CHAPTER 6 : PLASTIC<br />6.0 Plastic<br /><ul><li>Plastic : An organic polymer material (with carbon and hydrogen base) with the ability to flow into a desired shape when heat and pressure are applied to it and retain the shape when they are withdrawn
Polymer : A material consisting of long molecular chains or networks of low-weight elements to form a single compound</li></li></ul><li>
6.1 The History of Plastic<br />1. Regnault (1835)<br /><ul><li>a France chemical scientist
introduce a chemical material called vinyl chloride monomer transformed in white powder (polyvinyl chloride - PVC)
but not popular because of not enough raw materials at that moment</li></ul>2. John Wesley Hyatt (1866)<br /><ul><li>an American, finally came upon the solution with celluloid
produced billiard balls using celluloid as a substitute</li></li></ul><li>3. Alexander Parkes (1855)<br /><ul><li>origin from Birmingham, England
won a bronze medal at the 1862 Great International Exhibition in London
produced a hard but flexible transparent material, which he called Parkesine from an organic material derived from cellulose
claimed that this new material could do anything rubber was capable of, but at a lower price
Parkesinesoon lost its luster due to the high cost of the raw materials needed in its production</li></ul>4. Leo Hendrik Baekeland (1907)<br /><ul><li>a Belgian-born American living in New York state
developed a liquid resin that he named Bakelite</li></li></ul><li>6.2 Properties of Plastic<br />1. The general properties :<br /> i. light<br /> ii. processable<br /> iii. durable<br /> iv. resist corrosion<br /> v. colour stay<br /> vi. translucent<br /> vii. transparent<br /> viii. good electric insulator<br /> ix. good thermal insulator<br /> x. wear resistance<br />
6.3 Polimer<br />1. Polymer<br /><ul><li>a chemical substance made up of a lot of mer or repeating units or molecules to form a long flexible chain
a material consisting of long molecular chains or networks of low-weight elements to form a single compound</li></ul>2. Polymerization<br /><ul><li>a process of linking the monomers together to form a polymer
a chemical reaction in which high-molecular-mass molecules are formed from monomers</li></ul>3. Monomer : a single molecule or a small molecule that may become chemically bonded to other monomers to form a polymer.<br />4. Homopolymer : polymeric materials which consist of polymer chains made up of single repeating units.<br />
5. Copolymer : consist of polymer chains made up of two or more chemically different repeating units which can be in different sequences.<br />Linear polymer M – M – M – M<br />Alternating copolymer M – O – M – O – M – O<br />Random copolymer O – M – O – O – M – O – M<br />Block copolymer M – M – M – O – O – O – M<br />6. Amorphous (non-crystalline)<br /><ul><li>a substance having no specific space lattice, the molecules being distributed at random
a structure not having the long range repetitive pattern of atoms arrangement</li></li></ul><li>6.4 Polymer Structure<br /><ul><li>It is a long linkage structure.
The chains of molecules bonded by a weak force and flexible causes the carbon bondage to move and rotating in the chains.
Polymer exists in two structures : amorphous (non-crystalline) and semi-crystalline.</li></li></ul><li>6.4.1 Amorphous Structure (non-crystalline)<br /><ul><li>At a high temperature, the polymer became a viscous liquid where the chains moved and glide into one another in tangled state.
The chains arrangement are randomize and not in certain geometry pattern.
When the temperatures are reduced, the flexibility of molecule properties is blocked and it is similar to the glassy state.</li></li></ul><li>6.4.2 Semi-Crystalline Structure<br /><ul><li>With slow cooling, the molecules will have the certain structure.
Causing the molecule to packed together and increased the forces between them, with higher strength, rigid and brittle.
The polymer formed with 90% in crystalline chains and the remain in amorphous state.</li></li></ul><li>6.5 Types of Polymer Chains<br /><ul><li>There are 4 types of molecule chains by polymerization process as shown in the figure below :</li></ul> i. Linear chain polymers<br /> a. in the form of a long molecular chains<br /> ii. Branched chain polymers<br /> b. the linear chain with a series of branched<br /> iii. Cross-linked chain polymers<br /> c. with short links which connected the closer chains together<br /> iv. Network chain polymers<br /> d. molecular structure in 3 dimension networks<br />
6.6 Polymerization Processes<br />1. Polymerization : a chemical reaction in which high-molecular-mass<br />molecules are formed from monomers.<br />2. Two types of polymerization process :<br />a. Addition Polymerization<br /><ul><li>chain reaction involves the straight-forward addition of monomers of the same kind or of different kinds
it is occurs between molecules or monomers contains carbon bonding when temperature, pressure and certain catalyst are given
examples :</li></ul> i. Polyester are formed from ethylene polymerization<br /> ii. Polyvinyl Chloride (PVC)<br />b. Condensation Polymerization<br /> step reaction involves between two monomers with the elimination of a simple by product, such as water, hydrogen chloride, etc<br /> it is occurs when two monomer substances react together to produce polymer and small molecules (usually water) will be expelled<br />
6.7 Differences between Thermoplastic and Thermosetting<br /><ul><li>The plastic are divided into 2 large groups :</li></ul> a. Thermoplastics<br /> - group of plastics that can be softened every time they are heated<br /> - with no curing (chemical change) takes place during the moulding operations<br /> - they then can be reshaped<br /> - e.g. Polyethylene, Nylon, PVC (Polyvinyl Chloride)<br /> b. Thermosetting plastics<br /> - group of plastics that only can be heated and formed only once<br /> - undergo chemical change (curing) during moulding<br /> - can never again be softened by heating<br /> - e.g. Epoxy, Polyesters<br />
6.8 Thermoplastic<br /><ul><li>Thermoplastic materials belong to the linear and branched chain polymers that are obtained by addition or condensation polymerization of monomers.
They can be softened, hardened or resoftened repeatedly by application of heat.
Thermoplastics are solids at room temperature , melted or softened by heating, placed into a mold and then cooled to give the desired shape.
The monomer linear chains structure of thermoplastics :</li></ul>M – M – M – M – M – M<br />M – M – M – M – M – M<br />M – M – M – M – M – M M is molecule or monomer<br /><ul><li>When heated, the chains will move separately and took place to a new position and maintain to that position after the heat and pressure removed</li></li></ul><li> The additives of thermoplastics :<br />1. Filler<br />i. wood flour, calcium carbonate, glass fiber, asbestos, aluminium powder, mica granules<br />ii. to improve the impact strength and reduce shrinkage during moulding<br />2. Plasticizers<br />i. to make plastics softer and more flexible<br />ii. to improve flow properties<br />iii. to reduce rigidity and brittleness<br />3. Lubricants<br />i. substance that reduces friction when applied as a surface coating to moving parts to fix the processing and flowability<br />4. Colourants<br />i. subdivided into dyestuffs, organic and inorganic pigments<br />ii. pigments more able to resist the temperature and lights<br />5. Antioxidants<br />i. prevent oxidation, the polymer reacting with oxygen<br />ii. oxidation can cause loss of impact strength, elongation, surface cracks and discolouration<br />iii. antioxidants help prevent thermal oxidation reactions when plastics are processed at high temperatures and light-assisted oxidation when plastics are exposed to UV light<br />iv. stabilizers<br />
6.8.1 Properties of Thermoplastics<br /><ul><li>can be softened and hardened repeatedly by applying the appropriate thermal and pressure
can be attached/ jointed using heat and pressure
not liquidify but flow at appropriate pressure and heat for injection moulding
when blowed, acts like glasses, can be shape as bottles and round by using pressure or vacuum technique
are linear chain polymers form by a long carbon chains through covalent bonding</li></li></ul><li>6.9 Thermosetting Plastic<br /><ul><li>Thermosetting materials belong to the three-dimensional cross-linked or network polymers.
Once they have been set and hardened, they cannot be remelted and returned to their original state (cannot be recycled or reused)
The three-dimensional cross-linked structure :
Stronger material and insoluble to the solvents because impossible for the solvents to enter and breaks the chains.</li></ul>6.9.1 General Properties of Thermosets<br /><ul><li>melted material when the first time heated and then settled or preserved into hard and rigid shape at that temperature
after the first formation, no changes will happen eventhough heat and pressure applied
usually harder, stronger and more brittle than thermoplastic</li></li></ul><li>6.10 The Advantages Of Plastics<br />1. Excellent electrical insulation properties.<br />2. Good insulation properties.<br />3. Certain plastics has good chemical resistance.<br />4. Good resistance to shock and vibration.<br />5. Transparent and translucent.<br />6. Easily coloured.<br />7. Easily assembled or jointed.<br />8. Repairing the self lubricating.<br />9. Good wear and scratch resistance.<br />6.11 The Disadvantages Of Plastics<br />1. Measurement dimension changed by humidity and high thermal extension.<br />2. Low temperature duration for operation (150 - 600˚F).<br />3. Brittle at low temperature.<br />4. Softer and less elastic than metal.<br />5. Certain plastics are easily affected by ultra-violet.<br />6. Creeps occurs at any temperature.<br />7. Easily burnt.<br />
6.12 Factors For Plastics Usages<br />1. Processable and assembleability compared to metals<br /><ul><li>e.g. plastic moulding production</li></ul>- do not need any finishing or tidying process<br />2. Plastics materials are cheaper than metals<br /><ul><li>e.g. nylon replacing zinc</li></ul>3. Plastic properties<br /><ul><li>e.g. insulation, self lubrication, etc</li></ul>4. Plastic-metal composites can be produced to achieve better properties<br /><ul><li>e.g. plastic-metal reinforcement are good electrical conductors, lightness and easily produced</li></ul>5. Multiple functions in the same time<br /><ul><li>e.g. housing or covers, insulators and self lubricating bearing</li></li></ul><li>6.13 Plastic Manufacturing Process<br /><ul><li>In plastic products manufacturing process, varies of machines used depends upon the shape products and types of resins used.
Injection Moulding Process<br />1. The basic concept of injection moulding machine is a heat-softened thermoplastic material is forced by a screw-drive cylinder into a relatively cool mould cavity that gives the plastic the desired shape.<br />2. The mould usually made from tool steels or beryllium cuprum and can be multiple cavities so that more than one component can be injected in every cycle of process.<br />3. It was the fastest process and widely used.<br />4. Figure 5 shows the typical components of injection moulding machine.<br />
5. Work principles :<br />i. the raw material (resins or pellets) are inserted into the hopper<br />ii. under gravity manipulating, the resins enter the injection channel which surrounded by heater<br />iii. certain temperature applied depends upon types of material used, usually around 75˚C - 95˚C for smaller products and 120˚C - 260 ˚C for larger products<br />iv. when the resins becomes softer, it will be injected using weither the reciprocating-screw or torpedo plunger<br />v. the softened plastics will be enter directly to the nozzle and enters the mould cavity<br />vi. to ensure the mould was fully injected, pressure between 35MPa to 40 MPa are used<br />
vii. the product will be solidify in the mould cavity under water cooling cycle through the mould<br />viii. after that, the injector will be pulled back, the mould then opened and the products will be retrieved from the mould<br />ix. the connecting opener will retrieved the product where the ejector pins will ejected the product from the mould<br />
6. The advantages :<br />the injector mould usage are faster than pressure <br />mould(torpedo plunger)<br />ii. mould cost are lower because of the simplicity<br />iii. various shape, intricate or thinner products can be <br /> produced<br />iv. high quality parts can be produced at a high <br /> production rate<br />v. the process has relatively low labour costs<br />vi. good surface finishes can be produced on the <br />mouldedpart<br />vii. the process can be highly automated.<br />
7. The disadvantages:<br />sink = caused by moulding temperature or the <br /> pressure are higher<br />ii. flashing = caused by unfitted mould<br />iii. bubble = caused by cold mould or the pressure are <br /> too low<br />iv. uncompleted moulding = the material did not enter <br /> the mould fully, example : small gates<br />weld line = caused by cold mould, low moulding<br /> temperature and time for injection are too fast<br />vi. not economical for small production<br />vii. certain mould only for certain product<br />viii. high cost of machine means that a large volume <br /> of parts must be made to pay for the machine<br />ix. the process must be closely controlled to produce <br /> a quality products.<br />
Extrusion Moulding Process<br />1. A process that supplies a continuous stream of thermoplastic material/ products with equally cross-section where it is directly produced through a shaping tool or to some other subsequent shaping process (dies) placed directly on the end of the extrusion machine.<br />
2. Work principles :<br />i. the pellets or resins are inserted into the hopper<br />ii. the material then fed into the heated cylinder by rotating screw<br />iii. when the material become softer, it will be forced continuously by the rotating the screw ram<br />iv. the products or outcomes are formed into continuous shape<br />v. after the product exiting from the die, it will be cooled by air (air-blast system), water (water-bath system) or become cold by interact will cold roll surface where then it will be solidifies while rolling<br />vi. extruded products such as pipe, rod, etc, extrude this way because it can be curved or bended after extruded with hot water by sinking it in it<br />
3. The advantages :<br />i. continuous extrusion<br />ii. accurate measurement<br />iii. good finishing<br />iv. economical for mass production<br />4. The disadvantages :<br />i. the dies are easily wear out<br />ii. need to be closely controlled<br />iii. thermosetting materials are not suitable because it solidifies quickly<br />
Blow Moulding Process<br />1. It is suited for the manufacture of bottles and other simple hollow shaped parts, from a preformed plastic tube.<br />2. It is a process producing thermoplastic products using injection or extrusion method where the material are softened will be injected or extruded into the mould and then blowed with air.<br />3. It is a cylinder or tube of heated plastic called a parison is placed between the moulds. The mould is closed to pinch off the ends of the cylinder and compressed air is blown in, forcing the plastic against the walls of the mould.<br />
4. Work principles for extrusion blow moulding method :<br />i. the resins or pellets are inserted into the hopper<br />ii. with gravity manipulating, the materials enter the injection or extrusion chamber or cylinder which surrounded with heater<br />iii. when the materials become softened (plasticizers), it will be injected or extruded using screw rotator or torpedo plunger<br />iv. the hot and softened material called parison<br />v. the materials will be flow straight to the nozzle and enter the mould cavity<br />vi. top of the parison will be cutted off using a blade<br />vii. the air then blowed into the parison through a cavity at the top of the mould<br />
ix. after that the mould will opened and the product will be retrieved<br />x. the connecting opener will retrieved the products using ejector pin to eject the product out from the mould<br />viii. the plastic will be cooled, then solidified in the mould under water cooling system cycling in the channels in the mould<br />
5. Work principles for stretch blow moulding method :<br />i. the plastic is first moulded into a "preform" using the injection molded process<br />ii. these preforms are produced with the necks of the bottles, including threads (the "finish") on one end<br />iii. these preforms are packaged, and fed later (after cooling) into an blow molding machine<br />iv. in this process, the preforms are heated (typically using infrared heaters) above their glass transition temperature, then blown using high pressure air into bottles using metal blow moulds<br />v. usually the perform is stretched with a core rod as part of the process<br />
vi. the stretching of some polymers, such as PET (polyethylene terephthalate) results in strain hardening of the resin, allowing the bottles to resist deforming under the pressures formed by carbonated beverages<br />6. The advantages :<br />i. it does not need the finishing or tidying<br />ii. various of products, intricate or thinner can be produce<br />7. The disadvantages :<br />i. not economical for small production<br />ii. certain mould only for certain product<br />
Transfer Moulding Process<br />1. It is a process where the moulding compound is first softened by heat in a transfer chamber and then is forced under high pressure into one or more mould cavities for final curing.<br />2. In this process, the resin is not fed directly into the mould cavity but into a chamber outside the mould cavities. When the mould is closed, a plunger forces the plastic resin from the outside chamber through a system of runners and gates into the mould cavities.<br />3. It is also used for moulding thermosetting plastics such as the phenolics, ureas, melamines and alkyds.<br />4. The plastic material flowing process into the mould causing the heat transferred equally and quickens the chemical reaction for cross linking or curing.<br />
5. Work principles :<br />i. the resins were placed in a chamber (preclosedmould) outside the mould cavity forming a preformed plastic shape<br />ii. then heated at certain temperature where it will be plasticize<br />iii. when it softened, it then forced into the mould cavity (closed mould) as hot melt plastic material by a plunger through a system of runners and gates<br />iv. then let it cooled to solidified<br />v. after the plastic has cured, the plunger is removed and the mould cavity then opened<br />vi. the parts is then ejected by ejector pin and removed the plastic parts from the mould<br />
6. The advantages :<br />i. the solidifies timing for transfer moulding are faster than compression moulding<br />ii. the inserting timer are shorter because of the usage of bigger preformed material which can be heatened quickly<br />iii. this process is suitable to form parts that need small metal insertened because the plastic material entering the mould cavity in stages without using higher pressure<br />iv. intricate shape and with variation of bigger cutting can be produced<br />v. no flash is formed during moulding and the moulded part requires less finishing<br />vi. many parts can be made at the same time by using a runner system<br />7. The disadvantages :<br />i. losing material in flowing channel<br />ii. mould cost are higher than compression mouldingmould<br />
Compression Moulding Process<br />1. It is a process where two parts mould combined together under one compression and then formed a cavity accordingly to the component shape.<br />2. In the process, before the combined, the resin (thermosets) may be preheated, is loaded into a hot lower part of the mould containing one or more cavities. The upper part of the mould is forced down on the resin and then applied pressure and heat melts the resin and forces the liquefied plastic into filling the cavities forming the needed component.<br />3. Many thermosetting resins such as the phenol formaldehyde, urea formaldehyde and melamine formaldehyde resins are formed into solid parts by the compression moulding process.<br />4. The formation is prepared under certain pressure by using upper part of the mould. This is similar to forging process for metal and facing flashing problem where in need for trimming or cutting process.<br />
5. Work principles :<br />i. the materials inserted into the mould as preformed powdered shape because of heat and pressure<br />ii. the preheated material is loaded into a hot mould containing the cavity<br />iii. the upper part of the mould is forced down on the plastic resin, pressure and heat are then applied, forces the liquified plastic to fill the cavity<br />iv. continued heating is required to complete the cross-linking of the thermosetting resin<br />v. then the part is ejected from the mould<br />
6. The advantages :<br />i. mould costs are low<br />ii. short flow of material reduces wear and abrasion on moulds<br />iii. production of large parts is more feasible<br />iv. more compact moulds are possible<br />v. expelled gases from the curing reaction (cross-linking process) are able to escape during the mouldingprocess<br />7. The disadvantages :<br />i. complicated part configurations are difficult to make with this process<br />ii. inserts may be difficult to hold to close tolerances<br />iii. flash must be trimmed from molded parts<br />
Rotational Moulding Process<br />1. It is a process to forming big hollow component in small or average quantities where a thin metal mould consists of two separated parts and designed to rotating in two axis which 90 to each other.<br />2. Work principles :<br />i. loading a measured quantity of polymer (usually in powder form) into the mould<br />ii. the mould then heated in an oven whilst it rotates, until all the polymer has melted and adhered to the mold wall<br />iii. by that homogeneous layer with similarity/ accuracy thickness will be formed accordingly to the mould shape<br />iv. the material/ product are cooled while the machine rotated before removed it from the mould by bringing the mould to the cooling station where water are sprayed to the mould<br />v. the mould then brought to the last station where the product will be removed from the mould<br />
3. This process only limited for materials that has low melting temperature in small quantities and protype production such as :<br />a) Polyethylene<br />b) Polypropylene<br />c) Polyvinyl Chloride<br />
Thermoforming Process<br />1. It is a process when a heated plastic sheet is forced into the contours of a mould by heat and pressure or vacuum.<br />2. Mechanical pressure may be used with mating dies or a vacuum may used to pull the heated sheet into an open die. Air pressure may also be used to force a heated sheet into an open die.<br />3. There are several categories of thermoforming, including vacuum forming, pressure forming, twin-sheet forming, drape forming, free blowing, and simple sheet bending.<br />
4. Work principles for vacuum forming method :<br />i. the plastic sheet/ material was placed in between the opened mould and the heating coils by clamping it to the moveable clamper<br />ii. then the sheet was brought closer to the coils to softened it, usually placed on top of the machine<br />iii. when the sheet are softened enough, the clamper with sheet back to its place and in the same time the mould which usually placed below the clamper moving up towards the softened sheet<br />iv. the air between the sheet and the mould then are gasped out or vacuumed causing the sheet to fit tightly to the mould which the air holes can be seen in the opened mould<br />v. then air are blowed to separated the forming sheet and mould and in the same time cooled the product<br />vi. the mould then moved downward to its original place and the formation that occurs from the sheet are the product that need to be trimmed or cut off<br />
5. The advantage :<br />low in mould cost because it can be formed using plaster, thermosetting plastic, wood or aluminium<br />6. The disadvantage :<br />hollowed or opened space components cannot be produced because of the different pressure when the forming process occurs.<br />7. Materials that can be used :<br />a) Polystyrene<br />b) Acrylonitrile-Butadiene-Styrene (ABS)<br />c) Polyvinyl Chloride<br />d) Polypropylene<br />e) Polyethylene<br />
8. Applications :<br /> used to formed plastic sheets for certain shape such as :<br />- food packaging<br />- yogurt package<br />- motorcycle fairing<br />- vehicles bumper<br />
Casting Process<br />1. It is a process by the use of a liquid or powder material that is shaped without the application of significant pressure.<br />2. Casting is a manufacturing process by which a liquid material such as a suspension of minerals as used in plastic is introduced into a mould, allowed to solidify within the mould, and then ejected or broken out to make a fabricated part.<br />3. For thermoplastics, liquid monomer is poured into the mould and, with heat, allowed to polymerize in place to a solid mass.<br />4. For thermosets, they are poured into a heated mold wherein the cross-linking reaction completes the conversion to a solid.<br />5. Casting is used for making parts of complex shape that would be difficult or uneconomical to make by other methods, such as cutting from solid material.<br />6. Encapsulation and potting are terms for casting processes in which a unit or assembly is encased or impregnated, respectively, with a liquid plastic which is subsequently hardened by fusion or chemical reaction.<br />
Calendaring Process<br />1. It is a process to produced sheets or films by passing the soft or half gel thermoplastic materials through a number of pairs of heated rollers which the heat and pressure are applied to the materials and the rolls in combination are called calendars.<br />2. Usually this process applied in stages of rolling before final products with certain thickness achieved.<br />3. Materials used : PVC and copolymers that contains Vinyl Chloride.<br />4. The thickness of products between 0.05 – 0.7mm and with width upto 1m.<br />
5. Work principles :<br />i. polymer powders, adhesives and other additives were mix in normal condition<br />ii. stirred in closed heated mixture<br />iii. the softened or half gel materials from the mixture then masticated between two heated rolls that squeeze it out into a film or sheet<br />iv. the heated sheet or film then trimmed and then passes around one or more additional rolls (calendars) before being stripped off as a continuous film<br />6. The calendres contains with 3 to 4 rollers made of hardened steel, steel covered with fiber or cast irons.<br />7. The calendering rollers has polished surface and the cylinders have drilled holes to channeled heated liquid, oil or water.<br />