Anisotropy Scanning, Other Equipment Can Enhance Glass Fabricators’ Quality Control Process
Glass is a dominant feature in many beautiful, vibrant and sustainable buildings, showcasing an ever-widening design palette while driving aesthetics as well as energy performance. As a result, today’s glass fabricators are being asked to help produce increasingly complex window systems, such as custom-engineered curtain walls; highly customized faceted window profiles with all-glass outside corners; and building envelopes comprised of hundreds of thousands of square-feet of custom curtain wall.
As the scope and intricacy of building projects increase, the risk of something going wrong intensifies, too. Installed glass that does not meet expectations or exacting quality standards and ultimately requires replacement is a potentially major issue that carries huge financial implications for the entire glass supply chain.
Given the financial stakes and the inherent visibility of these structures, it is vital for glass fabricators to analyze their quality control processes and close any open gaps.
Due to its complexity and multitude of variables, the glass fabrication process is susceptible to a variety of conditions that can mar the quality of the final product. Fabricators must be vigilant in monitoring issues such as flatness and roller wave distortion, visual defects, non-uniform color appearance and installed aesthetics.
Most of these problems are detected in the plant either by visual inspection or by calibrated quality-control equipment. The goal is to catch glass defects sooner, prior to shipment to the customer or right at the time the imperfection occurs.
Unfortunately, even with rigorous monitoring, quality and glass distortion problems can go undetected until the finished product is installed on a building. When these problems arise so late in the process, glass fabricators and their customers are left to confront two uncomfortable questions: First, how did this happen? And, more to the point, how can problems like this be prevented?
What Are Those Lines?
One of the more vexing and difficult-to-detect issues associated with glass aesthetics is called anisotropy, more commonly known as “strain pattern.”
In layman’s terms, anisotropy is the phenomenon of specific geometric patterns and colorful areas appearing in heat-treated glass under certain light and viewing conditions—for instance, the pattern of white dots that a driver may see when looking through another car’s rear window through polarized sunglasses.
Anisotropy is accentuated when the glass is viewed with polarized light, such as in the presence of a sunny blue sky (clouds and haze are less polarized) or on reflective surfaces, such as adjacent glazed buildings, that will convert un-polarized light into polarized light by reflection.
The Causes of Anisotropy
Originating in the heat-treatment process, anisotropy results from subtle stress differences produced by either uneven heating or cooling during the overall heat treatment process. When glass enters the quench section of the furnace, it is rapidly cooled by a series of air nozzles. As the glass passes underneath the nozzles, the cold air causes subtle stress patterns in its surface that are not typically visible, but which cause light to reflect at different angles.
Sections of glass positioned directly in line with a nozzle are induced to develop a slightly different level of stress than those positioned between the nozzles. These resulting stress differences produce slight variations in the density of the processed glass. The condition is more evident in tempered glass because the quick-cooling cycle is more likely to produce a slightly greater stress differential.
Any non-uniformity in quenching, such as from the rollers causing cooler glass temperatures from conduction or the quench nozzles blowing air inconsistently, can produce more visible anisotropy. If the tempering process is not optimized for uniform cooling, it can negatively affect the amount of visible strain pattern (or anisotropy). Conversely, if the glass is not uniformly heated through its thickness and across its surface when it enters the quench, a similar stress differential can be generated even if there is uniform cooling.
Anisotropy caused by stress differences in tempered glass.
ASTM C1048, Section 7.4 states that anisotropy is inherent in all heat-treated glass and should not be mistaken as discoloration, non-uniform tint or color, or a defect in the glass. In addition, anisotropy pattern does not affect any physical properties or performance values of the glass.
Anisotropy can always be seen with a polarization filter (sunglasses), but sometimes can be visible with the naked eye, which confirms the occurrence of light polarization somewhere in the environment. Therefore, the final installation location of the finished glass has a significant impact on the visibility of anisotropy.
New Technology Aids in Detecting Anisotropy
As previously mentioned, anisotropy is not technically a defect. From a fabricator’s perspective, the condition can arguably be called an “acceptable imperfection.” Despite ASTM standards stating that anisotropy is not a defect (but an inherent by-product of heat treatment processes), building owners and architects generally expect anisotropy-free glass.
One rudimentary method that has been employed to check for the presence of anisotropy is a light box with polarized film that can be used to periodically inspect glass offline. Unfortunately, these occasional spot-checks lack the necessary rigor and accuracy required to consistently detect anisotropy.
There is good news, however. Recent breakthroughs in anisotropy-visualization equipment are enabling glass fabricators to detect and quantify the amount of polarized light in tempered and heat-strengthened glass that goes through the furnace.
Anisotropy Scanners/Visualizers
Today, sophisticated anisotropy scanners, featuring robust analyzing software, can be installed to analyze the surface of heat treated glass as it exits the furnace; calculate the percentage of non-critical anisotropy; then document the results—all in real time with no production delays.
Such equipment precisely calculates measures such as optical retardation and overall anisotropy quality and determines whether the presence of anisotropy is within internal tolerances. Since there are currently no glass industry standards for anisotropy, these tolerances are usually established by the glass fabricator.
To calculate these measurements, anisotropy scanners shine light through heat-treated glass, displaying the reflectivity of its various stress points. The calibration also shows true angles of reflection. For instance, a pristine piece of annealed glass (no visible anisotropy) reflects at a 30-degree angle. As the glass exits the furnace, the entire surface should theoretically have the same 30-degree angle of reflectivity. However, because it is virtually impossible to heat-treat glass without producing some deviation in the angle of reflection, the scanner measures the number of nanometers from which that original angle diverges.
A deviation in the angle of reflection of only a few nanometers likely isn’t detectable; but a change of 50 nanometers or higher may be visible to the naked eye depending on the conditions. When glass fabricators use anisotropy scanner systems, they may choose to measure this as the threshold against which they accept or reject glass. (Glass fabricators can ultimately prescribe their own acceptable tolerances for these measurements.)
Interpreting the Measurement
Despite the sophistication and accuracy of anisotropy scanning equipment, the acceptability of anisotropy/strain pattern on a panel of glass can still be a subjective decision.
One example of this can be percentage of glass that has visible effects of anisotropy strain pattern. If 20 percent of a glass surface is affected, the acceptability of the product may further be determined according to its location (i.e., a stripe down the middle of a panel might be rejected while a pattern in the edges or corners of a panel where it is not highly visible may be acceptable).
The numerous analytics and images that the anisotropy scanners provide enable fabricators to make more accurate and calculated judgments about which glass is acceptable.
An “Insurance Policy”
While there are many variables that can affect the level of anisotropy in heat-treated glass, there are other quality control procedures fabricators can use to review their processes and ensure that they are producing high-quality products and maintaining or improving yields.
One method is to examine the break pattern during a temper quality check. If the pattern is not uniform—that is, the sizes of particles in a given break area are different than the rest of the glass—this reveals an imbalance in the tempering process. The bottom line is that heating or cooling was not uniform through the thickness or across the surface of the glass.
Normally, if glass is uniformly heated and cooled, its break pattern will appear uniform. Conversely, the presence of a pattern of larger break particles in a given area of glass would normally indicate less heating or cooling (or vice versa), while a pattern of smaller break particles in the same area would indicate more heating or cooling.
Without recognizing any type of heating or cooling imbalance, glass fabricators might accept glass which, despite meeting break-pattern and internal roller wave distortion requirements, may have very visible anisotropy/strain pattern when installed on a project.
Anisotropy scanners are but one important tool in a fine-tuned, robust quality-control process. In addition to using an anisotropy scanner, Vitro Architectural Glass recommends:
- An online automated flatness scanner to automatically monitor 100 percent of the glass and produce flatness data
- A thermal exit scanning pyrometer to evaluate the glass exit temperature when it exits the furnace to ensure uniform heating throughout the glass
- A color-monitoring process to ensure that the final product being installed has uniform color within any one piece of glass and across all glass
- An online automated visual defect scanner to automatically monitor 100 percent of the glass to ensure it meets the visual defect standards
Although these four monitoring processes are becoming more standard in glass fabricators’ operations, the addition of an anisotropy scanner can provide an “insurance policy” against field issues, and serve as an effective tool for pinpointing gaps or flaws in their overall fabrication processes.
Conclusion
As building designs become increasingly elaborate, architects become more astute, and glazing is subject to higher levels of scrutiny, the issue of anisotropy/strain pattern can no longer be minimized.
Glass fabricators must determine to what extent their equipment produces this condition and learn the many variables that potentially contribute to it. Fortunately, many fabricators are investing in equipment to ensure that their glass is flat, has acceptable distortion, meets visual defect standards and has a uniform color appearance.
Momentum within the industry to more effectively measure and manage anisotropy is growing. In July 2018, Louis Moreau, head of technology and innovation at AGNORA, a Canadian fabricator of oversized architectural glass, and Rick Wright, director of technical services at Oldcastle BuildingEnvelope®, a North American supplier of related products and services, assembled an international group of stakeholders to develop a new ASTM standard test method for anisotropy measurement of glass.
This effort, along with ongoing research and development, has led to progressive solutions such as anisotropy scanners that enable fabricators to identify and control anisotropy before glass installation and thereby alleviate the huge financial consequences and time-consuming burden of replacing glass that has already been installed.
Anisotropy/strain pattern continues to be a challenging issue in the architectural glass industry, but fortunately more-robust measuring tools, increased knowledge, and new production technologies are helping to address this phenomenon.
Steve Marino is a manager of technical services at Vitro Architectural Glass (formerly PPG Flat Glass).
