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ME 350 – Lecture 10 – Chapter 13 SHAPING PROCESSES FOR PLASTICS • Properties of Polymer Melts • Injection Molding • Extrusion • Extrudate Production • Other Molding Processes • Thermoforming • Casting • Polymer Foam Processing • Product Design Considerations
Polymer Melt - Viscosity Shear rate and viscosity: thinner at higher shear rates Temperature & viscosity: thinner at higher temperature
Die Swell, aka: Viscoelasticity Extruded polymer "remembers" its previous shape when in the larger cross section of the extruder, tries to return to it after leaving the die orifice Swell ratio: rs = Dx / Dd Die swell, a manifestation of viscoelasticity in polymer melts.
Extrusion Heated plastic is forced to flow through a die orifice to provide a long continuous product (tube, sheet, etc.) whose cross‑sectional shape is determined by the die orifice. The extrudate is then cut into desired lengths. Three zones in an extruder: feed, compression, & metering.
Die End of Extruder • Progress of polymer melt through barrel leads ultimately to the die zone • Before the die, the melt passes through a series of wire meshes supported by a stiff plate containing small axial holes called a: screen pack • Functions: • Filter(remove contaminants and any hard lumps) • Build pressurein the metering section • Straighten flow- remove "memory" of circular motion from screw
Melt Flow in Extruder • Archimedian screw forces polymer melt toward die • Principal transport mechanism is Qd, resulting from friction between the viscous liquid and the rotating screw: drag flow • Compressing the polymer melt through the die creates a back pressure that reduces drag flow transport, Qb called: back pressure flow • Resulting flow in extruder is: Qx = Qd – Qb
Extruder Screw Melt Flow Qd = 0.5 π2 D2 N dc sinA cosA Where, D – flight screw diameter N – screw rotational speed dc – screw channel depth A – flight angle Qb ≈ Where, p – head pressure (die) η – melt viscosity L – length of the barrel Assumes leakage flow is negligible Qx = Qd - Qb Drag Flow Qd→ Back Pressure Flow Qb ← Flight angle ‘A’? tan A = p / πD
Extruder Screw Melt Flow • Boundary Conditions: • With no back pressure • Qx = Qmax = Qd • 2) With no flow • Qx = 0 = Qd – Qb, • pmax = Extruder characteristic Qx = Qmax – (Qmax/pmax)p Qmax Die characteristic Qx = Ksp Melt flow Ks = (for round opening) Dd – effective die opening Ld – effective die opening length Head pressure pmax Intersection (Qx,p) known as the operating point
Extrusion Die for Solid Cross Sections • The shape of the die orifice determines the cross sectional shape of the extrudate. • Solid shapes: rods, beams, bars, plates, sheets, etc.
Extrusion Die for Hollow Shapes • Hollow profiles require a “spider” mandrel: • Polymer melt flows around legs supporting the mandrel to reunite into a monolithic tube wall due to: die swell • Mandrel includes an air channel through which air is blown to maintain hollow form of extrudate during hardening
Extrusion Die for Coating Wire • Polymer melt is applied to bare wire as it is pulled at high speed through a die • A slight vacuum is drawn between wire and polymer to promote adhesion of coating
Sheet and Film Production via: Calendering Feedstock is passed through a series of rolls to reduce thickness to desired gage • Process is noted for good surface finish and high gage accuracy • Products: PVC floor covering, shower curtains, vinyl table cloths, pool liners, etc.
Injection Molding Polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity where it solidifies and the molding is then removed. • Produces discrete components almost always to net shape or near net shape • Typical cycle time ~1 to 30 sec • Mold may contain multiple cavities, so multiple moldings are produced each cycle • Some thermosets and elastomers are injection molded, but equipment and operating parameters must be modified to avoid: premature cross‑linking
Injection Molding Machine Two principal components: • Injection unit (operates similar to an extruder) • Melts and delivers polymer melt (plunger for injection) • Clamping unit • Opens and closes mold each injection cycle
1. Mold Closes 2. Inject Plastic 3. Cooling Time 4. Mold Opens
Two‑Plate Mold • Cavity – slightly oversized to allow for shrinkage • Distribution channel • Sprue - leads from nozzle into mold • Runners - lead from sprue to cavity (or cavities) • Gates - constricts flow to: decrease viscosity • Ejection system – pins built into moving half of mold • Cooling system – typically water • Air vents – at end of flow path
Three‑Plate Mold • Advantages over two-plate mold: • As mold opens, runner and parts disconnect and drop into two separate containers under mold • Allows automatic operation of molding machine • Allows material to be injected at the mold base or middle, rather than side injection, which a two-plate mold must do.
Hot‑Runner Mold • Heaters are located around the runner channels which eliminates solidification of the: sprue and runner • This type of mold improves mold flow as material is heated right up to when in enters the cavity.
Shrinkage • Polymers have high thermal expansion coefficients, so significant shrinkage occurs during solidification • Typical shrinkage values: PlasticShrinkage, mm/mm (in/in) Nylon‑6,6 0.020 Polyethylene 0.025 Polystyrene 0.004 PVC 0.005 • Dimensions of mold cavity must be larger than specified part dimensions: Dc= Dp + DpS + DpS2 where Dc = dimension of cavity; Dp = molded part dimension, and S = shrinkage value and the third term on right hand side corrects for shrinkage in the shrinkage
Shrinkage Factors • Fillers in the plastic tend to: reduce shrinkage • Injection pressure – higher pressures in the mold cavity tend to: reduce shrinkage • Compaction time – longer time tends to: reduce shrinkage • Molding temperature - higher temperatures lower polymer melt viscosity, which tends to: reduce shrinkage
Thermoplastic Foam Injection Molding Molding of thermoplastic parts that possess dense outer skin surrounding lightweight foam center • Part has high stiffness‑to‑weight ratio suited to structural applications • Produced either by introducing a gas into molten plastic in injection unit or by mixing a gas‑producing ingredient with starting pellets • A small amount of melt is injected into mold cavity, where it expands to fill cavity • Foam in contact with cold mold surface collapses to form dense skin, while core retains cellular structure
Injection Molding of Thermosets • Temperatures in the injector are generally: lower • The barrel length of the injection unit is generally: shorter • Melt is injected into a heated mold, where cross‑linking occurs to cure the plastic • The most time‑consuming step in the cycle: curing in the mold
Compression Molding • A widely used molding process for thermosets • Also used for rubber tires and polymer composites • Molding compound available in several forms: powders or pellets, liquid, or preform • Amount of charge must be precisely controlled (1) charge is loaded, (2) and (3) charge is compressed and cured, and (4) part is ejected and removed.
Molds for Compression Molding • Simpler than injection molds • As opposed to injection molding, there is no: sprue and runner system • Limited to simpler part geometries due to lower flow capabilities of TS materials • Mold must be heated, usually by electric resistance, steam, etc • Typical molding materials: phenolics, melamine, epoxies, urethanes, and elastomers • Typical compression-molded products: • Electric plugs, sockets, and housings; pot handles, and dinnerware plates
Transfer Molding TS charge is loaded into a heated chamber; pressure is applied to force the soft polymer into the heated mold. • Pot transfer molding: charge is injected from a "pot" through a vertical sprue channel into cavity • Plunger transfer molding: plunger injects charge from a heated well through channels into cavity
Compression vs. Transfer Molding • In both processes, scrap is produced each cycle as leftover material, called the: cull • The TS scrap cannot be recovered • Transfer molding is capable of molding more intricate part shapes than compression molding but not as intricate as injection molding • Transfer molding lends itself to molding with inserts, in which a metal or ceramic insert is placed into cavity prior to injection, and the plastic bonds to insert during molding
Blow Molding • Molding process in which air pressure is used to inflate soft plastic into a mold cavity • Material limited to: thermoplastics • Accomplished in two steps: • Fabrication of a starting tube, called a: parison • Inflation of the tube to desired final shape • Two methods: • Extrusion blow molding • Injection blow molding
Extrusion Blow Molding (1) extrusion of parison; (2) parison is pinched at the top and sealed at the bottom around a metal blow pin as the two halves of the mold come together; (3) the parison is inflated; and (4) mold is opened to remove the solidified part.
Injection Blow Molding (1) parison is injection molded around a blowing rod; (2) injection mold is opened and parison is transferred to a blow mold; (3) parison is inflated; and (4) blow mold is opened and product removed.
Vacuum Thermoforming • Starting material:thermoplastic sheetor film • To soften, heat is supplied by radiant electric heaters located on one or both sides Products: bathtubs, contoured skylights, door liners for refrigerators, boat hulls, shower stalls, advertising displays and signs, etc.
Polymer Casting Pouring liquid resin into a mold, using gravity to fill cavity, where polymer hardens • Both thermoplastics and thermosets are cast • Thermoplastics: acrylics, polystyrene, polyamides (nylons) and PVC • Thermosetting polymers: polyurethane, unsaturated polyesters, phenolics, and epoxies • Simpler mold • Suited to low quantities
Polymer Foams • A polymer‑and‑gas mixture, with gas added by: • physical mixing or injection (air, nitrogen, CO2) • chemical blowing agent that decomposes at elevated temperatures • Two types: • closed cell (a) • open cell (b) • Product methods: • make the beads first, then feed the beads into a cavity to be fused together (e.g. styrofoam cups) • injection mold using a chemical blowing agent • extrude sheets (least expensive method)
Product Design Guidelines: General • Strength and stiffness • Plastics are not as strong or stiff as metals • Avoid applications where high stresses will be encountered • Creep resistance is also a limitation • Strength‑to‑weight ratios for some plastics are competitive with metals in certain applications
Product Design Guidelines: General • Impact Resistance • Capacity of plastics to absorb impact is generally good; plastics compare favorably with most metals • Service temperatures • Limited relative to metals and ceramics • Thermal expansion • Dimensional changes due to temperature changes much more significant than for metals
Product Design Guidelines: General • Many plastics are subject to degradation from sunlight and other forms of radiation • Some plastics degrade in oxygen and ozone atmospheres • Plastics are soluble in many common solvents • Plastics are resistant to conventional corrosion mechanisms that afflict many metals
In-class Exercise Q in plastic extrusion The diameter, D, of an extruder barrel is 65 mm and its length, L, = 1.75 m. The screw rotates at 55 rev/min. The screw channel depth, dc, = 5.0 mm, and the flight angle = 18°. The head pressure, p, at the die end of the barrel is 5.0 x 106 Pa. The viscosity of the polymer melt is given as 100 Pas. Find the volume flow rate (cm3/s) of the plastic in the barrel. • D=65mm, L=1.75m, N=55rev/min, dc=5mm, A=18°, p=5x106Pa, η=100 Pas • Qx = Qd – Qb, where, • Qd = 0.5 π2 D2 N dc sinA cosA = 28.08×10-5 m3/s= 28.08 cm3/s • Qb = 5.803×10-6m3/s = 5.804 cm3/s • Qx = Qd – Qb = 28.08 – 5.804 = 22.28 cm3/s Qd = 0.5 π2 D2 N dc sinA cosA
Next Lecture – Composite Materials • Exam grade distribution • Exam 1 problem review