lecture_10_-_shaping_processes_for_plastics_-_chapter_13

lecture_10_-_shaping_processes_for_plastics_-_chapter_13 -...

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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
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Polymer Melt - Viscosity Shear rate and viscosity: thinner at higher shear rates Temperature & viscosity: thinner at higher temperature
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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 Die swell, a manifestation of viscoelasticity in polymer melts. Die Swell, aka: Viscoelasticity Swell ratio: r s = D x / D d
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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 e xtrudate is then cut into desired lengths. Three zones in an extruder: .
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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: 1. Filter (remove contaminants and any hard lumps) 2. Build pressure in the metering section 3. Straighten flow - remove "memory" of circular motion from screw
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Melt Flow in Extruder Archimedian screw forces polymer melt toward die • Principal transport mechanism is Q d , 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, Q b called: back pressure flow Resulting flow in extruder is: Q x = Q d Q b
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Extruder Screw Melt Flow Q x = Q d - Q b Drag Flow Q d Back Pressure Flow Q b Flight angle ‘A’? tan A = p / πD Q d = 0.5 π 2 D 2 N d c sinA cosA Where, D – flight screw diameter N – screw rotational speed d c – screw channel depth A – flight angle Q b Where, p – head pressure (die) η – melt viscosity L – length of the barrel Assumes leakage flow is negligible L A Dd p c η π 12 sin 2 3
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Extruder Screw Melt Flow 2 cot 6 c d A DNL η π Boundary Conditions: 1) With no back pressure Q x = Q max = Q d 2) With no flow Q x = 0 = Q d – Q b , p max = Q max p max Extruder characteristic Q x = Q max (Q max /p max )p K s = (for round opening) D d – effective die opening L d – effective die opening length d d L D 128 4 Die characteristic Q x = K s p Intersection (Q x ,p) known as the operating point Head pressure Melt flow
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The shape of the die orifice determines the cross sectional shape of the extrudate. Solid shapes:
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