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Chapter19
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Chapter19

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  1. Chapter 19: Forming and Shaping Plastics and Composite Materials Faculty of Engineering Mechanical Dept.
  2. Introduction Plastics ~ polymers Plastics are engineered materials  Made from natural or synthetic resins and compounds  Low density, low tooling costs, good corrosion resistance, low cost  Can be molded, extruded, cast, or used for coatings  Plastics are very versatile materials and are used more than steel, aluminum, and copper combined in the United States  Used as food and beverage containers, packaging, signs, housewares, foams, paints, toys, etc  An important group: reinforced plastics (composites)
  3. Molecular Structure of Polymers • Hydrocarbons CnH2n+2 Covalent bonding Double or triple covalent bonds may also be present • Monomer Smallest repeating unit Basic structure of some polymer molecules: (a) ethylene molecule; (b) polyethylene, a linear chain of many ethylene molecules; (c) molecular structure of various polymers. These molecules are examples of the basic building blocks for plastics
  4. Some Plastics Terminology Thermoplastic (TP) – Polymers that can be shaped when heated and regain original hardness & strength upon cooling Have a linear or branched structure (weak secondary bonds) Process is reversible Acrylics, cellulosics, nylons, polyethylenes, polyvinyl chloride Thermoset (TS) – Polymers that become permanently set when heated Have a cross-linked structure (strong secondary bonds) Process is irreversible Epoxy, polyester, urethane, phenolics, silicones Elastomer (Rubber) – Elastic; low elastic modulus Tires, footwear, gaskets, flooring, weatherstripping, hoses
  5. Polymer Additives • Fillers Improve strength, stiffness, and toughness Reduce shrinkage and weight Common fillers: wood flour, silica flour, clay, powdered mica • Plasticizers Added in small amounts to reduce viscosity • Stabilizers and antioxidants Retard the effects of heat, light, and oxidation • Colorant (organic dyes or inorganic pigments) • Flame retardants • Lubricants Reduce friction Improve moldability Facilitates part removal
  6. Forming and Shaping Processes for Plastics, Elastomers, and Composite Materials
  7. Extruder Schematic    Raw materials in the form “if thermoplastic pallets” granules, or powder, placed into a hopper and fed into extruder barrel. The barrel is equipped with a screw that blends the pallets and conveys them down the barrel Heaters around the extruder’s barrels heats the pellets and liquefies them Screw has 3-sections  Feed section  Melt or transition section  Pumping section.
  8. Extrusion Die Geometries Common extrusion die geometries: (a) coat-hanger die for extruding sheet; (b) round die for producing rods; and (c) and (d) nonuniform recovery of the part after it exits the die.
  9. Extrusion of Tubes Extrusion of tubes. (a) Extrusion using a spider die and pressurized air. (b) Coextrusion for producing a bottle.
  10. Production of Plastic Film and Bags (b) (a) Schematic illustration of the production of thin film and plastic bags from tube – first produced by an extruder and then blown by air. (b) A blown-film operation. This process is well developed, producing inexpensive and very large quantities of plastic film and shopping bags
  11. Injection Molding       Similar to extrusion barrel is heated Pellets or granules fed into heated cylinder Melt is forced into a split-die chamber Molten plastic pushed into mold cavity Pressure ranges from 70 Mpa – 200 Mpa Typical products : Cups, containers, housings, tool handles, knobs, electrical and communication components, toys etc.
  12. Injection Molding Sequence
  13. Products Made by Injection Molding (a) (b) Typical products made by injection molding, including examples of insert molding.
  14. Mold Features for Injection Molding Illustration of mold features for injection molding. (a) Two-plate mold with important features identified. (b) Four parts showing details and the volume of material involved.
  15. Types of Molds used in Injection Molding Injection molds have several components such as runners, cores, cavities, cooling channels, inserts, knock out pins and ejectors Three basic types of molds  Cold runner two plate mold  Cold runner three plate mold  Hot runner mold 
  16. Process capabilities  High production rates  Good dimensional control  Cycle time range 5 to 60 sec’s  Mold materials- tool steels, beryllium - Cu, Al  Mold life- 2 million cycles (steel molds) 10000 cycles ( Al molds) Machines :  Horizontal or vertical machines  Clamping – hydraulic or electric
  17. Injection-Molding Machine  Mixture of resin with 2 or more reactive fluids forced into the mold cavity at high speed .  Applications : Bumpers, tenders, thermal insulation, refrigerators and freezers, water skis, stiffness
  18. Injection-Molding Machine A 2.2-MN (250-ton) injection molding machine. The tonnage is the force applied to keep the dies closed during the injection of molten plastic into the mold cavities and hold it there until the parts are cool and stiff enough to be removed from the die. Source: Courtesy of Cincinnati Milacron, Plastics Machinery Division.
  19. Injection Molding: Sources of Defects  Weld lines (similar to cold shut in metal casting)  Unfilled die cavity if have premature solidification due to narrow runners  Form flash if dies do not mate properly  Sink marks form at thick sections due to uneven cooling causing local shrinkage Avoid defects by: Temperature control Proper pressures Simulate processes using computer software
  20. Reaction-Injection Molding Process In reaction-injection molding (RIM), a mixture of two or more reactive fluids is forced under high pressure into the mold cavity. Chemical reactions take place rapidly in the mold and the polymer solidifies, producing a thermoset part. Major applications are automotive bumpers and fenders, thermal insulation for refrigerators and freezers, and stiffeners for structural components. Initial injection pressures typically are much lower than traditional injection molding.
  21. Blow-Molding     Modified extrusion and Injection Molding process. A tube extruded then clamped to mold with cavity larger than tube diameter. Finally blown outward to fill the cavity Pressure 350Kpa-700Kpa Other Blow Molding processes  Injection Blow molding  Multi layer Blow molding Schematic illustrations of (a) the extrusion blow-molding process for making plastic beverage bottles; (b) the injection blow-molding process; and (c) a three-station injection molding machine for making plastic bottles.
  22. Rotational Molding Process Rotational molding is used for large plastic parts. The thin-walled metal mold is a split female mode made of two pieces and is designed to be rotated about two perpendicular axes. A premeasured quantity of finely ground plastic material is placed inside a warm mold. The mold is then heated, usually in a large oven, while it is rotated about the two axes. The action tumbles the powder against the mold where heating fuses the power without melting it. In some cases, a cross linking agent is added to the powder, and cross linking occurs after the part is formed in the mold by continued heating. Typical parts are tanks, trash cans, boat hulls, buckets, housings, toys, carrying cases, and footballs. Various metallic or plastic inserts may also be molded into the parts.
  23. Slush-molding  Plastisols are used in slush molding  Plastic materials are forced against the inside walls of the heated mold by tumbling action.  The part is cooled while it is still rotating and then removed by opening the mold
  24. Thermoforming Process In thermoforming, a plastic sheet is heated in an oven to the sag point but not to the melting point. The sheet is then removed from the oven and placed over a mold and through the application of a vacuum is pulled against the mold. Typical parts are advertising signs, refrigerator liners, packaging, appliance housings, and panels for shower stalls. The parts cannot have openings or holes or the vacuum cannot be maintained. Various thermoforming processes for a thermoplastic sheet. These processes commonly are used in making advertising signs, cookie and candy trays, panels for shower stall, and packaging.
  25. Compression molding      Pre-shaped charge ,pre-measured volume of powder and viscous mixture of liquid resin and filler material is placed directly into a heated mold cavity. Compression mold results in a flash formation which is an excess material. Typical parts made are dishes, handles, container caps fittings, electrical and electronic components and housings Materials used in compression molding are thermosetting plastics & elastomers Curing times range from 0.5 to 5 mins 3- types of compression molds are  Flash type  Positive type  Semi-positive
  26. Compression Molding Types of compression molding – a process similar to forging: (a) positive, (b) semipositive, and (c) flash, which is later trimmed off. (d) Die design for making a compression-molded part with external undercuts.
  27. Transfer Molding     Transfer molding is an improvement if compression molding Uncured thermosetting material placed in a heated transfer pot or chamber, which is injected into heated closed molds Ram plunger or rotating screw feeder forces material into mold cavity through narrow channels This flow generates heat and resin is molten as it enters the mold Typical parts : Electrical & electronic components, rubber and silicone parts
  28. Transfer Molding Sequence of operations in transfer molding for thermosetting plastics. This process is suitable particularly for intricate parts with varying wall thickness.
  29. Processes for Plastics and Electrical Assemblies Casting Conventional casting of thermo plastics :   Mixture of monomer, catalyst and various additives are heated and poured into the mould The desired part is formed after polymerization takes place. Centrifugal casting :   Centrifugal force used to stack the material onto the mold Reinforced plastics with short fibers are used
  30. Processes for Plastics and Electrical Assemblies Potting & Encapsulation  Casting the plastic around an electrical component to embed it in the plastic is potting  Used to coat transformers, transistors, etc.  Plastic can serve as a nonconductor  In both the plastic is Dielectric
  31. Foam Molding  Polystyrene beads are placed in a mold and heated  Beads will expand up to 50 times their original size  Changing the bead size will determine the density of the finished foam part  Used to make styrofoam cups, insulating blocks and packaging materials
  32. Foam Molding
  33. Cold forming  Processes such as rolling ,deep drawing extrusion closed die forging ,coining and rubber forming can be used for thermoplastics at room temperatures  Typical materials used : Poly propylene, poly carbonate, Abs, and rigid PVC Considerations :  Sufficiently ductile material at room temperature  Non recoverable material deformation
  34. Calendaring Schematic illustration of calendering. Sheets produced by this process subsequently are used in thermoforming. The process also is used in the production of various elastomer and rubber products. In calendering, sheets of plastic are laminated together by rolling through heated roller. Basically, warm or molten plastic (usually from an extruder) is fed through a series of heated rolls as in this figure. The gaps between the rolls determine the final sheet size. Each additional roll would reduce the sheet thickness further. Then, once the laminated sheet is the correct thickness, the sheet is then stripped off.
  35. Processing Polymer-Matrix Composites   PMCs have high strength/stiffness to weight ratio and excellent creep resistance. They consist of the polymer and reinforcing fibers, bonded together in various ways. Reinforced-plastic components for a Honda motorcycle. The parts shown are front and rear forks, a rear swing arm, a wheel, and brake disks.
  36. Polymer-Matrix Composites: Fiber Impregnation   Fibers can consist of fiberglass, graphite, boron, ceramic and kevlar. Prepregs are made by dipping continuous fibers in resin. (a) Manufacturing process for polymer-matrix composite tape. (b) Boron-epoxy prepreg tape. These tapes are then used in making reinforced plastic parts and components with high strength-to-weight ratios, particularly important for aircraft and aerospace applications and sports equipment.
  37. Polymer-Matrix Composites: Fiber Impregnation  Sheet-molding compounds are made by dropping randomly oriented pieces of fiber on a layer of resin paste, under which there is a thin sheet of polymer (carrier film). Schematic illustration of the manufacturing process for producing fiber-reinforced plastic sheets. The sheet still is viscous at this stage and later can be shped into various products.
  38. Polymer-Matrix Composites: Examples
  39. Polymer-Matrix Composites: Filament Winding    Fibers are dipped in resin bath They are then wrapped around an object by means of rotating mandrel Used to strengthen pressure vessels.
  40. Polymer-Matrix Composites: Pultrusion   Fibers are run continuously through a resin bath before being pulled through a set of dies. Used to make golf clubs, ski poles, ladders. (a) Schematic illustration of the pultrusion process. (b) Examples of parts made by pultrusion. The major components of fiberglass ladders (used especially by electricians) are made by this process. Unlike aluminum ladders, they are available in different colors but are heavier because of the presence of glass fibers. Source: Courtesy of Strongwell Corporation.
  41. Processing Metal-Matrix and Ceramic-Matrix Composites Liquid Phase Processing    Liquid metal-matrix and solid reinforcement are either cast or pressure-infiltration cast Metal-matrix is usually aluminum or titanium Solid reinforcement is usually graphite, aluminum oxide or silicon carbide. Solid Phase Processing   Powder-metallurgy techniques are used Example: tungsten-carbide reinforced tools.
  42. Design Considerations: Plastics and Composites     Compared to metals, plastics have lower stiffness and strength. Dimensional tolerances, except with injection molding, are higher than with metals For casting, ensuring proper flow into mold cavities is important Variations in section thicknesses or abrupt changes in geometry should be avoided.
  43. Design Modifications to Minimize Distortion in Plastic Parts Examples of design modifications to eliminate or minimize distortion in plastic parts: (a) suggested design changes to minimize distortion; (b) stiffening the bottoms of thin plastic containers by doming – a technique similar to the process used to shape the bottoms of aluminum beverage cans; and (c) design change in a rib to minimize pull-in (sink mark) caused by shrinkage during the cooling of thick sections in molded parts.

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