manufacturing processes

LECTURE NUMBER 13Glassworking

Manufacturing Processes, ME33x2Lab Engineer Rehan Zahid

NUST School of Mechanical and Manufacturing Engineering (© Fundamentals of Modern Manufacturing; Materials, Processes and Systems,

by M. P. Groover)

1Chapter 12

OBJECTIVES

1. Introduction2. Raw Materials Preparation and Melting3. Shaping Process in Glassworking4. Heat Treatment and Finishing5. Product Design Consideration

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1- Introduction

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• Glass falls in the category of ceramics and has non-crystalline structure

• Starting material is silica (SiO2)• Steps involved in glass working

• Starting material is heated to transform it from a hard solid into a viscous liquid

• Viscous liquid is shaped into the desired geometry• Heat treatment and solidification

• Examples of Glass products : light bulbs, beverage bottles, window glass, giant telescope lenses

Introduction

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Introduction

2- Raw Material Preparation & Preparation

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• Primary source of silica is quartz • Sand must be washed and classified

• Washing removes impurities (clay) and undesirable coloring of the glass

• Classifying means grouping the grains according to size (0.1 to 0.6 mm)

Raw Material Preparation and Melting

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• Recycled glass is also added to facilitate melting• Batch of starting materials to be melted is known as

charge• Procedure of loading charge into the melting furnace is

called charging the furnace• Glass melting is carried out at temperatures 1500˚C to

1600˚C• Molten glass is a viscous liquid

Raw Material Preparation and Melting

3- Shaping Processes in Glassworking

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• Similar to centrifugal casting of metals• Applications:

• Funnel-shaped components such as the back sections of cathode ray tubes for televisions and computer monitors

• Process: • Stack of molten glass is dropped into a conical

mold steel• Mold is rotated so that centrifugal force causes the

glass spread itself on the mold surface

Shaping Processes in Glassworking

Spinning

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Spinning

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• Used for mass producing glass pieces such as dishes, bake ware, headlight lenses etc.

• Large quantities of most pressed products justify a high level of automation


Pressing

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• Blowing is performed on highly automated equipment • Applications:

• Jars, beverage bottles, incandescent light bulb enclosures etc.

• Types of blowing sequences• Press-and-blow

• Consists of pressing and blowing operation• Split mold is used in the blowing operation for

easy part removal


Blowing

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Blowing

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Press and blow

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Blow and blow

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• Not much used in glassworking• Molten glass is poured into a mold• Relatively massive objects (astronomical lenses and

mirrors) are made by casting• To avoid internal stresses and cracking very slow

cooling is carried out • Finishing is done by lapping and polishing• Problems:

• Molten glass is relatively viscous at normal working temperatures

• Does not flow through small orifices or into small sections


Casting

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• Used for making plate glass and producing tube stock using continuous processes

• Long sections of flat window glass or glass tubing are made and later cut into appropriate sizes and lengths


Shaping of Flat and Tubular Glass

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• Starting glass in a plastic condition is squeezed through opposing rolls

• Separation between rolls determines the thickness of the sheet

• Rolled glass sheet must later be ground and polished for parallelism and smoothness


Rolling of Flat Plate

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• Produces smooth surfaces that need no subsequent finishing

• Glass flows directly from its melting furnace onto the surface of a molten tin bath

• Highly fluid glass spreads evenly across the molten tin surface, achieving a uniform thickness and smoothness

• Cooler region of the bath hardens glass• Glass is cut to size


Float Process

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Float Process

4- Heat Treatment and Finishing

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• Glasses are heat treated using annealing and tempering

• Annealing involves• Heating of glass to an elevated temperature

(500˚C)• Maintaining the temperature for some period of

time to eliminate stresses• Cooling

Heat Treatment and Finishing

Heat Treatment

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• Tempering increases the toughness of glass• Tempering involves

• Heating the glass to a temperature above its annealing temperature

• Quenching of the surfaces with air jets• Glass formed by tempering is known as Tempered

Glass• Properties of Tempered Glass

• Resistant to scratching • Resistant to breaking • Withstand large compressive stresses


Tempering

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• Applications of Tempered Glass• Tall buildings• Glass doors


Tempering

Can we use tempered glass in automobile windshields?

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Tempering

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• Finishing operations include• Grinding• Polishing• Cutting

• Glass produced by drawing and rolling contains defects and scratch marks due to usage of hard tooling on soft glass which can be removed by polishing

• Pressing and blowing operation produced by split dies left seam marks which can be removed by polishing

• Continuous sections produced during plate and tube production are cut into smaller pieces by glass-cutting wheel or cutting diamond


Finishing

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• Decorative and surface processes include• Sandblasting• Chemical etching • Coating


Finishing

How glass can be converted into mirrors?

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5- Product Design Considerations

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• Glasses are used when transparency, light transmittance, magnification etc is required

• Glasses should not be used in applications that involve impact loading because of brittleness

• Certain glasses have very low thermal coefficients and can tolerate thermal shock (Any example…???)

• Outside and inside corners should be rounded

Product Design Considerations

Why corners are generally rounded or chamfered…???

manufacturing processes

Documents

glass pieces

cmolten glass

plate glass

glass tubing

melting8recycled glass

glassworkingmolten glass

glass workingstarting

mold surface shaping