By applying different glass tempering methods, it is possible to change the shape and the structure of glasses. Consequently, tempered glasses can function as partitions and can realize safety functions opposed to glasses that do not undergo any heat treatment.
CE Glass applies 4 kinds of heat treatment methods:
- Tempering – ESG
- Heat Soak Test – HST
- Annealing– TVG
- Curving and bendingof glasses.
At CE Glass, these operations are realized in glass tempering and glass curving furnaces and also in glass laminating machines with the following size limits:
| Glass tempering furnace | ||
|---|---|---|
| Glass tempering furnace | Ming Te 1536 | EZD |
| Bavelloni FC 500 | EZD |
| Glass Curving Machines | ||
|---|---|---|
| Glass Curving Machines | ||
| Szilánk Glass cuving machine | HAJL | |
Table including the sizes for tempering and curving
| Type of working | Thickness mm |
The smallest size possible (mm) | The greatest size possible (mm) | Max weight Kg |
COmment | |||
|---|---|---|---|---|---|---|---|---|
| min | max | |||||||
| Tempering Float | 3 | 19 | 100 | 280 | 2450 | 4800 | 500 | Maximum thickness noted in the tempering table |
| Curving | 3 | 10 | 200 | 300 | 700 | 1200 | 21 | Necessary to discuss the curve |
| Temvpering soft-coated glasses over 6mm | 4 | 10 | 100 | 250 | 2400 | 4800 | 500 | |
| Heat-Soak Teszt | 3 | 12 | 300 | 600 | 2500 | 4500 | 800 | |
The size and the thickness of the glass determine whether the given glass can be tempered or not
Thickness
| Glass | 3 | 4 | 5 | 6 | 8 | 10 | 12 | 15 | 19 | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Min | Max | Min | Max | Min | Max | Min | Max | Min | Max | Min | Max | Min | Max | Min | Max | Min | Max | |
| Float | 100x280 | 700x1500 | 100x250 | 1700x2500 | 100x250 | 2000x3000 | 100x250 | 2450x4800 | 100x250 | 2450x4800 | 100x250 | 2450x4800 | 100x250 | 2450x4800 | 100x250 | 2450x4800 | 100x250 | 2450x4800 |
| Anyagában szinezett | 100x280 | 700x1500 | 100x250 | 1700x2500 | 100x250 | 2000x3000 | 100x250 | 2450x4800 | 100x250 | 2450x4800 | 100x250 | 2450x4800 | X | X | X | X | X | X |
| AGC Stopray család |
X | X | 100x250 | 1500x2500 | X | X | 100x250 | 2450x4800 | 100x250 | 2450x4800 | 100x250 | 2450x4800 | X | X | X | X | X | X |
| AGC Planibel Energiy család |
X | X | 100x250 | 1500x2500 | X | X | 100x250 | 2450x4800 | 100x250 | 2450x4800 | 100x250 | 2450x4800 | X | X | X | X | X | X |
| Planibel K | X | X | 100x250 | 1500x2500 | X | X | 100x250 | 2450x4800 | X | X | X | X | X | X | X | X | X | X |
| Planibel G | X | X | 100x250 | 1500x2500 | X | X | 100x250 | 2450x4800 | X | X | X | X | X | X | X | X | X | X |
| Guardian Solar család |
X | X | 100x250 | 1500x2500 | X | X | 100x250 | 2450x4800 | 100x250 | 2450x4800 | 100x250 | 2450x4800 | X | X | X | X | X | X |
| Guardian HP család |
X | X | 100x250 | 1500x2500 | X | X | 100x250 | 2450x4800 | 100x250 | 2450x4800 | 100x250 | 2450x4800 | X | X | X | X | X | X |
| Guardian HS család |
X | X | 100x250 | 1500x2500 | X | X | 100x250 | 2450x4800 | 100x250 | 2450x4800 | 100x250 | 2450x4800 | X | X | X | X | X | X |
| Guardian PremiumT |
X | X | 100x250 | 1500x2500 | X | X | 100x250 | 2450x4800 | 100x250 | 100x250 | 100x250 | 2450x4800 | X | X | X | X | X | X |
X not a common thickness Annealing only in case of float, 3-12mm
Tempered glass
Flat glass is a quite fragile material.
Its resistance against surface pressure is high, the tensile strength of the raw material is quite low.
Cracks may appear if the flat glass is curved to a very low extent or if an immediate change of temperature (50°C) occurs. These microscopic fractures result in the low tensile strength of the glass (only 1/100 of the tensile strength that should be possible in theory). The reason for this is the base-structure of glass: during its production when the liquid form becomes solid, a crystal structure cannot form inside which could would provide enough rigidity. The molecules themselves are stable, but the bindings between them are weak or do not exist at all.
If we wish to increase the tensile strength, we need to decrease the number and the size of cracks on the surface. This can be achieved by tempering.
Tempering is a kind of heat treatment during which the material is heated up to a given temperature (depending on the thickness of the glass) and then it is immediately cooled down (depending also on the thickness of the glass) with air of a given pressure.
During tempering, due to its heat conduction characteristics, a molecular grid structure develops, at a faster rate near the surface than in the core. The inner stress that forms between the layers creates the balance of tension and compression which is typical of tempered glasses, where the outer layer is under compression, and the inner one is under tension.
The heat treatment changes a number of characteristics of the glass:
- tempered glasses cannot be processed with glass cutters, drills and cannot be milled
- the edges of the tempered glass cannot be processed, except for some individual and costly cases
- tempered glasses are less likely to fracture
- the static indicators of tempered glasses will improve (a 4mm thick tempered is glass is almost equal to a 6mm thick non-tempered glass)
- tempered glasses are more resistant to sudden drastic changes in temperature (above 50°C)
- in case of fracture, the plate breaks into small, blunt pieces. The fracture's look is similar to a spider-web.
- the surface of the tempered glass is more wavy than that of a non-tempered glass'
The conditions of tempering
The processing of the glass must be carried out with utmost care before tempering because of the inner tension evolving in the glass.
- hairline cracks evolve on the edge of the glass during cutting. This makes tempering almost impossible since the glass would burst into pieces during the quenching
- it is indispensable to grind the edges before tempering.
The basic conditions of a glass to be tempered
-Ideally, the diameter of the chip-free holes should at least equal the thickness of the glass. (e.g: in case of a 10mm thick float, the diameter of the hole should be 10mm as well). Depending on the thickness of the glass, a hole smaller by1 or 2mm may be drilled on the glass, but in this case, countersinking must be done from both sides of the glass.
- The distance between the edge of the hole and the edge of the glass should be at least 1.5-2 X the thickness of the glass. If the distance is smaller, the hole must be opened.
-The distance between the holes must be at least twice the thickness of the glass.
-The ratio of the shorter and the longer edge of the glass cannot be greater than the ratio of 1:10. If the ratio is greater, the maximum deflection of 3 mm /m cannot be guaranteed.
- The distance between the corner and the hole must be at least six times the thickness of the glass
-in case of inner cutouts, the size of the inner radius must be at least equal to the thickness of the glass. If the below depicted processing is to be carried out on a 10 mm thick glass, then the inner radius in each case should be 10mm:
The tempering furnace has 4 main units:
-the loading table
- laminating oven/furnace
-quenching unit
-unloading table
The process of tempering
By pushing a button, the glasses roll into the laminating oven or furnace on the loading table. Inside the furnace, the glass lites are heated up to the right temperature in accordance with the save recipes (or programs) based on their glass type and thickness, while they make alternating movement inside the oven. There are upper and lower heating units inside the furnace that can operate independently. The steady heating up of the glasses is provided by ceramic rolls, and the forced air convection inside the furnace.
After the heating phase, the glasses are rolled into the quenching unit where tempering actually happens. During this phase, the glasses make alternating movement on Kevlar-coated rolls.. During the quenching phase the glasses are cooled to room temperature by cooling air of adequate pressure in accordance with the glass thickness, defined in the recipe.
After the quenching phase, the glasses are rolled out to the unloading table.
Quality control of tempered products
- "zebra" test
- overall bow test
- local distortion test
- mechanic tolerance test
- fragmentation test
These tests must be carried out at all times when new glass types or glasses of different thickness are put under tempering. Each of the above mentioned tests are needed in order to get the production started.
„Zebra test”:
The zebra test serves as a continuous check for local distortions during production. Also, it helps changing some parameters of the saved recipes within certain limits.
Local distortions can be measured by measuring instruments. According to the standard, the difference must be smaller than 0.5 mm along a 300 m length of glass surface.
The authorized maximum tolerance for local distortions according to the standard is:
- in case of tempered glasses 0.5 mm /300mm
- in case of annealed glasses 0.3 mm / 300mm
The maximum tolerance for overall bow is 3 mm/ m. In other words, a deflection of 3 mm is authorized in case of tempered and annealed glasses.
Fragmentation test
The fragmentation test is carried out after tempering, by fracturing a plate of 360 x 110 mm, via a point load placed on the glass along the symmetry axis of the longer side, 13 mm from the edge of the glass.
The control area of 50x50 mm must be marked (outside the area highlighted in black in the picture), where the largest pieces (deriving from fracture) can be found. In case of 4-12mm thick glasses, the number of pieces must be at least 40. In case of 3 mm thick glasses, there must be at least 15 pieces, and in case of 15-19mm thick glasses, there must be at least 30 pieces.
Mechanic tolerance test
During this test, the glasses undergo a given load in accordance with the type and the thickness of the glass.
Heat Soak Test
With HST, spontaneous breakage that can occur with tempered glasses can be detected or even prevented. During glass production, invisible nickel-sulfide inclusions may occur.
99% of the glasses with inclusions fracture during tempering. The remaining 1% with latent defects may cause some inconvenience. In the course of time (months, years), these glasses may break spontaneously.
HST is a suitable test to detect latent defects since it operates as an “artificial ageing”. The glass is heated up to 290°C under a given period of time, left on this temperature for a specific period, and then cooled down under a given period of time as well. With this method, the remaining 1% with latent defects can be detected up to 99% by HST, consequently, the chance of spontaneous fracture can be avoided by up to 99.99%, since glasses with latent defects fracture during the test.
Annealed glasses
Annealed glasses are manufactured quite similarly to tempered glasses. The difference is that the quenching phase is longer and thus less intense. The fragmentation of annealed glasses is similar to that of non-tempered glasses, however, their mechanic characteristics are between that of tempered and non-tempered glasses. The main difference compared to tempered glass is that the annealed glass does not break into small pieces which would fall apart, but into pieces which stay together due to the tension evolving between them after the fracture; as a result, the glass preserves its structure. If there is no external effect, the pieces do not fall apart, maintaining the safety of people below the glass. In case of annealed glasses, spontaneous fracture is excluded. As a consequence, there is no need for HST.
Curved glasses
During this process, the edges of the glass are placed on a metal frame and then put in the glass curving furnace. The glass is heated up to a temperature where it softens, becomes viscous and finally, the center of the glass (the part that is not supported by the frame) sinks. It is important to know and to take into consideration that the result will not be geometrically accurate. The outcome may differ aesthetically and geometrically among different glass types and thicknesses, and it may differ even in one single production run. Curved glasses can be installed in insulating structures, but other processing is impossible to carry out.
During this process, the edges of the glass are placed on a metal frame and then put in the glass curving furnace. The glass is heated up to a temperature where it softens, becomes viscous and finally, the center of the glass (the part that is not supported by the frame) sinks. It is important to know and to take into consideration that the result will not be geometrically accurate. The outcome may differ aesthetically and geometrically among different glass types and thicknesses, and it may differ even in one single production run. Curved glasses can be installed in insulating structures, but other processing is impossible to carry out.
