Process Description – Manufacturing of Float Glass

Introduction

Float glass refers to a sheet of glass produced through a process of floatation of glass in a molten state in a molten tin bed, a process that ensures that the glass achieves a perfectly flat surface and uniform thickness (Haldimann, Luible and Overend 1). The common name of float glass is window glass. People have used this type of glass for many decades and it is a vital product used in the process of glass fusing. In order for glass to be manufactured, the raw material, in the form of liquefied glass, is spread on a surface that does not allow any leakage of the molten material. A flat surface is aimed at achieving high-quality glass products. The resulting glass product is free from distortion and the process has been recognized as the standard procedure for producing glass. Indeed, more than 90% of the flat glass in the world results from the float glass manufacturing process (Haldimann et al 1). In order to understand the process, this paper examines the stages and steps involved in float glass production.

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Background

Before the adoption of the manufacturing approaches that are being utilized today, engineers used to cut raw material for making glass into many pieces. Using this process, engineers collected bottles or large disks together and cut them into pieces, which were flattened later. Flattening of the pieces resulted in a large surface of the glass from which constructors cut window-panes and glass for other purposes. After casting, both sides of the glass were ground and polished to achieve clarity and smoothness (Jones and Hitchman 453). Up to the early periods of the nineteenth century, many types of glass used for windows were products of roundels (Haldimann et al 2). Most parts of the nineteenth century saw a shift from using roundels to the bottle system for making glass for windows. Many sources in the glass-making industry link Alastair Pilkington as the main inventor of the process of producing float glass (Jones and Hitchman 453). In 1948, the art of making glass was protected from being misused by illegal entities. This was done through patenting the approaches in making glass. From the background, it is apparent that the float glass manufacturing process represents a major move in the industry that resulted in the development of high-quality float glass products of different properties. With the adoption of modern technologies with regard to manufacturing, glass and glass products are produced efficiently and quite fast.

Description of the float glass production process

The chart below shows the typical stages in float glass production. As seen in the diagram, the basic stages start at the batch plant, where there is batching of raw materials. The processes proceed to the furnace for melting and refining, then to the tin bath, coating, annealing, cutting and stocking in preparation of transportation.

An illustration of the seven steps that are involved in the production of glass.
Figure 1. An illustration of the seven steps that are involved in the production of glass. Adapted from Structural Glass Facades and Enclosures (p. 76), by Patterson, Mic. Hoboken, 2011, NJ: John Wiley & Sons.

It is important to note the idea of buoyancy of the molten material in the floatation concept of producing glass. In fact, high-density and low-density liquid-like substances have varied buoyancies, which affect their flow rates. In a controlled and automated system, the process enables the glass to float as a ribbon on the tin that remains liquid at a temperature of 600 degrees Celsius. After the glass goes through the furnace or the float bath, special rollers move the glass and take it to the next step, which is annealing. Afterward, the glass leaves the lehr stage at 200 degrees Celsius (Patterson 102). Engineers leave the glass to cool in order to achieve room temperature. The next steps are cutting the glass, packaging and storage for shipment. Furthermore, manufacturers can choose to develop the glass further into products such as self-cleaning glass, safety glass, mirrors, reflective glass, and other glazed units. The thickness of float glass can range from 1.5mm to 20mm. Manufacturers achieve different thicknesses of the glass through two manways. First, glass with the smallest thickness could be made by limiting the speed of rollers and reducing the size of float flow. Second, engineers make think glass using materials that limit the flow of molten glass materials so that they could on an area for some time. Even though various manufacturers use varying approaches to the basic process, the production of float glass goes through an integrated series of seven stages, each with steps that contribute to the final product (Haldimann et al 3).

Stages in the process

Stage one: Raw material batching

In the first stage of float glass production:

  1. Engineers mix the main raw materials of soda lime, silica sand, calcium oxide, soda and magnesium in the ratio of 73:9:13:4 percent properly weighed and placed into batches (Haldimann et al 4).
  2. The engineers add recycled glass to every batch.
  3. The addition of recycled glass is followed by testing of the materials and storage as the batches wait for mixing under a computerized system.

Stage two: melting and refining

  1. Having mixed the raw materials to achieve fine grains, the materials are checked for quality and balance to create the batch for processing.
  2. With the right mix and quality, the batch flows into the melter that turns it into a molten form at 1,500 degrees Celsius.

Melting and refining are important because the float produces glass that is almost of poor optical quality. At this stage, the material in the equipment is melted, homogenized, and refined (Patterson 102).

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  1. The long melting process goes on for approximately fifty hours, resulting in glass with temperatures as high as 1,100 degrees Celsius.
  2. In order to achieve a continuous flow of the molten glass, the process aims at ensuring that the material does not have bubbles and inclusions.

The stage that involves melting and refining are vital for achieving high-quality glass products. Furthermore, engineers at the melting stage can modify the composition of the molten glass based on the desired properties of the product (Patterson 102).

Stage three: The float bath

  1. The glass leaves the float at 600 degrees Celsius in the form of a solid ribbon.

Although the principle of manufacturing float glass remains fairly the same, the finished products have changed significantly. In fact, the current processes produce thinner glass, remove inclusions, striations and bubbles. Overall, the float bath stage creates a fine finish. Thus, the finish improves the appearance of the glass and makes it fetch good prices in the market.

Stage four: Coating

  1. At this level, engineers make profound transformations to the glass depending on the desired optical qualities.

For instance, engineers can use the technique of chemical vapor deposition, which is a major way of coating glass representing a major advancement in the process of producing float glass. For instance, engineers can give the glass infrared properties using the chemical vapor deposition technology to introduce micro tiny layers or coatings (Jones and Hitchman 454). Future development and advancement of the chemical vapor deposition technique should focus on altering the composition of the glass as a way of achieving different optical features of the final products.

Stage five: Annealing

  1. This step involves the heat treatment of the glass ribbon as it passes through a lehr, which is a long furnace. The objective is to relieve stress on the glass, which may break it. The process involves proper control of temperature across and along with the glass ribbon as it passes through the furnace (Patterson 102).

In spite of the smooth flow of glass from the first step, the process of cooling the glass ribbon develops substantial stresses that may break it. Technological advancements spearheaded by Pilkington designed ways of automatically detecting stress levels in the ribbon to effect automatic temperature adjustment in the long furnace (Jones and Hitchman 453).

Stage six: Inspection

  1. This step involves inspection of the float process to ensure everything is as expected. Although the manufacturing process of float glass is designed to achieve near perfection, inspection at every stage helps to confirm high-quality standards.

For instance, it has been shown that the processes involved in refining the glass could not efficiently remove all the inclusions. In addition, some other factors could lead to the introduction of bubbles into the liquid-like glass. The process has automated systems to detect flaws in real-time. The automated inspection system identifies potential problems upstream for appropriate correction. In case a flaw passes undetected, the inspection enables the automated system downstream to detect the flaw in the cutting process.

  1. The automated inspection system performs over 100 million measurements in one second over the glass ribbon, which enables the system to detect the smallest of flaws that could be difficult to detect using an unaided eye.
  2. Having identified the flaws, the ‘intelligent’ or automated cutters know where to cut to minimize wastage and deliver the highest quality of products to the end-users.

Stage seven: cutting to order

  1. This is a fundamental step because customers buy float glass measured in square meters. Therefore, customers often bring their orders with specific requirements, which the computer system converts into patterns for cutting in a way that minimizes wastage. In fact, modern systems of manufacturing are aimed at significantly reducing the number of raw materials lost during manufacturing processes.

Conclusion

The description of the manufacturing process of float glass reveals the magic of science evolving through a series of seven stages from raw materials to finished products of different properties. The steps in the production of glass include raw material batching, smelting and refining, the float bath, coating, annealing, inspection, and cutting based on specifications. The float glass, which is the final product, has applications in three major areas. First, it is utilized in the construction industry, especially in achieving structures with some high levels of transparency. Second, float glass is used to make different types of glass that have various applications. Some of these are reflective glass and safety glass, among others. Third, float glass is important for precision mechanics, particularly in cases where engineers require an extremely flat surface such as in visual displays.

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Works Cited:

Haldimann, Matthias, Andreas Luible, and Mauro Overend. Structural use of glass. Vol. 10. Zürich, Switzerland: Iabse, 2008. Print.

Jones, Anthony C., and Michael Hitchman, eds. Chemical vapor deposition: precursors, processes and applications. Cambridge, United Kingdom: Royal Society of Chemistry, 2009. Print.

Patterson, Mic. Structural Glass Facades and Enclosures. Hoboken, NJ: John Wiley & Sons, 2011. Print.

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