Refer K& S text, Chapter 7 - PDF Free Download

Refer K& S text, Chapter 7 Thermosets generally possess better mechanical, thermal, and chemical properties; electrical resistance; and dimensional stability than do thermoplastics 3

This diagram shows the basic processes for forming plastics & reinforced plastics For plastics, elastomers, composite materials Note three entry points: -Pellets, granules, powder -Liquid, paste -Fibres Reinforced plastics use fibres Fibres may be many materials 6

Extrusion Used to produce the largest volume of plastics TP (pellet, granular, powder) placed in hopper Fed into barrel of screw conveyor Three sections: -Feed section conveys material from feed to central region -Melt section heat applied externally and generated by viscous shearing of plastic pellets; melting begins -Pumping section pressure build up on die; additional shearing causing melting Molten plastic forced through die Breaker plate (plate with several small holes) helps improve mixing of polymer Extruded product air (blowers) or water cooled Controlled cooling (uniform) reduces shrinkage & distortion (b): Molten plastic flows in channel formed by screw blades; note profile dimensions change along the length of the screw. Plastic moves in a long, thin shape that actually changes shape as it moves along the screw channel - Pressure & flow rate are determined by the relationship between screw dimensions (change), the rotational speed and the viscosity of the polymer Continuous supply to hopper means long continuous extruded profiles are possible Extruders are generally rated by diameter, D, of the barrel; and the length-to-diameter (L/D) ratio of the barrel. 7

Some common die profiles - In general, the polymer undergoes much greater and uneven shape recovery than is common with metals (a) Coat hangar die (a flat extrusion die) - Used to extrude plastic sheet (>0.5 mm) and film (<0.5 mm) - The metal distribution manifold spreads the molten plastic evenly across the width of the die (& sheet) - Extruded sheet is taken up first on water-cooled rolls, then on rubber-covered pull off rolls - The amount of plastic extruded can be controlled by the adjustable lip (b) Round dies - Note that the extruded section has greater diameter than the die - The molten plastic also shrinks upon cooling - Can be sized (by drawing with a puller) after cooling, but still in a plastic state (c) Square dies - Note difference between die profile and the extruded profile - The polymer swells upon exit from the die Production parameters to control for product integrity: - Extruder-screw rotational speed, barrel-wall temperatures, die design, rate of cooling, drawing speeds Defects are similar to metals extrusions (therefore die shape is important) 8

There are variations on the basic extrusion process for some products (a) Plastic Tubes & Pipes Produced in an extruder with a spider die (similar to metals extrusion) Can also introduce reinforcing fibres at this time (b) Coextrusion Simultaneous extrusion of two or more polymers through a single die The polymers can be different Used to produce flat sheets, films, tubes Especially useful for food packaging; different functions/layer: -Inertness for food -Barriers for fluids such as oil, water -Product labeling 9

Thin polymer films ( Blown film ) Two stage process -First extrude the polymer (upward) as a continuous tube, using a mandrel -Then blow air to expand the film to desired film thickness (lateral stretching) Usually cooled by air in a cooling ring, which can also act as a barrier to further expansion, thus controlling product dimensions Note the pinch rolls (at top) which pull the plastic and thus the material is stretched and oriented in the longitudinal direction. The product therefore has undergone biaxial stretching, imparting strength & toughness to the product. Produce: - Plastic bags roll pinched & cut off Billy Boot process - Plastic film slit length wise Blow ratio the ratio of blown diameter to tube diameter (about 3:1 in (b)) 10

Spinning Process -Most synthetic fibres used in reinforced plastics are extruded in a spinneret (has small holes similar to a showerhead) to create continuous filaments of semi-solid fibre -The liquid polymer filaments are converted to a rubbery state and solidified -There are four spinning processes: melt, wet, dry, gel Melt-spinning process (This diagram) -Polymer is melted for extruding through spinneret, then immediately cooled (using forced air) and simultaneously pulled to stretch them into strands that are smaller in diameter than as extruded -Produce nylon, olefin, polyester, pvc (therefore the most important spinning process) The high strength of fibres results from the significant stretching 11

Injection Molding -Similar to hot-chamber die casting -Pellets fed into heated chamber -The melt is forced into the mold using either a plunger (a) or a rotating screw (b) -Extruder barrel can be heated externally; -Most heat results from frictional heating 12

In step 2 the non-rotating screw is pushed forward thus transferring a controlled quantity of plastic melt into the die 13

Insert molding: - Involves metallic components (screws, pins, strips) that are placed in the mold cavity prior to injection and then become an integral part of the molded product 14

Molds may have several components to accommodate part design -Runners -Cores -Cavities -Cooling channels -Inserts -Knockout pins -Ejectors Similar components as for metal casting 15

Three basic types of molds (a) Cold runner, two plate mold - The simplest and most common - As also shown in Fig. 19.10 (the previous slide) (b) Cold runner, three plate mold - The runner system is separated from the part when the mold is opened. - Use Stripper plate (c) Hot runner mold - The molten plastic is kept hot in a heated runner plate In cold runner molds, the solidified plastic remaining in the channels must be trimmed In hot runner molds (faster), there are no gates, runners, etc. to be trimmed. Thus cycle times are faster because there is less material to cool. Also, no time needed for trimming. 16

Injection molding machines -Usually horizontal -Generally use hydraulics to provide clamping force -Vertical machines for smaller parts, with close tolerances; inset molding -Molds generally made of tool steels -May have multiple cavities for multiple part production 19

Reaction-injection Molding RIM) -A monomer and two or more reactive fluids are forced at high speed into a mixing chamber -Mixture then poured into die cavity -Rapid chemical reactions in the mold -Polymer solidifies -Major applications: bumpers, fenders, steering wheels, instrument panels, thermal insulation for refrigerators, water skiis -May also add reinforcing fibres for stiffness 20

Blow moulding is a modified extrusion and injection moulding process (a) Extrusion blow moulding - First extrude a plastic preform or tube - Grip preform in a mould cavity much larger than the preform. - Blow the preform to expand it to fit shape of the mould - Blowing usually performed with hot air blast - May be set up as a continuous process (b) Injection blow moulding - A two stage process - May store (& transport) preforms between stages - Injection mould a short tubular piece parison into cool dies - Open dies and transfer parison to blow-moulding die - Inject hot air into the parison, expanding it to the walls of the mould cavity - Browning Harvey on Ropewalk Lane does the second stage from purchased preforms, before filling the bottles 21

Rotational Moulding -Use a split mould -The mould can be rotated about two perpendicular axes (primary & secondary) -For each part to be made, a premeasured quantity of powdered plastic is placed inside the warm mould -The heated mould is rotated simultaneously about both axes. -The action forces the material against the walls of the mould where heat fuses the powder without melting it. 22

Thermoforming A process for forming thermoplastic sheets or films over a mould through the application of heat and pressure. (a) Sheet is clamped and heated to the sag point (above glass transition temperature, T g ), usually by radiant heating. Note: T g is the temperature at which amorphous polymers undergo a distinct change in mechanical behaviour across a narrow range of temperatures: above- rubbery, leathery; below-hard, rigid, brittle, glassy. They do not have a melting point (b) Drape forming is similar to straight vacuum forming except that the part is stretched or pulled over the mould Mould is at room temperature; thus the part becomes shaped upon contact with the mould (C) Mould is inverted (d) Also ridge forming: part clamped in a ring and pressure applied. Do not need to have all surfaces of mould in contact with the part 23

Refer to Figure 19.16 (previous slide) Openings in parts would not permit proper application of vacuum The small holes in the moulds are usually <0.5 mm (0.02 ). If holes are too big they can leave marks on the part 24

Compression Moulding The charge is: - Pre-shaped - Premeasured volume - Powder or viscous mixture of liquid-resin and filler Charge is placed directly into heated mould cavity - 200 o C (400 o F) or higher Forming is done under pressure - Process is similar to closed die forging of metals Three types of compression mould available: (a) Positive For high density parts (b) Semi-positive For quality production (provision for land & overlap) (c) Flash For shallow or flat parts. Flash later trimmed Used mainly with thermosetting plastics; original material is in partially polymerized state Also some TP and elastomers Allow time to cure (0.5 to 5 min.) Thicker takes longer In general, part complexity is less than with injection moulding; but better dimensional control 25

Transfer Moulding A further development of compression moulding Uncured TS resin placed in heated transfer pot or chamber Heat the material Inject heated material into heated closed moulds Machinery may be: -Ram -Plunger -Rotating screw feeder Pressures up to 300 MPa (43.000 psi) Viscous flow generates heat, raising temperature of material, homogenizing the material Curing takes place by cross linking Capable of complex parts and dimensional control (like injection moulding) Particularly suitable for intricate shapes of varying wall thickness Complexity & precision adds to cost 26

Casting (a) A process for some TP ( nylons, acrylics), and some TS (epoxies, phenolics, polyurethanes, polyesters) Moulds may be rigid or flexible A relatively slow, but simple and inexpensive, process The polymer must have sufficiently low viscosity to flow easily into the mould Basic method for casting TP: - A mixture of monomer, catalyst, activators additives is heated above T m - Molten mixture poured into mould; cooled -Polymerization at ambient pressure to form the part -Intricate shapes are possible using flexible moulds which are peeled away and reused Potting and Encapsulation (b) & (c) -Both involve casting the (typically) liquid epoxy resin around an electrical component to embed it in the plastic -In both processes, the plastic material serves as a dielectric (non conductor) -Requires processing in vacuum to remove moisture Potting (b) -Carried out in a housing or case which becomes an integral part of the component and fixes it in position Encapsulation (c) - The component is coated with a layer of the plastic, surrounding it completely, solidifying Dielectrics (insulators) Materials with high electrical resistivity (p. 94) 27

Calendaring A process used to produce rubber and some plastic sheets. A warm mass of the compound is fed into a series of rolls and masticated Produces sheets typically 0.3-1 mm but can be stretched thinner Calendared rubber may be used in various products e.g. tires, machinery belts Calendared rubber or TP may be used to laminate a core of tape, paper, fabric, soft or firm plastics 29

Reinforced Plastics Aka PMCs polymer-matrix composites Products with unique mechanical properties: -High strength to weight ratio -High stiffness to weight ratio -Fatigue strength -Creep resistance -Directional properties Formed by various methods Complex, challenging production processes 31

(a) PMC tape production process Diagram shows the laminating process (b) The PMC tape 32

(a) Vacuum Bag Forming Prepegs (preimpregnated) laid on mould to desired shape Cover with a plastic bag Create vacuum Cure at room temperature or in an oven (b) Pressure-bag Moulding A variation of process - Flexible bag placed over the resin & reinforcing fibre mixture - Pressure applied over mould 35

Contact Moulding (Open-mould processing) A wet method Materials applied in layers Reinforcement impregnated with the resin at time of moulding Used to make laminated products with high surface area to thickness ratios (a) Hand layup Materials placed in proper order Brushed and shaped by hand with a roller to compact materials and eliminate porosity Can control orientation of the reinforcements (b) Spray layup Both resin & chopped fibres are sprayed over the mould surfaces Roll to remove pores Since chopped fibres are oriented randomly there is no control of directional properties (Roving = slightly twisted strands) 36

Filament Winding Process (a) A process in which the resin and fibres are combined at time of curing in order to develop a composite structure Can produce axis-symmetric and some non symmetric parts on a rotating mandrel The reinforcing filament, tape or roving is wrapped continuously around the form (Roving = slightly twisted strands) The reinforcements are impregnated by passing them through a polymer bath The process can be modified by wrapping the mandrel with a prepreg material Parts made this way are very strong because of their highly reinforced structure (b) Filament winding process can also be used to strengthen cylindrical or spherical pressure vessels made of aluminum or titanium This example is for aircraft The presence of the metal inner lining make the part impermeable 37

Pultrusion Process Used to produce long parts with various uniform cross sections -rods, profiles, flat strips, tubing See (b) A continuous process; parts cut to length The reinforcement, glass roving, or fabric is supplied through several bobbins The bundle is pulled -First through a thermosetting polymer bath (usually polyesters), -Then through a preforming die -Then through a heated steel die (up to 1.5 m), where it sets Note: control speed (time in die) to provide sufficient time for setting Then cured and cut to length 38

Case Study Highlights the Intellifibers system of PZT fibres in the frame that act to reduce vibration thus reducing tennis elbow injuries 40

Case Study (continued) Highlights the Intellifibers system of PZT fibres in the frame that act to reduce vibration thus reducing tennis elbow injuries 41

Reinforced plastics -Are also known as polymer-matrix composites (PMC) and fibre-reinforced plastics (FRP) -Consist of fibres in a polymer matrix -Common firbres are: -Glass, Carbon, Conductive graphite, Ceramic, Polymer, Boron -Matrix -Support fibres in place; transfer stresses to fibres -Physically protect fibres -Reduce propagation of cracks in the composite -Usually TP or TS Higher density is a limitation 43

The metal is the matrix All matrix materials are metals Some of these fibre materials also appear in a list of fibre materials for reinforced plastics (PMC Polymer-matrix Composites) 44

Rheocasting (Text 11.4.7 p. 284) The metal is heated to just above its solidus temperature and poured into a vessel to cool it down to the semisolid state. The slurry is then mixed and delivered to mould or die During spray atomization, the reinforcing fibres are mixed with a matrix that contains both liquid and solid phases of the metal 48

In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions. In modern laboratory usage, this tends to imply that the process is reproducible, reliable, and established to work in multiple laboratories. A chemical synthesis begins by selection of compounds that are known as reagents or reactants. Various reaction types can be applied to these to synthesize the product, or an intermediate product. This requires mixing the compounds in a reaction vessel such as a chemical reactor or a simple round-bottom flask. Many reactions require some form of work-up procedure before the final product is isolated. The amount of product in a chemical synthesis is the reaction yield. Typically, chemical yields are expressed as a weight in grams or as a percentage of the total theoretical quantity of product that could be produced. A side reaction is an unwanted chemical reaction taking place that diminishes the yield of the desired product. Chemical vapor infiltration (CVI) is a variant on Chemical Vapor Deposition (CVD). CVD implies deposition onto a surface, whereas CVI implies deposition within a body. Chemical vapor infiltration is widely used as a means of fabricating Ceramic Matrix Composites (CMC) such as alumina-alumina, in which a chemical vapor consisting of AlCl3-H2-CO2 is deposited onto porous alumina fibers or preforms. This process was designed and first experimented by Professor Roger Naslain from the University of Bordeaux on SiC composites for aerospace applications. 51

Similar to considerations for casting of metals Selection of appropriate materials requires consideration of: -Service requirements -Long-term usage -End-of-use disposal 52