Back in the 1800s, automobiles were manufactured without any windshield, forcing their drivers to wear goggles to protect their eyes from outdoor elements, such as debris, insects, and the wind, causing quite a lot of inconvenience. However, the open-air design gave its place to enhanced versions that included a windshield and more powerful engines, in the early 1900s. But, since the early windshields were manufactured from easy-to-break glass, every impact was dangerous, even life-threatening to the car’s passengers. This led automobile manufacturers of the time to seek methods that could make the glass harder, safer, and stronger; glass that could withstand an impact without being shattered that easily. With that in mind, tempered glass was used, which is glass undergoing a special heat treatment to gain the valuable properties mentioned above.
As things evolved, tempered glass was replaced by multi-layered laminated glass (two sheets of glass surrounded by a plastic layer), which bore the same qualities as laminated glass but had an additional benefit to bend upon impact (even just slightly), offering extra protection to drivers and passengers against shattering. To this day, laminated glass is the safest type of glass used to manufacture windshields. In fact, in many countries (USA and Canada included) it is required by law that automobile glass is made of laminated glass. The only difference is that today’s laminated glass is composed of three layers, with its inner layer constructed of about .98 inch thick polyvinyl butyral surrounded by clear tempered glass on each side. This helps any damage caused by small objects striking the windshield (i.e. rocks) to be limited to the outside layer of the windshield. Although glass will still need to be repaired or replaced when such incidents happen, multi-layered laminated glass is the most durable and a much safer glass for vehicles.
Main Product Categories
Glass is manufactured from a wide array of ceramic materials with oxides being their main components. The main categories of this versatile material are:
Flat glass (aka float glass)
Automotive glass, including windshields, belongs to the flat glass category, consuming ¼ of the flat glass production in the country.
What is Auto Glass Made of
The essential components of auto glass are (1) calcium oxide, (2) sodium oxide, (3) and silica, which are derived from limestone, soda ash, and sand and are all heated together to form a glass. In short, limestone is used to improve the chemical durability of the glass and its hardness while soda ash helps lower the melting point of the batch materials. Small quantities of magnesium oxide, potassium oxide, and aluminum oxide are also often added to glass used for windshields.
How is Windshield Glass Made
1. Mixing Materials and Manufacturing Glass
The raw materials (included broken waste glass – aka cullet) are collected and weighed in the appropriate amounts. Then, they are mixed together and fed to a tank so the melting process can begin, using the float glass process. To prevent segregation of the batch’s ingredients, a small amount of water is added at this point.
During the float glass process, the materials are heated until liquefied before they are fed into the float chamber (a large tank 13ft to 26ft long) and a bath of molten tin, whose temperature is about 1,835 degrees Fahrenheit, which helps clean the glass of impurities. Since the tin is perfectly flat, the glass floats on top of it and becomes flat itself.
When the glass exits the chamber, its temperature has dropped to around 1,115 degrees Fahrenheit, which allows it to harden enough for rollers to pick it up and move it into the furnace of the next chamber (aka lehr). It should be noted that if the manufacturer needs to apply solar coatings, they do so before the glass enters the oven. In the lehr, the glass cools gradually to about 400 degrees Fahrenheit and is then left to cool to room temperature.
2. Cutting, Shaping and Tempering Glass
Now it’s time to cut the glass to the desired shape and dimensions using a tool with diamond-dust-containing metal points (aka diamond scribe).
With the help of the harder-than-glass diamond and automated procedures, cut lines are marked into the glass, which is then either snapped or broken along these lines.
Next, the glass is fed to metal molds that are heated in an oven until it takes the shape of the mold.
To harden the glass after the shaping process, it is quickly heated to around 1,500 degrees Fahrenheit and then blasted with cold air. This procedure is also known as tempering.
After that, the glass needs to toughen, which is why it’s inside surface is put into tension while its outside into compression. This process is called quenching and gives the glass the ability to break into small (or larger) pieces, when damaged, that have no sharp edges, allowing the driver of the vehicle to have a good vision until they can get their windshield replaced or repaired.
3. Laminating Glass
After exiting the tempering phase, the glass is cleaned and bonded to another sheet of glass (approx. .03 inch thick each). Inside the two sheets of glass, there is a layer of plastic, which often also acts as a UV filter (it is tinted to achieve that). The bonded glass enters a special furnace called autoclave that uses pressure and heat to allow the separate sheets of glass and the plastic layer to become a unit, with high tearing resistance. This means that when the glass is broken, the pieces of the glass do not scatter or obstruct the driver’s ability to have good visibility of the road. Instead, they remain bound to the inner plastic layer that has become tear resistant through the laminating process, while the broken sheet of glass maintains its transparency.
4. Encapsulating Glass
The windshield assembly process involves the framing of the glass with plastic moldings. To achieve that the outermost part of the windshield is set in a mold cavity that it then injected with plastic which forms a frame around the glass, when it cools. The assembly is ready to be shipped to the car manufacturer, who will use a polyurethane adhesive to install the windshield in a vehicle (glazing process).
There are specifications that safety glass used to windshields should meet regarding strength, impact resistance and other properties. The American Society for Testing of Materials has developed some standards for measuring these properties while there are also specifications developed regarding windshield performance, too.
During each phase, the raw materials are tested and monitored. If there is a defect in any of the processes mentioned above, photoelectric devices that inspect the glass at each phase will detect it. The same applies to checking whether the glass has the right radius of curvature and dimensions after the formation of the windshield.
As people’s requirements change, demanding more and more enhanced driving experiences, new glass compositions come to life, and new ways to manufacture laminated-glass windshields are being explored. For example, we already enjoy the benefits of angle-selective glazings and optical switching films that reject direct sunlight and change transmittance properties respectively. Or the polymer multilayer solar control film, where the plastic film is replaced by a polymer (made in any color) in conventional windshields and can transmit up to 90% of the light. Or a glaze (another coating) that can reduce IR (infrared) energy by almost 60%. Lately, we also see bi-layer windshields developed that consist of one sheet of glass joined with a sheet of polyutherane, which in turns consists of another 2 layers, each with different properties; one with high surface resistance and the other with high absorption attributes. This bi-layer type of windshield also meets the demands of those in need of more complex shapes, and amazes with its self-healing properties once it is scratched, among others. However, it wouldn’t surprise us to see something new and evolved emerging in the next few months or years!