Float Process

This process was introduced by Pilkington Brothers Ltd in the 1950s.

Firstly, the raw materials (sand, soda, lime) are continuously introduced

into the furnace, melted at 1500˚ C, homogenized by convection and fined to eliminate bubbles. The furnace contains typically 2000 tons of glass and produces every day 500 tons of glass. The viscous liquid travels onto the float at a temperature of 1100˚ C under a nitrogen atmosphere in order to prevent corrosion of the tin bath. Under these conditions the equilibrium thickness of the glass sheet is about 6mm so that the sheet has to be expanded or contracted by top rolls (or rollers working from the top of glass) to produce thinner or thicker glass sheets respectively.

Several pairs of top rolls are used; they are made of steel and are water cooled. Their rotation axis is horizontal and shifted out of the float axis to draw the glass ribbon. The range of commercial thickness is between 2 and 19mm. Glass with lower thickness (<2mm) is difficult to produce and fusion draw is preferred. The glass sheet is extracted from the float at 600˚C. At such temperature the glass ribbon is viscous enough to be drawn upward out of the tin bath.

This process has been a revolution for the glass industry because it offers very good optical quality without requiring any further operation, while polishing before the invention of the float process. Therefore, most flat glass is produced through this process. This implies that to produce curved, tempered, laminated glazing it is necessary to heat the glass again close to its transition temperature.

An important characteristic of float glass is the intrinsic difference between the surface in contact with the bath (tin side) and the opposite side (atmospheric side).

Schematic representation of horizontal tempering furnace

The tin side of the glass is in contact with the bath (and enriched with tin) and it is contacted by the rollers. None of this happens to the top side.

Concentration and penetration of tin have been measured extensively on both faces (tin and atmosphere sides.

A major change to the float process was the introduction of on-line coatings. As the glass ribbon moves continuously, high deposition rates are required (60–100nms).

The first manufactured coatings (low-E glazing) were tin oxide (SnO₂) which was deposited from an organotin compound and oxygen.

Fusion Draw

Fusion draw was invented by Corning in the 1960s for automotive applications.

With this process, the glass sheet is formed from a continuous glass flow delivered by a refractory piece called an ‘isopipe’. Glass flows out laterally along the two sides of the pipe. These two flows later join downwards to form the glass sheet. The control of the thickness is carried out with rollers that do not contact the central part of the sheet.

An important characteristic of this process is that the external faces of the sheet are not mechanically damaged since they are not in contact with any tool. In fact, the two glass surfaces that were in contact with the refractory are fused together when they join. This process allows production of sheets of 1m2 with an excellent control of thickness down to 0.5mm.

This is an important point to reduce flat display panel weight. Moreover, no polishing of the glass is necessary since the process delivers excellent glass surface quality.