Production Technology PDF

664 A Textbook of Production Technology Nickle Plating : is done as a protective coating as it has excellent adhesive properties which form good base for chrome plating. Chrome plating is corrosion resistant and also enhances the look...

664 A Textbook of Production Technology Nickle Plating : is done as a protective coating as it has excellent adhesive properties which form good base for chrome plating. Chrome plating is corrosion resistant and also enhances the looks of the component. (D) Paint Coating and Slushing. Metal parts are painted to protect their surfaces against corrosive action of the surrounding medium and also to improve their appearance. The process of coating with paints and varnishes is carried out in three stages : preparation of the surface to be coated, painting, and drying with finishing. These coatings are known as ‘‘Organic Coatings’’. Preparation for painting consists in cleaning and degreasing surfaces. The surface so prepared is then primed for better adhesion of the subsequently deposited coatings. Use is made of oil- varnish, Oleo- bituminous, water - soluble and nitro - soluble primers. The primed surface is then treated with a filler, whose layer should be as thin as possible. Oil- varnish fillers and quick - drying pyroxylin fillers are commonly used. In the case of machines, the clearance surfaces (those that do not mate with surfaces of other parts during operation) are painted and the preparation for painting consists in cleaning and degreasing the surfaces, priming, luting (puttying) and smoothing down the luted surfaces with emery cloth. Painting is done by applying one or several layers of paint. Oil and enamel paints and varnishes are used for this purpose. Enamel paints include oil enamels, nitro-enamels and spirit enamels. Nitro-enamels dry in 30 to 40 min ; after drying these form a hard glossy layer. Oil and spirit enamels dry for 24 to 48 hours. Methods of Painting : The various methods of industrial painting are: Brush painting, Spray painting, Dip coating, Flow coating and painting in drums. (1) Brush painting. Brush painting is used in piece and small lot production. It is done by hand and is a slow and cumbersome method, where quick-drying paints are used. The method requires minimum of equipment but maximum of labour. The point losses are upto 5%. (2) Spray painting. The method consists in applying fluid paint in the atomized form. This method is the most common and productive, but requires premises equipped with exhaust devices and spraying equipment. There are various ways of spray painting : mechanical spraying, air spraying, airless spraying, and electrostatic spraying. The method allows coating with quick- drying paints (nitrolacquers and enamels) and formation of smooth coated surface. The method can be easily automated by the aid of special installations and industrial robots. (i) Mechanical spraying. In this method, the paint is delivered to a spray gun by a pump. (ii) Air spraying. In this method, the paint is sprayed by a jet of compressed air which carries the paint mist to the surface being painted. The method is capable of coating 30 to 80 m2 of surface per hour, but the losses are high (40 to 50%). (iii) Air-less spraying. In this method, point heated to 70 – 90°C is forced through a nozzle at a pressure of 2 to 4 N/mm2 and so sprayed. The method allows the use of highly viscous paints, which cuts the solvent requirements and drying time. The production rate can be 50 – 200 m2 of surface per hour and paint losses amount to 25 – 50%. (iv) Electro-static spraying. In this method, a negatively charged sprayer delivers paint which gets onto the surface of a positively charged metal part being painted. The charging is provided by a high-voltage constant-current source. The method can also be used for non-metallic parts by setting up metal screens behind the parts. The paint losses are less than 5%. The method makes it possible to improve working conditions, to provide for fairly high productivity (50 m2 of surface per hour) and for the possibility of developing a fully automated painting process. (3) Dip coating. Dipping or dip - coating is used in automatic production. The method consists in dipping the parts suspended from a chain conveyor, in a paint bath. The method is used in large - lot and mass production for painting parts of simple shape. Operating conditions present no Special Processing Methods 665 hazard to health and are fire - safe. The method offers high productive output and low cost. Paint losses are less than 5%. (4) Flow coating. In this method, the part being painted is put for some time under the jets of paint, whereby the paint flows over the part surface, forming a smooth, dense and uniform coat. (5) Drum painting. Painting in drums is employed for small parts in mass production. The parts may be either stationary or may travel on a conveyor. The method is used to form a single - layer coat of quick drying paints on small single - type parts. Roller coating is used to paint sheet material. Drying. After being painted, the parts, units or machines undergo drying. The quality of paint coatings depend on the method of drying. Drying is a complex chemical process involving evaporation of the solvent and oxidation or polymerization of the coating film. Drying can be natural or artificial. The natural or air drying is carried out at a temperature of 18 to 25°C over a long time. The artificial drying makes it possible to speed up solidification of the coat and also to greatly improve its quality. The most common method of artificial drying is by convective heating. The drying is carried out in a closed chamber which is air - heated by gas, electricity or steam to a temperature of 55° to 220°C or by means of a reflector equipped with banks of special electric lamps. The latter method takes only from one fourth to one half as much time as hot - air drying. other drying methods utilize high - frequency currents (induction heating) and infrared rays. The latter are used in drying parts coated with lacquers and enamels. Finishing. The finishing of a painted surface includes : varnishing, polishing and decorative design. Varnishing increases stability of the paint and makes it glossy; the varnish is applied on the painted surface in one or several layers. Polishing is used to obtain a shiny surface by means of felt wheels or bands with special polishing pastes. Decorative design involves the application of narrow decorative lines, ornaments and trade marks to the painted surface. Note. Sometimes parts are painted without preliminary preparation of the surfaces and without finishing. This simplified procedure is used for parts such as automobile rear axles, gearbox cases, etc. Machine components are usually painted before assembly, but in some cases painting is done after the product has been fully assembled and tested, for example, machine tools and other manufacturing equipment. After coating the parts with the slushing compound they are wrapped in waste paper. Varnishes. Varnishes are homogeneous mixtures of synthetic or natural resins in a solvent. The commonly used resins are : amber, copal, shellac or lac. The common solvents are : turpentine oil, methylated spirit, alcohol and linseed oil. A dryer is also added to hasten the drying of varnish. The varnish is applied as a protective and decorative coating. When it dries, it leaves a hard, transparent, glossy and hard lustrous film on the surface of the part. When the solvent is methylated spirit, we get “Spirit varnish”. This varnish is very quick drying. However, the film produced on the surface has a tendency to crack and peel off. Such varnishes are usually used for polishing wooden surfaces. When the solvent is linseed oil or turpentine oil, we get “oil varnishes”. These varnishes take longer to dry, but the coating is hard, lusturous and durable. Lacquers. Lacquer is a “film forming material which dries very quickly by the evaporation of solvent. Most lacquers are made of nitrocellulose dissolved in volatile organic solvents, with pigments added for colour. Because of their quick drying property, they find a great application in auto industry. A coat of primer is a must because of the poor adhesive property of lacquers on metal surfaces. Clear lacquers are used for protection against indoor atmospheres (painting of indoor woodwork, metal surfaces, furniture etc.), while pigmented lacquers are useful for outdoor applications as well. 666 A Textbook of Production Technology Vinyl lacquers are impermeable to water, are chemically resistant and are free from odour, taste and toxicity. So, they find applications in linining food and beverage containers. Shellac. It is a solution of natural resin (lac) in an alcohol. It hardens by the evaporation of the thinner used. It does not penetrate wood surfaces deeply. So, it is often used as a sealing coat on wood and will give a durable film. Since it is soluble only in an alcohol, varnishes and lacquers can be used over it without the two running together. (E) Inorganic Coatings. These coatings are made up of refractory compounds. They are harder, more rigid, and have greater resistance to elevated temperature than organic coatings (paints, varnishes, Lacquers, Shellac etc.) Inorganic ocatings include : Porcelain enamels Ceramic coatings For porcelain enamels see Art. 13.6. Ceramic coatings are viterous and metallic oxide coatings. They contain a higher percentage of alumina than porcelain enamels and are more refractory than these. These coatings protect the metal from oxidation and corrosion and increase its strength and rigidity and hence its wear resistance. The chief ingredient of these coatings is silicate powder. However, coatings are based on oxide materials, carbides, silicides and phosphates. They are applied by : dipping, spraying, flow coating etc. 12.5.4. Temporary Coatings or Corrosive Proofing:- The protective coatings discussed under Art. 12.5.3 are known as ‘‘permanent coatings’’ and these are used for parts in service, so that they do not get corroded while in service. These coatings remain on the surface permanently. However, parts intended for long-term storage or transportation are subjected to ‘‘Temporary Coatings’’ or ‘‘Corrosive Proofing’’. These coatings can be removed from the surface without damaging it. Such coatings are produced by applying a slushing compound (Vaseline, Rifle grease): With brushes, by dipping in a hot slushing solution or by spraying. Use is also made of anti corrosive varnishes, which can be removed by de-slushing with petrol or other solvents. Effective corrosive proofing is also done by dipping parts for 2-3 min in a tank with 30% solution of sodium nitride heated to 40-50°C, and by wrapping in paper impregnated with a 10% solution of sodium nitride and other corrosion inhibitors. 12.6. ADHESIVE BONDS Permanent joints can be made by means of adhesives applied in a thin layer between the connected parts. They are employed to fasten together metal elements, metal and non-metallic elements (textolite, foam plastic etc.) and two non-metallic elements. Advantages 1. Reliable connection of parts made of very thin sheet materials. 2. Dissimilar materials can be joined. 3. The process is used to reduce production costs. 4. It is used to lessen the mass of parts. 5. Highly skilled labour is not required for bonding. 6. The process provides for tight and corrosion - free joints. 7. Smooth bonded surfaces. 8. Exterior surfaces remain smooth. 9. Only low temperatures are involved, therefore, absence of stresses or their lower concentration. 10. Heat sensitive materials can be joined without any damage. 11. Complex assemblies can be made at low cost. 12. Adhesives contribute towards absorption of shocks and vibrations. Special Processing Methods 667 13. Adhesive bonds can tolerate the thermal stresses of differential expansion and contraction. 14. Because the adhesives bond the entire joint area, good load distribution and fatigue resistance are obtained. 15. The joints are sufficiently strong in shear and withstand dynamic and variable loads. 16. Compared to welded, soldered and riveted joints, adhesive bonded parts have uniformly spread stresses and do not tend to warp. 17. The process is very fast. 18. Stress concentrations are reduced or entirely absent. 19. Adhesives are indispensable where welding, brazing or soldering fails or where bolts and rivets can not be used for fear of stress concentration, e.g., honeycombings structures. Disadvantages 1. Comparatively low operational temperatures (maximum upto about 100°C for most adhesives). 2. Low resistance to tear - off. 3. Reduced strength of some adhesives in the course of timing (ageing). 4. Tendency to creep, if subjected to long standing and heavy loads. 5. The need for extended polymerization time. Applications Adhesive bonds have been particularly developed in the air-craft industry : The appearance of honeycomb elements is entirely due to this method. The method is used in critical structures such as control surfaces in air craft and entire aircraft body. In machine tools, adhesives are employed to bond carriage - guide - ways to beds, and in automobile industry to fasten friction linings to clutch - disks and brake - bands. Adhesive bonds are also used in appliance and consumer goods fields and also for sealing, vibration damping and insulating etc. 12.6.1. Adhesive-bonded Joints. Adhesive-bonding of parts is effected on the following types of surfaces : 1. On cylindrical surfaces, for example, placing bushings into holes in housing - type parts and discs onto shafts, coupling pipes together, fitting plugs, and fastening linings to brake blocks etc. 2. On flat surfaces, for example, lap-type joining of sheet parts with one or two straps and so on. Typical adhesive-bonded joints are shown in Fig. 12.2. (a) (b) (c) (d) (e) (f) (g) (h) Fig. 12.2. Main Types of Adhesive-bonded Joints. The strength of an adhesive joint is dependent on the amount of clearance in it, which is normally kept at 0.05 to 1.5 mm. With increased clearance, the strength of the joint decreases. As the length of over lapping of the joint increases, the force needed to break down the joint in- creases asymptotically approaching a certain limit. Surface roughness of the parts bonded should be held to within 6.3 to 1 μmRa. Increase in curing time has a favourable effect on the strength of the adhesive - bonded joint. With cold - curing, the strength grows continuously over a long 668 A Textbook of Production Technology period of time. The strength of joints bonded with cold curing adhesives increases if the polymerization process is accompanied by heating. Heating also greatly reduces the curing time. Making an adhesive-bonded Joint An adhesive-bonding process comprises the following steps : (i) Preparation of part surfaces. (ii) Preparation and application of the adhesive. (iii) Assembly of parts under a pressure determined by the grade of the adhesive. (iv) Heating of the assembled product. The surfaces to be bonded must be cleaned and degreased. Cleaning is done with wiping wastes, brushes or in a sand blaster. The substances used for degreasing are : acetone, trichloroethylene, Carbon tetra chloride and other organic solvents. Aluminium- alloy parts are prepared by pickling. Where necessary, the surfaces to be joined are machined to obtain a surface finish that provides for better holding of the adhesive. Adhesives are prepared in special polychloroethylene or metallic containers ; the latter need to be chrome-plated or coated with silicone varnish. Hot curing adhesives can be stored in closed containers for a long time. Cold - curing adhesives are prepared just before use as their pot life is only 30 to 40 min. The method of application of an adhesive depends on its viscosity. Liquid adhesives, that can be applied with brushes or sprayers, are used most commonly. Some grades of adhesives are convenient to apply with spatulas, rollers or injectors. The adhesive is spread in an even thin layer (0.1 – 0.2 mm) with a bristle brush or a spatula. To prevent frothing, the adhesive must be applied moving the brush in one direction. In hand pneumatic injectors, (Fig. 12.3), compressed air is supplied through an inlet connection. The air extrudes the adhesive by means of a piston through a nozzle having a diameter of 1 mm. After the application of adhesive, the parts are Piston assem-bled in special fixtures and clamped by means of lever mechanisms, springs, or pneu-matic clamping devices. Clamping force must ensure a unit pressure of 0.05 to 1.0 MN/m2. Lastly, heating is effected in cabinets equipped with electric or gas heaters. The heating temperature Nozzle and curing time depends on the composition of the adhesive. For instance, a curing temperature of 150 to 160°C and a curing time of 1.5 h is needed for a cold-curing adhesive based on epoxy resin. For a Inlet connection hot - curing adhesive based on a grade of epoxy resin, a curing time of 3 to 4 h at 150° to 160°C or 1.5 Fig. 12.3. Hand Pneumatic Injector. to 2 h at 180° to 190°C is recommended. Adhesives should be handled very carefully as their constituents are toxic. The work, therefore, should be done with gloves on, under proper exhaust ventilation. 12.6.2. Adhesives. There is a large variety of adhesives available for bonding metals with metals and metals with structural non - metallic materials. They can be classified into the following main groups : 1. Adhesives based on epoxy resins. The available epoxyresin- based adhesives are both cold and hot curing. These are used for cold and hot joining of metals, ceramics, plastics, wood and other materials. In cold curing adhesives, a curing agent, such as polyethylene polymide (8 to 10 parts by mass) or hexamethylenediamine (20 parts by mass) is added to 100 parts by mass of the resin. Special Processing Methods 669 Maleic anhydride (40 parts by mass) is added as a curing agent to the resin in making hot curing adhesives. The various epoxy resins used as adhesives are given below with the curing temperatures given within brackets : Epoxy (room temperature cure, 16 to 32°C) Epoxy (elevated temperature cure, 93 to 177°C) Epoxy nylon (121 to 177°C) Epoxy phenolic (121 to 177°C). 2. Phenol-resin based resins. These are modified by various compounds. Curing takes place at a temperature of about 150°C with the jointed components being held against each other. Phenol polyvinyl acetate adhesives are available ready-made without subsequent introduction of curing agent. These adhesives can sustain temperatures upto 70°C. Phenolic rubber and phenolic resin- based adhesives modified by organic polymers and silicone compounds feature high temperature resistance. Phenol formaldehyde is used to bond foam plastics, textile laminate etc. The other common adhesives in this group are : Neoprene - phenolic (135 – 177°C) Nitrile - phenolic (135 – 177°C) Butyral - phenolic (135 – 177°C) 3. Polyurethane adhesives. These adhesives have resistance to temperature of 100 to 120°C and the same strength as Phenol polyvinyl acetate adhesives. 4. Special grade adhesives. These are used for higher temperature resistance and possess high shearing strength. Bonding Plastic parts. The above mentioned adhesives and special- purpose adhesives are used to bond plastic parts. For many thermoplasts, their solvents serve as adhesives, for example, dichloroethane for organic glass, benzol for polystrene, acetone for viniplast etc. The scope of automation of adhesive-bonding processes is the application of the adhesive to mating surfaces, assembly and accurate location of the parts bonded, and subsequent curing. Adhesives can be applied with rollers, or fed with an injector into the clearance between the mating parts ; dipping the mating parts into it is also practicable. 12.7. SURFACE COATINGS FOR TOOLING In almost every type of production tooling, the most desirable feature to have is a very hard surface layer on a low strength but tough body. Toughness is needed to survive mechanical shocks, that is, impact loading in interrupted cuts. Shocks occur in even continuous chip formation processes, when the tool encounters a localized hard spot. The examples of such tooling include : metal cutting tools, rock drills, cutting blades, forging dies, screws for extrusion of plastic and food products and saw mills and so on. Other applications include : parts for earth moving machinery, valves and valve seats for diesel engines, and many such parts involving high heat applications and in general, applications requiring wear resistance. The various techniques employed for this purpose are discussed below : 1. Hard Facing. This is a welding technique and has already been discussed in Chapter 5 on “Welding Process”, under Art. 5.11. 2. Nitriding Case Hardening. Discussed in Chapter 2. 3. Hard Chrome Plating. Hard chrome plating is done by the Electrolytic electro - plating technique (See Art. 12.5.3). It is the most common process for wear resistance. 4. Flame Plating. Flame plating is a process developed to prolong the life of certain types of cutting tools and for severe wear applications. By this process, a carefully controlled coating of tungsten carbide, chromium carbide (Cr3 C2) or aluminium oxide is applied to a wide range of base metals. The more common materials which have been successfully flame - plated include : aluminium, brass, bronze, Cast iron, ceramics, copper, glass, H.S.S., magnesium, molybdenum, nickle, steel and titanium and their alloys. 670 A Textbook of Production Technology The process uses a specially designed gun into which is admitted metered amounts of oxygen and acetylene. A change of fine particles of the selected plating mixtures is injected into the mixture of oxygen and acetylene. Immediately after this, a valve opens to admit a stream of nitrogen to protect the valves during the subsequent detonation. The mixture is now ignited and an explosion takes place, which plasticises the particles and hurls them from the gun barrel at 750 m/s. The particles get embedded in the surface of the component and a microscopic welding action takes place, which produces a highly tenacious bond. Each particle in the coating is elongated and flattened into a thin disc. The coating has a dense, fine - grain laminar structure with negligible porosity and an absence of voids or visible oxide layers. The layer of the plated material is about 0.006 mm, and this layer can be built up, by repeating the explosions, to thicknesses ranging from 0.05 to 0.75 mm, according to the requirements of any subsequent operations. The resultant coating is dense, hard and well bonded. Because of the hard dense structure of the coatings, flame - plating has provided industry with a valuable tool for the solving of many abrasion, erosion and wear problems. For example, bushes for many applications, core pins for powder metallurgy, dies, gauges, journals, mandrels and seals for high - duty pumps, have all been given much longer lives. The process has influenced considerably certain types of cutting processes, especially in the glass, leather, paper, rubber, soap and textile industries and has proved to be of great advantage for components involving high heat applications such as “hot-end” of gas turbines. The coatings show an excellent resistance to galling and corrosion. Flame-plated coatings can be ground and lapped, if necessary. Resultant surface finish can be within the region of 0.025 Another advantage is that the components can be masked to enable the coatings to be placed precisely where required. The mixture of tungsten carbide coating material consists of cobalt ranging from 7 to 17% and the balance of tungsten carbide. Aluminium oxide plating mixture is almost of Al2O3 (Above 99%). Chromium carbide plating mixture consists of about 75 to 85% of Cr3C2 and balance of (Ni - Cr). 5. Chemical Vapour Deposition. Chemical vapour deposition (CVD) uses volatile metal compounds which are carried as vapours in a gas stream and deposited as metal upon any surface that is hot enough to produce the desired reaction. Vapour phase deposition may be done by two methods : (i) In decomposition method, a metal halide is vapourised, metered and transported by means of an inert carrier gas to the heated component, where it decomposes on the surface to yield pure metal. (ii) In the second method, a reduction process, Induction or Resisters Heating hydrogen is used as the carrier gas through a purifier Mixing Discharged Gas and dry hydrogen chemically reduces the halide to Chamber Part pure metal on the part surface, as shown in Fig. 12.4. For Carbide tools, single coat- ings can be Dryer given of Al2O3 TiC, TiN, HfC or HfN. Multiple Flow coatings of Al2O3 or TiN can also be given on top Meter of Al2O3 or TiC. Coating thickness is in icrometres. Vaporiser Purifier For depositing a layer of TiC on Carbide tool inserts, a mixture of hydrogen, methane and Titanium tetrachloride gases is formed in the mixing chamber. Metal Hydrogen The mixture of these gases then flows to another Gas chamber in which carbide inserts (WC) are heated I Heat II upto about 1000°C by induction heating or by Fig. 12.4. Reduction Method of C.V.D. Special Processing Methods 671 resistance heating. The following chemical reaction takes place near the surface of the parts : TiCl 4 + CH 4 TiC + 4HCl  TiC so produced gets adhered to the surface of the substrate, that is, WC. The main advantages of CVD process is its ability to produce : (a) High - density coatings because the coating is built up atom by atom. (b) High - purity materials (c) High - strength materials (d) and complex shapes An emerging coating technology, used particularly for multiple-phase coatings, is Medium- temperature CVD (MTCVD). It is being developed to machine ductile iron and stinless steels and to provide higher resistance to crack propagation than conventional CVD. 6. Physical Vapour Deposition (PVD). In the basic form of PVD method, metal or an oxide is evaporated by applying sufficient heat with the help of one of the many techniques. The atoms or molecules so produced move in all directions. When they come into the atomic or molecular attraction of the component, that is, the substrate, they condense on it to form a uniform coating. In a variation of the method, a cathode target (substance to be deposited) is bombarded by accelerated ions (usually of an inert gas such as Argon). This impact dislodges or drives off (Sputtering) single atoms or small clusters into the surrounding gas for deposition on a nearby substrate surface. To increase coating adhesion and improve film structure, the substrate surface is heated to temperature from about 200°C to 500°C. PVD process is particularly suited to TiN coating of H.S.S. tools, because it being a relatively low temperature process, the tempering temperature point of HSS is not reached. So, after the PVD process, the heat treatment is not needed. 7. Diffusion Coatings. The surface hardness of low carbon steels (with C < 0.2%) can be increased by making them hardenable by diffusing carbon or nitrogen into the surface. On heating and quenching, the carbon- nitrogen enriched surface becomes very hard, but the core remains tough. The surface can also be hardened by ‘ion nitriding’ method, where the steel surface is bombard by low energy nitrogen ions produced in a plasma. 8. Ion-plantation. In this method, high energy ions are penetrated into the surface. For cutting tools, nitrogen ions are most commonly used. There is virtually no change in dimensions in the last two processes. 12.8. FINE BLANKING In sheet metal working, there is a great demand for blanks with very clean-cut edges, perpendicular to the sheet surface and of a surface finish sufficiently smooth to allow immediate use of the parts, for example, as gears in lightly loaded machinery and close-tolerance contacting members in instruments. Clearly sheared edges square to the stock surface can also be advantageous in wleding operations and for accurate location of parts in an assembly fixture. Fine-blanking (Fine- edge blanking, Smooth-edge blanking, Fine-flow blanking) process produces precision blanks in a single operation, without the fracture edges characteristically produced in conventional blanking or piercing. The process eliminates the need for in-process secondary operation, such as `shaving for conventionally blanked or pierced parts. A quick touch up on an abrasive belt or a short treatment in a vibratory finisher may be used to remove the small burr on the blank. We know that fracture can be delayed by the imposition of a high hydrostatic pressure. This principle is exploited in the process of fine blanking. A specially shaped blank holder (A V-shaped impingement ring) is forced into the stock, to lock it tightly against the die, just prior to the beginning of the cut. The deformation zone is kept in compression and the whole thickness is 672 A Textbook of Production Technology plastically deformed. The material being sheared is not structurally separated, until the punch has fully penetrated the stock thickness. This results in the production of precise blanks. Die clearance is extremely small and punch speed is much slower than in conventional blanking. A counter punch operates with the main punch, eliminating any curvature of the part. A specially designed triple-action hydraulic press or a combination hydraulic and mechanical press is used. The outer slide holds the stock firmly against the die ring and forces a V-shaped impingement ring into the metal surrounding the outline of the part. An ineer slide carries the main blanking punch. A lower slide furnishes the counteraction to hold the blank flat and secure it against the blanking punch, (Fig. 12.5) The counter punch also ejects the blank. The stripping and ejection actions are delayed, until after the die has opened at least to twice the stock thickness, to provent the blank from being forced into the stock strip or slugs from being forced into the blank. The process is completed in the following steps : Main Punch (i) The stock to be blanked is held against the die-ring, with the help of the ring indentor. This superimposes V-ring compressive stresses on the stock. According to Siebel, for a suffi- ciently high, superimposed compressive stress, the shear fracture stress will become Stock larger than the shear flow stress, thereby, Strip reducing the chances of crack formation. In conventional blanking, the shear Die ring Blank fracture stress is lower than the shear Counter Punch flow stress. Bridgeman has also shown that by super- imposing a pure hydrostatic compressive stress fully smooth sheared surface can be obtained. Fig. 12.5. Fine Blanking. (ii) The stock is then pressed by a counter punch against the main blanking punch, and the two punches move down- wards. (iii) After shearing, the main blank- ing punch and the impingement ring move upward. Simultaneously, the counterpuch moves upwards and ejects the blank out of the Top die die-ring. Fine blanking is not restricted to compund dies only, but also utilizes progressive and transfer tooling technology. Forming, bending and coining are some of the features that can be combined into Shell fine blanking tools and applications. Advantages of fine-blanking process are : 1. Clearly sheared edges over the whole Bottom die material thickness. 2. Improved flatness. 3. Maintenance of close tolerance dimensions. 4. Part repeatability for the life of the tool. 12.9. COUNTER-BLANKING In this recently developed method, two punches are employed for completing the blanking: Fig. 12.6. Nosing. main blanking punch and a counter punch. Blanking Special Processing Methods 673 is completed in three stages : 1. The main punch (top punch) penetrates into the stock only slightly. 2. Counter-blanking takes place, that is, the bottom punch (Counter punch) comes into action (starts moving upwards). 3. In the last stage, the top punch completely penetrates the stock and completes the blanking process. The process is carried out in a transfer tooling and three sets of punches are employed. For every stage, clearances are matched. This process does not produce blanks of the same quality as produced by fine blanking process, but, sheared surfaces, completely free from burr, with an increased smooth portion can be obtained. 12.10. NOSING Nosing is a hot or cold forming process in which the open end of a shell or tubular component is closed by axial pressure with a shaped die, Fig. 12.6. PROBLEMS 1. Why is ‘hot machining’ needed for some materials ? 2. Write a short note on “Hot machining”. 3. Write a short note on ‘Unit heads’. What are the advantages of unit heads ? 4. Give the various applications of Plastic tooling. 5. Name the materials for plastic tooling. How are plastic toolings produced ? 6. What is “Electro-forming” process Give its advantages. 7. Give the various types of mandrels used in electro-forming process. 8. Give the applications of electro-forming process. 9. What are the needs of surface cleaning and surface treatments ? 10. Discuss the following methods of Surface cleaning : (a) Alkaline cleaning (b) Solvent cleaning (c) Vapour cleaning (d) Pickling (e) Ultra-sonic cleaning 11. What are the advantages of ultra-sonic cleaning ? Write the applications of this process. 12. What is Surface polishing ? 13. Discuss the following methods of surface polishing : (a) Chemical polishing. (b) Electrolytic polishing. 14. What is the use of surface coatings ? 15. What is “electro - deposited coating” ? 16. What is the function of the following types of electro - deposited coatings: Copper coating, chrome plating, cadmium plating, Nickle plating, Lead plating, Zinc plating, Silver plating, Tin plating, Gold plating, borating, phosphating and lead-indium plating ? 17. Discuss the following types of Coatings : (a) Phosphate coating (b) Oxide coating (c) Plastic coating (d) Metallization (e) Anodizing. 18. Discuss the various hot-dip coatings. 19. Write a short note on “Paint coating and Slushing”. 20. Write the advantages, disadvantages and applications of Adhesive bonded joints. 21. Write about the various adhesive bonded joints. 22. Discuss the steps of making an adhesive joint. 674 A Textbook of Production Technology 23. How the adhesives are applied at the joint ? 24. Discuss the various adhesives used for adhesive joints. 25. What is the significance of ‘surface coating’ for tools ? 26. Discuss the process of “flame plating” of tool surfaces. 27. What are CVD and PVD processes of surface coatings ? Write their applications. 28. What are : Diffusion Coating and Ion - plantation ? 29. Write short notes on:- (a) Fine blanking (b) Counter blanking (c) Nosing 30. Discuss the machanical methods of cleaning and finishing of surfaces. 31. What is emulsion cleaning of surfaces ? 32. Write the advantages of chemical polishing. 33. What is brass plating ? 34. What is “thermo-electro-plating” or “thermo-diffusion” ? 35. What are conversion coatings ? 36. Discuss the various methods of paint coating. 37. Write on the “Drying” and “Finishing” processes after the operation of painting. 38. Write briefly on: Varnishes, Laquers and Shellac. 39. Write briefly on “Inorganic Coatings”. 40. Name the common conversion coatings. 41. What are primers ? 42. What is preparation of the base ? 43. What is the principle of electro-plating ? 44. Compare the dipping process with electro-plating. 45. Describe the main characteristics of an anodized surface. 46. List the advantages of fine blanking. 47. Compare fine blanking and Counter blanking. 48. List the main advantages of CVD process. 49. Compare diffusion and ion plantation. 50. Why parts have to be coated with ceramics ? 51. How is hot dipping performed ? 52. Where is metallizing used ? 53. List the main drawback of lacquers. 54. What products have made use of vinyl lacquers ? 55. What are the main ingredients of an oil paint ? Explain their functions. 56. What is electro-less plating ? Write its advantages and disadvantages. 57. Explain the difference between electro-plating and electro-less plating. 58. List the similarities between electro-plating and anodizing. 59. How is hot dipping performed ? 60. Why is galvanizing important for automotive body sheet ? 61. List some applications of mechanical surface treatment. 62. Give the main specifications of different Unit Heads. Unit Head Main Specifications (i) Unit Heads, Boring : Bore , Power (kW) (ii) Unit Heads, Drilling : Drill , Power (kW) (iii) Unit Heads, Milling: Cutter , Power (kW) (iv) Unit Heads : Combination/SPM: No. of heads, Product.  Chapter 13 Ceramic Materials and Their Processing 13.1. GENERAL The word “Ceramic” is derived from the Greek “Keramos”, which means potter's earth or clay. Therefore, ceramics may be considered as materials made from naturally occuring clay or earth. Thus, to the common man, ceramics mean pottery, Chinavares and like materials. In a narrower definition, ceramics are compounds of metallic and non-metallic elements. However, the above definition of ceramics is not complete, because it leaves out such manufactured ceramic materials as diamond, SiC and Si3N4 and so on. Thus, in modern applications, a broader definition regards ceramics as everything that is not a metal or organic material. That is, ceramics are inorganic and non-metallic materials and which are processed or used at high temperatures. The American Ceramic Society has defined ceramic products as those manufactured" by the action of heat on raw materials, most of which are of an earthy nature (as distinct from metallic, organic etc.) while of the constituents of these raw materials, the chemical element silicon, together with its oxide and the compounds thereof (the silicates), occupies a predominant position.” Ceramic materials are used in industry for many purposes. They account for nearly 7% of the tonnage of major engineering materials, compared to about 11% of that of the steel. Ceramic articles of industry are : Dinner ware, electrical and chemical porcelain, refractory bricks and tiles, glass, porcelain enamels, abrasives, cutting tools, bricks and tiles, cements and concretes, whitewares, mineral Ores, slags and fluxes and insulators etc. 13.2. CLASSIFICATION OF CERAMICS A general classification of ceramics is difficult to make, because of the great versatility of these materials. However, ceramics may be classified as given under : - (1) Ceramics can be natural or manufactured. (a) Natural Ceramics : The most frequently used, naturally occuring Ceramics are : Silica (SiO2), Silicates and Clay minerals. (b) Manufactured Ceramics : Such ceramics include : SiC, Al2O3, Silicon Nitride (Si3N4) and many varieties of Oxides, Carbides, Nitrides, Borides and more complex ceramics. Some of the naturally occuring ceramics are also, manufactured which results in their enhanced properties, as compared to natural ceramics. For example, Magnesia (Magnesium Oxide, MgO) also occurs in nature, but for industrial use, it is made from the Carbonate or Hydroxide. However, all the ceramics not available naturally, are manufactured. The natural ceramics are also called as “traditional ceramics”, while manufactured ceramics are usually called as “High-tech ceramics” or “fine ceramics”. 675 676 A Textbook of Production Technology (2) Functional Classification : This classification indicates particular industries and industrial applications of the ceramics, as given below : (a) Abrasives : Alumina, Carborundum (b) Pure Oxide Ceramics : MgO, Al2O3, SiO2, Zirconia (ZrO2) and Berylia (BeO) etc. (c) Fired-clay products : Bricks, Tiles, Porcelain etc. (d) Inorganic glasses : Window glass, lead glass etc. (e) Cementing materials : Portland cement, Lime etc. (f) Rocks : Granites, Sandstones etc. (g) Minerals : Quartz, Calcite etc. (h) Refractories : Silica bricks, Magnesite etc. (3) Structural Classification : This classification indicates the structural criteria as given below: (i) Crystalline Ceramics : Single phase like MgO or multiphase from the MgO to Al2O3 binary system. (ii) Non-Crystalline Ceramics : Natural and synthetic inorganic glasses e.g., win- dow glass. (iii) “Glass-bonded” Ceramics : Fired clay products - Crystalline phases are held in glassy matrix. (iv) Cement : Crystalline or Crystalline and non-Crystalline phases. The American Ceramic Society has classified “Ceramics” into the following groups : 1. Whitewares 2. Glass 3. Refractories 4. Structural clay products 5. Enamels. 13.3. PROPERTIES OF CERAMICS The co-valent bonding of ceramic materials, alongwith their high melting point and relative resistance to oxidation, make ceramics good candidates for high temperature applications. In addition, they are relatively cheap and abundant and are not dependent on import for supply. In general, ceramics are hard, brittle and high melting point materials with :- — desirable electrical, magnetic and optical properties, i.e., low electrical and thermal conductivity. — good chemical and thermal stability, that is, high hot-strength and high corrosion resistance, and freedom from oxidation. — good creep resistance, and — High compressive strength and excellent resistance to wear — Their low density is also an attractive feature to minimise centrifugal stresses in parts rotating at high speed. Many ceramics retain strength to much higher temperatures than metals. Despite the generally excellent high temperature strength, many of the ceramics are susceptible to thermal shock (Due to brittleness). Porous ceramics (for thermal insulation) are resistant to thermal shock, whereas the same ceramic, in dense form, for structural use may be susceptible. Since ceramics are brittle materials, their use in the past has severely been limited in structural applications. They have no yield strength and fail when the local stress exceeds the material fracture