for soda‐lime silicate). In the second step, the more viscous molten glass cooled down to a temperature at which the viscosity is 103−106.65 Pa·s (i.e. at 1050–700 °C for soda‐lime silicate) is stretched in the longitudinal direction, while minimizing simultaneous narrowing of the glass ribbon.
In both glass delivering and stretching systems, properties of the molten glass such as surface tension, gravitational force, and tensile stresses are critical factors, whereas the kinetic aspects of the process are tightly controlled by means of drawing chambers, débiteuses, draw bars, rolls, float baths, slots, fusion pipes, etc. Besides, heat management through variously devised heaters and coolers is another fundamental aspect because glass properties vary very strongly with temperature. In addition to physical effects, one must also take into account chemical factors such as possible devitrification and chemical reactions between the glass and other materials, which may themselves depend on glass composition.
Since annealing follows forming, the conditions of the former process are greatly influenced by those of the latter. The formed glass ribbon is cooled down and conveyed to an annealing lehr where the decreasing glass temperature is carefully controlled so that residual stresses caused by viscoelasticity are relaxed between the annealing and strain points (1012 Pa·s, 570 °C, and 1013.6 Pa·s, 530 °C, respectively, in soda‐lime silicate), and breakage caused by thermal stresses is minimized upon cooling down to room temperature. Finally, optical devices are installed at the downstream part of production line to detect in the glass at room temperature any visible defects such as bubbles, stones, streaks, etc., which originate either from the melting tank or the forming process. These defects are at once clearly marked on the glass ribbon and their positions are usually electronically recorded for optimized cutting either at the end of the production line or by the user according to its specific application. For flat glass to be used in buildings, any 10 m2‐sheet must, for instance, have fewer than three defects with a maximum size of 1 mm (which would be tantamount to finding fewer than three coins on a football field!).
Table 1 Comparison of forming processes.
| Category | Process | Mechanism | Advantage | Disadvantage | Current situation | |
|---|---|---|---|---|---|---|
| Step 1: Preliminary forming (Molten glass delivering) | Step 2: Main forming (Stretching) | |||||
| Updraw process | Fourcault process | Glass flow toward débiteuse and upward flow through débiteuse slot | Upward drawing against gravity by pairs of rolls | Earliest continuous production Smaller investment | Quality (Draw lines) Cyclic operation | Almost obsolete for commodity applications Customized/modified process operating for specialty glass |
| Colburn process | Glass flow toward drawing point and upward flow from free surface | Upward drawing against gravity by pair of knurled rolls and bending roll | Higher output Wide range of thickness | Low surface quality Complex operation | ||
| Pittsburg Pennvernon process | Glass flow around draw bar and upward flow above draw bar | Upward drawing against gravity by pairs of rolls | Better surface quality Longer cycle | Distortion Thickness deviation | ||
| Asahi process | Glass flow toward Asahi blocks and upward flow through gap between Asahi blocks | Upward drawing against gravity by pairs of rolls | Smaller investment Longer cycle | Cyclic operation | ||
| Roll out process | Continuous double roll process | Horizontal glass flow through forehearth | Pressing by pair of rolls | Value added with patterns and wires Versatility Smaller investment | Limited applications | Popular for patterned glass, wired glass, and specialty glass |
| Float process | For thinner sheet (Top roll process) | Viscous flow with equilibrium thickness in upstream area of bath | Horizontally stretching by conveyor rolls and top rolls | Large‐scale production (productivity) Quality (flatness) Flexibility for thickness and width | Large investment Constraint of chemical elements in glass | Widely operating in the world for various applications |
| For thicker sheet(Fender process) | Viscous flow with restricted width by pair of fenders | Cooling to appropriate temperature in fender area (without stretching) | + Thicker and larger sheet | |||
| Downdraw process | Slot downdraw process | Glass flow toward slot and downward flow through slot | Stretching by pairs of rolls and gravity with anchored to slot | Thinner glass Small‐scale production | Flatness and surface quality Limited width | Customized/modified process operating for specialty glass |
| Fusion downdraw process | Glass flow through trough and over weirs, downward flow on both sides of fusion pipe | Stretching by pairs of rolls and gravity with anchored to root | Thinner glass High surface quality | Minute control required (temperature, glass flow) Constraint of liquidus viscosity | Popular for specialty glass |
3 Updraw Processes
3.1 Fourcault
The first manufacturing method successfully industrialized and commercialized was invented from 1901 in Belgium by É. Gobbe and É. Fourcault and finally implemented industrially in 1912 by É. Fourcault in his family company in Charleroi. With this process, the molten glass was drawn vertically through a débiteuse into a continuous glass ribbon. The débiteuse, a rectangular refractory piece with a spindle‐shaped slot at the center, was immersed into the molten glass. The molten glass flowing up from the slot was drawn upward and immediately cooled by the coolers while conveyed upward by pairs of rolls in a such a manner that its width was kept constant. The formed glass was annealed along the way and finally cut off at the top of the 8–10 m drawing tower (Figure 2). The flow rate was controlled by the immersion depth of the débiteuse, the shape of the slot, and the cooling exerted, whereas the thickness of the sheet was determined by the drawing speed.
The advantages of this process were many. Not only could production be made with several drawing machines
