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Heat Strengthened Glass

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Heat strengthened Glass is not regarded as a safety glass and when it breaks it resembles close to that of annealed glass and is generally twice in strength as compared to annealed glass of similar thickness or half that of tempered glass. Although the manufacturing process is very similar to that of tempered glass, the cooling cycle is slower as compared to the tempered glass.

Benefits

1) Strength:- Heat Strengthened glass as required in ASTM C 1048 is generally twice as strong as annealed glass of the same thickness, size and type. A Heat Strengthened glass has minimum surface compression is 4000-7000 psi for 6 mm glass.

2) Safety:- When broken by impact, heat Strengthened glass break nearly the same way as annealed glass breaks.

3) Edge Strength:- The heat strengthened glass has a high edge strength as compared to normal annealed glass which gives freedom to the designers to use the heat strengthened glass in spider glazing and point fixed glazing.

4) Thermal Breakage:- When the 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 get more sunlight and heats up faster but the edges are relatively cooler and this creates temperature difference inside the same pane of glass and when it crosses certain limit then there is a chance of thermal breakage. But a Heat Strengthened glass has significantly higher egde strength to withstand and chances of thermal breakage.

Production Process

The tempering can be done using two different types of systems using the same principle. The two different type 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 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 all the kinds of edge treatment, holes, cutout are then drilled onto the glass. Then the glass is washed by deionised water and loaded onto the transportation bay of the furnace.

Step III
The glass is then transferred into the heating chamber of the tempering furnace in which 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 Heat Strengthened glass its strength.

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 strengthen glass depending upon the operating cycle set by the operator in accordance with the clients 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. A furnace and its quench must be adjusted by its operator for one or the other 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.

So a heat strengthen glass has nearly the same production process but the strength of both varies significantly. A heat strengthened glass is not regared as a safety glass, and the breakage patern is similar to annealed glass. It don’t have the same strength as that of fully tempered glass. Tempered glass is 4to5 times stronger than anelalled glass and heat strengthen glass is 2 times stronger than an 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 machines.
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 the 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 have high degree of optical distortion.

Forced Convection furnace:- They are the latest generation machine in which the heating is done through forced converction 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 quiet 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 galss in uniform way. This furnace is designed to temper all kinds of glasses be it clear, tinted, reflective and Low e.And so the glass that comes out is relatively of superior quality and is tempered in accordance to the standards.

Key Quality Areas

1. Optical Distortion – Heat-strengthened 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.

2. Flatness – Heat-strengthened glass products are not as flat as flat as annealed glass due to processes used in manufacturing . For heat–treated glass products, the deviation for flatness is a function of thickness. Width, length and other factors. Usually, increasing thickness yields flatter products.

3. Strain Pattern - For heat-strengthened, a strain pattern or iridescence, which is not normally visible, can become visible under certain light conditions especially polarized sky conditions. This is an inherent characteristics of heat –processed glass and should not be mistaken as discoloration or non uniform tint or color. Strain pattern is a result of air quenching (cooling) of the glass and is not considered a defect.

4. Bow & Warp- Since heat-strengthened are reheated to their softening points and rapidly cooled, a certain amount of wrap 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 instances, it will be more noticeable in reflective glass. And, it is an inherent characteristics of heat-treated glass and is not considered a defect.

7. Scratches – Inspect glass from a distance of 10 ft.(3 m). Scratches up to 3” (76mm) are allowed. Scratches from 3” to 5” (76 mm to 127 mm) are only allowed within 3” (76 mm) from the edge of the glass. Concentrated scratches or abraded areas are not allowed.

8. Fragmentation:- Heat Strengthened glass breaks in similar fashion as an annealed glass breaks so it is not possible to gauge the quality of Heat Strengthened glass by doing a fragmentation test.

9. Spontaneous Breakage:- Heat Treated glass on rare occasion shatters for no apparent reason. The main reason for this are the presence of a microscopic inclusion known as Nickel Sulfide (NiS). NiS in annealed float glass do not pose an treat to the glass because during annealing the NiS gets sufficient time to loose its energy and reach its stable condition. Once the raw glass containing NiS is tempered the NiS stone gains energy and become unstable. So when it is installed at site it get impetus with temperature difference and various pressure to release its energy and break. Many research is done to eliminate the NiS stone from the float tank but till this time it is impossible to remove it from the raw material. Spontaneous breakage don’t depend upon the quality of tempering, if there is a presence of NiS in tempered glass then there is a possibility of breakage.

Typically the NiS induced breakage will be in the first 2-3 years and then the rate drastically reduce. Research reveals that 1 to 2 % of tempered glass panes break due to NiS inclusions. NiS induced fracture reduces by the factor of 20 for heat strengthen glass as the cooling rates is slower allowing NiS particles to return back to their stable state.

Properties & specification

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 %

AIS Stronglas Heat Strengthened
Thickness	Max Size (mm)	Min size (mm)	Remarks
4 mm	2135 x 1525	300 x 300
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
10 mm	2440 x 3660	300 x 300
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

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