CHAPTER FIVE- final.pdf
- 3. Introduction to plastic engineering • It would be difficult to imagine our modern world without plastics • Its integral part of everyone’s lifestyle with applications varying from common place domestic articles to sophisticated scientific and medical instruments. • Plastics offer advantages such as lightness, resilience, resistance to corrosion, colour fastness, transparency, ease of processing, etc.,
- 4. Introduction to plastic engineering • although they have their limitations, their exploitation is limited only by the ingenuity of the designer • Few designers would simply specify metal as the material for a particular component so it would be equally unsatisfactory just to recommend plastic. • This analogy can be taken still further because in the same way that there are different grades of steel there are also different grades of, say, polypropylene. • In both cases the good designer will recognize this and select the most appropriate material and grade on the basis of process ability, toughness, chemical resistance, etc
- 6. Introduction to plastic engineering • The words polymers and plastics are often taken as synonymous but in fact there is a distinction. • polymer is the pure material which results from the process of polymerisation and is usually taken as the family name for materials which have long chain-like molecules. • when additives are present that the term plastic is applied • Polymers contain additives for a number of reasons…... • Antistatic Agents: Most polymers, because they are poor conductors of current, build up a charge of static electricity. • Antistatic agents attract moisture from the air to the plastic surface, improving its surface conductivity
- 7. Introduction to plastic engineering Coupling Agents: Coupling agents are added to improve the bonding of the plastic to inorganic filler materials, such as glass fibres. A variety of silanes and titanates are used for this purpose. Fillers: Some fillers, such as short fibres or flakes of inorganic materials, improve the mechanical properties of a plastic. Others, called extenders, permit a large volume of a plastic to be produced with relatively little actual resin. Calcium carbonate, silica and clay are frequently used extenders.
- 8. Introduction to plastic engineering Flame Retardants: Most polymers, because they are organic materials, are flammable. • Additives that contain chlorine, bromine, phosphorous or metallic salts reduce the likelihood that combustion will occur or spread. Lubricants: Lubricants such as wax or calcium stearate reduce the viscosity of the molten plastic and improve forming characteristics. Pigments: Pigments are used to produce colours in plastics.
- 9. Introduction to plastic engineering Reinforcement: The strength and stiffness of polymers are improved by adding fibres of glass, carbon, etc. Stabilisers: Stabilisers prevent deterioration of the polymer due to environmental factors. Antioxidants are added to ABS, polyethylene and polystyrene. Heat stabilisers are required in processing polyvinyl chloride. Stabilisers also prevent deterioration due to ultra-violet radiation. Plasticisers: Plasticisers are low molecular weight materials which alter the properties and forming characteristics of the plastic. An important example is the production of flexible grades of polyvinyl chloride by the use of plasticisers.
- 10. PLASTIC PROCESSING • One of the most outstanding features of plastics is the ease with which they can be processed. • The processing stages of heating, shaping and cooling may be continuous (eg production of pipe by extrusion) or a repeated cycle of events (eg production of a telephone housing by injection molding
- 11. Extrusion In most cases the choice of method is based on the shape of the component and plastic type (thermoplastic or thermosetting) Screw used for plastic processing categorized as single screw and multiple screw depending on the raw material used and product type General Features of Single Screw Extrusion • One of the most common methods of processing plastics is Extrusion using a screw inside a barrel
- 12. Extrusion • Feed in the form of granules or powder from a hopper on to the screw conveyed along the barrel • where it is heated by conduction from the barrel heaters and shear due to its movement along the screw flights
- 13. Extrusion • At the end of the extruder the melt passes through a die to produce an extrudate of the desired shape • an extruder screw has three different zones (a) Feed Zone: The function of this zone is to preheat the plastic and convey it to the subsequent zones • The design of this section is important since the constant screw depth must supply sufficient material to the metering zone so as not to starve it • on the other hand supply so much material lead overrun to the metering zone is • optimum design is related to nature and shape of the feedstock
- 14. Extrusion • frictional properties of the screw and barrel in relation to the plastic should be considered (b) Compression Zone: In this zone the screw depth gradually decreases so as to compact the plastic squeezing any trapped air pockets back into the feed zone and improving the heat transfer through the reduced thickness of material. (c) Metering Zone : In this section the screw depth is again constant but much less than the feed zone.
- 15. Extrusion • in the metering zone the melt is homogenised so as to supply at a constant rate, material of uniform temperature and pressure to the die. • The lengths of the zones on a particular screw depend on the material to be extruded
- 16. Extrusion venting zone: in Some extruders. • This is principally because a number of plastics are hygroscopic - they absorb moisture from the atmosphere. • One possibility is to pre-dry the feedstock to the extruder but this is expensive and can lead to contamination. • Vented barrels were developed to overcome these problems. • in the first part of the screw the granules are taken in and melted, compressed and homogenised in the usual way.
- 17. • The melt pressure is then reduced to atmospheric pressure in the decompression zone. • This allows the volatiles to escape from the melt • then melt conveyed along the barrel to a second compression zone which prevents air pockets from being trapped
- 18. Extrusion gauze filter: after the screw and before the die. This effectively filters out any inhomogeneous material which might otherwise clog the die. • normally filter the melt to 120-150 μm. However, even smaller particles than this can initiate cracks in plastics extrudates
- 19. Extrusion Mechanism of Flow • Initially a thin film of molten material is formed at the barrel wall. • As the screw rotates, it scrapes this film off and the molten plastic moves down the front face of the screw flight. • When it reaches the core of the screw it sweep up again. • As the screw rotates more and more solid material is swept into the molten pool until eventually only melted material exists between the screw flights.
- 20. Extrusion Mechanism of Flow • the movement of the plastic depend on whether or not it adheres to the screw and barrel. a) the material sticks to the screw only and therefore the screw and material rotate as a solid cylinder inside the barrel. b) the material slips on the screw and has a high resistance to rotation inside the barrel. • The external heating and cooling on the extruder also plays an important part in the melting process. • In all extruders, barrel cooling is essential at the feed pocket to ensure an unrestricted supply of feedstock
- 21. Extrusion Mechanism of Flow • Extruders may be run without external heating or cooling but they are not truly adiabatic since heat losses will occur.
- 22. Extrusion Twin Screw Extruders have two screws rotating in a heated barrel. permit a wider range of possibilities in terms of output rates, mixing efficiency, heat generation etc compared with a single screw extruder The output of a twin screw extruder can be typically three times that of a single screw extruder of the same diameter and speed.
- 23. Comparison of single screw co-rotating and counter- rotating twin-screw extruders
- 24. Extrusion • Extrusion is an extremely versatile process in that it can be adapted, by the use of appropriate dies, to produce a wide range of products. Some of the more common of these production techniques (a) Granule Production/Compounding In the extruder feedstock is melted, homogenised and forced through a capillary shaped die. continuous lace (tie) formed then may be chopped into short granules and packed into sacks. It is more convenient for use in other processing methods, such as injection moulding.
- 25. Extrusion (b) Profile Production the simple operation of a die change can provide a wide range of profiled shapes such as pipes, sheets, rods, curtain track, edging stsips, window frames, etc The successful manufacture of profiled sections depends to a very large extent on good die design. (c) Film Blowing The molten plastic from the extruder passes through an annular die and emerges as a thin tube. A supply of air to the inside of the tube prevents it from collapsing and indeed may be used to inflate it to a larger diameter
- 26. Extrusion (d) Blow Moulding method for producing hollow plastic articles (such as bottles and barrels) Initially a molten tube of plastic called the Parison is extruded through an annular die. A mould then closes round the parison and a jet of gas inflates it During moulding, the inflation rate and pressure must be carefully selected so that the parison does not burst.
- 27. The conventional extrusion blow moulding process may be continuous or intermittent. In most cases the mould assembly moves relative to the die. When the mould has closed around the parison, a hot knife separates the latter from the extruder and the mould moves away for inflation, cooling and ejection of the moulding. Meanwhile the next parison will have been produced and this mould may move back to collect it
- 28. Extrusion (i) Co-Extrusion: it is Recent Developments in Extrusion Technology In co-extrusion two or more polymers are combined in a single process to produce a multilayer film. These co-extruded films can either be produced by a blown film The main reason for producing multi-layer co-extruded films is to get materials with better barrier properties - particularly in regard to gas permeation.
- 29. Co-extrusion of plastic film
- 30. Injection Moulding it is a simple process. A thermoplastic, in the form of granules or powder, passes from a feed hopper into the barrel where it is heated so that it becomes soft. It is then forced through a nozzle into a relatively cold mould which is clamped tightly closed. When the plastic has had sufficient time to become solid the mould opens, the article is ejected and the cycle is repeated.
- 31. Injection Moulding The major advantages of the process its versatility in moulding a wide range of products high production rates manufacture of articles with close tolerances.
- 32. Injection Moulding In order to split up the mass of material in the barrel and improve the heat transfer, a torpedo is fitted in the barrel moulding material drops from the feed hopper into the barrel. The plunger then conveys the material along the barrel where it is heated by conduction from the external heater. The material is thus plasticised under pressure so that it may be forced through the nozzle into the mould cavity.
- 33. Injection Moulding Disadvantages There is little mixing or homogenisation of the molten plastic. It is difficult to meter accurately the shot size. the pressure at the nozzle can vary quite considerably from cycle to cycle. The presence of the torpedo causes a significant pressure loss. The flow properties of the melt are pressure sensitive and since the pressure is erratic, this amplifies the variability in mould filling.
- 34. Injection Moulding reciprocating screw type of injection moulding An extruder type screw in a heated barrel performs a dual role. it rotates in the normal way to transport, melt and pressurize the material in the barrel
- 35. Injection Moulding Screws the most common L/D ratios are in the range 15 to 20 injecting the plastic at pressures up to 200 MN/m*2 important difference from an extruder screw is the presence of a back-flow check valve at the end of the screw Barrels and Heaters These are also similar to those in extruder machines. vented barrels have become available to facilitate the moulding of water sensitive plastics without the need for pre-drying.
- 36. Injection Moulding generally extrusion requires a lower moisture content than injection moulding to produce good quality products
- 37. Injection Moulding Nozzles :The nozzle is screwed into the end of the barrel and provides the means by which the melt can leave the barrel and enter the mould • Contact with the mould causes heat transfer from the nozzle. in cases where this is excessive it is advisable to withdraw the nozzle from the mould during the screw- back part of the moulding cycle. Otherwise the plastic may freeze off in the nozzle. types of nozzle • open nozzle • shut-off nozzle(if the melt viscosity)
- 38. Injection Moulding Clamping Systems: In order to keep the mould halves tightly closed when the melt is being injected under high pressures. • (a) hydraulic • (b) mechanical (toggle) • In the hydraulic system, oil under pressure is introduced behind a piston connected to the moving platen of the machine • The toggle is a mechanical device used to amplify force. • Toggle mechanisms tend to be preferred for high speed machines and where the clamping force is relatively small.
- 39. Injection Moulding Moulds consists of two halves male and female • the surfaces of the mould halves are accurately machined so that no leakage of plastic can occur. • The mould cavity is joined to the machine nozzle by means of the sprue. • The sprue is the channel along which the molten plastic first enters the mould • It delivers the melt from the nozzle to the runner system • and ensuring that the moulded part remains on the moving half of the mould, when the mould opens.
- 40. Injection Moulding • For multi-cavity moulds the impressions are joined to the sprue by runners - channels cut in one or both halves of the mould through which the plastic will flow without restriction • The sprue is incorporated in a hardened steel bush which has a seat designed to provide a good seal with the nozzle. • A narrow constriction between the runner and the cavity allows the moulding to be easily separated from the runner and sprue. • This constriction is called the gate (small orifice which connects the runner to the cavity). • It has a number of functions.
- 41. Injection Moulding • convenient weak link by which the moulding can be broken off from the runner system. • The gate also acts like a valve in that it allows molten plastic to fill the mould but being small it usually freezes off first. • Runners: The runner is the flow path by which the molten plastic travels from the sprue (i.e. the moulding machine) to the gates (i.e. the cavity). • To prevent the runner freezing off prematurely, its surface area should be small
- 42. Injection Moulding • so as to minimize heat transfer to the mould. However, the cross sectional area of the runner should be large. • so that it presents little resistance to the flow of the plastic but not so large that the cycle time needs to be extended to allow the runner to solidify for ejection. • good indication of the efficiency of a runner is, therefore, the ratio of its cross-sectional area to its surface area. Venting: Before the plastic melt is injected, the cavity in the closed mould contains air. When the melt enters the mould, if the air cannot escape it become compressed.
- 43. Injection Moulding • At worst this may affect the mould filling, but in any case the sudden compression of the air causes considerable heating. • This may be sufficient to burn the plastic and the mould surface at local hot spots. • vents are machined into the mating surfaces of the mould to allow the air to escape. • The vent channel must be small so that molten plastic will not flow along it and cause unsightly flash on the moulded article. .
- 44. Injection Moulding • Typically a vent is about 0.025 mm deep and several millimeters wide • Away from the cavity the depth of the vent can be increased so that there is minimum resistance to the flow of the gases out of the mould. • Mould Temperature Control: For efficient moulding, the temperature of the mould should be controlled and this is normally done by passing a fluid through a suitably arranged channel in the mould. • In most cases the mould temperatures used are a compromise based on experience.
- 45. Injection Moulding The rate at which the moulding cools affects total cycle time surface finish tolerances distortion and internal stresses of the moulded article. • High mould temperatures improve surface gloss and tend to eliminate voids. • However, the possibility of flashing is increased and sink marks are likely to occur • If the mould temperature is too low then the material may freeze in the cavity before it is filled.
- 46. Injection Moulding Mould Clamping Force: • In order to prevent ‘flashing’ i.e. a thin film of plastic escaping out of the mould cavity at the parting line • it is necessary to keep the mould tightly closed during injection of the molten plastic. • Practical experience suggests that the clamping pressure over the projected area of the moulding should be between 10 and 50 MN/m2 depending on factors such as shape, thickness type of material
- 47. the force on the shaded element is given by Po is the pressure at the gate and m is a constant which is usually between 0.3 and 0.75 Consider the moiilding of a disc which is centre gated
- 48. Injection Moulding Gas Injection Moulding • This offers many advantages including greater stiffness weight ratios and reduced moulded-in stresses and distortion. • (caused by laminar flow and variation in pressure throughout the moulding) • The first stage of the cycle is the flow of molten polymer into the mould cavity through a standard feed system. • Before this flow of polymer is complete, the injection of a predetermined quantity of gas into the melt begins through a special nozzle located within the cavity or feed system.
- 49. Injection Moulding • the timing, pressure and speed of the gas injection is critical • By controlling the amount of gas injected into the hollow core, the pressure on the cooling polymer is controlled an maintained until the moulding is packed. • The final stage is the withdrawal of the gas nozzle, prior to mould opening, which allows the gas held in the hollow core to vent.
- 50. Injection Moulding Reaction Injection Moulding • In Reaction Injection Moulding (RIM), liquid reactants are brought together just prior to being injected into the mould. • In-mould polymerisation then takes place which forms the plastic at the same time as the moulding is being produced. • • In some cases reinforcing fillers are incorporated in one of the reactants and this is referred to as Reinforced Reaction Injection Moulding (RRIM) • range of plastics lend themselves to the type of fast polymerisation reaction which is required in this process - polyesters, epoxies, nylons and vinyl monomers
- 51. •The components A and B are an isocyanate and a polyol and these are kept circulating in their separate systems until an injection shot is required. •At this point the two reactants are brought together in the mixing head and injected into the mould.
- 52. Injection Moulding • Since the reactants have a low viscosity, the injection pressures are relatively low in the RIM process. • Thus, comparing a conventional injection moulding machine with a RIM machine having the same clamp force, • the RIM machine could produce a moulding about 10 times greater projected area • Therefore the RIM process is particularly suitable for large area mouldings such as car bumpers and body panels
- 53. Injection Moulding Injection Blow Moulding • Initially a preform is injection moulded. This is subsequently inflated in a blow mould in order to produce the bottle shape. • In most cases the second stage inflation step occurs Immediately after the injection moulding step. • but in some cases the preforms are removed from the injection moulding machine and subsequently re-heated for inflation.
- 55. Injection Moulding The advantages of injection blow moulding (i) the injection moulded parison may have a carefully controlled wall. thickness profile to ensure a uniform wall thickness in the inflated bottle. (ii) it is possible to have intricate detail in the bottle neck. (iii) there is no trimming or flash (compare with extrusion blow moulding).
- 56. Thermoforming • When a thermoplastic sheet is heated it becomes soft and pliable and the techniques for shaping this sheet are known as thermoforming. • Some articles: Aircraft window reveals, refrigerator liners, baths, switch panels, car bumpers, motorbike fairings etc. • thermoforming a process of shaping flat thermoplastic sheet softening the sheet by heat, followed by forming it in the mold cavity. • Widely used in food packaging industry
- 57. Thermoforming • Thermosets can not be formed by the thermoforming because of their cross linked structure • thermoforming’incoroporates a wide range of possibilities for sheet forming but basically there are two sub-divisions - vacuum forming and pressure forming. (a) Vacuum Forming • shaping a preheated thermoplastic sheet by means of vacuum produced in the mold cavity
- 58. • In this processing method a sheet of thermoplastic material is heated and then shaped by reducing the air pressure between it and a mould • A sheet of plastic, which may range in thickness from 0.025 mm to 6.5 mm, is clamped over the open mould. • A heater panel is then placed above the sheet and when sufficient softening has occurred the heater is removed and the vacuum is applied. For the thicker sheets it is essential to have heating from both sides.
- 59. Thermoforming • Materials which can be vacuum formed satisfactorily include polystyrene, ABS, PVC, acrylic, polycarbonate, polypropylene and high and low density polyethylene (b) Pressure Forming • pressure is applied above the sheet rather than vacuum below it. • advantage of this is that higher pressures can be used to form the sheet.
- 60. Thermoforming • attractive as an alternative to injection moulding for moulding large area articles such as machine housings.
- 61. Thermoforming (c) Matched Die Forming • does not involve gas pressure or vacuum is matched die forming. • The plastic sheet is heated and is the sandwiched between two halves of a mould. • Very precise detail can be reproduced • the moulds need to be more robust than for the more conventional process involving gas pressure or vacuum.
- 62. If a thermoplastic sheet is softened by heat and then pressure is applied to one of the sides so as to generate a freely blown surface found that the shape so formed has a uniform thickness.
- 63. Thermoforming volume balance between the original sheet and the final shape could provide the wall thickness of the end product. where A = surface area, and h = wall thickness (‘i’and ‘f’ refer to initial and final conditions)
- 64. Recycling of plastic What does recycling mean? Recycling involves BOTH collecting a material, AND having an economic incentive to convert it into a product recycling of plastics has become paramount in preserving the environment. About 7% of all household waste is plastic. Annually, 3 million tonnes of plastic rubbish are produced. 57% of litter found on beaches is plastic. In 2001 only 7% of all plastic was recycled.
- 65. Why recycle plastic? Conservation of non-renewable fossil fuels - Plastic production uses 8% of the world's oil production. Reduced consumption of energy. Reduced amounts of solid waste going to landfill. Reduced emissions of carbon-dioxide (CO2), nitrogen- oxide (NO) and sulphur-dioxide (SO2)
- 66. How are polymers recycled? Mechanical recycling of plastics refers to processes which involve the melting, shredding or granulation of waste plastics. Plastics must be sorted prior to mechanical recycling. Avoid contamination. Contamination reduces the value of a recycled product At the moment in the UK most sorting for mechanical recycling is done by trained staff who manually sort the plastics into polymer type and/or colour.