In My Opinion
All Glass is Not Created Equal
by Monica Mathews and Blair Imbody
Editor’s Note: This column is making its debut this month as a forum for differing opinions. What’s your opinion? E-mail
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All windshields are not created equal. And creating the highest quality automotive windshield starts with a strong attention to detail, manufacturing expertise, substantial OE (original equipment) supply experience, leading technology, quality raw materials and, most importantly, great people to bring it all together.
Float Glass Production
The quality difference starts with the very beginning of part manufacture—the raw materials. Everyone may know that glass is made from sand, but to transform it into high-quality, automotive-grade glass requires significant expertise and technology. Sand, along with several other raw materials, such as soda ash and dolomite, are checked for just the right physical characteristics before they are released to put into the melting furnace. Even tiny contaminants in sand can create seriously flawed glass.
The highest grade of mass-produced automotive glass is manufactured from what is commonly referred to in the industry as the float process. The float process is the standard in the industry and was developed originally by Pilkington. It has been licensed by all the major glass manufacturers throughout the world.
The investment to build a float plant is significant, with costs ranging from $85 million to $135 million dollars. Today’s advancements in technology regarding the float process center around automation and inspection capabilities. For example, Pilkington is developing inspection capabilities that target a hundred million measurements per second to check for extremely small flaws in the glass. Float glass is made in one never-ending ribbon or sheet, typically 12 feet wide, and with the machine running 24 hours a day, seven days a week, for about ten years continuously. As the glass ribbon comes off the end of the float line, it is cut into smaller useable sheets (referred to as block size). These sheets are then sent on to be used in windshield production.
Glass Design Considerations
OE windshields must be produced to meet vehicle manufacturers’ design expectations and to maximize optical quality and fit to the vehicle. One of the OE glass manufacturer’s biggest challenges with windshield production is to maintain excellent optical quality and fit on parts with complex bends. It’s like trying to wrap a plastic sheet around a ball without getting wrinkles. In order to manufacture glass at the highest quality levels, a supplier must have broad experience in working closely with multiple leading vehicle manufacturers. Without this experience, many required glass specifications, tolerances and design issues will not be known.
OE glass suppliers work with the vehicle designers and engineers to minimize the amount of wrinkles in bent glass. Tools such as computer-aided design and simulation are used to predict the final properties of a given design shape. Without ever having to bend that first prototype windshield for a new vehicle design, computer analysis models can provide accurate information regarding the bend consistency, glass fit performance, optical quality and surface contour. This information is used to work with the vehicle designers to develop the best design for performance and ability to manufacture. This process can be used for both laminated windshield and tempered automotive applications.
As mentioned above, in addition to satisfying the vehicle manufacturers’ design expectations, glass for automotive OE applications needs to be nearly flawless. An OE glass supplier’s manufacturing expertise and technology must be capable of achieving these high specifications.
To start, windshields are made of two pieces of glass, an inboard and an outboard ply. These two flat-glass plies are cut to slightly different patterns to ensure that their sizes match precisely after bending. The ceramic paint borders are silk-screened to the flat glass before going into the furnace. The composition of the ceramic paint includes glass particles, known as frit, which are designed to fuse to the glass surface in the furnace. Improved product performance comes with the ability to coordinate the melting temperature of the frit closely with the operating temperature of the furnace. The physical properties of glass change very rapidly over a small temperature range. Between 930ºF and 1150ºF, the viscosity of glass decreases by a factor of 1000, which changes it from a hard, brittle material to one that is formed easily (like warm taffy).
This rapid rate of change means that the heating profile in a glass-bending furnace must be controlled carefully to produce the shape and quality of glass required for today’s vehicles. The major challenge with complex windshields is to produce double curvature (glass bent in two directions—top to bottom and side to side) from a flat sheet of glass without creating wrinkles or optical defects. Without manufacturing expertise and leading technology, glass will wrinkle and buckle just as the plastic sheet does when wrapping it around the ball. The glass used for windshields is heated to enable shaping and then “annealed” (cooled slowly) to ensure very low internal stress for the best stone impact performance. Some compressive stress (a result of bending the glass in the furnace with a mold) is needed around the edge of the windshield to make it strong for handling and fitting.
Laminating And Assembly
Laminated glass consists of a glass/plastic (“polyvinyl butyral” or PVB)/glass sandwich. The widest use of laminated products is for windshields. After the glass is shaped, PVB is placed between the two glass plies in a controlled, clean-room environment. This step ensures contaminant-free windshield assembly and is critical to achieving high quality. The resulting glass/PVB/glass sandwich is subjected to high temperature and pressure (570ºF with pressures above 100 PSI = pounds per square inch) in an autoclave or pressurized oven to manufacture the finished product.
The finished product must meet exacting OE standards. For example, OE tolerances for the glass surface, shape and size allow for only 2 millimeters of deviation from the original design. Windshields are measured to design on a check fixture in both a horizontal position and at the installation angle, which predicts the glass performance during installation. This precise fit between the glass surface and the vehicle body results in installation ease, improved appearance of required mouldings and reduces the potential for stress cracks thereby enhancing overall occupant safety.
Today’s vehicle manufacturers expect the OE glass suppliers to support their design intentions and an OE glass supplier’s technology must meet those expectations head-on. Modern car glazing is a prominent feature in a vehicle and, as mentioned, a key element in design, styling and the overall structural integrity of the vehicle. Automotive glass or “glazings” are now more than ever regarded as transparent body panels. Steep installation angles for greater aerodynamics, thinner glass for vehicle weight reduction, larger window areas, flush glass mountings and shapes that blend into the bodywork are becoming more common. These trends present challenges to OE glass suppliers in terms of achieving complex shapes combined with high levels of optical quality and performance.
Windshields that are manufactured to OE specifications have these types of benefits:
• defect/contaminant-free glass meeting automotive grade requirements;
• leading edge-bending technology for stress-crack reduction;
• optical quality for high driver visibility;
• ceramic paint quality to improve appearance and block ultraviolet (UV) light to ensure long-term adhesive-to-glass bonding;
• a good fit between the glass surface and the vehicle body ensuring quality installations, improving moulding appearance, reducing wind noise levels and enhancing overall occupant safety;
• windshield assembly with consistent PVB-to-glass adhesion for structural integrity and durability; and
• solar control to reduce the heat and UV transmission into the vehicle to improve climate control and passenger comfort.
We are able to meet specifications such as these with a strong attention to detail, manufacturing expertise, cutting-edge technology, quality materials and, of course, great people. Thus, it is very clear that all windshields are not created equal.
Blair Imbody is the director of marketing for Pilkington AGR. He has worked for Pilkington for ten years in a number of global roles in sales, marketing, operations, technical product management and quality. Monica Mathews is the technical support manager for Pilkington AGR. She has worked for Pilkington for 17 years in the automotive group in R & D, engineering, technical support/service and sales.
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