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Lecture 15

GLASSWORKING

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Introduction

Glass is an amorphous solid with a non-crystalline

structure. There are three major categories of glass

products:

glass piece ware

flat glass glass fibers

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Process sequence in glass working

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Preparation of raw materials

Preparation of silica sand

Silica sand (SiO2) is the principal component of all glasses.Preparation of silica sand includes washing, drying and classification.

AdditivesVarious other materials, such as soda ash,potash, limestone etc. are

added in the correct proportions to achive the desired composition.

Recycled glassesRecycled glasses are crushed and added to the mixture in a proportion ofup to 100%.

MeltingMelting of starting raw materials is usually carried out at around1500~1600o C in a melting cycle of 24~48 hours. Melting is performed indifferent furnaces, usually electric furnaces.

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Shaping Processes

The shaping processes can be classified into

three categories:

Processes for piece ware

Continuous processes for glass making Fiber making processes

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SHAPING OF PIECE WARE

CastingThis process is similar to metal casting. Relatively massive parts, such asastronomical lenses and mirrors, are made by this process. The parts arecooled slowly after casting to avoid internal stresses and cracking.Finally, products must be polished.

SpinningSpinning is similar to centrifugal casting. It is used to produce funnel-

shaped components such as tubes for televisions and computer monitors.

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PressingPressing is a process for mass production of relatively flat items, such asdishes, bake ware, lenses, TV tube faceplates (the faceplate is assembled

to the funnel using a sealing glass of low melting point).

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BlowingThe blowing process is used to make thin-walled hollow glass items, suchas bottles and glass containers.

Stages in manufacturing an ordinaryglass bottle

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SHAPING OF FLAT GLASSES

Drawing

The Fourcault process uses a special die made of fireclay and called

debiteuse to draw the sheet from the molten glass. In Colburn process,the sheet is bent while the glass is still plastic so that it can be cooled inhorizontal position.

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Float processThe glass flows directly from the melting furnace onto the surface of amolten tin bath

The float process for producing sheet glass

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In all processes for flat glasses, the continuous sheet is cut into standardsizes after the glass has hardened significantly. Subsequent grinding andpolishing removes tool marks are produces smooth and parallel surfaces.

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Danner processThis process is used for glass tubing:

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PRODUCING OF GLASS FIBERS

Drawing

This process is used to produce continuous glass fibers of high quality

and small diameter used in fiber-optics communication lines.