GLASS - by Atri Sikdar

GLASS

TYPES OF GLASS & GLASS MANUFACTURING PROCESS

TYPES OF GLASS 

Glass comes in many shapes and forms. The names given to different types of glass often depend upon the products used in glass bath and process used to form the glass.

Soda - lime glass is the most common term used for the majority of flat glass product formulations; while ceramic and borosilicate glasses are typically specialty glass composition.

Soda – lime glass, based on the process of drawing of molten glass, are of two major types: Sheet Glass and Float Glass. When Sheet Glass is drawing by use of roller or debiture, Float Glass is produced by pouring molten glass into a chamber that contains a bed of molten tin. The atmosphere inside the chamber is carefully controlled. The molten glass floats on the tin and forms itself in the shape of the container. It spreads 90 to 140 inches wide at a thickness determined at the time of manufacture.

The main difference between Float Glass and Sheet Glass is surface. Float Glass surface is absolutely flat without any distortion and waviness and almost uniform thickness in the entire width of the glass ribbon, where as Sheet Glass surface is wavy and with distortion. Thickness variation occurs in the width of sheet glass ribbon.

When an ultra violet ray beam is focused on float glass the bottom surface show blue coloration due to thin tin layer, where as sheet glass remains transparent.

GLASS MANUFACTURING PROCESS

Soda-lime glass, also called soda-lime-silica glass, is the most prevalent type of glass, used for windowpanes, and glass containers (bottles and jars) for beverages, food, and some commodity items. Soda-lime glass, since it constitutes 77 percent of total glass production, is discussed here.

Soda-lime glass is prepared by melting raw materials, such as soda ash, limestone, silica sand, alumina, cullet ( broken glass )  and small quantities of fining agents (e.g., sodium sulfate, sodium chloride) in a glass furnace at temperatures locally up to 1675°C. The temperature is only limited by the quality of the furnace superstructure material and by the glass composition. Green and brown bottles are obtained from raw materials containing iron oxide. For lowering the price of the raw materials, pure chemicals are not used, but relatively inexpensive minerals such as silica sand and feldspar are used. The mix of raw materials is termed batch.

 

Soda-lime glass is divided technically into glass used for windows, called flat glass, and glass for containers, called container glass. Both types differ in the application, production method (float process for windows; blowing and pressing for containers), and chemical composition. Float glass has a higher magnesium oxide and sodium oxide content as compared to container glass, and lower silica, calcium oxide, and aluminium oxide content. From this follows a slightly higher quality of container glass concerning the chemical durability against water, which is required especially for storage of beverages and food.

Float Glass Production Process

Introduction

Float glass is produced by floating a continuous stream of molten glass onto a bath of molten tin. The molten glass spreads onto the surface of the metal and produces a high quality, consistently level sheet of glass that is later heat polished. The glass has no wave or distortion and is now the standard method for glass production and over 90% of the world production of flat glass is float glass.

Production Process

The basic science:

If molten glass is poured onto a bath of clean molten tin, the glass will spread out in the same way that oil will spread out if poured onto a bath of water. In this situation, gravity and surface tension will result in the top and bottom surfaces of the glass becoming approximately flat and parallel.

The molten glass does not spread out indefinitely over the surface of the molten tin. Despite the influence of gravity, it is restrained by surface tension effects between the glass and the tin. The resulting equilibrium between the gravity and the surface tensions defines the equilibrium thickness of the molten glass.

Raw Materials:

• Raw Material % • Sand 72.6 • Soda Ash 13.0 • Limestone 8.4 • Dolomite 4.0 • Alumina 1.0 • Others 1.0

Process Details:

The batch of raw materials is automatically weighed and mixed and then continuously added to the melting furnace where it is taken to around 1050oC using gas fired burners. The mix then flows over a ‘dam’ where the continuous stream of molten glass flows onto the bath of molten tin. The stream of glass is pulled along the top of the molten tin by haul-off conveyors at the end of the float area which transports the glass into the annealing lehr.  

At the start of the float area the molten glass spreads outwards with flat top and bottom surfaces and the thickness decreases towards the equilibrium thickness.

The thickness can then be further controlled by the stretching effect of the conveyors as it cools until it reaches 600oC when it exits the float area and enters the annealing lehr. Whilst the equilibrium thickness is approximately 7 mm the process has been developed to allow the thickness to be controlled between 0.4 mm and 25 mm.

For thin sheets, the exit conveyor speed can be increased to draw the glass down to thinner thicknesses. This drawing will also result in a decrease in the sheet width and to prevent unacceptable sheet width decreases edge rolls are used. Edge rolls grip the outer top edge of the glass and not only reduce decrease in width but also help to reduce the thickness even further.

For thick sheets, the spread of the molten glass is limited by using non-wetted longitudinal guides. The glass temperature allows the spread to remain uniform and is reduced until the ribbon can leave the guides without changing dimensions.

 

Float glass offers the quality of plate glass combined with the lower production cost traditionally associated with sheet glass manufacturing.  Float glass is virtually distortion and defect free, making it ideal for various premium glazing applications in buildings and homes or for automotive glass along with hundreds of other glass fabrications.

 

Float glass manufacturing is not unlike the manufacturing of commodities like steel or plastic. Each of the processes requires raw materials to be weighed, mixed, melted at high temperatures, formed into continuous ribbons, cooled and cut into a size that fits its use.

Float glass is made from a combination of several ingredients such as sand, soda ash, dolomite, limestone, salt cake, and cullet.

The raw materials are received and stocked in the silos or on/in the optional storage areas. If the raw materials are stocked in the optional storage areas they are then moved as needed to the silos. The raw materials are then drawn down from the silos for batch weighing and mixing. Cullet, which is crushed glass from edge trim of the cut lites or from broken cut lites, is blended with the mixed batch to make from up from 15% to 30% of each batch. The mixture is then delivered to the melting furnace by belt conveyor.

The batch house consists of, silos, hoppers, conveyors, chutes, dust collectors, and the necessary controls to properly handle the raw materials and mixed batch. The raw materials storage and handling is designed to suit the types of glass which will be produced along with the availability and cost of the raw materials.

The mixed batch is delivered from the batch house to the furnace storage bin, by a belt conveyor system, where it's stored and then fed into the furnace at a controlled rate by the batch charger. As the batch is fed into the furnace melter area it's heated by the natural gas burners to approximately 2900 degrees F. From the melter the molten glass flows through the refiner then through the waist area, where stirrers homogenize the glass, then into the working end where the glass is allowed to cool slowly to the proper temperature for delivery to the float furnace.

The melting furnace consists of refractory bricks and special shapes, support and binding steel, insulation, a fossil fuel firing system, temperature sensors and a computerized process control system. The design of the furnace is carefully made to meet the plant's specific gross daily glass production tonnage goals.

The insulation, special airflow features, and combustion air heating enable the furnace to operate at maximum fuel efficiency with negligible pollutant emissions. The furnace is sized and designed to provide high quality glass with the smallest amount of energy per ton of glass melted.

From the working end of the melting furnace, the glass flows through the canal area and then into the float furnace (tin bath or float bath) onto molten tin. The float furnace atmosphere is controlled by a mixture of nitrogen and hydrogen gas to prevent the tin from oxidizing.

The molten glass, which by now has dropped to 2000 degrees F, forms a continuous ribbon that floats on the molten tin. The desired width and thickness is obtained through an operator controlled program which sets the speed of the annealing lehr and knurl machines. The ribbon thickness can range from 2 to 12 mm. As the continuous ribbon moves through the float furnace its temperature is gradually reduced allowing the glass to become flat and parallel.

The float furnace consists of support steel, upper and lowers welded steel casings, a refractory liner, tin, electric heating elements, oxygen eliminating atmosphere system, temperature sensors, and a computerized process control system. Each customer's float furnace is specifically designed to respond to heat flow balance, desired ribbon width, glass thickness, glass color and the gross daily production tonnage.

The glass leaves the float furnace at about 1100 degrees F and enters the annealing lehr, which controls the cooling of the glass. The glass ribbon is carried through the lehr by a series of rollers driven by motors and a mechanical drive system. The temperature of the glass is reduced according to a precise time/temperature gradient profile, established for each annealing lehr, to produce glass that meets industry standards. The time/temperature profile, for all of the glass thicknesses and colors produced by the plant, is installed in the computerized annealing lehr process control system.

The components of the annealing lehr include a steel lined inner chamber and a steel outer shell separated by insulation, steel alloy and insulated steel rollers, a mechanical drive system, air circulation fans and ducts, heating elements, and temperature sensors and computerized process control system. The design of the annealing lehr is adapted to meet the critical cool down requirements of each float plant's gross daily production tonnage and glass colors.

The cooled glass ribbon exits the annealing lehr and is conveyed to the cutting area by a system of rollers and drives linked to the lehr drive system. The glass is scored by carbide cutting wheels, parallel and perpendicular to the ribbon travel, into sizes that meet the plant's customer requirements. The scored glass ribbon is then separated into lites for packaging by unloading personnel or automatic equipment for transfer to the wareroom for storage or shipment to the customer.

The cutting system consists of steel support frames, a mechanical roll drive system, "X "and "Y "cutting system, a glass trim and scoring system, manual unloading conveyors and/or semiautomatic or automatic unload systems and a computerized process control system.