The tin bath’s wonders
The float process is based on a revolutionary idea by Sir Alistair Pilkington where molten glass is poured onto a bed of molten tin. This method is the basis of industrial flat glass manufacturing up until today.
The molten glass is fed into the tin bath, which contains molten tin in an inert gas atmosphere. The glass mass spreads over the molten tin and forms a continuous ribbon. The underside of the glass floats onto tin in the tin bath and is heated from above to achieve a distortion-free and exceptionally high-quality glass ribbon and planarity on both sides. In the tin bath, the temperature of the glass drops from 1,050 degrees Celsius or 1,922 Fahrenheit at the inlet to around 600 degrees Celsius or 1,112 degrees Fahrenheit at the outlet and the glass ribbon leaves the tin bath in a formed and pre-hardened condition.
Get all your components from one supplier
CNUD EFCO GFT delivers the complete tin bath with everything that goes with it. In addition to design and construction, this includes intelligent equipment combination to ensure optimal plant operation. All components of the plant are manufactured in-house: the tin bath with its highly complex equipment, the top roller and the dross box. Quality-relevant plant components such as the roof, dross box, venting and de-drossing pocket have been further developed on the basis of process simulations in order to optimize the tin bath’s overall efficiency.
This is how we plan and design a tin bath: you tell us the desired glass composition, the thicknesses and the tonnage; we will advise you accordingly and develop a tin bath that exactly meets your needs. In concrete terms, that means:
- Individual design based on the desired viscosity curve of the glass.
- The tin flows are analyzed in advance and optimized by means of different refractory material and graphite barrier structures – all for the highest glass quality.
- Individual tin bath bottom profiles can be developed on the basis of our simulation to avoid any flaws in the optical glass quality.
In addition to tin baths for standard glass applications in the automotive industry and architecture, CNUD EFCO GFT also offers special solutions for high-temperature glasses such as high-aluminum or borosilicate glasses.
Tin bath roof
The tin bath roof completes the tin bath; it seals it and generates overpressure in the tin bath to shield the process from the outside world. Among other things, this is where the uniform temperature distribution is determined, which is a critical factor for optimal glass production. In addition, the protective gas is fed in to reduce atmospheric contamination and prevent defects in the glass.
EFCO, which is now part of CNUD EFCO GFT, was a supplier from the very beginning: the British company Pilkington – one of the world’s leading manufacturers of glass and glass products – made the float glass process market-ready in the 1960s and initially manufactured all components of the production line itself. The first external component in this process was supplied by EFCO: it was the tin bath roof. Since then, this important process component has been continuously optimized. This knowledge base contributes significantly to CNUD EFCO GFT’s all-round expertise.
The top roller machines are designed so that the final glass thickness is already set in the tin bath. The standard at CNUD EFCO GFT is that all five degrees of freedom of the top rollers are automated: rotation, parallel stroke, nip angle, slew angle and penetration. The process is controlled remotely from the tin bath control room – no manual intervention is required.
There is a lot of potential for greater efficiency in this process, and CNUD EFCO GFT is already working on it: the top roller machines are an important part of camera-supported process automation, namely the real-time monitoring of process parameters and, if necessary, readjustment by the top roller machines to stabilize the glass ribbon.
Venting is used for the timely removal and disposal of volatile tin components. It is not necessarily mandatory in the tin bath, but it definitely contributes to its efficiency: when volatile tin returns from a gaseous to a metallic state in the cooler zone, the metal particles (tin drips) can contaminate the glass surface and cause defects. This can be prevented by directing the flow direction of the atmosphere away from the exit end and toward the venting tubes.
The special feature of CNUD EFCO GFT’s venting system is that the extraction is controlled and thus has no negative influence on the sensitive pressure conditions in the tin bath.
The de-drossing pocket at the end of the tin bath prevents dross from sticking to the underside of the glass ribbon while it is being lifted out of the tin bath toward the dross box. It is mounted laterally to remove the dross that accumulates in the outlet area.
The contaminated tin is passed through a refractory channel. The dross floating on the tin gets stuck on a weir, is dammed up and can be removed. The system is usually driven by a linear motor that generates electromagnetic fields and thus a flow in the direction of the de-drossing pocket. Alternatively, a paddle system can be used, which reduces the cooling effect in the process – important in the production of special glass where cooling inputs must be avoided.
Just like the venting system, the de-drossing pocket is not absolutely necessary for the tin bath, but it contributes immensely to the quality of the glass. Together with the dross box, it helps guarantee the best possible quality.
The dross box is the interface and sluice between the tin bath and the annealing lehr. Here, for the first time, the glass floating on the tin is lifted out by three motorized rollers and transferred to the annealing lehr. The dross box also has the important task of sealing the tin bath atmospherically. This is done by means of graphite pressing below the driven rollers and curtains above the glass.
The special features of the CNUD EFCO GFT dross box include adjustable pressure on the graphite blocks and an adjustable roller height. In addition, the dross box is not welded but attached by means of a clamping device, which is firmly secured after the tin bath has been heated to 620 °C. In this way, physically induced thermal expansion is taken into account without deforming the construction. Additional advantages include lower heat losses and a more homogeneous heat distribution within the dross box.
It would be ideal if you could monitor the production process with your own eyes. Since this is not possible due to the high temperatures and the protective atmosphere, our camera systems in the tin bath give you insight into the process and make it visible. The spread of the glass ribbon in the tin bath entrance, the top roller machines, the narrowing of the glass ribbon at the shoulder level and the lift-out curve before the dross box – everything can be tracked visually.
For each tin bath, 18 to 20 camera periscopes with HD technology are used; depending on the number of top rollers, there might be even more. They are important components of process automation; in particular, they help optimize the top roller line and thus reduce edge losses.
We developed a shorter tin bath that delivers the following benefits:
- Lower investment costs, due to the requirement for a smaller building, fewer steel structures and less tin.
- Lower running costs as fewer resources, like energy and inert gas, are needed.
- If space is available, the tonnage of the entire system can therefore be increased.