2005 Pearson Education South Asia Pte Ltd Forming and Shaping Processes and Equipment Manufacturing Engineering and Technology 13.Rolling of Metals 14.Forging

2005 Pearson Education South Asia Pte Ltd

Forming and Shaping Processes and Equipment

Manufacturing Engineering and Technology

13.Rolling of Metals14.Forging of Metals15.Extrusion and Drawing of Metals16.Sheet-Metal Forming Processes17.Processing of Metal Powders18.Processing of Ceramics, Glass and

Superconductors19.Forming and Shaping Plastics and Composite

Materials20.Rapid-Prototyping Operations

18. Processing of Ceramics, Glass and Super-conductors


Chapter Objectives

• Preparation of raw materials as fine ceramic particles.

• Casting, pressing, extrusion, and molding to produce discrete product shapes.

• Drying and firing to induce strength and hardness.

• Finishing operations to improve size and surface finish.

• Production of superconductors into wire and tapes.



Chapter Outline

1. Introduction2. Shaping Ceramics3. Forming and Shaping of Glass 4. Techniques for Strengthening and Annealing Glass 5. Design Considerations for Ceramics and Glasses 6. Processing of Superconductors



18.1 Introduction

• Ceramics and glass have important characteristics, such as high-temperature strength and hardness, low electrical and thermal conductivity, inertness to chemicals, and resistance to wear and corrosion.

• Fig 18.1 shows the example of typical glass parts.



18.2 Shaping Ceramics

• Several techniques are available for processing ceramics into useful products depending on the type of ceramics involved and their shapes.

• Table 18.1 shows the general characteristics of ceramics processing.




• The procedure involves the following steps:

1. Crushing or grinding the raw materials into very fine particles

2. Mixing them with additives to impart certain desirable characteristics

3. Shaping, drying, and firing the material• Fig 18.2 shows the processing steps involved in making ceramic parts.







• The ground particles are then mixed with additives—the functions of which are one or more of the following:

1. Binder: for holding ceramic particles together.

2. Lubricant: to reduce internal friction between particles during molding and to help remove the part from the mold.

3. Wetting agent: to improve mixing.

4. Plasticizer: to make the mix more plastic and formable.

5. Agents: to control foaming and sintering.



18.2.1 Casting

• The most common casting process is slip casting (also called drain casting).• Fig 18.3 shows the sequence of operations in slip-casting a ceramic part. After the

slip has been poured, the part is dried and fired in an oven to give it strength and hardness.

• A slip is a suspension of colloidal (small particles that do not settle) ceramic particles in an immiscible liquid (insoluble in each other), which is generally water.



18.2.1 Casting



18.2.1 Casting

Doctor-blade process• Thin sheets of ceramics (less than 1.5 mm thick) can be made by a casting

technique called the doctor-blade process.• The slip is cast over a moving plastic belt while its thickness is controlled by a blade.• Ceramic sheets also may be produced by other methods, including: (a) rolling the

slip between pairs of rolls and (b) casting the slip over a paper tape which subsequently burns off during firing.



18.2.1 Casting

Doctor-blade process• Fig 18.4 shows the production of ceramic sheets

through the doctor-blade process.



18.2.2 Plastic forming

• Plastic forming (also called soft, wet, or hydroplastic forming) can be carried out by various methods, such as extrusion, injection molding, or molding and jiggering.

• Fig 18.5(a) shows Extruding and (b) jiggering operations.



18.2.3 Pressing

Dry Pressing• Dry pressing is used for relatively simple shapes, such as

whiteware, refractories for furnaces, and abrasive products.• Density can vary significantly in dry-pressed ceramics (as in P/M

compaction) because of friction among the particles and at the mold walls.



18.2.3 Pressing

Dry Pressing

• Several methods may be used to minimize density variations, including:

a) proper design of tooling,

b) vibratory pressing and impact forming (particularly for nuclear-reactor fuel elements), and

c) isostatic pressing.



18.2.3 Pressing

Wet Pressing• In wet pressing, the part is formed in a mold while under high pressure in a hydraulic or

mechanical press. • This process generally is used to make intricate shapes. Moisture content usually ranges

from 10 to 15%. • Production rates are high; however, (a) part size is limited, (b) dimensional control is

difficult to achieve because of shrinkage during drying, and (c) tooling costs can be high.



18.2.3 Pressing

Isotatic Pressing• Used extensively in powder metallurgy, isostatic

pressing also is used for ceramics in order to obtain uniform density distribution throughout the part during compaction.



18.2.3 Pressing

Jiggering• A combination of processes is used to make ceramic plates. • In this process, clay slugs first are extruded and formed into a bat over a plaster mold. • They then are jiggered on a rotating mold .• Jiggering is a motion in which the clay bat is formed by means of templates or rollers. The part then

is dried and fired. • The jiggering process is confined to axisymmetric parts and has limited dimensional accuracy. • The operation is automated for improved productivity.



18.2.3 Pressing

Injection molding• Injection molding is used extensively for the precision forming of ceramics in

high-technology applications, such as for rocket-engine components.• The raw material is mixed with a binder, such as a thermoplastic polymer

(polypropylene, low-density polyethylene, or ethylene vinyl acetate) or wax. The binder usually is removed by pyrolysis (inducing chemical changes by heat); the part then is sintered by firing.



18.2.3 Pressing

Hot pressing• In this process (also called pressure sintering), the pressure and the heat are

applied simultaneously. • This method reduces porosity and, thus, makes the part denser and stronger. • Graphite commonly is used as a punch and die material, and protective

atmospheres usually are employed during pressing.



18.2.4 Drying and firing

• Drying is a critical stage because of the tendency for the part to warp or crack from variations in the moisture content and in the thickness of the part.

• Control of atmospheric humidity and of ambient temperature is important in order to reduce warping and cracking.

• Firing (also called sintering) involves heating the part to an elevated temperature in a controlled environment.




• Some shrinkage occurs during firing. • Firing gives the ceramic part its strength and hardness. This improvement

in properties results from (a) the development of a strong bond between the complex oxide particles in the ceramic and (b) reduced porosity.

• Fig 18.6 shows the shrinkage of wet clay caused by the removal of water during drying. Shrinkage may be as much as 20% by volume.




• Nanophase ceramics can be sintered at lower temperatures than those used for conventional ceramics.

• They are easier to fabricate, because they can be compacted at room temperature to high densities, hot pressed to theoretical density, and formed into net-shaped parts without using binders or sintering aids.



18.2.5 Finishing operations • The finishing processes employed can be one or more of the following operations:

1. Grinding (using a diamond wheel)

2. Lapping and honing

3. Ultrasonic machining

4. Drilling (by using a diamond-coated drill)

5. Electrical-discharge machining

6. Laser-beam machining

7. Abrasive water-jet cutting

8. Tumbling (to remove sharp edges and grinding marks)



18.2.5 Finishing operations

• Process selection is an important consideration because of the brittle nature of most ceramics and the additional costs involved in some of these processes.

• The effect of the finishing operation on the properties of the product also must be considered.

• To improve their appearance and strength and to make them impermeable, ceramic products often are coated with a glaze or enamel, which forms a glassy coating after firing.



Example 18.1 Dimensional changes during the shaping of ceramic components

A solid, cylindrical ceramic part is to be made with a final length, L, of 20 mm For this material, it has been established that linear shrinkages during drying andfiring are 7 and 6%, respectively, based on the dried dimension, Calculate (a) the initial length, of the part and (b) the dried porosity, if the porosity of the fired part, is 3%.




Solutiona. On the basis of the information given and noting that firing is preceded by drying, we can write




Solutionb. Since the final porosity is 3%, the actual volume, of the solid material in the part is

Therefore, the porosity, of the dried part is 19%.



18.3 Forming and shaping of glass

• Glass is processed by melting and then shaping it, either in molds and various devices or by blowing. • Glass shapes produced include flat sheets and plates, rods, tubing, glass fibers, and discrete products such

as bottles and headlights.• Glass products generally can be categorized as follows:

1. Flat sheets or plates ranging in thickness from about 0.8 to 10 mm, such as window glass, glass doors, and table tops.

2. Rods and tubing used for chemicals, neon lights, and decorative artifacts.

3. Discrete products such as bottles, vases, headlights, and television tubes.

4. Glass fibers to reinforce composite materials and for use in fiber optics.



18.3.1 Flat-sheet and plate glass

• Flat-sheet glass can be made by the float-glass method or by drawing or rolling it from the molten state.

• All three methods are continuous processes.

1. In the float method molten glass from the furnace is fed into a long bath in which the floats over a bath of molten tin. The glass then moves at a temperature of about 650°C over rollers into another chamber (lehr), where it solidifies. Float glass has smooth surfaces (fire-polished), and further grinding or polishing is not necessary.




2. The drawing process for making flat sheets or plates involves a machine in which the molten glass passes through a pair of rolls in an arrangement similar to an old-fashioned clothes wringer. The solidifying glass is squeezed between these two rolls (forming it into a sheet) and then moved forward over a set of smaller rolls.




3. In the rolling process (Fig. 18.8b), the molten glass is squeezed between rollers, forming a sheet. The surfaces of the glass may be embossed with a pattern by using textured roller surfaces. In this way, the glass surface becomes a replica of the roll surface. Thus, glass sheet produced by drawing or rolling has a rough surface appearance.




• Fig 18.7 shows the float method of forming sheet glass.

• Fig 18.8(a) shows the drawing process for drawing sheet glass from a molten bath, and (b) Rolling process.



18.3.2 Tubing and rods

• Glass tubing is manufactured by the process shown in Fig. 18.9.

• Molten glass is wrapped around a rotating hollow mandrel (cylindrical or cone-shaped) and is drawn out by a set of rolls.



18.3.3 Discrete glass products

Blowing• Hollow and thin-walled glass items (such as bottles, vases, and flasks) are made by blowing

—a process that is similar to the blow molding of thermoplastics.• Fig 18.10 shows the steps in manufacturing an ordinary glass bottle.• The mold usually is coated with a parting agent (such as oil or emulsion) to prevent the glass

from sticking to the mold.• Blowing may be followed by a second blowing operation for finalizing product shape, called

the blow and blow process.




• The surface finish of products made by the blowing process is acceptable for most applications, such as bottles and jars.




Pressing• In the pressing process, a gob of molten glass is placed into a mold and pressed

into a confined shape with a plunger.• Fig 18.11 shows the manufacturing a glass item by pressing glass into a mold.• Fig 18.12 shows the pressing glass into a split mold.• Pressing in one-piece molds cannot be used for (a) shapes of products from

which the plunger cannot be retracted, or (b) thin-walled items.







Centrifugal casting• Also known in the glass industry as spinning, this process is similar to that used for

metals. • The centrifugal force pushes the molten glass against the mold wall, where it solidifies. • Typical products made are TV picture tubes and missile nose cones.• Fig 18.13 shows the centrifugal casting of glass. Large telescope lenses and television-

tube funnels are made by this process.




Sagging

• Shallow dish-shaped or lightly embossed glass parts can be made by the sagging process.

• A sheet of glass is placed over the mold and heated.

• The glass sags by its own weight and takes the shape of the mold.

• The process is similar to the thermoforming of thermoplastics, but no pressure or vacuum is involved.

• Typical applications are dishes, sunglass lenses, mirrors for telescopes, and lighting panels.




Glass ceramics manufacture• Glass ceramics (trade names: Pyroceram, Corningware) contain large proportions

of several oxides. • Thus, their manufacturing involves a combination of the methods used for

ceramics and glasses. • Glass ceramics are shaped into discrete products (such as dishes and baking

pans) and then heat treated, whereby glass becomes devitrified (recrystallized).



18.4 Techniques for strengthening and annealing glass

Thermal tempering

• In this process (also called physical tempering or chill tempering), the surfaces of the hot glass are cooled rapidly by a blast of air.

• As a result, the surfaces shrink, and (at first) tensile stresses develop on the surfaces.

• As the bulk of the glass begins to cool, it contracts.

• The already solidified surfaces of the glass are then forced to contract, and consequently, they develop residual compressive surface stresses, while the interior develops tensile stresses.




Thermal tempering• Fig 18.14(a) shows the stages involved in inducing

compressive surface residual stresses for improved strength. (b) Residual stresses in a tempered glass plate




Chemical tempering• In this process, the glass is heated in a bath of molten

nitric or sulphuric acid depending on the type of glass.• The time required for chemical tempering is about one

hour longer than that for thermal tempering.




Laminated glass• Laminated glass is a product of another strengthening method called laminate

strengthening.• It consists of two pieces of flat glass with a thin sheet of tough plastic in between. • When laminated glass is cracked, its pieces are held together by the plastic sheet—a

phenomenon commonly observed in a shattered automobile windshield.




Bulletproof glass• Laminated glass has considerable resistance and can prevent the full penetration of solid

objects because of the presence of a tough polymer film in between the two layers of glass.

• Bulletproof glass (used in some automobiles, armored bank vehicles, and buildings) is a more challenging design.

• This is due to the very high speed and energy level of the bullet and the small size and the shape of the bullet tip—a small contact area and high localized stresses.




Bulletproof glass• A more recent design for bulletproof glass consists of two adjacent layers of thermoplastic

polymer sheet over the same surface of the glass and is based on a somewhat different principle.

1. The outermost layer (the side where the bullet enters) is an acrylic sheet (polymethylmethacrylate, PMMA). This sheet dulls the tip of the bullet—thus slowing down the bullet’s speed and its ability to penetrate easily because of the now blunt tip. Additionally, the acrylic film has high weather resistance, making it suitable as the outer layer, which is exposed to the elements.




Bulletproof glass

2. The next layer is a polycarbonate sheet. Because it has high toughness, the polycarbonate layer stops the bullet, which has already been dulled when penetrating the acrylic sheet first. The glass shatters in the same manner as in other designs.



18.4.1 Finishing operations

• In all finishing operations on glass and other brittle materials, care should be exercised to ensure that there is no surface damage, especially stress raisers such as rough surface finish and scratches.

• Because of their notch sensitivity, even a single scratch can cause premature failure of the part, especially if the scratch is in a direction where the tensile stresses are a maximum.



18.5 Design Considerations for Ceramics and Glasses

• Ceramic and glass products require careful selection of composition, processing methods, finishing operations, and methods of assembly with other components.

• The control of processing parameters and of the quality and level of impurities in the raw materials is important.

• Dimensional changes and warping and the possibility of cracking during processing and service life are significant factors in selecting methods for shaping these materials.



18.5 Design Considerations for Ceramics and Glasses

• Ceramics and glasses undergo a phenomenon called static fatigue, whereby they can suddenly break under a static load after a period of time.

• A general guide is that, in order for a glass item to withstand a load of 1000 hours or longer, the maximum stress that can be applied is about one-third of the maximum stress that it can withstand during the first second of loading.



18.6 Processing of Superconductors

• Two basic types of superconductors are:

1. Metals, called low-temperature superconductors (LTSC), include combinations of niobium, tin, and titanium.

2. Ceramics, called high-temperature superconductors (HTSC), include various copper oxides. Here, “high” temperature means closer to ambient temperature, and hence HTSCs are of greater practical use.



18.6 Processing of Superconductors

• The basic manufacturing process for superconductors consists of the following steps:

1. Preparing the powder, mixing it, and grinding it in a ball mill down to a grain size of 0.5 to 10 micrometers

2. Forming the powder into shape

3. Heat treating it• The most common forming process is the oxide powder in tube (OPIT) method.



Case Study 18.1 Production of High-Temperature Superconducting Tapes

Significant progress has been made in recent years in understanding high temperature superconducting materials and their potential use as electrical conductors. Two bismuth-based oxides are superconducting ceramic materials of choice for various military and commercial applications, such as electrical propulsion for ships and submarines, shallow water and ground minesweeping systems, transmission cable generators, and superconducting magnetic energy storage (SMES). A variety of processing methods have been explored to produce wires and multifilament tapes. The powder-in-tube process (Fig. 18.15) has been used successfully to fabricate long lengths of bismuth-based wires and tapes with desirable properties. The following example demonstrates this method for the production of high-temperature superconducting multifilament tapes.



Case Study 18.1 Production of High-Temperature Superconducting Tapes

Production of the multifilament tapes involves the following steps:

1. A composite billet first is produced using a silver casing and ceramic powder. The casing is an annealed high-purity silver that is filled with the bismuthceramic powder in an inert atmosphere. It is compacted in several increments to a 30% relative density using a steel ram. In order to minimize density gradients (such as those shown in Fig. 17.11), about one gram of powder is added to the billet for each stroke of the ram. Each billet is weighed and measured to verify the initial packing density. The billet ends then are sealed with a silver alloy to avoid contamination during subsequent deformation processing.



Case Study 18.1 Production of High-Temperature Superconducting Tapes 2. The billet is extruded and drawn to reduce its diameter and increase the powder density. Billets are drawn to a final diameter of 1.63 mm on a draw bench using 12 passes with a 20.7% reduction per pass. The dies have a semi-cone angle of 8°, and the drawing speed is approximately 1.4 m/min. A semi-soluble oil and zinc-stearate spray are used as lubricants.

3. Following the drawing process, the wire is transformed progressively into tape in a single-stand rolling mill in two-high and four-high configurations.For the four-high case, the diameter of the backup rolls (which are the work rolls for the two-high configuration) is 213 mm, and the diameter of the work rolls is 63.5 mm. The final tape dimensions are 100 to 200 micrometer in thickness, 2 to 3 mm in width with a ceramic core ranging from 40 to 80 micrometer in thickness and 1.0 to 1.5 mm in width.



• Ceramic products are shaped by various casting, plastic forming, or pressing techniques. The parts then are dried and fired to impart strength and hardness. Finishing operations (such as machining and grinding) may be performed to give the part its final shape and dimensional accuracy or to subject it to surface treatments. Because of their inherent brittleness, ceramics are processed with due consideration of distortion and cracking. The control of raw-material quality and processing parameters also is important.

Concept Summary



• Glass products are made by several shaping processes that are similar to those used for ceramics and plastics. They are available in a wide variety of forms, compositions, and mechanical, physical, and optical properties. Their strength can be improved by thermal and chemical treatments.

• Continuous methods of glass processing are drawing, rolling, and floating. Discrete glass products can be manufactured by blowing, pressing, centrifugal casting, or sagging. The parts subsequently may be annealed to relieve residual stresses.

Concept Summary



• Design considerations for ceramics and glasses are guided by such factors as their general lack of tensile strength and toughness and their sensitivity to external and internal defects. Warping and cracking during production are important considerations.

• Manufacturing superconductors into useful products is challenging because of the anisotropy and inherent brittleness of the materials involved. Although new processes also are being developed, the basic process consists of packing the powder into a silver tube and deforming it plastically into desired shapes.

Concept Summary

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2005 Pearson Education South Asia Pte Ltd Forming and Shaping Processes and Equipment Manufacturing Engineering and Technology 13.Rolling of Metals 14.Forging