How automobile windshield is made - material, making, history, used, components, dimensions, composition, procedure, product
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.
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