PLASTICS AND ELASTOMERS

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1 PLASTICS AND ELASTOMERS

2 Application Examples Household appliances For packing, containers For housing & bodies of machines Insulator for electrical devices Fillers foams Household articles, furniture Composites for aircraft body parts (boats, cars, toys, etc.) Optical fibers Plastic conductors Plastic computer chips Nanotechnology Plastic MEMS

3 Plastics: Material of the future, Emerged over last 50 years Definition: Group of engineered materials characterized by large molecules that are built by joining smaller molecules. Cover a wide range of materials They are neither ceramics nor metals Usually made from hydrocarbons; also, made from natural or synthetic resins or their compounds NATURAL or SYNTHETIC NATURAL PLASTICS: Shellac & cellulosics-known for long time SYNTHETIC PLASTICS: plenty are around and new are emerging

4 Molecular structure of plastics usually hydrocarbon origin C atoms with free links (un-saturated covalent bonds) can make long chains having the tendency to the polymerisation process : i.e. joining together in large molecules the molecular weight increases as a result of polymerisation

5 Principle of polymerisation - By addition or condensation mechanism In addition, number of basic units (Monomers) are added: Ethylene (Monomer) Polyethylene (Polymer) For polymerisation heat + pressure + catalysts (to accelerate the transformation) are required Polymerisation helped by adding special substrates (hardeners) having catalytic effect MER: The basic repetitive unit of polymer Degree of polymerisation: The number of MERS, typical In condensation, reactive molecules combine to form polymers:

6 Two types of plastic (Based on response to Temperature): Thermoplastic plastics (thermoplasts) -Softens when heated. Can be molded into shapes -Recyclable Thermosetting plastics (thermosets) -Hardens when heated. Burn or char on further heating. -Stronger The different temperature behaviour of these two groups of plastics affects the manufacturing process

7 Bonding Stronger Co-Valent bonding between atoms and monomers in chain (primary bonds) Weaker van der Waals forces between chains (secondary bonds) Thermoplastics: Strength is determined by secondary bonds Thermosets: Strength is determined by primary bonds

8 Thermoplasts: Addition polymers Contain molecules of different sizes range of melting temperature Weaker secondary bonds become softer with increasing temperature Become harder and stronger at lower temperature Do not undergo chemical changes with the change of temperature (when molded ) has only physical changes. Large plastic deformation is possible suitable for molding and extrusion Possible to recycle

9 Thermosetting Plastics (thermosets) Condensation polymers Stronger and more rigid than thermoplasts Material can become one large molecule can be formed and kept in shape using heat/ sometimes pressure) result a permanently hard material as a result of chemical changes (when reheated remains hard ) The heat first softens the material but, as additional heat or other chemicals (hardeners) are added, the plastic is hardened by a chemical reaction polymerisation The material is not anymore temperature dependant

10 GENERAL PROPERTIES OF PLASTICS Advantages Disadvantages Easily Producible Low cost Low Density Electric Insulators (not all) Good heat insulators Colorable Low strength and hardness Less heat dissipation Low dimensional stability Thicker walls than metals Good surface finish (easy to achieve)

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12 TYPES OF THERMOPLASTICS ABS- Acrylonitrile + Butadiene + Styrene Cellulosics - natural from wood & cotton fibers Helmets, tubes, laminates, construction materials Acrylics Plexiglas (light transmitting auto parts) Nylon strong fibers (fabrics, valves, bearings) Teflon high temperature resistant, low friction, chemicals resistant (gaskets, bearings, pans) Polyethylene kitchen ware, electric wire insulators Vinyls rubbery, tear resistant (floor/wall coverings, gloves)

13 THERMOSETTING PLASTICS Used mainly as binders in composite materials 1. Phenol Resins Filters Insulators Impregnation Fiberglas fillers 2. Epoxy Resins (adhere to glass & metal adhesive, glues) Electrical insulator parts 3. Silicones (expensive) base polymers good properties as addition for oils, greases, rubbers, resins Resistant to high temperature, humidity Sealants. 4. Polyesters Boat/car bodies, textiles, adhesives. 5. Urea-formaldehyde lighting fixtures, containers.

14 COMMON ADDITIVES fillers, lubricants, coloring agents, antioxidants, etc. Fillers: wood flour cloth fibers composites glass fibers mica, asbestos

15 PLASTIC PRODUCTION PROCESS Plastic compounds differ greatly from each other different methods of processing Thermoplasts: Heated to become formable solid or liquid cast or injection molded or extruded Thermosets: Polymerization and forming have to occur simultaneously Process: Casting, Blow-Molding, Compression Molding, Transfer Molding Cold Molding, Injection Molding, reaction injection molding, rotational molding, foam molding, Extrusion and thermoforming Raw material for processing is usually in powder or in granular form, sometimes in sheets or as liquids Desired to transform material into product in a single process FIRST OPERATION compounding and pre-forming, i.e., the ingredients have to be mixed before molding

16 CASTING Thermoplasts: acrylics, nylons, urethanes and PVC plastisols thermoplastic materials are in the form of pellets melted and continuously formed in a rotary press Thermosets: phenolics, ployesters, epoxies, silicones and urethanes - material - is usually in form of liquid (ex. resins) - castings like metals, but low temperature - needs an additional process - steel pattern is dipped in molten lead lead shell is formed resin is held in the mold and cured

17 BLOW MOLDING Typical plastic parts production (eg. soda bottles) Thermoplastic polyethylene, PVC, polypropylene bottles and hollow containers Produces thin wall hollow containers from thermoplastic resins A two step process: first a parison (preform) is formed hollow tube with solid bottom the parison is placed in form die when is still hot then, expanded by air pressure (forcing the material against the mold surfaces) after cooling, the product is removed from the mold

18 Plastic bottles are produced by continuous blowing in a pinch tube process Tube of thermoplastic material is extruded from a plasticiser into an open mold Each end of the tube is pinched by closing of the mold The air pressure forms the plastic bottle

19 COMPRESSION MOLDING for thermosets Heated metal mold is used Pellet/Granules are heated in die Compression in hydraulic presses the press applies pressure and also introduces heat to the molded material. pressures between 10 and 5000 bars (150 to 7500 psi) temperature C IMPORTANT! Mainly for thermosetting materials Thermosets: Compressed and heated Thermoplasts: Melted, formed, cooled

20 TRANSFER MOLDING for THERMOSETS The thermosetting materials placed into separate pressure chamber above the mold cavity material is plasticised (molten) by heat and pressure and is injected into the cavities of mold as a hot liquid After curing & hardening, the part is removed from the mold The preheating time & the curing time is shorter than in compression molding. This process is convenient for parts with metal inserts Mold cost is higher than for compression molding Complicated and larger parts

21 COLDMOLDING PROCESS Raw material is first pre-formed while cold then is cured in an oven low quality finish surface not expensive specially good for thermosets

22 INJECTION MOLDING For thermoplastics rather than thermosets Plunger type injection molding machine similar to die casting machine material is changed from solid to liquid phase by heating, then, it is injected into the mold, where it solidifies injection molding machines are developed to produce large plastic parts size designation the closing force of the die ( tons) the amount of plastic injected per cycle up to 10Kg Heating chamber has a torpedo Spreader to keep the incoming material in a layer thin enough to be heated uniformly and instantly Temp: C reached by means of electrical resistance coil injection pressure reaches 2000 bars to speed up the solidification of pieces

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24 ADVANTAGES OF INJECTION MOLDING much faster than compression molding (prod. Cycle 2-6 injections/min) Ready to use very fine pieces can be produced by injection molding practical only for thermoplasts Injection molding for the thermosets Called jet molding For jet molding, every injection molding machine can be converted process requires: both cooling & heating of the nozzle during the molding cycle heating, to make the material plastic (not to polymerise it) injected into the mold and heated to complete polymerisation reciprocating screw injection machine can be also used as shown previously similar to transfer molding Because thermosets require more time to cure the product, not much injection time slow process

25 Reaction Injection Moulding No heat is required for polymerization Polymerization is due to chemical reaction Different chemicals are mixed by the impingement liquid streams The mixture polymerizes in the mold Large parts can be made due to no-heating Easier for thermosets

26 EXTRUSION continuous process some thermoplastics have to be made as bars or profiles a screw mixes heated plastic first, to become a thick viscous mass extruded through dies into simple shapes of any length melted polymer is pushed through the die after leaving the die it is cooled on the conveyor to solidify similar machines for thermosets utilised a rod instead of a screw and a heated conveyor instead of a cooling one. sectional tolerances (0.5 %) are obtained

27 THERMOFORMING Single cavity mold; less expensive Thermoplastic sheet metal is heated to working temp. and formed into shapes by heat, pressure or vaccum Can be discrete sheet or continuous sheets used mainly for heated thermoplastic sheets and can be divided into: Free forming no mold required (just pattern) Vacuum forming with help of heat Positive pressure molding

28 CLOSED MOLD PROCESS Two parts mold more expensive for machine housings

29 LAMINATED PLASTICS THERMOSETS Sheets of paper, fabric, wood, etc are first coated with resin and then combined under heat and pressure to form a sandwich like structure Thermosetting liquid resin is mainly used Ex. Plywood, laminated boards as table tops etc. there is the plastic layer added between each reinforcing layer bonding the sheets or providing a fine and protective surface Laminated plastic materials are hard, strong, impact resistant and unaffected by heat and water Final product can be made of many sheets it can be machined to make the machine parts like (noiseless gears)

30 Manufacturing processes of composite laminates Resin material is dissolved and coverted into Liquid in a bath through which the strips of paper or fabric are passing They are next processed together in a molding press and then they are cured laminated plastic tubes can be also produced in a tube mold. - Ex. Plywood for construction - laminated plates for machine parts - safety glass for windshields - curved surface bodies boats and car bodies REINFORCED PLASTICS Made from thermosetting resins with fibers of glass, cotton or synthetic fibers fiber glass is a known example.

31 single threads or fibers are fed through a bath of resin and wound on a mandrel to make pressure vessels and also missile bodies, with exceptional strength to weight ratios large objects can be produced with unique shape do not require molds FILAMENT WINDING very flexible process can be machined (resin bath) expensive

32 Foam Molding Plastic material is mixed with foaming agent which releases gas when being heated during molding product of very low density rigid for structural applications flexible for cushioning, insulation thermosetting and thermoplastic materials are used rigid foam core can be produced Foaming agent used (CFCs and more recently, H 2 0)

33 MOLD FOR PLASTICS For injection and compression processes, the dies are made of heat treated steel (as for pressure die casting) allowance for shrinking along with the draft for compression molding, the molds are heated or cooled by adjacent plates for injection molding, the molds have cooling channels in both parts of the mold the molding process has to take in to consideration: right filling of the mold with material evacuation of air (a problem) removal of parts from the mold the material selection is related to the function of the produced part the technology Fillets to reduce stress concentration wall thickness controls the curing time mm usually wall thickness tolerances max 0.2 mm

34 Surface finish in order to avoid machining of plastics as bodies parting surfaces (lines) problem flat surface should be stiffened using ribs or by dooming

35 INSERTS Threaded inserts are used insert made of brass or steel inserts require grooves or knurling, to avoid loosening in device for places where inserts are installed the wall thickness have to be increased to half of the die of the insert Circumferential Grooves to take axial load Axial grooves to prevent rotation.

36 RUBBER & ARTIFICIAL ELASTOMERS Elastomers are elastic over a wide range of deformation and this is related to their structure They can store energy (but they not very compressible) natural rubber still very much used obtained as latex from trees after drying it is formed into sheets of crude rubber crude rubber is mixed with additives (mainly sulphur) depending on additives, rubber can be soft or very hard when reinforced by textile and fabrics, very high strength can be obtained (tires) steel wires are used for reinforcement of rubber tires and conveyor belts Natural rubber does not resist petroleum products and temp.

37 ARTIFICIAL ELASTOMERS Synthetic Rubber form of them are superior than Natural rubber (BUNA invented in 1942) Can have very good resistance to the petroleum products and to temperature specially with silicone additives Production processing two procedures for elastomers FOR LATEX: from liquid crude two ways; Immersion form immersed into latex compound repeatedly with the latex film drying after every immersion cyclic process latex can be applied by anodic process which deposits latex particles on the form, It is a continuous process (like galvanising or metal plating) FOR RUBBER SHEET PRODUCTS- from solid Mix all the additives with elastomers into a mixer the mixture is put in a mill (roll in opposite directions) than, on calenders where the rubber is formed into sheets also, in calenders the fabric tapes or sheets are covered with rubber as shown. Some rubber products are made by extrusion (tubes and hoses)

38 Dipping eg. Boots, gloves Rubber Processing Natural rubber, Neoprene, Silicone Sheets using calenders