windshield

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Panoramic (wrap-around) windshield on a 1959 Edsel Corsair.

The windshield or windscreen of an aircraft, automobile, bus, motorcycle, or tram is the front window. Modern windshields are generally made of laminated safety glass, a type of treated glass, which consists of two (typically) curved sheets of glass with a plastic layer laminated between them for safety, and are glued into the window frame.

Motorcycle windshields are often made of high-impact acrylic plastic. As the name implies, their main function is to shield the driver from the wind, though they do not do so as totally as those of a car, whereas on sports and racing motorcycles the main function is improving the drag coefficient when the rider assumes the most aerodynamic optimal configuration of his body in unison with the machine and does not shield the rider from wind when sitting upright.

Usage

In daily use, windshields mainly protect the vehicle's occupants from wind, temperature extremes, and flying debris such as dust, insects, and rocks, as well as providing an aerodynamically formed window towards the front. UV Coating may be applied to screen out harmful ultraviolet light.

Safety

Early windshields were made of ordinary window glass, but that could lead to serious injuries in the event of a crash. A series of lawsuits led up to the development of stronger windshields. The most notable example of this is the Pane vs. Ford case of 1917 that decided against Pane in that he was only injured through reckless driving. They were replaced with windshields made of toughened glass and were fitted in the frame using a rubber or neoprene seal. The hardened glass shattered into many mostly harmless fragments when the windshield broke. These windshields, however, could shatter from a simple stone chip. In 1919, Henry Ford solved the problem of flying debris by using a new technology founded in France called glass laminating. Windshields made using this process were actually two layers of glass with a cellulose inner layer. This inner layer held the glass together when it fractured. Between 1919 and 1929, Ford ordered the use of laminated glass on all of his vehicles.^[1]

Split and raked windshield on a 1952 DeSoto. Note the panes of glass are flat.

The modern, glued-in glass contribute to the vehicle's rigidity, but the main force in innovating the windshield has historically been the need to prevent injury from sharp glass fragments. Modern windshields, now almost universally required in all nations, do not fragment, but tend to stay in one piece even if broken, except if pierced locally by a strong force. Properly installed automobile windshields are also essential to safety; along with the roof of the car, they provide protection in the case of a roll-over accident in the vehicle.

Other aspects

In many places, laws restrict the use of heavily tinted glass in vehicle windshields; generally, laws specify the maximum level of tint permitted. Some vehicles have noticeably more tint in the uppermost part of the windshield to block sunglare.

In aircraft windshields, an electric current is applied through a conducting layer of tin(IV) oxide to generate heat to prevent icing. A similar system for automobile windshields, introduced on Ford vehicles as "Quickclear" in Europe ("InstaClear" in North America) in the 1980s, uses very thin heating wires or conductive-film layer embedded between the two laminations.

Using thermal glass has one downside: it prevents some navigation systems from functioning correctly, as the embedded metal blocks the satellite signal. This can be resolved by using an external antenna.

Automobile windshield displaying "spiderweb" cracking typical of laminated safety glass.

How a windshield is made

First, a sheet of glass is scored by an automated plotter. Then a robotic arm completes the cut with a propane torch.

Sanding belts take off the sharp edges in an operation called seaming. In the Manufacturing Processes Reference Guide, this operation is referred to as band filing, which is “a multipoint cutting process in which a workpiece is fed into a continuously moving file, and chips are removed by cutting teeth that are arranged in succession along the file surface.” Silicon carbide sandpaper, used in this process, is a type of file that has an abrasive materials pasted onto primarily cloth backings.

Following the seaming, the glass is then washed in water and then sprayed with talcum powder to prevent glass sheets from sticking to each other. Next, another glass sheet is placed on top of the first sheet. A silkscreen is painted around the glass which will show where the frame for the vehicle will be relative to the windshield.

The samples are inspected visually, and then transferred to rollers to align the two glass sheets with each other. The glass is then put onto four pins, which recede, leaving the glass on a bending iron so it can be formed into a specific shape of windshield. A conveyor takes the sheets to a bending leer, a type of oven that heats the glass and allows the glass to fit the shape of the bending iron. Following that, the glass goes through a slow cooling process to strengthen it.

To laminate the glass, a sheet of milky white vinyl is placed between the two sheets of glass. The windshield then is transferred to a nipper, where rubber rollers press the pieces of glass together, pushing out any air bubbles in the vinyl. This allows the windshield to become transparent once again. The windshield is then put into a pressure cooker where the remaining air bubbles are removed. After that, the windshields are inspected by a human.

A sample windshield is taken and a 2.25 kg steel ball, representing a driver’s head, is dropped from 4 meters above the windshield to test the quality of the windshield. If the ball hits the glass, but doesn’t travel through the glass, the safety test is successful. Often windshields may undergo a stress test using blunt force trauma. Baseball bats, large stones, and PVC pipes are common objects used to stress-test windshields.

The windshields are then examined for stress defects by viewing the windshield through polarized light. Once it has been determined that the windshield is road ready, the windshields can be put into the various vehicles that require them.

Terminology

The term windshield is used generally throughout North America. The term windscreen is the usual term in the UK and Australia/New Zealand for all vehicles. In Japanese English, it is called "front glass". In the USA, windscreen refers to the mesh or foam screen placed over a microphone to minimize wind noise, while a windshield refers to the front window of a car. In the UK, the meaning of these terms is reversed.

Today’s windshields are a safety device just like seat belts and air bags. The installation of the auto glass is done with an automotive grade urethane designed specifically for automobiles. The adhesive creates a molecular bond between the glass and the vehicle. If the adhesive bond fails at any point on the glass it can reduce the effectiveness of the air bag and substantially compromise the structural integrity of the roof. (Raymond Clough)

Brookland aeroscreen on a 1931 Austin Seven Sports.

Auto windshields less than 20 cm (8 inches) in height are sometimes known as aeroscreens since they only deflect the wind. The twin aeroscreen setup (often called Brooklands) was popular among older sports and modern cars in vintage style.

A wiperless windshield is a windshield that uses a mechanism other than wipers to remove snow and rain from the windshield. The concept car Acura TL features a wiperless windshield using a series of jet nozzles in the cowl to blow pressurized air onto the windshield.

Stone chip and crack damage

Many types of stone damage can be successfully repaired. Bullseyes, cracks, starbreaks or a combination of all three, can be repaired without removing the glass, eliminating the risk of leaking or bonding problems sometimes associated with replacement.

Repair

Windshield repair is a process that combines modern technology and skill to fill a damaged area on a windshield with special clear adhesive resin. The resin is then cured with an ultraviolet light. When done properly, the damaged area’s strength is restored, as is most of the clarity.

When repairing a windshield, it is important to start with a clean work area. Any dust, dirt, or contaminants in or on the glass can result in scarring or trapped particles that will permanently be visible in the final repair. Any moisture can cause future cracks when the glass cools or heats. Many chips in automotive safety glass will never grow but insurance companies in the United States often waive the deductible to ensure they do not have to pay for the replacement of the auto glass.

See also

• Driver visibility
• Windshield wiper
• Motorcycle windshield
• BS 857

References

Manufacturing Processes Reference Guide: Robert H. Todd, Dell K. Allen, and Leo Alting How It's Made: Windshields

External links

• Car Windshields A website devoted to windshields, including markings.
• Long crack repair A video of the crack repair process.
• UNECE Reg. 43 Safety glazing material
• BS 857:1967 Specification for safety glass for land transport
• [2] National Glass Association
• [3] Auto Glass Replacement Safety Standards Council
• [4] Auto Safety Expert
• [5] How It's Made: Windshields