Unit 5-MANUFACTURING OF PLASTIC COMPONENTS

Manufacturing Technology 1-Unit 5-MANUFACTURING OF PLASTIC COMPONENTS-Anna University-KIT

1. UNIT-V MANUFACTURING OF PLASTIC COMPONENTS

2. INTRODUCTION Plastics are belonged to the family of organic materials. Organic materials are those materials obtained directly from carbon and chemically combined with oxygen, hydrogen and other non-metallic compounds. These organic materials are classified into two types.

3. They are 1. Natural organic materials: The wood, coal, petroleum and natural rubber are under the categories of natural organic. 2. Synthetic organic materials The plastics, synthetic rubbers, ceramics glass are under the categories of synthetic organic. Technically, these organic materials are called as Polymers.

4. TYPES OF PLASTICS : All plastics are broadly classified into two main groups Thermosetting plastics Thermo plastics

5. Thermosetting Plastics: The plastics which are hardened by heat effecting a non-reversible chemical change are called thermo-setting. Thermo setting plastics do not soften on reheating and cannot be reworked. Thermosetting molecules are formed by condensation polymerization . The molecules of such type of plastics have three dimensional network and very strong binding force between molecules.

6. The raw materials for thermosetting plastics is in the form of liquid or solid . These types of plastics are polymerized when moulded or formed. It consumes more time for formation. The various types of thermosetting resin are discussed below. Phenol formal dehyde : It is also named as bakelite . It is made by the reaction of phenol with formaldehyde.

7. It is generally produced in dark colour and it has high strength, stability, and rigidity. It can be easily cast or laminated . Uses : Plugs, knobs, pulleys, bottle caps, tooling and forming dies. 2. Polyester resin: It has low moisture, good electrical resistance and variety of colours . It is used in paper mat, TV parts and car bodies. The main drawback of the polyester is high cost .

8. 3. Melamines : It has excellent electrical and heat resistance . It has good stability and low moisture absorption . The melamines are available under various names of melmac , catlin , melantine and plaskon . It is widely used for moulded parts. Uses: Telephone sets, circuit breakers, switch panels and lighting fixtures.

9. 4.Phenol furfural: It has good flowability at low moulding temperatures and sets quickly at correct temperature. The phenol furfural has good resistance to moisture and electricity. Examples: Brake linings, electrical parts and instrumental cabinets. Uses: It is used as a binder in resinoid abrasive wheels, laminating varnishes and adhesives.

10. 5.Epoxy resins: The most popular variety of epoxy resins is araldite . It has a good chemical and electrical resistances . It is mostly available in the form of liquid. They also have good resistance to wear and impact. They are quite expensive. Uses: Tools and dies, jigs and fixtures, housing for electrical parts and enamels.

11. 6. Silicones: Silicones have high resistance to high temperature upto 260º C and posses execellent dielectric strength at high temperatures. In liquid form, they are used as water repellants. They can be compressed and reinforced . Uses : It is used in coatings, laminates, foam products and induction heating apparatus. In rubber form it is used in gaskets for providing high heat resistance.

12. 7. Urea formaldehyde (Amino resin): It is obtained by the condensation of urea and aqueous formaldehyde. It cannot be cast. But, it can withstand temperature up to 77º C only. It is widely used as an adhesive and binding material. Uses: It is used in toilet seats, table ware, buttons, clock cases, electric switches and plugs.

13. 8. Alkyds: It is also known as oil-modified polyesters. Alkyds are used in synthetic enamels and lacquers. It is used in solid form where high electrical and heat resistances are required. Example: Automobile ignition parts. 9. Polyurethanes: It is mainly used for cushions in transportation seats for insulation and electronic equipment as a packing material.

14. Thermoplastics: The thermoplastics have separate long and large size molecules arranged side by side. It does not have any cross linking in their molecular structure. Some of the thermo plastic structure is amorphous in nature other than that all are crystalline structure in nature. It is formed by addition polymerization process. When thermo plastics are heated, it becomes very soft and rehardens on cooling.

15. During heating, the linear bonding links between molecules breakup and molecules are seperated . Relinking takes place on cooling and retains their hardness. It is easily remoulded or extruded to any shape. These plastics do not have a definite melting temperature. The various thermo plastics are discussed below. It is classified into: 1. Cellulose derivatives 2. Synthetic resins.

16. Cellulose Derivatives : Cellulose nitrate: It is obtained by treating the cellulose with a mixture of nitric and sulphuric acid . It has high toughness, good resistance to moisture and highly inflammable. Uses: Spectacle frames, toilet articles, pen bodies and table tennis balls.

17. (ii) Cellulose acetate: It is obtained by treating the cellulose with acetic acid. It can be injected and compressed in the mould for obtaining better stability and high mechanical strength. It is lighter than cellulose and tendency to absorb moisture. Uses: Photographic films, buttons, radio panels, toys and extruded sheets, tubes and rods.

18. (iii) Ethyl cellulose: The ethyl cellulose is the lightest of all cellulose derivatives. It has good electrical properties , chemical resistance, surface hardness and strength. Uses: Jigs, fixtures, forming dies, hose nozzles and moulded articles.

19. (iv) Cellulose acetate-butyrate: It is obtained by treating cellulose with acetic acid and butoric acid. It has good stability against light and heat and moisture absorption tendency. It can also be injection moulded and extruded . Uses: Radio cabinets, pipes and tubing, steering wheels, insulating tapes, handles and coatings.

20. (v) Cellophane: It is available in extruded form. It has attractive appearance and good resistance to moisture, fire and solvents. Uses: Curtains, drapers wrapping and packaging. (vi) Cellulose propionate: It has low tendency for moisture absorption and can easily be moulded . The cellulose propionate can withstand temperature upto 93º C. Uses: Fountain-pens, telephones and flash light cases.

21. 2. Synthetic Resins: Polyethylenes : It has very high resistance to acids, alkalizes and solvents can be made flexible, tough and good insulators. It has low water absorption. The polyethylenes are softened at 93º C. Uses: Fabrics, trays, pipes and tubing chemical containers and corrosion resistant coatings.

22. (ii) Polystyrenes: It has dimensional stabilities and strain resistances. It is easily mouldable and has a tendency to crack under load. The polystyrenes are easily joined by cementing . It can be produced in any form and colours . Uses: Battery boxes, radio parts, tableware, toys and high frequency insulation parts.

23. (iii) Acrylic resins: It has high transparency tendency. It can be made in any colour with dielectric properties , resistance to moisture, good strength and excellent light transmitting power. It can also be cast, injection moulded , extruded and stretch formed into sheets. Uses: Tubes, plates, coatings and adhesives, laminates, display cases, lenses, valves and helmets.

24. (iv) Vinyles : Its trade name is PVC. Vinyl plastics are made in the form of flexible or rigid. It has good electrical and weather resistance . The vinyls are water resistance and produced in various colours . Uses: Tarpalin , water roofing, raincoats, tubes and insulation.

25. (v) Polytetra fluoroethylene : Its trade name is Teflon. It has maximum chemical resistance , can withstand temperatures upto 288º C , and cannot be dissolved in any solvent. It has high electrical resistance, low friction and very low adhesion to other substances. It is available in forms such as rods, sheets and tubes. Uses: Gaskets, greaseless bearing, electrical insulators and chemical containers.

26. (vi) Polyamide: It is popularly known by its trade name Nylon . It has high strength, toughness and elasticity. It can be moulded and extruded into rods. The powder metallurgy methods can also be used for this type of plastics. It is a good insulator and has good wear resistance. Uses: Yarn for cloth, bearings and coupling, gears, wire insulation and combs.

27. (vii) Methyl methacrylate: It trade name is Lucite and plexiglass . It can be formed easily at temperatures around 120º C. It is marked by its clear colour and high light transmission capability. Uses: Aircraft parts, transparent bowls, contact lenses and various surgical instruments.

28. INJECTION MOULDING: Working principle: The injection moulding is used to achieve high speed moulding of thermoplastics. The working principle of this process is that the molten thermoplastic is injected into a mould under high pressure. For achieving high pressure , the plunger system is used.

29. Operation: The moulding material is loaded into a hopper from which it is transferred to a heating section by a feeding device where the temperature is raised to from 150 o C to 370 o C . The material melts and is forced by an injection ram or by plunger through a nozzle and sprue in a closed mould which forms the part. There are two types of injection moulding and it is given below.

30. ( i ) Ram or plunger type injection moulding : The ram and plunger type injection moulding has two units. (a) Injection unit, and (b) Clamping unit. So, it may be split in order to eject the finished component. Initially, the polymer is filled in a hopper . Then, it goes to the heating section where the polymer is melted and the pressure is increased. The heated material is injected by the ram under pressure.

31. So, the heated material is forced to fill in the mould cavity through the nozzle to get the required shape of the plastics. Here, the mould is water – cooled type.

32. (ii) Screw type injection moulding : In this type also, are two units to split and eject the finished component such as (a) Injection unit, and (b) Clamping unit . The injection unit has hopper, screw , and heating section. In clamping section , it has mould. In a screw type moulding machine, the pellets are initially fed into the hopper. The resins are pushed along with the heated reciprocating screw.

33. The screw is moved forward to force the plastic material into the mould. The screw itself is moving backwards and allowing the accumulation of enough material to fill the mould. The rotation of the screw provides the plasticizing action by shearing and frictional effects. The axial motion of the screw provides the filling action.

34. The jet moulding process is used to find the problems occurred in injection moulding process. The reaction moulding is the recent development in injection moulding . In reaction moulding , the low viscosity monomers are used in the mould. A chemical reaction takes place between resins at low temperature and a polymer is created. In jet moulding , the plastic is preheated about 93 o C in the cylinder surrounding the nozzle. The reaction moulding is suitable for the production of polyurethane moulding . The injection capacity of injection moulding machines ranges from 12,000 mm 3 to 2.2 X 10 6 mm 3 .

35. Advantages of injection moulding : High production capacity and less material losses are possible. The cost is low and it needs less finishing operation. It is used for making complex threads and thin walled parts. Accuracy becomes ±0.025mm. Wide ranges of shapes can be moulded .

36. Applications: It is used in making parts of complex threads . Intricate shapes such as thin walled parts can be produced. Typical parts such as cups, containers, tool handles, toys, knobs and plumbing fittings can be produced. Electrical and communication components such as telephone receivers can be produced. Limitations: Equipment of cylinder and die should be non – corrosive. The reliable temperature controls are essential.

37. Compression Moulding : Working principle: The compression moulding is widely used for thermosetting polymers and it is also used to thermoplastic polymers. It involves a pre-measured quantity of plastic in the form of particles or briquettes which is placed in a heated mould and compressed at suitable pressure and temperature. The charge is placed in the heated mould cavity and mould is closed.

38. The desired compression is given by compression press thereby resulting an immediate contact of the polymer charge with all parts of the mould . Both the pressure and heat ensure the flow of resin , filling of all parts and corners of the cavity. For thermosetting systems, the pressure is maintained till the linking is obtained to an optimum level. Finally, the mould is opened and ejected from the cavity.

39. The mould is cooled below the transition temperature before the mould is opened while making thermoplastics. This is the cyclic process . The time interval covering the mould is known as cycle time. A slight excess material is placed in the mould and squeezed out between mating surfaces of the mould . The use of perfumes against moulding powder gives low compression ratio and breathing is helpful in eliminating voids and improves the quality of the mould.

40. The moulding temperature of thermosetting materials ranges from 150 o C to 180 o C. The time required to harden the mould piece ranges from 1 to 15 minutes. The compression moulding has four basic types. ( i ) Flash type (ii) Landed positive type (iii) Positive type (iv) Semi positive type. The compression moulding is the equivalent of closed – die forging.

41. Hydraulic presses are usually employed to provide the pressure which may ranges from 20 to 30 Mpa or even higher upto 80 Mpa . The main objective of compression moulding is to bring the plastic virtually to a molten state. When the plastic is completely trapped between the male and female dies, it is called as positive mould.

42. S.No . Type of Compression mould Uses 1. Flash type This is the widely used method for making plastics. 2. Landed positive type It is used for high impact material 3. Positive type It is used for high impact material and deep draw. 4. Semi positive type For deep drawing this method is used

43. Applications: It is used to make dishes , handles, container taps and fittings. Electrical and electronic components , washing machine agitators and housings are made by this process.

44. 2. TRANSFER MOULDING: Working principle: Transfer moulding is a modification of compression moulding in which the material is first placed in separate chamber called transfer pot. Then the material is pushed in sprue through the orifice and into the mould cavity by the action of a punch.

45. The pressure is used in transfer moulding is from 50 to 100 % higher than the compression moulding and the mould is kept at a high temperature so that the heat transmission is easier when compared to compression moulding . Transfer moulding cycles are shorter than compression moulding and moulding is done at high temperature and pressure. The material to be moulded is often pre – heated by radio - frequency methods.

46. Where it is desired to improve toughness and strength , the reinforcing fillers may be used. Transfer moulding is generally employed for thick sections and also useful for incorporating metal parts in the moulding . The pre-melted polymer charge flows easily into interior parts with inserts. It is not possible in compression moulding .

47. Transfer moulding design: The following points are to be considered while designing transfer moulding 1. Flow of materials should be easy. 2. Ejecting of mould should be easy. 3. Heating of all the parts should be uniform.

48. Advantages: Before completely filling the plastic in the mould cavity, little pressure is maintained inside the mould which will create full liquid pressure inside the cavity. When the plastic flows through the orifice into the cavity, the temperature will increase. Cold presses can be used. The viscosity of flow material reduces.

49. Applications: It is used for batch production . Short runs of mould metal during moulding . Shape of mould can be readjusted

50. BONDING OF THERMOPLASTICS: It is done by the application of pressure and heat. It consists of layers, such as paper, cellulose, glass fiber etc. Synthetic resins are used as binders. The thermoplastics are bounded by the process of lamination .

51. The lamination process is classified into two categories: ( i ) High pressure laminates (ii)Low pressure laminates. In high – pressure laminates, the pressure applied is upto 7 Mpa and temperature of about 150 C. In low pressure laminates, the pressure requirement is very less when compared to high pressure laminates. Materials such as asbestos, cotton, fibres are fabricated by this process.

52. The low- pressure laminations are also called “ Reinforced Plastics ”. Figure shows the lamination process in which the paper and glass are immersed in the resin solution using rollers and then resin mixed plastics are dried in the drying oven. The dried plastics are cut in the cutting section. After cutting, it will be pressed by the press.

53. The stages involved in lamination are: 1. Saturation of the base with resin solution. 2. Wet drying 3. Size cutting 4. Pressing The laminated plastics are used in electrical and electronic components and also for mechanical devices. In furniture industry, the decorative laminations are used.

54. The reinforced plastics have the characteristics of: 1. Elastic stability, and 2. Less weight The reinforced plastics are used in: 1. Making thin sheets 2. Making aircraft panels 3. Making horns 4. Making storage bins.

55. 3. BLOW MOULDING: Working principle: In this process, a hot extruded tube of plastic called parison is placed between two parts of open moulds. The two valves of the mould move towards each other so that the mould closes over the tube.

56. The bottom end of parison is sealed . The compressed air is used to blow the molten plastic into the mould and the tube gets pinched off and also welded at the bottom by closing the moulds . The air pressure is about 0.7 to 10 kg/cm 2 . This air pressure will force the tube against the walls of the mould . Finally, the component is cooled and the mould opens to release the components.

57. The blow moulding method ranges from simple manual operation to complicated automatic ones .

58. The various types of blow moulding process are: 1. Injection blow moulding 2. Extrusion blow moulding 3. Multi larger blow moulding The figure shows the injection blow moulding . In multiplayer blow moulding , multi layer structure are used. Typical examples for multiplayer blow moulding are plastic packing for food and beverage .

59. Applications: 1. It is used in making plastic bottles and toys. 2. The hollow cont ainers are produced by this process. 3. The multiplayer blow moulding is used in cosmetics and pharmaceutical industries .

60. Rotational moulding Working Principle: Rotational moulding process is used to make thin walled hollow parts. In this method, a measured quantity of polymer powder is placed in a thin-walled metal mould . The mould is closed and it is rotated about two mutually perpendicular axes as it is heated.

61. The rotation will cause the powder to sinter against the mould walls. After heating and sintering, the mould is cooled while it is still rotating. The cooling of mould is done by using water and air. Then the rotation is stopped when the moulded component is removed. In this rotational moulding , thin walled metal mould is made of two pieces and is rotated in perpendicular axis.

62. A measured quantity of powdered plastic material is placed inside the mould . Then, the mould is heated and rotated. This action tumbles the powder against the mould where the heating fuses the powder without melting it. Most thermoplastics and some thermosets can be formed into large hollow parts by rotational moulding . In some parts , chemical agents are added to the powder and cross-linking after the part is formed in the mould by continuos heating.

63. Rotational moulding can also produce parts with complex hollow shapes with wall thickness of 0.4 mm minimum. Large size parts are 1.8m x 1.8m x 3.6m can also be formed by this process. The surface finish of the mould is same as that of surface finish of walls. The temperature-time relationship during the oven cycle is very important.

64. Applications: It is used to produce toys in P.V.C. It is used to make large containers of polyethylene. It is used to make petrol tanks for motocars from polyethylene and nylon. Metallic or plastic inserts are moulded by this process. The buckets, housings, boat hulls and trashcans are made by this process. It is used to produce tanks of various sizes, boat hulls and footballs.

65. Film Blowing Working Principle: Crystalline sharp melting polymers such as nylon or PET are very much suited for the film productions by melt casting techniques. Initially, the heated plastic powder is extruded by using extrude machines called extruder. In this extruding process, the thin film is produced . After extruding the thin film, it is stretched by pulling rollers through the chilled drum in the reeling wheel. The thin film is cooled in the chilled drum and the rollers are used to pull the film from chilled drum. The reeling wheel is used to make the film roll.

66. Extrusion Process Working Principle: The process consists of feeding the powdered plastic from the hopper into the heated chamber. A rotating screw carries the material forward and forces it out through the heated orifice of the die. The required shape is obtained through the die. The mould is suitably cooled by water or air – blast and carried away by a running belt. The raw material is in the form of pellets , granules and powder. The thermosetting plastics are not suitable for extrusion. For the extrusion of plastics, a single screw machine has completely replaced the ram type machine.

67. The screw has three distinct sections . ( i ) Feed section connects the material from hopper into the central region of the barrel. (ii) Melt section in which plastic starts to melt. (iii) Pumping section in which additional shearing and melting occur. In this, the material is fed from the hopper and the screw is rotated in a barrel . The screw imparts both axial and rotary motions. The restricting effect of the die will build up a pressure until it is in a plastic state and can be extruded.

68. Applications : It is used to make tubes, sheets, films, pipes, ropes and other profiles. Complete shapes with constant cross sections can be extruded with relatively inexpensive tooling.

69. Vacuum Forming Process (Thermoforming) Working Principle: It is a process in which a heated plastic sheet is changed to a desired shape by causing it to flow against the mould surface by reducing the air pressure between one side of the sheet and the mould surface. Figure shows the vacuum forming process in which the plastic sheet is heated in a heater and the sheet is fixed in a clamp in the first stage.

70. In the second stage, the heated sheet is placed on the die where the air between the sheet and mould is removed. In the third stage, increasing intensity draws the sheet against the surface of the mould where it cools and solidifies. The vacuum forming process is also called “ Thermoforming ” . If large surface area moulds are used, it will be difficult to stretch the plastic into the mould. In this case, the mechanical assist is given to stretch the plastic into the mould.

71. The main advantages of this process are low cost and quick process of making the sheet. In this, the air pressure acts as a cushion and temperature of the air delays the sheet cooling. A wide variety of plastic products are made by this thermoforming.

72. Applications : It is very much useful for making trays, drink cups, refrigeration door lines. It is used for making panels for shower stalls and advertising signs.