In the early days of automobiles, cars were manufactured with an open-air design. Since they didn’t have windshields, drivers wore goggles to protect their eyes from wind and debris. This was inconvenient, so in 1904 the windshield was invented. The windshield became very important in protecting passengers from outdoor elements, particularly as cars became more powerful and could ravel at faster speeds. The early windshields, unfortunately, were manufactured from glass that shattered upon impact. Since this led to an increase in injuries, an alternative was sought.
Manufacturers began making windshields out of tempered glass. Tempering is a special heat treatment that makes the glass stronger and harder, so that it resists shattering. This was the preferred type of windshield until the middle of the century, when multi-layered laminated glass began to rise in popularity. Laminated glass is strong like tempered glass, but bends slightly upon impact, allowing for additional protection against shattering. Laminated glass is now considered the safest type of auto glass, and is actually required by law in many countries including the United States.
What are windshields made of
The essential components of windshield glass are silica sand (SiO2), soda ash (Na2 CO3), dolomite ((CaMg)(CO3)2), limestone (CaCO3) and cullet. Small quantities of potassium oxide and aluminium oxide are often added.
Silica, comprising 60 to 70 percent of the total, is the key ingredient; it is what makes glass, glass. (Quartz, a glass-like substance occurring in nature, is made of silica.) Soda Ash reduces the melting point of the admixture. Dolomite allows easier working of the melted material while limestone improves the finished product’s durability.
These ingredients are mixed with a small quantity of water and heated at high temperature. The process of chemical fusion results in these ingredients fusing into a new substance – glass.
How Windshield Glass is Made
The materials used to create a windshield have several important properties. Obviously, the glass needs to retain its clarity over a long period of time. It also needs to be constructed of strong materials that will provide structural support to the roof of the vehicle in the event of a rollover collision. The windshield needs to be tough enough that it resists damage during minor collisions, so that the owner of the vehicle does not have to replace it after every minor fender bender. The most important characteristic is that it must be shatterproof, so that shards of glass are not sent flying in a major accident.
A material called laminated safety glass fulfills all of these requirements. Even though to the naked eye a windshield appears to simply be a sheet of regular glass, laminated safety glass is actually composed of three layers. The inner layer is constructed of poly-vinyl butyral, and it is sandwiched on either side by clear tempered glass.
This design serves a very specific purpose. When small objects such as rocks strike the windshield, it may chip or crack the glass. However, because of the inner poly-vinyl butyral layer, the damage is limited to the outside layer of the windshield. Imagine how dangerous driving would be, if every rock or piece of debris flung toward the windshield could shatter the glass. Aside from being dangerous, car owners would constantly spend money to replace such inferior windshields. The current shatterproof, layered design prevents these problems. The glass still needs to be repaired or replaced from time to time, but this innovation provides much safer and durable glass for our vehicles.
Most windshield glass is made using the ‘Float Method’ so named because the glass literally ‘floats’ inside a chamber during one of the manufacturing steps.
In the first step, the ingredients and water are mixed together in a refractory tank where they are subjected to very high temperature and melted. The resulting molten admixture is then passed through a very short but broad opening into a second tank, the ‘float chamber’. This chamber has a thin layer of molten tin upon which the admixture floats, hence the name of the process and the chamber.
From this chamber, the long sheet of molten glass is transported on rollers into another chamber, the lehr, which is a special furnace where the glass is annealed – gradually cooled. This is the ‘raw’ glass that will now be used to make windshields from.
This annealed glass is now cut to the required dimensions using a diamond cutting tool called a ‘scribe’. Next, it is shaped during and tempered though this is hardly the last step.
The glass sheet is placed on or into a mould with the desired curvature and shape and then is heated just to the point where it becomes soft but does not melt and, as a result, sags on or into the mould. Then it is cooled extremely quickly by shooting jets of cold air on it. This tempering process greatly strengthens and hardens the glass.
The last step to make a windshield occurs when two sheets of tempered glass are put on each side of a very thin layer of Poly-Vinyl Butyral (PVB). This sandwich is heated in an autoclave and pressed between rollers. This step makes the PVB transparent and fuses the glass sheets and PVB into a single slab of laminated safety glass. PVB lends anti-shatter protection as, in case of collision, the PVB layer holds back glass shards from travelling at high velocities.
Different sensors, attachments and mouldings may be integrated or fabricated into the glass. These include everything from rain sensors to multi-use antennas to hydrophobic coatings.
The windshield is now ready to be fitted and bonded into the metal frame of the vehicle. Special primers, adhesives and sealants containing polyurethane – commonly known as ‘urethane’ – are used for this purpose.
Car windshield sizes
The size of the windshield will ultimately depend upon the design of the vehicle. During the manufacturing process, sheets of tempered glass are cut with a diamond scribe to fit the car manufacturer’s specifications. The inner poly-vinyl butyral layer is also cut to the same size. After all three pieces are separately cut and molded to the correct shape, they can be assembled to form one cohesive unit of layered glass. This process is completed in an autoclave, a special oven that uses heat and pressure to bond the layers together. The windshield emerges ready to install in the correct size and shape as designated by the car manufacturer.
Since windshield size is dependent upon the car’s design specifications, there are as almost as many windshield sizes as there are different models of cars. For this reason, it is next to impossible to replace a windshield with one from another vehicle that is considered inoperable. For safety reasons, this would not be a good idea anyway. If a windshield needs to be replaced, a new one that fits the vehicle will have to be ordered.
Since the curve of the windshield is also precisely manufactured to design requirements, it is not possible to cut a larger windshield to fit a vehicle. This could result in a windshield that does not have the correct degree of curve and does not fit the vehicle properly. A windshield should only be replaced with the correct size of windshield according to the car manufacturer’s specifications. This is one area in which there is no shortcut repair that can be safely implemented.
Due to design standards, the size of windshields is increasing even as tiny smart cars are invented. The popular aerodynamic designs of the future allow for greater visibility, and hopefully, increased safety on roadways. It is predicted that the size of windshields will continue to increase in relation to the overall size of vehicles.