetron blower motor; others use a separate stirrer motor attached directly to the fan. Once the stirrer fan is attached, a stirrer shield is screwed on top of the fan assembly. The shield prevents dirt and grease from entering the waveguide, where they could produce arcing and damage the magnetron.
Control switches, relays, and motors
•9 The cook switch provides power to the transformer by energizing a relay and a timer. The relay is mounted close to the power transformer, while the timer is mounted on the control board. The defrost switch works like the cook switch, activating a motor and timer to operate the defrost cycle. Also mounted on the control board are a timer bell that rings when the cooking cycle is complete and a light switch that allows viewing of the cavity. A number of interlocking switches are mounted near the top and bottom of the door area. The interlocking switches are sometimes grouped together with a safety switch that monitors the other switches and provides protection if the door accidently opens during oven operation.
Front panel
•10 A front panel that allows the operator to select the various settings and features available for cooking is attached to the chassis. Behind the front panel, the control circuit board is attached. The board, which controls the various programmed operations in their proper sequence when the switches are pushed on the front panel, is connected to the various components and the front panel by means of plug-in sockets and cables.
Making and assembling the case
•11 The outer case of the microwave is made of metal and is assembled on a roll former. The case is slipped onto the preassembled microwave oven and bolted to the main chassis.
Testing and packaging the oven
•12 The power cords and dial knobs are now attached to the oven, and it is sent for automatic testing. Most manufacturers run the oven from 50-100 hours continuously as part of the testing process. After testing is complete, a palletizer robot records the model and serial data of the oven for inventory purposes, and the oven is sent for packaging. This completes the manufacturing process.
Quality Control
Extensive quality control during the manufacture of microwave ovens is essential, because microwave ovens emit radiation that can burn anyone exposed at high levels for prolonged periods. Federal regulations, applied to all ovens made after October 1971, limit the amount of radiation that can leak from an oven to 5 milliwatts of radiation per square centimeter at approximately 2 inches from the oven surface. The regulations also require all ovens to have two independent, interlocking switches to stop the production of microwaves the moment the latch is released or the door is opened.

In addition, a computer controlled scanner is used to measure emission leaks around the door, window, and back of the oven. Other scanners check the seating of the magnetron tube and antenna radiation. Each scanner operation relays data to the next-on-line operation so that any problems can be corrected.

The Future
Because of their speed and convenience, microwave ovens have become an indispensable part of modern kitchens. Many developments in the microwave market and allied industries are taking place fairly rapidly. For example, foods and utensils designed specially for microwave cooking have become a huge business. New features will also be introduced in microwaves themselves, including computerized storage of recipes that the consumer will be able to recall at the touch of a button. The display and programmability of the ovens will also be improved, and combination ovens capable of cooking with microwaves as well as by conventional methods will become a standard household product.

Where To Learn More
Books
Davidson, Homer L. Microwave Oven Repair, 2nd edition. Tab Books Inc., 1991.

Gallawa, J. Carlton. The Complete Microwave Oven Service Handbook: Operation, Maintenance. Prentice Hall, 1989.

Microwave Oven Radiation. U.S. department of Health and Human Services, 1986.

Pickett, Amold and John Ketterer. Household Equipment in Residential Design. John Wiley and Sons, 1986.

Raytheon Company. Appliance Manufacturer. Cahners Publishing, 1985.

Periodicals
Klenck, Thomas. "How It Works: Microwave Oven." Popular Mechanics. September, 1989, p. 78.

Roman, Mark. "The Little Waves That Could." Discover. November, 1989, p. 54.

— Rashid Riaz



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Laser Guided Missile

Laser Guided Missile
Background:
Missiles differ from rockets by virtue of a guidance system that steers them towards a pre-selected target. Unguided, or free-flight, rockets proved to be useful yet frequently inaccurate weapons when fired from aircraft during the World War II. This inaccuracy, often resulting in the need to fire many rockets to hit a single target, led to the search for a means to guide the rocket towards its target. The concurrent explosion of radio-wave technology (such as radar and radio detection devices) provided the first solution to this problem. Several warring nations, including the United States, Germany and Great Britain, mated existing rocket technology with new radio- or radar-based guidance systems to create the world's first guided missiles. Although these missiles were not deployed in large enough numbers to radically divert the course of the World War II, the successes that were recorded with them pointed out techniques that would change the course of future wars. Thus dawned the era of high-technology warfare, an era that would quickly demonstrate its problems as well as its promise.

The problems centered on the unreliability of the new radio-wave technologies. The missiles were not able to hone in on targets smaller than factories, bridges, or warships. Circuits often proved fickle and would not function at all under adverse weather conditions. Another flaw emerged as jamming technologies flourished in response to the success of radar. Enemy jamming stations found it increasingly easy to intercept the radio or radar transmissions from launching aircraft, thereby allowing these stations to send conflicting signals on the same frequency, jamming or "confusing" the missile. Battlefield applications for guided missiles, especially those that envisioned attacks on smaller targets, required a more reliable guidance method that was less vulnerable to jamming. Fortunately, this method became available as a result of an independent research effort into the effects of light amplification.

Dr. Theodore Maiman built the first laser (Light Amplification by Stimulated Emission of Radiation) at Hughes Research Laboratories in 1960. The military realized the potential applications for lasers almost as soon as their first beams cut through the air. Laser guided projectiles underwent their baptism of fire in the extended series of air raids that highlighted the American effort in the Vietnam War. The accuracy of these weapons earned them the well-known sobriquet of "smart weapons." But even this new generation of advanced weaponry could not bring victory to U.S. forces in this bitter and costly war. However, the combination of experience gained in Vietnam, refinements in laser technology, and similar advances in electronics and computers, led to more sophisticated and deadly laser guided missiles. They finally received widespread use in Operation Desert Storm, where their accuracy and reliability played a crucial role in the decisive defeat of Iraq's military forces. Thus, the laser guided missile has established itself as a key component in today's high-tech military technology.

Raw Materials
A laser guided missile consists of four important components, each of which contains different raw materials. These four components


The missile body is die-cast in halves: molten metal (either aluminum or steel) is poured into a metal die and cooled to form the proper shape. The two halves are then welded together.
The principal laser components—the photo detecting sensor and optical filters—are assembled in a series of operations that are separate from the rest of the missile's construction. Circuits that support the laser system are then soldered onto pre-printed boards. The circuit boards for the electronics suite are also assembled independently from the rest of the missile. If called for by the design, microchips are added to the boards at this time.
are the missile body, the guidance system (also called the laser and electronics suite), the propellant, and the warhead. The missile body is made from steel alloys or high-strength aluminum alloys that are often coated with chromium along the cavity of the body in order to protect against the excessive pressures and heat that accompany a missile launch. The guidance system contains various types of materials—some basic, others high-tech—that are designed to give maximum guidance capabilities. These materials include a photo detecting sensor and optical filters, with which the missile can interpret laser wavelengths sent from a parent aircraft. The photo detecting sensor's most important part is its sensing dome, which can be made of glass, quartz, and/or silicon. A missile's electronics suite can contain gallium-arsenide semiconductors, but some suites still rely exclusively on copper or silver wiring. Guided missiles use nitrogen-based solid propellants as their fuel source. Certain additives (such as graphite or nitroglycerine) can be included to alter the performance of the propellant. The missile's warhead can contain highly explosive nitrogen-based mixtures, fuel-air explosives (FAE), or phosphorous compounds. The warhead is typically encased in steel, but aluminum alloys are sometimes used as a substitute.

Design
Two basic types of laser guided missiles exist on the modern battlefield. The first type "reads" the laser light emitted from the launching aircraft/helicopter. The missile's electronic suite issues commands to the fins (called control surfaces) on its body in an effort to keep it on course with the laser beam. This type of missile is called a beam rider as it tends to ride the laser beam towards its target.

The second type of missile uses on-board sensors to pick up laser light reflected from the target. The aircraft/helicopter pilot selects a target, hits the target with a laser beam shot from a target designator, and then launches the missile. The missile's sensor measures the error between its flight path and the path of the reflected light. Correction messages are then passed on to the missile's control surfaces via the electronics suite, steering the missile onto its target.

Regardless of type, the missile designer must run computer simulations as the first step of the design process. These simulations assist the designer in choosing the proper laser type, body length, nozzle configurations, cavity size, warhead type, propellant mass, and control surfaces. The designer then puts together a package containing all relevant engineering calculations, including those generated by computer simulations. The electronics suite is then designed around the capabilities of the laser and control surfaces. Drawings and schematics of all components can now be completed; CAD/CAM (Computer-Aided Design/Manufacture) technology has proven helpful with this task. Electronics systems are then designed around the capabilities of the aircraft's laser and the missile's control surfaces. The following step consists of generating the necessary schematic drawings for the chosen electronics system. Another computer-assisted study of the total guided missile system constitutes the final step of the design process.

The Manufacturing
Process
Constructing the body and attaching the fins
•1 The steel or aluminum body is die cast in halves. Die casting involves pouring molten metal into a steel die of the desired shape and letting the metal harden. As it cools, the metal assumes the same shape as the die. At this time, an optional chromium coating can be applied to the interior surfaces of the halves that correspond to a completed missile's cavity. The halves are then welded together, and nozzles are added at the tail end of the body after it has been welded.
•2 Moveable fins are now added at predetermined points along the missile body. The fins can be attached to mechanical joints that are then welded to the outside of the body, or they can be inserted into recesses purposely milled into the body.
Casting the propellant
•3 The propellant must be carefully applied to the missile cavity in order to ensure a uniform coating, as any irregularities will result in an unreliable burning rate, which in turn detracts from the performance of the missile. The best means of achieving a uniform coating is to apply the propellant by using centrifugal force. This application, called casting, is done in an industrial centrifuge that is well-shielded and situated in an isolated location as a precaution against fire or explosion.
Assembling the guidance system
•4 The principal laser components—the photo detecting sensor and optical filters—are assembled in a series of operations that are separate from the rest of the missile's construction. Circuits that support the laser system are then soldered onto pre-printed boards; extra attention is given to optical materials at this time to protect them from excessive heat, as this can alter the wavelength of light that the missile will be able to detect. The assembled laser subsystem is now set aside pending final assembly. The circuit boards for the electronics suite are also assembled independently from the rest of the missile. If called for by the design, microchips are added to the boards at this time.
•5 The guidance system (laser components plus the electronics suite) can now be integrated by linking the requisite circuit boards and inserting the entire assembly into the missile body through an access panel. The missile's control surfaces are then linked with the guidance system by a series of relay wires, also entered into the missile body via access panels. The photo detecting sensor and its housing, however, are added at this point only for beam riding missiles, in which case the housing is carefully bolted to the exterior diameter of the missile near its rear, facing backward to interpret the laser signals from the parent aircraft.
Final assembly
•6 Insertion of the warhead constitutes the final assembly phase of guided missile

Current laser guided missiles work in one of two ways. The first type, a "beam rider,' reads the laser light emitted from the launching aircraft and rides the beam toward the target. The second type uses on-board sensors to pick up laser light sent by the aircraft and reflected from the target. The sensors measure the error between the missile's flight path and the path of the reflected light, and the electronics suite alters the control surfaces as necessary to guide the missile toward the target.
construction. Great care must be exercised during this process, as mistakes can lead to catastrophic accidents. Simple fastening techniques such as bolting or riveting serve to attach the warhead without risking safety hazards. For guidance systems that home-in on reflected laser light, the photo detecting sensor (in its housing) is bolted into place at the tip of the warhead. On completion of this final phase of assembly, the manufacturer has successfully constructed on of the most complicated, sophisticated, and potentially dangerous pieces of hardware in use today.
Quality Control
Each important component is subjected to rigorous quality control tests prior to assembly. First, the propellant must pass a test in which examiners ignite a sample of the propellant under conditions simulating the flight of a missile. The next test is a wind tunnel exercise involving a model of the missile body. This test evaluates the air flow around the missile during its flight. Additionally, a few missiles set aside for test purposes are fired to test flight characteristics. Further work involves putting the electronics suite through a series of tests to determine the speed and accuracy with which commands get passed along to the missile's control surfaces. Then the laser components are tested for reliability, and a test beam is fired to allow examiners to record the photo detecting sensor's ability to "read" the proper wavelength. Finally, a set number of completed guided missiles are test fired from aircraft or helicopters on ranges studded with practice targets.

Byproducts/Waste
Propellants and explosives used in warheads are toxic if introduced into water supplies. Residual amounts of these materials must be collected and taken to a designated disposal site for burning. Each state maintains its own policy pertaining to the disposal of explosives, and Federal regulations require that disposal sites be inspected periodically. Effluents (liquid byproducts) from the chromium coating process can also be hazardous. This problem is best dealt with by storing the effluents in leak-proof containers. As an additional safety precaution, all personnel involved in handling any hazardous wastes should be given protective clothing that includes breathing devices, gloves, boots and overalls.

The Future
Future laser guided missile systems will carry their own miniaturized laser on board, doing away with the need for target designator lasers on aircraft. These missiles, currently under development in several countries, are called "fire-and-forget" because a pilot can fire one of these missiles and forget about it, relying on the missile's internal laser and detecting sensor to guide it towards its target. A further development of this trend will result in missiles that can select and attack targets on their own. Once their potential has been realized, the battlefields of the world will feel the deadly venom of these "brilliant missiles" for years to come. An even more advanced concept envisions a battle rifle for infantry that also fires small, laser guided missiles. Operation Desert Storm clearly showed the need for laser guided accuracy, and, as a result, military establishments dedicated to their missions will undoubtedly invent and deploy ever more lethal versions of laser guided missiles.

Where To Learn More
Books
Bova, Ben. The Beauty of Light. John Wiley and Sons, 1988.

Hallmark, Clayton L. and Delton T. Horn. Lasers: The Light Fantastic. TAB Books, 1987.

Hecht, Jeff. Optics: Light for a New Age. Charles Scribner's Sons, 1987.

Iannini, Robert. E. Build Your Own Fiberoptic, Infrared, and Laser Space-Age Projects. TAB Books, 1987.

Laurence, Clifford L. The Laser Book: A New Technology of Light. Prentice Hall, 1986.

Von Braun, Wernher, Frederick I. Ordway III, and Dave Dooling. Space Travel: A History. Harper & Row, 1985.

Wood, Derek. Jane's World Aircraft Recognition Handbook. Jane's Information Group, 1989.

Wulforst, Harry. The Rocketmakers. Orion Books, 1990.

Periodicals
"A Dull Scalpel for the Surgical Strike on Libya." Discover. June, 1986, p. 8.

Lenorowitz, Jeffrey M. "F-1 17s Drop Laser-Guided Bombs in Destroying Most Baghdad Targets." Aviation Week and Space Technology. Feb. 4, 1991, p. 30.

Magnusson, Paul. "American Smart Bombs, Foreign Brains." Business Week. March 4, 1991, p. 18.

— Robert A. Cortese



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Automobile Windshield

Automobile Windshield

Background:
Glass is a versatile material with hundreds of applications, including windshields. Glass has a long history and was first made more than 7,000 years ago in Egypt, as early as 3,000 B.C. Glass is found in a natural state as a by-product of volcanic activity. Today, glass is manufactured from a variety of ceramic materials (main components are oxides). The main product categories are flat or float glass, container glass, cut glass, fiberglass, optical glass, and specialty glass. Automotive windshields fall into the flat glass category.

There are more than 80 companies worldwide that produce automotive glass, including windshields. Major producers in the United States include PPG, Guardian Industries Corp., and Libby-Owens Ford. According to the Department of Commerce, 25 percent of flat glass production is consumed by the automotive industry (including windows) at a total value of approximately $483 million. In Japan, 30 percent of flat glass goes to the automotive industry, valued at around $190 billion in 1989. Major Japanese flat glass manufacturers include Asahi Glass Co., Central Glass Co., and Nippon Sheet Glass Co. Little growth is expected for the flat glass industry overall in both countries. Germany has a more positive outlook, with high growth rates expected from the automotive industry.

Glass windshields first appeared around 1905 with the invention of safety glass—glass tempered (tempering is a heat treatment) to make it especially hard and resistant to shattering. This type of windshield was popular well into the middle of the century, but it was eventually replaced by windshields made of laminated glass—a multilayer unit consisting of a plastic layer surrounded by two sheets of glass. In many countries, including the U.S., auto windshields are required by law to be made of laminated glass. Laminated glass can bend slightly under impact and is less likely to shatter than normal safety glass. This quality reduces the risk of injury to the automobile's passengers.

Raw Materials
Glass is composed of numerous oxides that fuse and react together upon heating to form a glass. These include silica (SiO 2 ), sodium oxide (Na 2 O), and calcium oxide (CaO). Raw materials from which these materials are derived are sand, soda ash (Na 2 CO 3 ), and limestone (CaCO 3 ). Soda ash acts as a flux; in other words, it lowers the melting point of the batch composition. Lime is added to the batch in order to improve the hardness and chemical durability of the glass. Glass used for windshields also usually contains several other oxides: potassium oxide (K 2 O derived from potash), magnesium oxide (MgO), and aluminum oxide (AI 2 O 3 derived from feldspar).

The Manufacturing
Process
•1 The raw materials are carefully weighed in the appropriate amounts and mixed together with a small amount of water to prevent segregation of the ingredients. Cullet (broken waste glass) is also used as a raw material.
•2 Once the batch is made, it is fed to a large tank for melting using the float

The glass for automible windshields is made using the float glass process. In this method, the raw material is heated to a molten state and fed onto a bath of molten tin. The glass literally floats on top of the fin; because the fin is perfectly flat, the glass also becomes flat. From the float chamber, the glass passes on rollers through an oven (the "annealing lehr"). After exiting the lehr and cooling to room temperature, the glass is cut to the proper shape and tempered.
glass process. First, the batch is heated to a molten state, and then it is fed into a tank called the float chamber, which holds a bath of molten tin. The float chamber is very large—from about 13 feet to 26.25 feet (4 to 8 meters wide and up to almost 197 feet (60 meters) long; at its entrance, the temperature of the tin is about 1,835 degrees Fahrenheit (1,000 degrees Celsius), while at the exit the tin's temperature is slightly cooler—1,115 degrees Fahrenheit (600 degrees Celsius). In the float chamber, the glass doesn't submerge into the tin but floats on top of it, moving through the tank as though on a conveyor belt. The perfectly flat surface of the tin causes the molten glass also to become flat, while the high temperatures clean the glass of impurities. The decreased temperature at the exit of the chamber allows the glass to harden enough to move into the next chamber, a furnace.
•3 After the glass exits from the float chamber, rollers pick it up and feed it into a special furnace called a lehr. (If any solar coatings are desired, they are applied before the glass enters the lehr.) In this furnace, the glass is cooled gradually to about 395 degrees Fahrenheit (200 degrees Celsius); after the glass exits the lehr, it cools to room temperature. It is now very hard and strong and ready to be cut.
Cutting and tempering
•4 The glass is cut into the desired dimensions using a diamond scribe—a tool with sharp metal points containing diamond dust. Diamond is used because it is harder than glass. The scribe marks a cut line into the glass, which is then broken or snapped at this line. This step is usually automated and is monitored by cameras and optoelectronic measuring systems. Next, the cut piece must be bent into shape. The sheet of glass is placed into a form or mold of metal or refractory material. The glass-filled mold is then heated in a furnace to the point where the glass sags to the shape of the mold.
•5 After this shaping step, the glass must be hardened in a heating step called tempering. First, the glass is quickly heated to about 1,565 degrees Fahrenheit (850 degrees Celsius), and then it is blasted with jets of cold air. Called quenching, this process toughens the glass by putting the outer surface into compression and the inside into tension. This allows the windshield, when damaged, to break into many small pieces of glass without sharp edges. The size of the pieces can also be changed by modifying the tempering procedure so that the windshield breaks into larger pieces, allowing good vision until the wind-shield can be replaced.


A finished windshield consists of two glass layers sandwiched around a plastic interlayer. Although very thin—about .25 inch thick—such laminated glass is very strong and is less likely to shatter than normal safety glass. In the United States, windshields are required by law to be made of laminated glass.

Laminating
•6 After the glass is tempered and cleaned, it goes through a laminating process. In this process, two sheets of glass are bonded together with a layer of plastic (the plastic layer goes inside the two glass sheets). The lamination takes place in an autoclave, a special oven that uses both heat and pressure to form a single, strong unit that is resistant to tearing. The plastic interlayer is often tinted to act as an ultraviolet filter. When laminated glass is broken, the broken pieces of glass remain bound to the internal tear-resistant plastic layer, and the broken sheet remains transparent. Thus, visibility remains good. Unlike traditional safety glass, laminated glass can be further processed—cut, drilled, and edge-worked, as necessary. A typical laminated windshield is very thin: each glass layer is approximately .03 inch (.76 millimeter) thick, while the plastic interlayer is approximately .098 inch (2.5 millimeters) thick.
Assembly
•7 After laminating, the windshield is ready to be assembled with plastic moldings so it can be installed on the car. Known as glass encapsulation, this assembly process is usually done at the glass manufacturer. First, the peripheral section of the windshield is set in a predetermined position in a mold cavity. Next, molten plastic is injected into the mold; when it cools, it forms a plastic frame around the glass. The windshield assembly is then shipped to the car manufacturer, where it is installed in an automobile. The installation is done by direct glazing, a process that uses a polyurethane adhesive to bond the windshield and automobile body.
Quality Control
Process control includes testing of raw materials and monitoring such process variables as melting temperature, furnace atmosphere, and glass level. As the glass is formed, photoelectric devices are used to inspect for defects automatically. Other automatic devices have been developed to measure dimensions and radius of curvature after the windshield has been formed.

Safety glass used in windshields must meet certain specifications regarding properties such as chemical durability, impact resistance, and strength. Standards have been developed by the American Society for Testing of Materials (ASTM) for measuring these properties. Specifications have also been developed for windshield performance by SAE International, an organization of automotive engineers.

The Future
Despite the recent downturn in the automotive industry, long-term prospects are more optimistic. Motor vehicle production markets will be stronger than in recent years, raising demand for flat glass products such as windshields. Windshields are also increasing in size in order to accommodate newer aerodynamic designs, and thus the use of glass is increasing relative to the total surface area of vehicles. (In fact, some models are incorporating glass roofs as well.)

Such increase in glass area, in turn, has a negative impact on comfort systems, namely air conditioners, which must be able to adjust the higher interior temperatures to a comfortable level. To avoid having to use larger air conditioning systems, new glass compositions, coated glasses, and aftermarket films are being evaluated. These include angle-selective glazings that reject high-angle sun, and optical switching films that actively or passively change transmittance properties.

One recently developed film, a polymer multilayer solar control film, can also act as a deicing device. The coated plastic substrate simply replaces the laminated plastic film in conventional windshields. The film can be made in any color and can transmit up to 90 percent of the visible light. Another coating is a glaze that consists of silver coating used in combination with other metal oxide layers. This glaze can reject up to 60 percent of the total solar energy, reducing the infrared energy by 56 percent.

In addition, new types of laminated-glass windshields are being researched. A bi-layer windshield has been developed that only requires one outer sheet of glass, .08 to .16 of an inch (2-4 millimeters) thick, joined to a .254 of an inch (1 millimeter) sheet of polyurethane. The polyurethane sheet consists of two layers, one having high absorption properties and the other high surface resistance. Unique features of this bi-layer windshield include ultraviolet resistance, self-healing of scratches, weight savings, more complex shapes, increased safety due to retention of glass splinters, and anti-fog capability.

Recycling of windshield components may also become a standard practice. Though traditionally recycling has been difficult because of the plastic laminated films, one manufacturer has recently developed a cost-effective process to remove these layers. The recycled glass can be used in several applications, including glassphalt for road repair. Legislation may also speed up recycling practices, with the introduction of the Municipal Solid Waste and Hazardous Waste Research Act of 1992. This bill seeks to determine the obstacles to increased automotive components recycling and find ways to overcome these obstacles. This may eventually require using fewer resins during manufacturing or making sure these resins are compatible for recycling.

Where To Learn More
Books
McLellan, George W. and E. B. Shand, eds. Glass Engineering Handbook. 3rd ed., McGraw-Hill, 1984.

Pfaender, Heinz G. and Hubert Schroeder. Schott Guide To Glass. Van Nostrand Reinhold, 1983.

Scholes, Samuel R. Modern Glass Practice. CBI Publishing Company, 1975.

Periodicals
"Bill To Overcome Recycling Obstacles," Autoglass. July/August, 1992.

"Guardian Produces Largest Production Car Windshield," Autoglass, March/April, 1992.

Leventon, William. "Press and Vacuum Form Complex Windshields," Design News. November 9, 1992, p. 159.

Olosky, M. L. and M. J. Watson. "Silicon Film Adhesives: Bonding Automotive Fixtures to Glass," SAE Paper No. 931013. SAE International, 1993.

Peters, G. M. and T. W. Karwan, et al. "A Cost Effective Quality Improvement for Automotive Glass Encapsulation," SAE Paper No. 931012. SAE International, 1993.

Sheppard, L. M. "Automotive Performance Accelerate with Ceramics," Ceramic Bulletin. 1990, pp. 1012-1021.

— L. S. Millberg



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