Overview
Tempered glass is made from normal annealed glass via a thermal tempering process in which the glass is subjected to heat till its softening point and then rapidly cooled. This gives the glass its strength. A fully tempered glass is 4 to 5 times stronger then an annealed glass of similar thickness. A fully tempered glass is regarded as a safety glass and when it breaks it disintegrates into small blunt pieces which greatly reduces the chances of injuries and if there are any then they are superficial in nature.
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A fully tempered glass is used in most modern glass facade, where glass strength is required. A fully tempered glass is recommended for windows that are on high floors or skylights where people are required to stand on top for cleaning. Also areas where risk of thermal breakage or impact breakage is high, fully tempered glass should be used so as to avoid risk of injury. Glass strength is also required in point fixed glazing, bolted and patch fittings. |
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| Benefits
1) Strength: - A fully tempered glass as required in ASTM C 1048 is generally 4 to 5 times stronger than annealed glass and twice as strong as heat strengthened glass of similar thickness, size and type. For a fully tempered glass the minimum surface compression is 10000 psi and for a heat strengthened glass is 4000-7000 psi for 6 mm glass.
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2) Safety - When broken by impact, fully tempered glass immediately disintegrates into relatively small pieces thereby greatly reducing the likelihood of serious cutting or piercing injuries in comparison with ordinary annealed glass. |
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3) Edge Strength: - The fully tempered glass has a high edge strength as compared to normal annealed glass. This gives freedom to designers to use the tempered glass in spider glazing and point fixed glazing. |
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4) Thermal Breakage: - When direct sunlight falls on a pane of glass then the glass surface tends to heat up. This heating is not uniform in nature. The central part that is exposed gets more sunlight and heats up faster while the edges are relatively cooler. This creates temperature difference inside the same pane of glass and when it crosses a certain limit there is a chance of thermal breakage. But a fully tempered glass has significantly higher edge strength to withstand chances of thermal breakage. |
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1) Optical Distortion: - Heat-strengthened or fully –tempered glass that is manufactured in a horizontal tempering furnace may contain slight surface waves caused by contact with the rollers. This waviness or roller distortion can be detected when viewing reflected images from a distance. Orientation of the glass in the furnace is critical in order to minimize the appearance of the roll wave distortion. It is recommended that the roller wave be oriented parallel to the horizontal glass dimension.
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2) Roller Marks: - Roller marks are small whitish hue or marks along the line of glass movement. The only reason for this is poor maintenance inside the furnace chamber. Whenever the glass breaks during the production process the small glass fragments remain inside the furnace on the surface of the rollers. Infrequent cleaning and negligible preventive maintenance are the main reasons for roller marks on the glass. |
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3) Waviness and Bending: - Since heat-strengthened and tempered glass are reheated to their softening points and rapidly cooled, a certain amount of warp and bow is associated with each glass piece due to the resulting stress. Although warp and bow is not generally a significant factor to the design professional, it may appear as distorted reflected images under certain viewing conditions. For instance, it will be more noticeable in reflective glass. However, it is an inherent characteristic of heat-treated glass and is not considered a defect. |
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4) Edge Strength: - Edges of a tempered glass are the weakest point but significantly better as compared to the edges of annealed glass. Edge strength is important to avoid thermal breakages and for point fixed glazing. Quality of edge improves performance by up to 3 times giving glass superior edge strength. As the edges heat up and cool down faster operator skills and operating procedures are key. Grinding is not only about looks but it also serves a very important purpose. |
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5) Fragmentation: - Tempered glass is primarily used for strength and safety. Although fragmentation is a destructive test, it reveals significant information about the tempered glass. The fragmentation should be even and should not have large chunks because a large piece means weaker glass. So it is important to conduct fragmentation for determining the quality of tempering. |
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5) Coating Burns: - Coating Burns are a major cause of concern, during tempering especially in reflective glasses and Low e glasses. As each manufacturer has different variants of coatings, which behaves differently when exposed to heat inside the furnace, some are better and more heat resistant than others. So, if there are high temperature pockets inside the furnace then the probability of coating burns are quite high in those areas. Reflective glass are designed to reflect radiation (heat) and take longer to temper. As a result, many people increase the temperature throughout and in turn inflict damage on the coating. A protective method that can be adopted to reduce the chances of coating-burns which primarily occur on the edges is to remove a small area of the coating by the CNC machines before scoring and cutting coated glass. |
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Manufacturing Process
The tempering can be done using two different types of systems and the same principle. The two different types of furnaces are: -
A) Radiation Furnace
B) Forced Convection Furnace
And they can produce: -
I) Tempered Glass
II) Heat Strengthen Glass
III) Tempered Low e Glass
Steps of Manufacturing
Step I
First and foremost the right glass combination is selected according to the clients' requirement. The choices are limitless, for e.g. the glasses can be Clear, Tinted and/or reflective.
Step II
The glass is then cut to size and then its edges are treated. Holes and cutouts are then drilled onto the glass. Then the glass is washed by deionised water and loaded onto the transportation bay of the furnace. |
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Step III
The glass is then transferred into the heating chamber of the tempering furnace. Here the glass is heated till its softening point.
As soon as the glass temperature reaches its softening point the glass is transported to the next part of the furnace where it is rapidly cool or quenched.
The quenching of the hot glass produces different stress that results in inducing the various stress zones and hence gives the tempered glass its strength. Heat-treated glass has two compression layers or zones, one starting at each surface, plus an interior tension zone centered in the middle of the glass. Each of the two compression zones is approximately 20% of the glass thickness. The middle 60% of the glass thickness is the tension zone. So to break the tempered glass one has to penetrate the compressive zones and hence enable the glass to release its energy and disintegrate into small pieces.
The color, clarity, chemical composition and light transmission characteristics of glass remain unchanged after heat-treating. Likewise, hardness, specific gravity, expansion coefficient, softening point, thermal conductivity, solar transmittance and stiffness remain unchanged. The only physical properties that change are improved flexural and tensile strength and improved resistance to thermal stresses and thermal shock. Under uniform loading, heat-treated glass is stronger than annealed glass of the same size and thickness. Heat-treating glass does not reduce the deflection of the product for any given load.
Step IV
The glass that comes out at the other end of the furnace is either a fully tempered glass or heat strengthened glass depending upon the operating cycle set by the operator in accordance with the client's order.
Heat-Treated Glass is separated into two products, heat-strengthened glass and fully tempered glass, by definition of the degree of residual surface compression or edge compression. Most furnaces can produce both. Its operator must adjust a furnace and its quench for one or the other type of a product “run.” The adjustments may include changes in furnace temperature, exit temperature of the glass, residual time in the furnace, and volume and pressure of the quench air.
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So a heat strengthened glass has nearly the same production process but the strength of both varies significantly. A heat-strengthened glass is not regarded as a safety glass, and the breakage pattern is similar to annealed glass. It doesn’t have the same strength as that of fully tempered glass. Tempered glass is a 4 to 5 times stronger than annealed glass and heat strengthened glass is 2 times stronger than annealed glass.
Low E tempering
There are some glasses that cannot be tempered in normal furnace, like Low e glasses so they have to be tempered in a different type of machine.
The two types of furnace that are used at present to temper the glass are: -
I. Radiation Furnace (conventional machines)
II. Forced convection Machines (newer generation machines)
Radiation Furnace: - They are the first generation machines that are used to temper most of the glasses. The principle is very simple. In this kind of machine the glass is heated directly using through heating elements due to which the heating is slow and as a
result the glass spends more time on the rollers in its soften state and hence has a high degree of optical distortion.
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Forced Convection furnace: - They are the latest generation machines in which the heating is done by a forced convection technique. By this technique the heating is more even and is nearly 25 % faster. As a result the glass spends less time on the rollers in its soften state and has significantly less optical distortion. In this type of furnace the glass is indirectly heated, the elements first heat the plates and then a turbulence of air is created which transfers the heat to the glass in a uniform way. This furnace is designed to temper all kinds of glasses whether it be clear, tinted, reflective or Low e. As a result, the glass that comes out is of relatively superior quality and is tempered in accordance to the standards. |
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| Properties of Tempered Glass
1. Density (approximate): 2.42-2.52 g/cubic cm
2. Tensile Strength: 120 to 200 N/sq .mm
3. Compressive Strength: 1000 N/sq.mm
4. Modulus of Elasticity: 70Gpa-
5. Coefficient of linear expansion: 9 x 10-^6 m/Mk
6. U Value: 5.7 W/sq .m.K for 6mm thick clear
7 SF for 6 mm clear: 81 %
8 Shading coefficient of 6 mm clear: .93
9 Selectivity: 1
10 Visible light transmission of 6 mm clear: 87 % |
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AIS Stronglas Flat Tempered |
Thickness |
Max Size
(mm) |
Min size
(mm) |
Remarks |
4 mm |
2135 x 1525 |
300 x 300 |
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5 mm |
2440 x 1830 |
300 x 300 |
thicker glass recommended for max size |
6 mm |
2440 x 1830 |
300 x 300 |
thicker glass recommended for max size |
8 mm |
2440x 1830 |
300 x 300 |
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10 mm |
2440 x 3660 |
300 x 300 |
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12 mm |
2800 x 4800 |
300 x 300 |
Max size depending upon raw glass stocks |
15 mm |
1830 x 3300 |
300 x 300 |
Max size depending upon raw glass stocks |
19 mm |
1830 x 3300 |
300 x 300 |
Max size depending upon raw glass stocks |
AIS Stronglas Cylindrical Bend Tempered |
Thickness
(mm) |
Max Arc length
(mm) |
Min
Arc
length
(mm) |
Min Radius (r)
(mm) |
Max straight length
(mm) |
Remarks |
5 mm |
1950 |
500 |
800 |
2800 |
Arc length is lesser of quarter circle (2*pi*r/4), or 1950mm |
6 mm |
1950 |
500 |
800 |
2800 |
Arc length is lesser of quarter circle (2*pi*r/4), or 1950mm |
8 mm |
1950 |
500 |
800 |
2800 |
Arc length is lesser of quarter circle (2*pi*r/4), or 1950mm |
10 mm |
1950 |
600 |
1000 |
2800 |
Arc length is lesser of quarter circle (2*pi*r/4), or 1950mm |
12mm |
1950 |
600 |
1000 |
2800 |
Arc length is lesser of quarter circle (2*pi*r/4), or 1950mm |
15 mm |
1950 |
800 |
1500 |
2800 |
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19 mm |
1950 |
800 |
1500 |
2800 |
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