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Manufacturing Processes
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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
2
4
• 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
7
• 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
8
• 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
10
• 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
12
• 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
Shaping Processes in Glassworking
Pressing
13
• 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
Shaping Processes in Glassworking
Blowing
17
• 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
Shaping Processes in Glassworking
Casting
18
• 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 Processes in Glassworking
Shaping of Flat and Tubular Glass
19
• 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
Shaping Processes in Glassworking
Rolling of Flat Plate
20
• 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
Shaping Processes in Glassworking
Float Process
23
• 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
24
• 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
Heat Treatment and Finishing
Tempering
25
• Applications of Tempered Glass• Tall buildings• Glass doors
Heat Treatment and Finishing
Tempering
Can we use tempered glass in automobile windshields?
27
• 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
Heat Treatment and Finishing
Finishing
28
• Decorative and surface processes include• Sandblasting• Chemical etching • Coating
Heat Treatment and Finishing
Finishing
How glass can be converted into mirrors?
31
• 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…???