Processing of Plastics PDF

Processing of Plastics 639 deformed at room and elevated temperatures. Even at their flow temperature, they behave like a highly viscous liquid. Therefore, some sort of pressure has to be applied to make a welded joint for p...

Processing of Plastics 639 deformed at room and elevated temperatures. Even at their flow temperature, they behave like a highly viscous liquid. Therefore, some sort of pressure has to be applied to make a welded joint for plastic articles. As we know that heating has no softening effect on thermo-setting plastics, therefore, they can not be welded by conventional processes. Thermo-setting plastics are available as semi-finished products, which can be either joined mechanically or cemented. On the other hand, when heated, thermo-plastics can repeatedly pass into a highly elastic and then into a plastic state, without losing their original properties on cooling again. These plastic materials are worked by the application of heat and pressure and so can readily be welded. For making permanent connections between plastic parts and components, welding is superior to cementing and riveting, in many respects, as it offers : high speed, low labour requirements, economy, improved working conditions, stronger and tighter joints. The various welding processes for plastics can be grouped into two broad groups, depending upon the source of heat used : (a) Welding processes, utilizing heat from an external source, such as, a stream of hot gas, a hot extruded filler metal, or a hot tool. In all these processes, heat is transferred to the surfaces being welded by convection, conduction and partly by radiation. (b) Process in which heat is generated within the workpiece through conversion of some other form of energy, such as, r.f. current, ultrasound, friction, infrared light, chemical reactions, or nuclear welding. The common welding processes for plastics are discussed below. The application of heat and pressure during welding, causes the plastic workpieces to undergo autohesion. 1. Hot-Gas Welding. In this process, the edges to be joined are heated by a stream of hot gas from a torch, [Fig. 11.10 (a)]. The hot gas can be : air, nitrogen, products of combustion of some fuel gas, such as acetylene or hydrogen. The hot gas heats the edges and the filler rod to viscous fluid state. As the filler rod is forced down by hand, it welds to the softened edges and forms a weld. This technique is like manual gas welding of metals. The drawbacks of the process are : Low joint strength, reduced plasticity in the weld and near-weld area, low welding rate, especially in thick sheets, danger of overheating, and dependence on the operators skills. Pressure Torch Tip Pressure Filler rod Weld Hot gas Sheets Heated area Rollers (a) Heated area (b) Fig. 11.10. Hot Gas Welding. Hot gas welding, without filler rods, [Fig. 11.10 (b)] speeds up the process and enhances the mechanical properties of the point. The edges are scarfed and sheets are fitted up for welding, and the edges are uniformly heated by hot gas. The hot gas jet is followed by cold rollers, which complete the weld by pressure. The process is most often applied to make lap joints in films. 2. Hot Tool Welding. In this process, a hot tool transfers heat to the plastic workpiece by 640 A Textbook of Production Technology direct contact. There are many variations of this process. In Fig. 11.11 (a), a hot blade is placed between the surfaces to be joined. After the hot blade has softened the surfaces, it is rapidly withdrawn. The joint is then completed by applying pressure. Lap and but joints over the entire surface of contact can be obtained at the same time. Hot blade Roller Pressure Direction of weld Start of weld Heating Top strips wedge End of weld bottom sheet (a) Resistance (b) element Pressure Resistance Plastic Hot plate element pieces Films Press Ducts Work Plate (c) (d ) Fig. 11.11. Hot Tool Welding. In Fig. 11.11 (b), a heating wedge is placed between the surfaces to be joined and is moved along the line of welding as the edges are softened. A roller applies pressure to the top strip which is then welded to the bottom sheet. This technique is applicable to elastic materials. The technique is used to weld thin rigid sheets or attach straps upto 5 mm to a thicker sheet. In Fig. 11.11 (c), a hot plate heated by a resistance element is moved over the films to be lap joined, raises them to welding temperature, and applied pressure to complete the joint. In a variant of this technique, a strip heater is advanced over the strips with the help of rollers. In hot press welding, [Fig. 11.11 (d)] heat is transferred to the area of welding by the hot platen of a welding press. The plastic pieces with their edges scarfed are champed in press having platens heated by resistance element. After the workpieces have been raised to the welding temperature, they are allowed to stay under pressure, as the press is cooled by water circulating in ducts. Presses usually make butt joints. Hot tool welding can be manual and semi-automatic. The process produces strong welds and the welding rate is sufficiently high. It is applicable to plastics, which can not be joined by r.f. induction heating (such as PTFE, polyethylene and polystyrene). 3. Friction Welding. Plastics can be friction welded, in much the same way as metals (See Fig. 5.37, chapter 5). Mechanical energy is directly converted to heat on the surfaces being welded. Friction welding has the following limitations : (i) One of the pieces must be a body of revolution, and its section at the joint must be a circle or an annuals : (ii) When pressure is applied to the pieces to complete the joint, flash is formed at the connection. 4. Ultrasonic Welding. An ultrasonic plastic welder has the same elements as one for metals (See Fig. 5.40 chapter 5). Ultrasonic welding is applicable to acrylics, PVC, polystyrene and synthetic textile. Lap and tee spot joints are made best of all. Satisfactory joints are also made in the case of lap welds in static jigs. In all cases, neither edge preparation nor filler material is needed. Ultrasonic welds can also be made in dissimilar plastics. Processing of Plastics 641 5. High Frequency Induction Welding. The high-frequency induction welding of plastics is a widely used process. The principle is the same as of the process of induction welding of metals (See Fig. 5.36 b, Chapter 5). The process consists in that the workpiece is placed in a high-frequency electric field, set up between two metal electrodes. The arrangement is similar to Fig. 5.28 (Chapter 5). The electrodes can be in the form of rollers, (see Fig. 5.31.) for seam welding. After the plastics have been heated in the r.f. field, the electrodes apply pressure to complete the joint. 6. Extruded-filler Welding. In this method, the filler fed into the joint is in a viscous fluidic state. The hot filler material melts the edges of the plastic being joined, and a strong bond is formed between the filler and parent material. The process makes satisfactory welds in both films and heavy-gauge sheets. 7. Nuclear Welding. In this method, the workpieces to be joined are irradiated with a steam of neutrons. The surfaces to be welded are bombard these elements, nuclear reactions take place, resulting in the generation of heat. Due to this, the surfaces to be joined turn into a polyethylene, polystyrene, quartz, aluminium and some other metals. The method can not be applied to materials which become strongly radioactive, when irradiated with neutrons. 8. Infra-red Welding. In this process, welding heat is supplied by a source of infra-red light, such as, a sylite glower, a chrome-steel is carried out on a black backing plate of a formed plastic, sponge rubber or thick rubberized fabric. The resistence of the back up plate held firmly against the workpiece supplies the necessary welding pressure. The process is satisfactory for joining polyethylene films. As noted above since the thermoplastics soften and melt as the temperature is increased, the methods discussed above, welding (fusion methods) and other methods are used to join thermoplastics. Thermoplastics can also be joined by adhesive bonding. Joining of Thermosets Thermosets do not soften on heating. So, the above methods can not be used to join thermosets. Thermosets are usually joined by using :– (i) thereaded or other moulded in inserts. (ii) Mechanical fasteners. (iii) Solvent bonding. This method involves the following steps :– (a) The surfaces to be joined are roughened with an abrasive. (b) Wiping the surfaces with a solvent. (c) Pressing the surfaces together and holding the pressure until sufficient bond strength is obtained. Solvent bonding is also applicable for certain types of thermo-plastics resins (acrylics, polystyrenes, cellulosics and some vinyls). 11.12. DESIGN OF PLASTIC PARTS In addition to providing necessary functional requirements, the optimum moulded part design should give consideration to factors involving mould convity making (economy of manufacturing and strength of mould members) and simplification of processing problems. While designing the moulded plastic parts, the following design rules should be followed : 1. Allow for shrinkage after moulding. All moulded parts are subject to some after shrinkage upon ageing. 1 2. Allow atleast a minimum draft of to 1 degree for easy withdrawal of the parts from the 2 mould. 642 A Textbook of Production Technology 3. Avoid undercuts whenever possible. They prevent removal unless special mould sections are provided that move at right angles to the opening motion of the main mould halves. Such moulds are costly to construct and to maintain. Alternatively, the undercuts will need cores or split cavity moulds. 4. If possible, the parting line should be located in one plane. 5. Design corners with ample radii or fillets. Provide adequate fillets between adjacent sections also. This will assure smooth flow of the molten material into all sections of the mould and will also eliminate stress concentration at sharp corners. Such a mould will be less expensive to build and also less prone to breakage where thin, delicate mould sections are encountered. 6. The curing time of the product is determined by its thickest section. Thus, thick sections should be preferably kept as nearly uniform in thickness as possible. The minimum wall-thickness depends upon the size of the product and the type of plastic used. It is also limited by the difficulty of removing very thin parts from the mould and also by the high pressures needed to fill at a high width-to-thickness ratio. The minimum recommended wall-thickness is 0.65 mm and it can be 3.2 mm for large parts. Variation in wall thickness of the moulding should not be over 30%. The large variations is cross-section sizes and abrupt changes in geometry should be avoided for better product quality and increasedd mould life. 7. Ribs should be provided to increase strength and rigidity and to reduce distortion. When extra strength is needed at corners, it is better to provide ribs there, than to have thick corners. which are likely to lead to gas pockets, under curing or creaking. Rib height should not be more than twice its thickness, which should be 0.6 to 0.8 that of the adjoining wall. The ribs should be arranged in the direction of the material flow on the mould. 8. Plastics have low modulii of elasticity. Therefore, large flat surfaces will not be rigid and should be avoided, whenever practicable. However, their strength can be increased by ribbing or doming. 9. Through holes are limited only by the strength of the core pin and are usually held below a length-to-diameter ratio of 8. Blind holes are also made with the help of core pins and these are limited to a depth-to-diameter ratio of 4 for diameter greater or equal to 1.5 mm and to a ratio of 1 for smaller holes. Threaded holes of diameter equal to and greater than 5 mm can be moulded directly. It is better to drill smaller holes. Smaller threads of reasonable strength are best provided by metal inserts. Binding posts, electrical terminals anchor plates, nuts and many other metallic components are conveniently obtained by moulded-in inserts. Metal inserts (usually made of steel or brass), are held in the plastic only by a mechanical bond, since there is no adhesion between metals and plastics. Therefore, the metal inserts are suitably knurled or grooved so that they are gripped firmly and do not become loose in service. Plastics have much greater thermal expansion as compared to metals. This helps to shrink the plastic onto the insert, but could also cause cracking of a brittle plastic. The wall-thickness around the insert, therefore, must be made large enough to sustain the secondary tensile stresses. 10. Mouldings should be simple in shape for easy removal from moulds. 11. Back tapered features in parts should be avoided since they require complicated moulds and improper moulding conditions. 12. It is advisable to reinforce the moulding end faces with shoulders, which prevent the part from cracking. The shoulders are arranged along the end face periphery without interruption. 13. If plastic parts have holes, then the minimum thickness of the wall between the holes should not be less than 0.5 mm for holes 2.5 mm in diameter and not less than 2.5 mm for holes 16 mm in diameter. Minimum distance from the part side edge of a hole = 1 mm for holes 2.5 mm in Processing of Plastics 643 diameter = 4.5 mm for holes 16 mm in diameter. 14. The l/d ratio for reinforcement elements in plastic parts (bushing cores, inserts) should not be less than 2 for their reliable fit. 15. Tolerances of Moulded Plastic Parts : - (Courtesy Kents Hand book) Tolerances are necessary in moulded parts because fo unintentional veriations in tools, materials or processing techniques. The average meulded part has three types of tolerance, Fig 11.12. (i) Dimensions like A : may be considered as fixed mould dimensions since they are independent of the two mould halves. A Cavity Parting Live A A B B C C A Fig. 11.12. Tolerances on Moulded Parts (ii) Dimensions like B : are known as built-up dimensions. They are influenced by the degree of mould closure. (iii) Dimensions like C : - exist where points being measured are on different halves of the mould. Any misalignment between the upper and lower mould halves is reflected in such dimensions. Rule of Thumb : The amount of telerances provided should be just essential. Tighter tolerances result in high tool cost, increased product cost, annoying production delays and higher percentage of rejects. Tolerances on Fixed dimension like A:- Normal dimensions mm Tolerances, mm,  Preferred Close 12.7 0.125 0.050 25.4 0.200 0.075 50.8 0.30 0.125 101.6 0.400 0.250 152.4 0.500 0.375 Tolerances on Build-up Dimension Like B:- (i) Small parts, transfer moulded :  0.125 mm (ii) Compression moulded, wood flour filled, small parts :  0.250 mm (iii) Compression moulded, learger parts, wood flour filled :  0.375 mm (iv) Compression moulded, small rag filled :  0.375 mm (v) Multiple cavity, wood flour filled :  0.675 mm Note :- If depth of moulded part exceeds 25 mm, it may be necessary to allow fixed mould tolerance in addition to above. Tolerances for Mould Mialignment (Dimension C) : - For average mouldes an additional tolerance of  0.150 mm should be allowed. more allowance shonld be made on very large moulded parts. 644 A Textbook of Production Technology Warpage Allowance :- A good average value is  0.075 mm per mm, measured from an average plane through the warped surface. The above tolerances are for phenolic parts. Note :- (a) Tolerances on Cold moulded parts are about double the values mentioned above. (b) Tolerances on Injection moulded parts usually should be somewhat larger than those for phenolic parts. (c) Grinling or machining allowance should be provided if extremely close dimensional accnracy is to be obtained. 11.13 BASIC PRINCIPLES OF DESIGN OF MOULDS FOR PLASTIC PARTS :- After the plastic part is designed the next step is the design of mould for fabricating the plastic part. The design of plastic mould should be such that it will not cause problems concerning shape generation, dimensional control and surface finish. 11.13.1. Mould Design :- Basically, a mould consists of : a plunger and a cavity which actually forms the moulded part, Supplementing these are the frame components which provide support and guidance and also the operating members which facilitate removal of the fabricated parts from moulds. Fig. 11.13 shows a schematic view of a compression mould and Fig. 11.14 shows a schematic view of an Injection mould which is a two plate mould. It is a mould with a single parting line. Note that the temperature control channels are in both the cavity and core. Note also the support Sprue Bushing Locating Ring Clamping Plate Mould Plate Cavity Plastic Sprue Parting Line Runner Temp. Core Control Channles Support Plate Knockout Pin Knockout Plates Ejector Housing Knockout Bar Fig. 11.13. A Schematic View of a Compression Mould. Processing of Plastics 645 pillars that prevent the support plate from buckling under the pressure of the injection material (fig. 11.14). The knorkout bar is atached to the machine and is the actuator for the knockout plates. A sprue puller of the “Z” type is used (Fig. 11.14) Cleanout slot Flat head screw (No. 1) Top plate Plunger Butt plate Steam core Guide pin Mould pin Retainer entering top Bushing Flash relief Shoe Gas relief Bottom set Pushback rod Steam plate Moulded part Knockout pin Flat head Parallel screw (No. 2) Support pin Pin plate Knockout bar Flat head screw Bottom plate Fig. 11.14. A Schematic View of an Injection Mould. 11.13.2. Mould Materials :- The main factors influencing the selection of material for a mould are : (i) Type of plastic to be moulded. (ii) Method of moulding (iii) Design of part to be moulded, i.e., size and complexity. (iv) Quantity of parts to be fabricated. (v) Cost The common materials used for moulds are :- (a) Steels : Alloy steels (Cr, Mo; Cr, V; Cr, Ni; Cr, W, Mn; Cr, W, V) Stainless steels Prehardened steels Hardened steels Chrome plated steels A typical composition of steel used for moulds is :- C : 0.42%; Cr : 1.45%; V : 0.25%; Mn : 0.30%; Si : 1.45% (b) Berrylium copper (c) Aluminium (d) Soft zinc-Aluminium alloy (Kirksite) (e) Cobalt-Nickle alloy (f) Copper (g) Iron Where possible, the mould pins are commonly made from commercial drill rods or music wire. Drill rod (hardened to Rockwell 48-50 C) is usually preferred. Plate steel is usually utilized for many of the miscellaneous frame components, as it is both adequate and economical for this purpose, since such components normally do nto require hardening. If needed, about 1.6 mm depth of case can be obtained by carbusisation, although, considerable correction of distortion may then be required. Examples of such components are : Retainer shoes, steam plates, parallels and knock-out bars. 11.13.3. Cavity Design :- The following types of cavity design are available for cavity design :- 1. Flash Type : Here, the cavity depth is just equal to the part size. The excess moulding compound overflows in a hosizontal plane, forming a thin flash edge, fig. 11.15 (a), 646 A Textbook of Production Technology Plunger PL Part (a) Plunger Part PL Mould (b) (c) (d) (e) Fig. 11.15. Cavity Designs. Processing of Plastics 647 The advantages of this design are :- — Simplest plunger and cavity relationship and so most economical. — No precise weight control of mould material as the excess material escapes quite readily over edges of the cavity — Low maintenance costs due to absence of any vertical rubbing action between plunger and cavity members. The disadvantages of the design are :- — Wastage of expensive moulding material in the form of flash — It is difficult to achieve high density mouldings on intricate part designs, since, the material flows out before the intricate cavities are filled. — Since the mating members depend entirely on mould pins or builtin interlocking guides, these should be maintained diligently. This design is exclusively used for injection and transfer moulding. However, within limitations listed above, the design is also used for compession moulding. 2. Positive Type :- This design is similar to closed-die forging porcess for metals. The moulding compound is completely trapped and forced into the cavity, fig. 11.15 (b) As such, there is no flash surrounding the mould cavity. Advantages — Uniformaly good resultant density of the moulded part, since the full moulding pressure is exerted against the plastic compound. — The only flash that escapes the cavity is the vertical burr between the fit of the plunger and cavity walls, which is uniform. Limitations — the plastic compound should be very accurately measured, otherwise it will not be possible to maintain close tolerances on vertical dimensions. — Mould wear due to the rubbing action that usually develops between plunger and the walls of the cavity. 3. Semi Positive Type :- This design. Fig. 11.15. (c) is a variation of the basic truly positive design. This type behaves somewhat as a flash type mould until the last fraction of the closing stroke, when the short positive portion of the plunger enters the cavity depression. — This design permits overflow of slight excess of compound charge while retaining good alignment between plunger and cavity. — The uniform vertical burr is especially benefitial for finishing operations, particularly if belt sanding can be utilized. 4. Landed Plunger Type :- Fig. 11.15 (d). Due to the design of the plunger as shown, wear between the plunger and the cavity walls does not affect the moulded part. Reasonable control of compound weight is required, although the situation is not so critical as in the truly positive design. The design is somewhat expensive, but gives excellent performance. This design is extensively used for compression moulds. 5. Sub-Cavity Type : - Fig. 11.15 (e) This design is used for high production, multiple cavity moulds for simple and small parts. A thin flash will be there between the various individual cavities. However, the extremities of the common cavity area are of landed plunger type. There is operating economy in the form of ease of loading compound into such a common cavity. Instead of loading each cavity individually, the entire lot may be loaded with one large preform. To get thin flash, high moulding pressures are envisaged. 11.13.4. Manufacture of Cavity :- The various methods for getting the desired carity in the mould material are :- 648 A Textbook of Production Technology (a) machining (b) Hubbing or Hobbing :- This is the process of pressing a hardened and polished punch (hob/master of tool steel) into a softer metal block (See Art 4.9). This method is used for softer steels, Berrylium-copper, Aluminium and kirksite etc. (c) Casting :- for Be-Cu alloy, Phosphor bronze, C.I., Al, Zn-Al alloy (d) Electro-forming :- for Co-Ni alloy, Cu, Iron (e) E.D.M. (Spark Erosion) : complex cavities can be machined in electrically conducting materials. References : 1. Kent’s Handbook 2. Plastics Engineering Handbook PROBLEMS 1. Define the term “Polymer”. 2. What is “polymerization” ? Discuss the various methods of polymerization. 3. What is degree of polymerization (D.P.) ? 4. What are the various agents added to polymers to modify their properties ? Discuss their functions. 5. What are plastic materials ? 6. What are thermoplastics and thermosetting materials ? 7. Discuss the properties of plastic materials and give their limitations. 8. Sketch and explain injection moulding process. 9. Why is screw injection moulding machine better than a ram type injection moulding machine ? 10. Sketch and explain extrusion process for plastics. 11. Sketch and explain the principle of vacuum forming process for plastics. 12. Sketch and explain the “Compression moulding” and “Transfer moulding” processes. 13. What special cares are taken when machining plastics ? 14. Sketch and explain calendering process for plastics. 15. What is “rotational moulding” of plastics ? 16. Sketch and explain “Blow moulding” process for plastics. 17. Discuss the various design factors for plastic parts. 18. Write short notes on : (i) RIM (ii) Cold forming (iii) Solid state forming (iv) Spinning (v) Thermo-plastic stamping. 19. What are “Potting” and “Encapsulation” ? 20. What is foam moulding ? 21. Discuss the various methods of welding of plastics. 22. Discuss the factors which have led to the rapid growth of polymers. 23. Write the product applications of engineering plastics. 24. List the typical product applications of compression moulding process. 25. Write the advantages of Transfer moulding process. 26. What is the difference between “Fillers” and “Additives” as used with polymers ? 27. List the advantages of cold forming of plastics. 28. What is the difference between potting and encapsulation ? Processing of Plastics 649 29. List several products that can be produced by Injection moulding process. 30. Give the advantages of thermo-plastic forming process. Where do the thermoformed parts find their largest market ? 31. How are the injection moulding machines rated ? 32. What is “Parison” ? 33. How is thin plastic film produced ? 34. What are the differences among three types of compression moulding methods ? 35. List the advantages of “Transfer Moulding” process. 36. What is “Wet extrusion” of plastics ? 37. What is the drawback of ram type extrusion machine ? 38. What are the forms of raw materials for processing plastics into products ? 39. List the product applications of “RIM”. 40. List the product applications of Calendering process. 41. List several products that can be made by Rotational moulding process. 42. List the product applications of Blow moulding process. 43. Disuss : Design of Plastic parts. 44. Sketch and label a schematic of mould for compression moulding. 45. Sketch and label a schematic of mould for injection moulding. 46. Discuss the various cavity designs for plastic parts. 47. Write a note on “Materials for Moulds” for plastic parts. 48. How the moulds for plastic parts are manufactured ? 49. What additives are used in plastics and why ? 50. What are the differencs between thermoplastic and thermo-setting plastics ? 51. List the effects a plasticizing agent have on a polymer. 52. What is an elastomer ? 53. How can you tell whether a component is made of a thermo-plastic ? 54. Explain why thermoplastics are easy to recycle than thermo-setting plastics? 55. The various plastic components in an auto-mobile are made of thermo-plastics or thermo- setting plastics? 56. List the limitations of injection moulding process. 57. List the products made by thermo-forming process. 58. List the similarities between compression moulding and closed-die forging. 59. Write the advantages of thermo-forming process. 60. List the product applications of foam moulding process. 61. List the advantages of casting process for plastics. 62. Compare the various methods for processing plastics. 63. Give the specifications of Plastic Processing Machines :- Machine Main Specifications (i) Plastic Injection Moulding Machine : Tonnage (ii) Plastic Compression Moulding Machine : Tonnage (iii) Plastic Blow Moulding Machines : Tonnage (iv) Plastic Extruders : Screw Size  Chapter 12 Special Processing Methods 12.1. HOT MACHINING A considerable percentage of the parts of up-to-date machinery are made of heat-resisting stainless steels, heat-resisting super-alloys and similar materials. This is due to the increased production of machines operating at high loads, pressures, speeds and temperatures, as well as in chemically active media. The machining of workpieces of such materials, by conventional methods, is extremely difficult and, in many cases, impossible. Very low cutting speeds and feeds will have to be employed, resulting in heavier loads on machine bearing and slides. Also, it will be quite a problem to correctly select cutting tool materials, tool life or tool geometry. Heat- resistant materials contain considerable amount of alloying elements, have a tendency to weld onto the cutting tool, lose very little of their strength, even when heated to temperatures as high as 800°C, have a very high shear strength, combine high tensile strength with high toughness, are susceptible to considerable work-hardening and have low thermal conductivity. All these features lead to the development of high cutting forces, and temperatures, and to intensive cutting tool wear. In addition, the surface finish obtained in machining is poor. Consequently, tools for machining heat-resistant materials should be very carefully sharpened and lapped. Tool geometry should be properly selected. To overcome these problems, entirely new machining methods have been developed. Some of these : ECM, EDM and USM have already been discussed in chapter 9. The method of “Hot machining” basically consists of applying localised heat, ahead of cutting tool, to reduce the shear strength of the workpiece metal (thus improving its machinability), and to permit the easy formation of the cutting chip. The chip is usually produced in the form of a long smooth chip, with lessened shock to the tool. The application of correct amount of heat, in the required place, is of maximum importance. Hence, the type of heat and its application needs to be studied with care. Heating of the workpiece also influences tool wear. Therefore, heating in the cutting process improves machinability, when the increase in tool life, due to the reduction of the work done in cutting, is greater than the detrimental effect of the high temperature on the tool, leading to increased wear. It has been established that the temperature- interval in machining with heating of the workpiece should be taken 35 to 40°C lower than the temperature- interval for annealing and aging. The heating temperature depends upon the cutting speed and the rate of feed, since the amount of heat generated in cutting increases with the speed and feed. Thus, in turning a particular grade of stainless steel, heating temperature is > 500°C at cutting speed of 19 m/min = 350°C at cutting speed of 300 m/min = 230°C at cutting speed of 375 m/min 650 Special Processing Methods 651 Advantages 1. The process is economical and in many case has reduced the operating costs. 2. Production gets increased. 3. Good surface finish can be obtained, superior to that obtained on these materials at room temperature. 4. Little evidence of any adverse microstructural change. Heating Devices : The work piece can be heated by various methods of heating as : high- frequency induction heating or electric-arc heating devices mounted on the carriage, by resistance heating with the application of an electric current in the cutting zone, by flame heating, by Plasma arc heating. Sometimes, the blank is preheated in a furnace before being loaded into the machine tool. 12.2. UNIT HEADS Basically a unit head is a power operated slide with provisions for advancing different types of cutting tools to the component. Unit heads are mounted on standardised bases. A unit head consists of a cast iron body which houses the gears driven from the motor to rotate the spindle. The body has longitudinal movement along the base which is effected from the main motor, (see Fig. 12.1), through a lead screw and nut. Body Spindle Motor Dead Spot Electrical Board Fast-traverse Motion Base Trip Stops Fig. 12.1. A Unit Head. The idle motions are carried out with the help of a fast traverse motor and its electrical brake. Depths of cut and various intermittent motions are controlled by a series of trip stops secured to the head, while a dead stop can be used to ensure the accuracy of cutter depth. The longitudinal movement of the head can be actuated also through the rotation of a plate or cylindrical cam, or when required for arduous duties, by hydraulic power. The “unit head” has opened up avenues of multiple-operation machines for the completion of components which would need a line of machine tools, each of which would need to be fully tooled and manned. Also interstage handling and storage have been eliminated. It is possible to load one or more components and not to remove them from the fixture on the machine until the completion of a wide range of operations. On the completion of the machining of these parts, the heads can be dismantled from the bases, and these with the bases, passed into stores until required for the machining of other components. The “unit heads” have made considerable headway in the production of medium to large-scale components. Advantages. 1. Unit heads allow for maximum versatility. 652 A Textbook of Production Technology 2. They can be mounted and remounted in a variety of positions on standard interchangeable bases. 3. High production rates, along with consistent high accuracy. 4. Number of handling times reduced. 5. Less floor space needed for machines and for spring. 6. Operators more fully employed. 7. Physical efforts of operators reduced. 8. Good economical recovery rate. The unit head is available in a wide range of sizes. Power rating of driving motors ranges from about 0.2 kW to 22 kW, with spindle speeds from 41 to 200 rev/min, and with feeds of 0.025 to 3.50 mm/rev. Each unit head needs a control panel and such panels can be housed in separate cabinets or enclosed within a standard base. When a machine setup includes several heads, a combined control board can be enclosed within the framework of the base. The majority of the unit heads are designed for boring, broaching, chamfering, counter boring, countersinking, drilling, end milling, face milling, gang milling, reaming, sawing, shot - facing, tapping, thread rolling and turning. The front faces of most of the heads are provided with means to allow the fixing of multiple - spindle drilling heads, to permit the drilling of more than one hole simultaneously. The versatility of drilling unit heads has been discussed under Art. 8.3.9. (Fig. 8.50). 12.3. PLASTIC TOOLING Certain types of non cutting tools are made of nonmetallic materials including plastics. The most common materials are epoxide resins, because of their better mechanical properties (excellent properties when loaded in compression) than other plastic materials. Epoxide resins are more costly than anyother tool material. But they are lighter than other 1 1 materials, being th weight of Zinc alloys and less than th weight of cast iron. Also, the cost 4 4 saving due to the reduction in time and labour involved in making plastic tools, outweighs the material cost. Other properties of plastics have been discussed in Chapter 11. Their tensile strength can be increased by reinforcing with fibre glass. Epoxide resins can be poured, cast, laminated or moulded into intricate shapes with negligible shrinkage, and finish with a minimum amount of surface finishing. Consequently, the greatest saving in cost is obtained with tools of complex shape, for which the cost of machining and final finishing will be very high. Compared with any of the tooling metals, plastics are soft and have a much shorter life than comparable tools in steel. It is not economical, therefore, to use plastic tools when : tool shapes are conventional, the component material is thick and quantities are large. Plastic tools, too, can be damaged more easily by faulty handling. Applications. Plastic tools are being used in many industries : (1) For drilling jigs, Routing jigs and fixtures for assembling, brazing and welding. In the majority of cases inserts are provided to prevent undue wear. (2) In plastic industry for the production of moulds for both thermo-setting and thermoplastic materials, for vacuum forming and for the injection blow moulding of polythene products. (3) In foundries for the production of patterns and core boxes. The entry orifice of the latter is normally fitted with a hardened steel insert to counteract the abrasive effect of the blown sand. Special Processing Methods 653 (4) Metal forming tools for drop hammers, hammer blocks, multipart press tools, piercing - punch plates, rubber press tools, spinning chucks and stretch press formers. Most of these tools can be given extra support by the inclusion in the mould of metal or fibre glass frames or supports. (5) Plastic tools also offer many advantages where short runs and prototypes are required or where a set of tools is required very quickly. General tapers and blending radii assist in producing a strong tool. The thickness of component metal should rarely exceed about 1.5 mm while radii less than 4.75 mm are to be avoided. The metal formed by plastic tools include : Aluminium alloys, brass and other copper alloys, mild steel, nimonic, stainless steel and titanium. Production of Plastic tools. Plastic tools can be produced by two methods : by Casting and by building up reinforced layers of resin and glass fibre. The casting process is used for the production of tools of large mass, such as forming dies and punches. Selected fillers are added to the resin to reduce the cost of the mass and to provide the properties required in the tool. Inserts and supports can be embodied in the casting to provide strength where required. Casting is least time - consuming and the more reproducible of the two methods. 12.4. ELECTRO-FORMING Electro-forming is a process of producing precision metal parts, that are usually thin in section, by electro-deposition on to a form (variously called as mandrel, mould, matrise or die) which is shaped exactly to the interior form of the product and which is subsequently removed. In the process, a slab or plate of the material of the product is immersed into electrolyte (an aqeous solution of a salt of the same metal) and is connected to the positive terminal of a low voltage, high current d.c. power. So, it becomes an anode. A correctly prepared master mandrel or pattern of correct shape and size is immersed at some distance from the anode and is connected to the negative terminal (cathode). The mandrels are made from a variety of materials, both metallic or non - metallic. If the material is non - conducting, a conductive coating must first be applied in order to perform electroplating. The mandrel should possess mirror like finish. When the circuit is closed, metal ions are removed from the anode, transported through the electrolyte towards the cathode (master) and deposited there. After the deposition, the master is removed or destroyed. A metal shell is left, which conforms exactly to the contours of the master. It may take hours or days to get a deposit of sufficient thickness. The thickness of electro - forms ranges from 0.25 to 25 mm. The process is very much similar to electro - plating, with the difference that whereas in electro- plating, the deposit stays in place (on the cathode), in electro - forming, it is stripped from the form. The electro-formed products are typically made from Nickel, Iron, Copper or Silver, and more recently from copper - tin, nickle - cobalt and nickle - manganese alloys. Advantages 1. Low plant cost, cheap tooling and absence of heavy equipment. 2. Low labour operating costs. 3. The process can be designed to operate continuously through out day and night. 4. Electro - deposition can produce good dense deposits, and compared with castings, electroforming offers high purity, freedom from porosity with a homogeneous structure. These important qualities are seldom obtained to such a degree in machined parts, stampings or forgings. 5. There is no restriction on the internal complexity of electro- forms, and this advantage eliminates in many instances, the costly joining processes. 6. The process has no equal for the reproduction of fine or complex details. 654 A Textbook of Production Technology 7. The use of inserts has widened the application of the process. Metal inserts are attached to or are embedded in a wax or fusible alloy master, and, when the master is melted, the inserts remain attached to the electroform. 8. A high quality surface finish is obtained on both internal and external surfaces of the electro-forms. Accuracies as close as 0.005 mm with surface finishes upto 0.125 μm can be produced. 9. Complex thin-walled parts can be produced with improved electrical properties. 10. Shell-like parts can be produced quickly and economically. Note. Some care must be exercised to minimize residual stresses. Mandrels. The mandrel, the mould or the master, is the most expensive item in the elector - forming process. The type of master and its precision are major factors in the economics of the process. As noted above, the mandrels can be made of a variety of metallic and non - metallic materials. The common metallic materials are : aluminium, brass, carbon steel, chromium - plated steel, stainless steel and titanium. A common feature of all these materials is their oxide passivation film, which facilitates their separation from the electro - form without any special surface treatment. Depending upon the shape of the electro - forms, the mandrels are of three types : permanent, semi - permanent and expendable. 1. Permanent Mandrels. These mandrels are usually made of metals or of glass or rigid plastic. The surface of the non - metallic mandrels is made conductive by metallizing by electro - plating or a chemical deposition technique. For close tolerances work, such as gears and gauges, stainless steel is recommended. Such mandrels can be used indefinitely, with a minimum of treatment to preserve the smooth surface. Adhesion is minimized by the application of a thin coating of a parting compound. Mandrels are also slightly tapered to facilitate stripping. permanent mandrels are used for long runs and convex surfaces. 2. Semi-permanent Mandrels. For straight-sided components, or components which have undercuts upto about 0.013 mm, semi - permanent mandrels are used. These are made from steel with fusible coatings, compounded usually from wax and graphite. To remove the electro - form the fusible layer is melted, and after removal the mandrel is cleaned and rebuilt to form. 3. Expendable Mandrels. Expendable mandrels for complex electro - forms are made from : (i) plaster, which after electro-forming are removed by breaking. (ii) plastic resins and fusible alloys (Sn-Zn alloy) which are melted. (iii) aluminium and zinc which are dissolved chemically. (iv) brass Plastic resins are commonly used for decorative work where tolerances are wide without undercuts. The surface finish of mandrels made from the fusible metal alloys can be improved by electroplating a layer of copper 0.025 to 0.050 mm thick. This copper layer is dissolved from the electro - form after the fusible alloy is melted. Applications. There is a wide range of applications of electroforming process : 1. Moulds and dies feature high in the list. Moulds for the production of artificial teeth, rubber and glass products, and high - strength thermosetting plastics are now commonplace. The moulds can be made with undulating parting lines which have made a considerable impact upon the production of thermoplastic toys and novelties. 2. Radar and electronic industry : Radar wave - guides, probes, complicated grids, screens and meshes can be produced much more easily, to fine accuracies and at lesser cost. 3. Spline, thread and other types of form gauges. 4. Cathodes for ECM and electrodes for EDM. Special Processing Methods 655 5. Electro-formed core boxes with inbuilt heating elements. Electro-formed moulds for the wax patterns. 6. Electro-formed precision tubing, parallel and tapered, formed to different shapes to eliminate the need for bending which distorts the bore. 7. Electrotypes, floats, bellows, venturi tubes, fountain pen caps, reflectors, heat exchanger parts, honeycomb sandwich, parts for gas appliances and musical instruments, radio parts, spraying masks and stencils, seamless screen cylinders for textile printing, filters and dies for stamping of high-fidelity records. Electro-forming is particularly useful for : (a) High-cost metals. (b) Low production quantities. (c) Quantity of identical parts, for example a multi - impression mould. (d) The possibility of using a single master for the production of a number of electro - forms. (e) Whereas intricate female impression is required, so that is would be much easier to produce a male form, that is, the master. 12.5. SURFACE CLEANING AND SURFACE TREATMENTS During the manufacture of virtually all metal parts, filings, fine metal chips, pieces of chips, remnants of waste or abrasive grit may get into holes or channels of parts. Also, oil, dirt, grease, scale and other foreign materials remain adhered to the part surface. The purpose of surface cleaning and getting rid of all the above materials is two fold : Firstly, they may get into holes or channels of parts. Subsequently, in operation of the finished machine, they may be carried by the lubricants into the bearings, where they may lead to overheating and premature wear of the bearings and even to a breakdown of the whole machine. This can be avoided by properly cleaning the parts and units. Thorough cleaning of parts is essential for high quality of their assembly. The second purpose of surface cleaning is to prevent corrosion, and to combine a decorative appearance with the protective coating. All metals will oxidize and corrode, when exposed to certain environments, unless protected with an antirust coating. Before application of any protective or decorative coating, it is essential that the surfaces of the part be prepared by proper cleaning to assure good adhesion. Neglect of this preparation can be the cause of poor quality coatings. Removal of oxide scales, dirt, grease, oil, and temporary coatings is absolutely necessary. The various surface cleaning methods are discussed below : 12.5.1. Mechanical Cleaning and Finishing Methods. These methods have already been discussed in Chapter 3, Art. 3.10 (Cleaning and finishing of Castings) and in Chapter 4, Art. 4.3.8. (Cleaning and finishing of forgings). “Barrel tumbling” may be employed for any of the following purposes : 1. Removing fins, flashes and scales from parts. 2. Cleaning of forgings, stampings and castings. 3. Deburring. 4. Improving micrometer finish. 5. Finishing high precision work to a high lustre. 6. Forming uniform radii. 7. Finishing gears and threaded parts without damage. 8. Removing paint or plating. 656 A Textbook of Production Technology After tumbling, the parts must be thoroughly washed and dried by sawdust or infrared lamps and then oiled to prevent the formation of rust. Tumbling is an inexpensive cleaning method. It can be done dry or wet. Other methods under mechanical cleaning and finishing methods include : Abrasive blast cleaning, such as sand blasting or shot blasting, airless shot blasting and hydro-blasting. 12.5.2. Chemical Cleaning Methods : 1. Alkaline Cleaning. In this method, the parts are cleaned by dipping them in aqueous solution of alkaline silicates, caustic soda, or similar cleaning agents. Some type of soap is added to aid in emulsification. Wetting agents may also be added to the solution to help in thorough cleaning of the parts. The method satisfactorily removes grease and oil. The cleaning action is by emulsification of oils and greases. Special washing machines are employed in lot and mass production, Washing machines may be of the single-, two- and three chamber types. (a) In a single chamber washing machine, the washing chamber is equipped with a bank of pipes with nozzles. A pump delivers the cleaning fluid, drawn up from the drain tank, to the pipes. The nozzles are arranged so that the part or unit is washed from all sides simultaneously with powerful streams of fluid. The parts may be traversed in the washing machine by a chain conveyer. The cleaning fluid is heated by a steam coil to 60 – 80°C, and therefore the parts ejected from the machine dry fairly soon. After washing, the cleaning fluid drains into a collector, wherefrom, passing a filter, it comes back to the pump. (b) A two-chamber washing machines have two washing chambers. The parts are cleaned in the first chamber as explained above and then rinsed of the washing solution in the second chamber. (c) In three-chamber machines, the third chamber is used for drying. This method should not be used on aluminium, zinc, tin or brass. 2. Solvent Cleaning. Small parts are cleaned of oils, dirt, greases and fats etc. by dipping in commercial organic solvents, such as, naphtha, acetone, trichloro - ethylene, or carbon tetrachloride. The parts are then rinsed once or twice in a clean solution of the same solvent. The vapours of these solvents are toxic and therefore require ventilation. The method is particularly suitable for parts of aluminium, lead and Zinc, which are chemically active and might get attacked by alkaline cleaners. 3. Emulsion Cleaning. In this method, the action of an organic solvent is combined with that of an emulsifying agent. The solvent is generally of petroleum origin and the emulsifying agents, which are soap or a mixture of soap and kerosene oil, include : nonionic polyethers, high molecular weight sodium or amino soaps of hydrocarbon sulphonates, amine salts of alkyl aryl sulphonetes, fatty acid esters of polyglycerides, glycerols and polyalcohols. Cleaning (removal of oils, greases etc.) is done either by spraying or dipping the part (metals and non-metals) in the solution (at room temperature) and then rinsing and drying. The method is particularly suitable for parts having deep pockets that will trap the solution. Again, the method is particularly suitable for parts of aluminium, lead or zinc while are attacked by alkaline solutions. 4. Vapour Degreasing. Vapour degreasing is a similar process, except that the solvent vapours are used as the cleaning agent. The solvent is heated to its boiling point and the parts to be cleaned, are hung in its vapours. The vapours condense on the surface of the parts and washe off the oil and grease. 5. Pickling. As discussed in chapter 4, the pickling process is used to remove dust or oxide scale from the surface of the components. For this, the parts are immersed in a tank filled with an acid solution, which is 12 to 15% concentrate of sulphuric acid in water, and is at temperatures from 65°C to 85°C. The solution acts to loosen the hard scale from the component surface and remove Special Processing Methods 657 it. The acid solution should not react with the clean metal while removing the scale. For this, an inhibitor agent is added to the acid solution. In should be noted that the pickling process only removes the oxide scale. It will not clean the dirt or oil from the part surface. Therefore, the parts should first be cleaned by alkaline cleaning method before doing pickling process. 6. Ultra-sonic Cleaning. Very dirty small parts, especially those of intricate shape with hard- to-access inside surfaces, are difficult to clean in ordinary washing facilities. Such parts are cleaned much more efficiently by the “ultra-sonic cleaning method”. The method is effected in three stages : preliminary washing, ultrasonic cleaning, and rinsing of parts in a clean washing medium (kerosene, trichloroethylene, tetra- chlorated carbon, etc.). Ultrasonic energy is produced by a high-frequency generator which feeds high-frequency electric energy to transducers that transform the electric energy into inaudible sound energy (at 20 to 25 kHz). The transducers are fixed to the bottom or sides of a stainless steel tank designed to afford the optimum acoustic conditions. High velocities are imparted to the particles of cleaning liquid in the tank. Cavitation bubbles of microscopic dimension are formed on the surface of the component. The cleaning action is caused by the formation and bursting of bubbles which practically blast all contaminants from all types of materials in seconds, penetrating every crack or crevice and removing all loose matter. The cleaning liquids include water, water based solutions, mild acids and caustic solutions which are thermostatically controlled to operate at temperatures of about 45°C. The effectiveness of ultrasonic cleaning is 99%. Components cleaned by this method can be immediately plated without further treatment. Rinses are required in all cases to ensure thorough removal of the cleaning agent before coating. After being washed, machine parts should be carefully dried. This is usually accomplished with compressed air. Such air blasting proves expedient in assembly before each operation. Special care should be taken to blow through holes, grooves, slots and other places, where dust and dirt may readily accumulate. Advantages 1. The process can be manned easily by trained labour. 2. The process reduces the time element. 3. The process produces cleaner surfaces and eliminates many manual operations and the quality hazards associated with the human element. 4. The cleaning of intricate assemblies after final assembly can reduce testing time. Applications. The process is used in all types of engineering factories, cafes, dairies, hospitals and hotels, and by manufacturing jewellers. Components which have been cleaned by the process include : ceramics, cutlery, electronic equipment, engine parts, jewellery, machine-tool assemblies, porcelain and watch parts. Other components which need to be scrupulously cleaned include air craft parts, ball-race assemblies, engine components, fuel gauges, gas turbine parts, gears, glass components, hydraulic devices, jet-engine parts, opthalmic frames and lenses, parts to be plated, refrigerator parts, Satellite components and parts for semi-conductors, teleprinter parts and timing devices. 7. Surface Polishing. Mechanical polishing of pressed or extruded metal products and many such articles (done manually or automatically) is by using a wide range of wire brushes or mops, in conjunction with specially blended greases and waxes. The two non-mechanical techniques for this purpose have economic advantages over mechanical polishing are: “Chemical polishing” and “Electrolytic polishing”. Chemical polishing has made greater progress due to the increased use of aluminium for a variety of applications. Electrolytic polishing has made slower progress, because it 658 A Textbook of Production Technology is more expensive to instal and operate, as compared to chemical polishing. (a) Chemical Polishing. In this process, the metallic objects are immersed in baths of selected acids. During the process, certain amount of metal, mainly from the peaks is dissolved, producing a bright surface without the formation of an etched pattern. For chemical polishing of aluminium alloys the most successful of the solutions used contain phosphoric, nitric and sulphuric acids. The production cycle consists of the following steps : (i) Immerse for 1 to 3 min. at 100°C (ii) Remove and rinse in hot water (70°C) to remove the viscous film formation. (iii) Rinse in a mixture containing equal amounts of water and 1.42 sp. gr. nitric acid at room temperature. (iv) If anodising required, rinse in cold water. (v) If lacquering is called for, rinse in hot water. The resultant surface finish is of the order of 0.45 to 0.50 with a high reflection factor of about 88%. The polishing solution for copper alloys contain phosphoric and nitric acids to which water may be added, according to the alloy being polished. The polishing times range from 30 sec. to 5 min., at temperatures from 60 to 80°C. An improved specular reflectivity with superior surface leveling is obtained when either of the above two acids are added to nitric- arsenic acid mixtures. Advantages 1. The process is comparatively cheap, with low operating costs. 2. The equipment has a long life. 3. It is very suitable for delicate, thin - walled, embossed or fluted components. 4. Both the inside and outside surfaces are cleaned easily. 5. The process can be combined with barrel - polishing to reduce time and cost. 6. The process can be included in the aluminium anodising cycle. 7. Improved reflectivity usually is obtained. (b) Electrolytic Polishing. The principle of electrolytic polishing is the same as that of ECM. A surface layer of the workpiece is removed by anodic dissolution of the metal, leaving the component usually with a highly polished surface. This “deplating” process is known as electro- polishing. ECM is a highly accelerated version of this depleting process. When a metallic object is immersed in an electro-polishing electrolyte, the current line leads from the surface peaks, tangentially, causing a higher current density on the peaks than on the valleys. Thus greater metal dissolution takes place on the peaks to produce a smoother surface, and as the process proceeds, a viscous film is formed on the metallic surface. This viscous film protects the micro valleys from the action of the current, but permits the minute peaks to be dissolved. The rate of metal removal ranges from 3 to 10 microns per minute. The length of time required for polishing is 4 to 10 minutes for ferrous and non-ferrous metals, and from 3 to 5 minutes for light alloys. If an increased current density is produced at the cutting edge of a tool, thereby intensifying the process of dissolution, it is possible to effect electrolytically assisted sharpening of cutting tools. A wide range of metals and alloys can be electrolytically polished, but the main industrial uses of this process are for the polishing of alloy and stainless steel, Copper alloys, nickle and aluminium alloys. 12.5.3. Surface Coatings. The various surface coatings on machine parts are used for protective, decorative, wear resistant and processing purposes. The different types of surface Special Processing Methods 659 coatings used for this purpose are : Metallic coatings, Plastic coatings, Conversion coatings, Organic coatings and Inorganic coatings. (A) Metallic Coatings are applied by : Electro-deposition, hot immersion, chemical deposition or metal spraying (Metallizing). (a) Electro-deposited Coatings. This process of coating also known as “Electro-plating”, comprises preparation of the surface to be plated, plating itself, and polishing (where ever necessary). The preparatory operations include grinding, polishing and degreasing of the surface. The part to be electro - plated is made the cathode and the metal to be deposited is made the anode and both are placed in a tank containing an electrolyte. The process is carried out at a voltage of 10 V (D.C.) and current density of upto 10A/dm2. When the circuit is closed, metallic ions from the anode migrate to the cathode and get deposited there. Some characteristics of electrodeposited coatings are given below : (i) Copper plating. Used for masking of steel parts from carburi- zation in case of hardening heat treatment, plating for improved running- in of plated surfaces and as an underlayer for multi - layer coatings. Coating thickness : 5 to 25 μm. (ii) Chrome plating. Wear - resistant protective and decorative coating. It results in improved retention of lubricant and lower co-efficient of friction. Coating thickness : 30 - 40 μm. (iii) Cadmium plating. Coating for protection against corrosion of steel in moist atmosphere (marine corrosion) and for improved running in of mating surfaces. Plating thickness: 15 μm. (iv) Nickle plating. Undercoat of chrome, corrosion protection for steel, wear qualities and for decoration. Plating thickness : Upto 25 μm. (v) Lead plating. Resistance to chemical corrosion. (vi) Zinc plating. Low-cost protection of steel and iron against atmospheric corrosion and for decoration. Plating thickness : Upto 15 μm. (vii) Silver plating. Electrical contacts. Good antigalling and siezing qualities at high temperature. Plating thickness : 2.5 to 12.5 μm. (viii) Tin plating. Coating for protection against weak acidic media, non-toxic protection in food, for subsequent soldering and for masking in nitriding. Plating thickness : 3 to 12 μm. (ix) Gold plating. Infrared reflectors, electrical contacts, jewellery. (x) Borating. High hardness coating. (xi) Phosphating. Anti-corrosive coating. Plating thickness : 0.5 - 1 μm. (xii) Lead - indium plating. Electro-deposit of lead on a silver plated surface followed by indium plating forms a satisfactory bearing surface. (xiii) Brass plating. Brass plating is frequently used as a base for bonding rubber and rubberlike materials to the metal. It improves appearance, provides soldering surface which is abrasive resistant. Since brass tarnishes (it is satin yellow to bronze initially and then turns to black to green on exposure), it must be covered with lacquer, when used for decorative purposes. Brass plating is used on steel, zinc, aluminium, and copper plate. (ivx) The coatings of Nickle-Cobalt, Zinc-cadmium and Tin-lead are obtained by methods which are called ‘‘thermo electro-plating’’ or ‘‘thermo-diffusion’’. The latter consists in that individual metals are successively deposited on the part and in the course of subsequent heating these diffuse and form a plating of some alloy. Nickle-cobalt plating increases hardness, Zinc Cadmium plating upgrades corrosion resistance and tin-lead plating reduces porosity and improves appearance. 660 A Textbook of Production Technology Electro-less Plating. This method of plating differs from the conventional method of plating, that is, electro-plating,in that no external source of electricity is used in the process. The plating is obtained with the help of a chemical reaction. For example, for nickel plating, a metallic salt of nickel, nickel chloride is reduced with a reducing agent, such as sodium hypophosphite. Nickel metal so obtained is deposited on the workpiece. Nickel and copper are the two most commonly used metals for this process. Advantages : 1. The process can be used for plating nonconducting materials such as plastics and ceramics. 2. The process does not produce hydrogen embrittlement. 3. Cavities, recesses and inner surfaces of tubes can be plated successfully. 4. Coating thickness is uniform. 5. The coating has excellent wear and corrosion resistance. Disadvantage : The process is costlier as compared to electro-plating. Sometimes, some portions of the base of a workpiece are not to be electro-plated for decorative purpose or for the sake of economy. This is known as ‘‘Blacking out’’. For this the complete base is prepared for electroplating. The base is heated to 100°C. Paraffin wax is applied to the portions to be blocked out and then the workprice is completely cooled. After that the electroplating process is carried out. Electro-plated parts are usually dull and possess little or no metallic lustre. To provide finish, shine and lustre to the electro-plated parts, they are finished by `mops' and `compos' The mop should be moved slowly over the surface to avoid removal of any portion of electro-plated leyer, the final finish/colour is obtained by mopping with chalk. Compos which contain abrasives should not be used with soft metals. (b) Hot - Dip Coatings. Many metal parts are given anti - corrosion coatings by dipping them into certain molten metals. Most commonly used metal coatings are of tin, Zinc, lead and an alloy of lead and tin, before the parts are coated by hot - dip method, they are thoroughly cleaned. (i) Tin Coating. Tin coated sheets are used for making food containers due to the nontoxity of tin. To remove the excess tin, the sheets are passed through rollers (immersed in palm oil) after these come out of the bath. (ii) Zinc Coating. Giving a coating of Zinc is called ‘‘Galvanizing’’. One method of doing so is by ‘‘electro-plating’’. In the hot-dip method, the parts or steel sheets are fluxed by immersing them into a solution of Zinc chloride and hydrochloric acid. After that they are dipped into a molten Zinc bath. Again, to remove excess Zinc, the sheets are passed through rollers after they leave the bath. Galva- nized steel sheets (and more recently, also one sided galvanized sheet) find increased use in automo- tive and appliance industry in addition to their use for roofing. (iii) Lead Coating. Load-coated sheet provides anticorrosion properties in some media, where tin coated and Zinc coated sheets can not resist corrosion. However, lead coated sheets can not be used for food applications, because lead is toxic. An alloy of 15% to 20% tin and the remaining lead can also be used for this coating. Lead coating method is also called ©Terneª coating. (iv) Aluminium - Coating. Aluminium - coated sheets can resist corrosion by hot gases. Due to this, these are suitable for heat exchanges, automotive exhaust systems and grill parts etc. (c) Metallization. Metallization means spraying molten metal with the aid of compressed air. The metal particles moving at a speed of 100 to 150 m/s strike the surface of a part being coated and adhere to it, thus forming a layer of strong, finely porous metal coating. The layer has a fairly high compres- sive strength, even though it is brittle. The coating thickness varies from a few hundredths of a mm to 3 - 4 mm. Parts after being coated can be turned and ground. The method is used to obtain decorative, protective, antifriction and heat-resistant coatings, to restore worn out parts and correct defects of castings. The metal being sprayed is melted by OA flame (gas metallization) or by electric arc (electro- metallization). The initial material is metal wire. Sometimes use is made of equipment operating on meltable powders. The surface to be coated is cleaned of oil and oxides. Sand blasting and rough turning is employed for better adhesion of the metal being sprayed to the surface. Special Processing Methods 661 (d) Cementation. This is another method of obtaining metallic coatings. The method is similar to heat treatment method of hardening (pack carburizing), in that the parts are heated in a closed con- tainer having suitable metal powder. However, it differs from the hardening method in that where as in the hardening method, the chemical composition of the outer layers of the parts in changed, in cementation process, metal alloy is formed on the surface layers of the part. Depending upon the powders used in the container (commonly used powders are of : Zn, Al and Cr), there are variations of this process :- (i) Sheardizing : Powder used is of Zn. Temperature in the box is 365 to 375°C. The method is used for coating threaded work, chains and small parts. (ii) Calorizing : Powder is a mixture of Al powder, Al2 03 and Aluminium chloride powders. The temperature in the box : 850 – 920° C in an non-oxidizing atmosphere. Used for coating parts of iron and steel to protect from high temperature oxidation. (iii) Chromizing : Powder is of Cr. used for iron and steel parts. Temperature in the box : 800 – 1050°C. Powder is of ferro Cr in granular form or ferro – Cr with ammonium iodide and non-vitrified kaoline. Parts : taps, dies, files, hacksaws, bandsaws etc. For corrosion and high temperature resis- tance. B. Plastic coating. Plastics are used as decorative, anti - corrosive, and anti - friction coatings. These are applied in liquid and powder form. The primary materials used are thermo - plastics such as : polyethylene, polypropylene, polyamide, polyvinyl butyral, poly - urethane, fluoroplastic and Caprolactum etc. These are used in the form of fine powders which, on heating, change to plastic state. The coat thickness ranges from 0.15 to 0.35 mm. Before coating, the parts are heated to 180–300°C depending on the plastic to be used. The treatment itself lasts from 2 to 5 s. Plastic coatings make it possible to use carbon steel instead of alloy steels or nonferrous metals. C. Conversion Coatings. These are the coatings produced when a film is deposited on the base material as a result of chemical or electro-chemical reaction. Many metals particularly steel, aluminium and zinc can be conversion coated. The coatings can be phosphate coatings, chromate coatings and oxalate coatings. After degreasing and cleaning in alkali, the part is soaked in suitable acid bath, for example, for chromate coatings, in chromic acid bath. Conversion coatings are obtained for corrosion protection, prepainting and decorative finish. Another important application of this coating is as a lubricant carrier in cold forming operations, such as wire drawing. The various conversion coatings are :— Phosphate coatings Chromate coatings Oxide coatings Anodic coatings (i) Phosphate Coatings : A coating, mainly for steel, as a preparation for painting, adhesive bondings or rust proofing. It also reduces wear on the sliding surfaces and facilitates cold working of iron and steel. However, it can also be used for non-ferrous and light metals. The process involves the chemical development of a protective film containing ferrous insoluble crystalline phosphates of manganese and ferrum, or ferrum and zinc, by treatment with a dilute solution of phosphoric acid and other elements. Phosphate coatings are also applied for increasing inter laminar electrical resistance of laminations made of Si sheets for electric motors and transformers. Depending upon the coating structure and the method of surface preparation, the film may be 2 to 15 m thick. The process resulting in thin coatings is called " Bonderizing". The process is rapid and the coating may be applied as a solution at 57 to 85° C by dipping for about 2 minutes or by spraying at 65 to 85°. The method is widely used in automotive and electrical appliances industries for preparing motor car bodies, bicycles, washing machines, refrigerators and similar products, to receive an organic finish. The paint will stick tightly to the surface produed by bonderizing. 662 A Textbook of Production Technology The process of obtaining thick coatings is called " Parkerizing". It consists of the following steps : 1. Cleaning and Rinsing with water 2. Dipping in a bath containing solution (either powder or liquid chemical and water at 65 to 90°C) for 5 to 60 minutes. 3. Rinse again with water 4. Drying or water staining to get black finish by dipping in staining solution and then drying. 5. Finally oil is applied on the surface. Used for : nuts, bolts, washers, small coatings, (ii) Oxide Coating. Oxide coating of steel parts is done for decoration, rust proofing and to obtain low friction surfaces. The coating is obtained by thermal, chemical and electro-chemical meth- ods. The thermal methods involve heating the part in air, steam or molten nitre. An oxide film of 1 m thick is formed on the part surface ; the film colour varies with the process temperature. Heating in the air serves to form thin oxide films on electrical components. The chemical methods include alkaline and acidic oxidation. In the first method, steel parts are treated with a hot concentrated solution of caustic alkali containing oxidants. In the second method, solution contains ortho-phosphoric acid and oxidants. The acidic oxidation is much quicker as com- pared to alkaline oxidation and provides a stronger oxide film with improved corrosion resistance. The oxide films on steel parts are thin (0.8 to 3 m) and porous, and therefore do not reliably protect the parts from corrosion. Their corrosion - resistance can be increased by subsequent varnishing. The chemical methods are used to oxidate parts made of aluminium, magnesium, copper, Zinc and their alloys. The field of the process application is the manufacture of instruments, tools and con- sumer goods. Electro - chemical oxidation of parts made of ferrous and non- ferrous metals and alloys is carried out in solutions of caustic alkali. The parts being processed form an anode. The process runs at lower temperatures and requires less chemical agents than the chemical alkaline oxidation. Prior to treatment of parts, these are cleaned of corrosion spots and degreased, and after oxidation they are rinsed in water. Decorative oxidation takes from 30 to 40 min ; corrosion - resistant films require upto 1.5 – 2 hours for their formation. (iii) Anodizing. Anodizing or Anodic coating is a process of providing corrosion resistant and decorative films on metals, particularly aluminium. The process is the reverse of electro - plating, in that the part to be coated is made anode, instead of cathode, as in electro - plating. When the circuit is closed, a layer of aluminium oxide is formed on the anode (aluminium) by the reaction of aluminium with the electrolyte. The layer of aluminium oxide on the surface is highly protective. There are two processes used for anodizing : (a) Chromic acid process. In this process, 3% solution of chromic acid is made as the electrolyte at a temperature of about 38°C. This process is applicable only to those aluminium alloys containing 1 not more than 5% copper or a total alloy content of not more than 7 %. The process produces a 2 light grey colour. (b) Sulphuric acid process. The electrolyte is 15 to 25% solution of sulphuric acid. The process is applicable to aluminium alloys containing more than 5% copper or a total alloy content of more than 1 7 %. The process produces a light yellow colour. 2 These processes shall not be applied to parts having joints or recesses in which solution may be retained. Normal anodized coatings are 0.0050 to 0.0075 mm thick. Anodic coatings are also applied by : Oxalic acid and Boric acid bath processes. The steps for obtaining anodic coatings are : 1. Degrease 2. Rinse 3. Clean in alkali 4. Rinse 5. Apply anodic coating (3 to 10% solution of chromic acid). 6. Rinse 7. Dry. (iv) Chromate Coatings : These coatings are mainly used for corrosion protection of galvanized sheet. The result is added corrosion resistance and base for paint. The product is called "Passivated" Special Processing Methods 663 or " Stablized". These coatings which are very thin (0.0005 mm) are used on non-ferrous materials like Al. Mg and Zn - coated materials and cadmium coated parts. The steps involved are : 1. Degrease 2. Rinse 3. Clean in Alkali 4. Rinse 5. Soak in solution of chromic acid, Cr salts together with hydro-fluoric acid or hydro-fluoric acid salts, phosphoric acid, or other mineral acid 6. Rinse 7. Place in di-chromate bath (45 min. soak) 8. Rinse 9. Dry. Note : The process should not be used on galvanized steel parts which are to be resistance welded or phosphated and painted. A chromate coating is less expensive than an anodic coating, because it is faster and the overheads are less. It has greater resistance to corrosion. However, anodic coatings have superior wear-resistance. Bicycle Wheel Rim plating Plant Process Chart The rims are loaded on the fixture. The fixture can carry up to twenty rims at a time and after the loading is done, the robot is switched on which takes control of the whole process. The rims undergo various processes before being unloaded for use in Body Assembly shop. The various process of Electroplating of the Rims is as in the table below : Serial Process Chemical Concentration Temperature Density Checking Number Sequence (° C) (deg. Be) 1. Kerosene M.T. kerosene oil 100% Room – Once daily Oilcleaning 2. Abrasive Surclean-504 70-80 cc/litr. 60-80 8-10 Once daily Cleaning NAON-80-100-80-ltr 3. Water Rinse Running Water Room – – 4. Soak Cleaning Steelex80-100 gm/ltr. 60-80 8-10 Once daily 5. Water Rinse Running Water Room – – 6. ElectroCleaning Ginbond-80880-100 gm/ltr. 60-80 8-10 Once daily 7. Water Rinse Running Water Room – – 8. Acid Dipping HCl 30-40% Room 8-10 Once daily 9. Water rinse (I) Running Water Room – – 10. electro Ginbond-808 60-80 8-10 Once daily Cleaning 80-100 gm/ltr. 11. Water Rinse (II) Running Water Room – – 12. Acid Dip Sulphuric Acid10-15% Room 8-10 Once daily 13. Water Rinse (II) Running Water Room – – 14. Semi-Bright Nickel Sulphate 45-55 18-25 Once daily Ni Plating (II) 250-300 gm/ltr. 15. Tri-Nickel Nickel Sulphate 45-55 18-25 Once daily Plating 250-300 gm/ltr. 16. Bright Nickel Nickel Sulphate 45-55 18-25 Once daily Plating 250-300 gm/ltr. 17. Drag out D.M. Water Tank Room – Once daily 18. Water Rinse (III) Running Water Room – – 19. Chrome Plating Chromic Acid 40-50 24-30 Once daily 275-325 gm./ltr. 20. Drag out D.M. Water Tank Room – Once daily 21. Drag out D.M. Water Tank Room – Once daily 22. Water Rinse (IV) Running Water Room – – 23. Unloading of the rims