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The main features of past and current processes are summarized in Table 1. Although updraw processes are no longer used for commodity applications, it remains worthwhile to examine their mechanisms and forming principles from a technological viewpoint. For flat‐glass forming, the essential requirement is to achieve the constant desired thickness, which now ranges from around 25 mm to less than 50 μm, with the specified width at a commercially admissible cost. Additionally, a flatter and smoother surface is requested. The essential forming defects are mainly of two kinds depending on whether they are derived from locally uneven deformation or undesirable stress. The former is caused by viscosity heterogeneity of glass originating from chemical impurities or temperature irregularity, and the latter is caused by fluctuation of forming condition or stress imbalance.

Architectural glass must, for instance, satisfy appropriate transparency and reflection, but glass for automobiles and electronics products has to meet much more demanding quality specifications even for thinner glass whose production becomes increasingly difficult.

In all forming processes, two distinct steps are involved once the glass has been melted at about 1500 °C, refined, and homogenized in the melting tank. The first is its delivery under conditions at which the temperature, thickness, and flow rate must be as stable and uniform as possible throughout its whole width at a viscosity of about 10²–10³ Pa·s (i.e. at 1200–1000 °C for soda‐lime silicate). In the second step, the more viscous molten glass cooled down to a temperature at which the viscosity is 10³−10^6.65 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 10^13.6 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