Breakages due to impact or wind loading are less likely to stay in frame. In applications, where tempered glass cannot be avoided due to load considerations or code requirements, it is advised to get it HST. Therefore, when glass is produced in the furnace, it is overwhelmingly likely that any NiS inclusions will be in the alpha-phase. Tempered glass can be made from annealed glass via a thermal tempering process. Spontaneous breakage due to NiS is not due to any manufacturing defect but is an inherent risk or problem associated with use of tempered glass. This happens easily with thicker glasses (8 mm and above). A person must interpret the results of ultrasound and laser imaging, glass by glass and the margin of user error might be quite large when searching for such a small problem. For “trapped” inclusions, this process takes anywhere from months to years. Glass is fragile and can break for a variety of reasons. Maintaining an elevated temperature facilitates faster conversion of any particle of NiS, if present, from alpha-phase to beta-phase, and therefore the idea is that any panel, which has a possibility of failure from NiS, should fail in the HST oven rather than on the building. It is then heated in a furnace to a temperature of 600-700 degrees C. This temperature is held until the glass softens slightly, at which time it is rapidly cooled through the use of air jets, a process called quenching. JNS can heat temper glass as small as 2” x 2” and as large as 60” x 120”. On the other hand, if a glass container is placed on a very hot source of heat (500°C for example), it can gradually lose its shape and change from a permanent solid form to a plastic state. When it does break, tempered/toughened glass is designed to shatter into small pieces that are much less likely to … Heat-strengthened glass differs from tempered glass in surface compression; its mechanical strength is about 1.6-2 times that of annealed glass. Edge quality is critical. Tempered glass is also much more durable than ordinary, annealed glass. In the fast cooling process used in tempered glass, and also heat strengthened glass, there is insufficient time to complete the phase transition of NiS from alpha to beta. Required fields are marked *, C-55, Okhla Industrial Area, Phase-I,New Delhi - 110020 (India), Copyright © 2020 McCoy Mart, all rights reserved. The phenomenon was first acknowledged in 1940 but the first documentation happened in 1961. In the end, heat-strengthened glass is approximately twice as strong as annealed, or untreated, glass. The magnitude of this stress drops off sharply away from the face of the inclusion, but is sufficient at the face to cause micro-cracking. Thermal stress occurs when one area of a panel of glass becomes hotter than the other, such as the center becoming hotter than an edge. Your email address will not be published. The glass may be able to resist 200kg of evenly distributed load, but that is very different from a rock. Heat-treatment can generate subtle roller waves in glass, which are more likely to occur in tempered glass than in heat-strengthened glass. This would have no effect on glass whatsoever were it not for the fact that when the NiS changes from alpha-phase to beta-phase, it increases in volume by 2-4 per cent. Glass (with nickel sulphide inclusions) is tempered by heating and then rapidly cooled to room temperature. Float Glass or annealed flat glass is the most basic form of glass. There are quite a few which have experienced an occasional failure, but there are some examples, which have repeated failures. Under this condition, lateral loads may increase the tensile stresses at the center of the glass. Non-tempered glasses do also break and sometimes, without any visible reason. In compression zones, even this large of a stress is not a concern due to its extreme localization. Both of these processes have specification requirements, tolerances, and testing procedures which are defined by ASTM International document C-1048-04. Hot temperatures can cause the glass to break when the bottle is subject to excessive thermal variations. (See image 4 and 5). Alternately one should be prepared for such occurrences and for replacing NiS broken glasses. There are many buildings with no report of NiS breakages. The most logical solution to the problem is by avoiding the use of tempered glass where there are other options like. A large glass furnace can easily produce from 500 to 800 tons of glass each day. Tempered glass is about four times stronger than "ordinary," or annealed, glass. Tempered glass with higher stresses, will have higher probability of NiS breakage as compared to tempered glass with lower stresses, just as a more pressurized balloon will need lesser provocative force to trigger its bursting. The edges are ground and any holes or cutouts required in the panels are created. The origin of breakage is generally away from the edge of glass. as 1 critical occurrence in 13 tons of glass. Spontaneous breakage in tempered glass is however much more dramatic, as the whole pane of tempered glass breaks with a loud noise and high visibility as it breaks into thousands of small pieces. These windows are sought after by buyers, adding value to your property. Inclusions larger than this are typically referred to as “critical” inclusions, whereas smaller inclusions are classified as “sub-critical” inclusions. If it is allowed to cool rapidly or in uneven manner, it will result into cracking while manufacturing or in service due to uneven stresses. On closer examination, at the origin, there will be at least two fragments in the shape of wings of the butterfly. Heat-strengthened glass has outstanding thermal stability, its flatness and light transmission is close to that of annealed glass and much better than that of tempered glass. “Heat-strengthened glass and tempered glass are very similar,” Baier says. Glass may break at low temperatures, but this is because the contents freeze and their expansion cause the glass to crack (if the cap does not come off). This expansion creates localized tensile stresses that are estimated to be as high as 125,000 psi (860 MPa) at the glass-NiS Interaction surface. Another aspect of nickel sulfide failure is the fact that these failures rarely occur upon installation or even within the first few months following installation. It can happen in an instant; tiny pieces of glass can come raining down. If the glass experiences enough stress, it will crack.6 Jul 2018 At what […] At a processing plant, large sheets of annealed glass are cut to the appropriate sizes and shapes. Ordinary glass breaks at about 6,000 psi (pounds per square inch) while tempered glass generally breaks at 24,000 psi. Spontaneous breakage in tempered glass is however much more dramatic, as the whole pane of tempered glass breaks with a loud noise and high visibility as it breaks into thousands of small pieces. Standards have evolved and many extensive publications have been made on the subject. Thermal stresses [ edit ] Breakage due to thermal stress is most common in large pieces of sealed insulating glass with heavy heat-absorbing (reflective) coatings. The breakage pattern will be highly non-uniform with some large islands of non-fragmented glass, enclosed or surrounded by much smaller fragments. Benefits of Heat Tempering vs. Heat Strengthening of Glass Download this information as a pdf There are two types of heat-treated glass, Heat-Strengthened and Fully Heat Tempered. However, a summary of preventative measures already in use or development in the industry is discussed here. It is the breakages caused by Nickel sulfide inclusions. But it can be safely said that appropriately done HST, would eliminate 95 to 98.5 per cent of such glasses as could have caused a spontaneous breakage post installation. Comparatively better than tempered glass. Annealed Laminated glass will have no occurrence of NiS breakage. First, the nickel separates from the other materials in the alloy, then it bonds with sulfur in the high heat of the melting furnace, and finally is trapped in the glass as the glass cools to its sheet form. The inclusions therefore are trapped in the glass in their high temperature alpha-phase. Responsible float manufacturers are supposed to take every precaution to control this but it is very difficult to prevent it altogether and in spite of best efforts, small amounts of nickel will find its way through inclusions in raw materials itself or their handling/ storage equipment’s etc. There are costs associated to HST which are the processing costs, cost of glass breakages due to NiS in HST oven, and damage by broken glass to the neighboring panels in HST oven, potentially even causing them to break as well and propagating the problem. There are approximately 50 different types of dirt or other inclusions recognized, but almost all of them are completely harmless. Defective tempered glass. Hardness is huge. Second, the rock may have a hardness similar to the glass. How to choose the best glass for windows? While many people believe that the thermal expansion of the glass caused by the hot sun can cause it to crack and then shatter, it is nearly impossible for it … The relationship between stress and diameter of NiS inclusion has been scientifically proven and field studies have also examples of such small inclusions causing the spontaneous breakages. This puts stress on the glass (hence the name “stress crack”). Another issue is that the secondary heating of tempered glass relaxes surface compression slightly without a corresponding decrease in core tension, which reduces the strength of the glass, though marginally. Many other studies have come out with different numbers and though the numbers do not match, it seems clear that failures due to NiS inclusions are incredibly rare. This is made of five basic ingredients namely silica sand, soda ash, dolomite, limestone, and salt cake. The success rate of HST is hard to define because of the difficulty of accurate data collection on resulting nickel sulfide failures, which are very rare occurrences. Breakages are Easily Cleared Up. These are also known as "stones", and can also break the glass when the glass is heated, as they create thermal anomalies. Heat strengthened laminated glass is the best option for most applications, except for point fixed glazing. What Are The Different Types of Fasteners and Where to Use? These are heated in huge furnaces to 1400-1500 degrees C, and then made to float out on a pool of molten tin in controlled atmosphere. Breakages are less likely with tempered glass. Inclusions are microscopic particles that are incorporated into the structure of the glass in the initial melting process of silica sand and other ingredients. High-stress tension breaks share one characteristic with low-stress tension breaks: The break origin is not 90 degrees to the edge of the glass, suggesting a tension break caused by bending. So far, no solution has adequately fulfilled all three criteria, and thus the industry is still searching. Temperature is not the only factor in determining why glass breaks. end up in the center tension zone of tempered glass making it prone to breakage. A tempered laminated glass will almost always stay in place after breakage and makes it easier to identify and analyze the cause. You can limit the heating power in the beginning of the heating cycle, and/or you can just drop the temperatures down to make sure you don’t create such a strong heat shock. It is then left to the architect and building owner to assess the risks against the increased price of the glass. It might seem the simplest solution to nip the problem at its origin by ensuring that there is no nickel or its alloys entering the glass-melting furnace. How you handle and repair glass depends on first identifying the type of glass and diagnosing the reason for the breakage. As well as thermal stresses within the glass, damage during installation – such as a chipped edge developing into a larger break – is the most common cause of breakage. But, of course, it isn’t indestructible. 5. Research has created a theoretical equation that predicts the smallest diameter of a NiS inclusion that would cause failure as 50 microns (0.05mm) in diameter. Looking at the broken glass will give an initial impression of glass being hit by a sharp object and the point of origin of breakage will be obvious. At temperatures below 3800C, nickel sulfide is stable in the beta-phase form. Float glass can contain these invisible imperfections that increase in volume over time and cause stress to fully tempered glass. In order to be a successful solution, the method must be cost-effective to implement, eliminate the costs of replacement of panels upon breakage, and prevent any injury to bystanders in a failure. JNS is geographically positioned to support customers’ tempered glass needs throughout the entire United States. This was more evident in the 8 hours soaking specified in DIN than in the 2 hours soaking as per EN. Once the glass cools past the phase change temperature, the NiS inclusion seeks to re-enter its lower energy beta-phase. Pella offers several glass options, ranging from tinted insulating glass and tempered insulating glass to impact-resistant insulating glass and laminated insulating glass. In spite of all the issues with the process, it is still the only method the industry has, to eliminate a large portion of nickel sulfide inclusions in batches that are compatible with large-volume production. Major strides have been made in reducing the contamination of raw materials, and great care is taken to avoid the contact between the raw materials and any nickel-containing alloys but it is practically impossible to prevent some microscopic quantities finding the way into the melt, from other sources of Nickel. Buildings that have seen multiple instances of NiS failures often have huge expanses of glass, which automatically increases the odds of such a failure occurring. Its mechanical strength is twice that of normal annealed glass and half of fully tempered glass. When that piece of tempered glass is later exposed to varying temperatures in its final installed position, this tiny stone – which can measure from 0.003 to 0.015 of an inch in diameter – may grow in size, and cause the glass to shatter for no apparent reason. This standard requires glass to be heated to 290±10 C and held at this temperature for 2 hours, which is a shorter duration than 8 hours for DIN 18516. Tempered glass is a type of glass that, unlike standard glass, shatters into small, primarily blunt, pieces when it breaks. It has been shown that when glass undergoes deflections that are in excess of 75 per cent of the panel thickness, the stresses in the glass due to lateral loads change from a bending stress profile to a membrane stress profile. On the other hand, if a glass container is placed on a very hot source of heat (500°C for example), it can gradually lose its shape and change from a permanent solid form to a plastic state. The process was introduced in 1982 and the first standard that evolved was DIN 18516. One study puts the number of NiS occurring as 1 in 500 glass panes. To understand this phenomenon of spontaneous breakages due to nickel sulfide, we need to briefly understand the manufacturing process of float glass and its heat treatment namely tempering, heat strengthening and heat soak testing. The difference between heat-strengthened and tempered glass is the speed at which they are cooled, which results in different surface compressions and therefore different overall glass strengths and properties, which are compared as under: Some microscopic imperfections, known as inclusions, are inherent in the glass production process. There are two specific phases of NiS, known as the alpha-phase and the beta-phase. Another study by a prominent Indian float manufacturer puts the no. The entire community of designers, specifiers, manufacturers, fabricators and glaziers etc are now broadly aware of the problem. The reduction in time was based on recommendations from research, which indicated that less than 1 break in 10,000 panes of glass was expected to occur after 2 hours. This type of crack—called a stress crack—can happen when a swing in temperatures causes windshield glass to expand (when heated) and shrink (when cooled) rapidly. This can also have a positive impact on the sale price of your house. Any non- tempered glass will normally break as a single crack or multiple cracks, which develop or propagate from origin either spontaneously or gently. If you heat up a piece of glass very, very slowly, you can bring it to an extremely high temperature without any ill effects. Consequently, when tempered glass is broken, it shatters into thousands of tiny pebbles—this practically eliminates the danger of human injury caused by sharp edges and flying shards. None of the temperatures generally used in daily life cause glass to break. For More Details Please See Our Other Articles: © Copyright 2020 Berlin Packaging All Rights Reserved. Most of us have come across a situation where a tempered (toughened) glass has broken without a provocation or any apparent reason. The large sheets are pulled onto a conveyor, and taken through an annealing tunnel called lehr, where it is cooled very slowly for an evenly controlled heat dissipation rate. As a result, tempered glass is less likely to experience a thermal break. By simply looking at the point of origin of breakage and its pattern, the cause can be identified in most cases. Quenching reduces the temperature of the surfaces of the glass quickly and significantly, but due to the low thermal conductivity of glass, the core of the panel remains at a much higher temperature. Because heat soaking can represent a significant cost, when presented with a proposal request for tempered glass, many glass manufacturers break out the additional cost impact to heat soak the glass. It has been observed that the incidences of NiS breakages are more in thicker tempered glasses (8mm-19mm) as compared to thinner glasses (4mm-6mm). There is a need for float manufacturers to further strengthen the quality procedures to include necessary steps for its prevention and to subject their quality procedures to internal and external quality audits. Insights / Packaging Resources / Packaging Fundamentals / Glass Containers FAQs, By: Berlin Packaging SpecialistDate: January 26, 2020. This temperature change causes the center of the glass to expand, placing stress on the glass and causing breaks or cracks. Defective tempered glass is products that often have damages already. There are other methods such as laser-imaging and ultra-sound which are non destructive but these methods will need a scan of each panel of glass which will show up all inclusions and not only NiS and thereafter, it will separately need an assessment of each inclusion in each panel to decide which ones are fatal types of NiS and located in the risk zone. Heat-strengthened glass is not as strong as tempered glass because the compression strength is lower—about 24,130 to 51,710 kPa (3500 to 7500 psi) compared to 68,950 kPa (10,000 psi) or greater. This will happen only if the stress differences on the same pane of glass are very large and this can be easily identified from such breakage. (see image-2) This is often called ‘butterfly pattern’ or ‘double D Pattern’. With heat-strengthened glass, the cooling process is slower, which means the compression strength is lower. Heat strengthened glass has substantially lower stresses than tempered glass and therefore have very little possibility of NiS breakages. And when toughened glass breaks, it does it in … Tempered glass is made using a heat process (or in some cases a chemical process) that creates tension and compression in the glass. Heat Resistant Glass. For glass that is too thin to be heat tempered, JNS also has the ability to chemically strengthen glass. In such cases, the point of origin of breakage will generally give a clue. The Guardian heat-soaking process exposes NiS inclusions in fully tempered glass, then breaks them safely in a heat-soak chamber before the glass reaches the field. In typical annealed glass, the slow cooling process provided by the annealing lehr, allows the NiS, ample time to transform to its beta-phase as the glass cools. Heat Tempered Glass Therefore this is an issue which designers should be aware of, and take into account when deciding the use of tempered glass in their building and adopt smart specifications. Most of the times, a tempered glass will disintegrate and fall out of the fixing, but the cause of breakage can be assessed with reasonable fairness, if it stays in fixing. Use heat-strengthened glass instead of tempered glass where technically feasible. There have been a number of case studies on manufacturing processes and on glazed building as well. Assuming that the nickel enters in the form of a nickel-alloy metal (like stainless steel), which is the most commonly accepted explanation for its origin, nickel sulfide forms in a three-step process. Heat strengthened glass is a semi-tempered glass which has all properties same as annealed glass or float glass, except strength. Any of these can result in creating a concentrated point load on the edge or corner of the glass. Any non-tempered glass will normally break as a single crack or multiple cracks, which develop or propagate from origin either spontaneously or gently. 99% On Time Delivery - Why It's Important, 99% On-Time Delivery - Frequently Asked Questions, 99% On-Time Delivery - Why We Perform Well. B ased on their heat resistant qualities, there are two types of glass commonly used for a fireplace or wood stove: tempered glass and ceramic glass.. Tempered glass is the less expensive alternative, and is best for lower temperature applications. NiS breakage occurrences are not very significant if we consider the total volume of tempered glass produced globally or nationally, and the volume of glass broken due to NiS. It has not been possible to put exact numbers on frequency and timeline since the results of various studies are all different in units. Above this temperature, it is stable in the alpha-phase. It is important to note that sub-critical inclusions of size substantially lower than 0.05mm (say 0.02mm) are generally not capable of creating enough localized tensile stresses, to be able to break the glass, but these can cause a failure if the glass is placed under additional tensile stress due to bending or thermal loading. Looking for more information on glass containers for your packaging needs? There is a reasonable consensus in the industry as to how NiS is formed, from the compounds that are initially introduced into the glass. The exercise is prohibitively expensive and not feasible on production scales. For example, a tempered screen protector can handle low drops. Diagram showing heat-strengthened glass Also called toughened glass, tempered glass has been heat treated for a minimum surface compression of 10,000 PSI and impact resistance four times stronger than annealed glass. 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