Presentation on theme: "Manufacturing Processes, 1311 Dr Simin Nasseri"— Presentation transcript:
1SHAPING PROCESSES FOR PLASTICS Chapter 13- Part 1 Properties of Polymer Melts Extrusion Manufacturing Processes, 1311Dr Simin NasseriSouthern Polytechnic State University
2SHAPING PROCESSES FOR PLASTICS Properties of Polymer MeltsExtrusionProduction of Sheet, Film, and FilamentsCoating ProcessesInjection MoldingOther Molding ProcessesThermoformingCastingPolymer Foam Processing and FormingProduct Design Considerations
3Plastic ProductsPlastics can be shaped into a wide variety of products:Molded partsExtruded sectionsFilmsSheetsInsulation coatings on electrical wiresFibers for textiles
4More Plastic ProductsIn addition, plastics are often the principal ingredient in other materials, such asPaints and varnishesAdhesivesVarious polymer matrix compositesMany plastic shaping processes can be adapted to produce items made of rubbers and polymer matrix composites
5Trends in Polymer Processing Applications of plastics have increased at a much faster rate than either metals or ceramics during the last 50 yearsMany parts previously made of metals are now being made of plasticsPlastic containers have been largely substituted for glass bottles and jarsTotal volume of polymers (plastics and rubbers) now exceeds that of metals
6Plastic Shaping Processes are Important Almost unlimited variety of part geometriesPlastic molding is a net shape processFurther shaping is not neededLess energy is required than for metals due to much lower processing temperaturesHandling of product is simplified during production because of lower temperaturesPainting or plating is usually not required
7Two Types of Plastics 1. Thermoplastics 2. Thermosets Chemical structure remains unchanged during heating and shapingMore important commercially, comprising more than 70% of total plastics tonnage2. ThermosetsUndergo a curing process during heating and shaping, causing a permanent change (cross‑linking) in molecular structureOnce cured, they cannot be remelted
8Classification of Shaping Processes Extruded products with constant cross‑sectionContinuous sheets and filmsContinuous filaments (fibers)Molded parts that are mostly solidHollow molded parts with relatively thin wallsDiscrete parts made of formed sheets and filmsCastingsFoamed products
9Polymer MeltsTo shape a thermoplastic polymer it must be heated so that it softens to the consistency of a liquidIn this form, it is called a polymer meltImportant properties of polymer melts:ViscosityViscoelasticity
10Viscosity of Polymer Melts Fluid property that relates shear stress to shear rate during flow.Due to its high molecular weight, a polymer melt is a thick fluid with high viscosity.Most polymer shaping processes involve flow through small channels or die openings.Flow rates are often large, leading to high shear rates and shear stresses, so significant pressures are required to accomplish the processes.Ketchup, a highly viscoelastic fluid (like the polymer melt)
11Viscosity and Shear Rate Viscosity of a polymer melt decreases with shear rate, thus the fluid becomes thinner at higher shear rates.(Newtonian Material: Viscosity changes by temperature, not by time or shear rate)Figure Viscosity relationships for Newtonian fluid and typical polymer melt.Shear rate Viscosity
12Viscosity and Temperature Viscosity decreases with temperature, thus the fluid becomes thinner at higher temperaturesFigure Viscosity as a function of temperature for selected polymers at a shear rate of 103 s-1.Temperature Viscosity
13Viscoelasticity Combination of viscosity and elasticity. Possessed by both polymer solids and polymer melts.Examples: Shampoo, hand cream, mayonnaise, toothpaste, yoghurt.Temperature, time and shear rate are important and affect the properties of viscoelastic materials (here the polymer melts).Example: die swell in extrusion, in which the hot plastic expands when exiting the die opening.
14Die SwellExtruded polymer "remembers" its previous shape when in the larger cross section of the extruder, tries to return to it after leaving the die orifice.Figure Die swell, a manifestation of viscoelasticity in polymer melts, as depicted here on exiting an extrusion die.
15Rod ClimbingIn this video clip a dilute (0.025 wt%) solution of a high molecular weight polystyrene polymer is dissolved in a low molecular weight Newtonian viscous solvent (Piccolastic, Hercules Inc).Vortices: If you stir water or iced tea (Newtonian liquids) vigorously (but not too vigorously!), you may see tiny "swirls" parallel to the axis of stirring. The flow is downwards.
17ExtrusionCompression process in which material is forced to flow through a die orifice to provide long continuous product whose cross‑sectional shape is determined by the shape of the orifice.Widely used for thermoplastics and elastomers to mass produce items such as tubing, pipes, hose, structural shapes, sheet and film, continuous filaments, and coated electrical wire.Carried out as a continuous process; extrudate is then cut into desired lengths.
18Feedstock is moved from hopper and preheated. ExtruderFigure Components and features of a (single‑screw) extruder for plastics and elastomers.Polymer is transformed into fluid, air mixed is extracted, and material is compressed.Melt is homogenized and sufficient pressure developed to pump it through die opening.Feedstock is moved from hopper and preheated.
19Two Main Components of an Extruder BarrelScrewDie - not an extruder componentSpecial tool that must be fabricated for particular profile to be produced.
20Extruder BarrelInternal diameter typically ranges from 25 to 150 mm (1.0 to 6.0 in.)L/D ratios usually between 10 and 30: higher ratios for thermoplastics, lower ratios for elastomersFeedstock fed by gravity onto screw whose rotation moves material through barrel.Electric heaters melt feedstock; subsequent mixing and mechanical working adds heat which maintains the melt
21Extruder Screw Divided into sections to serve several functions: Feed section - feedstock is moved from hopper and preheated.Compression section - polymer is transformed into fluid, air mixed with pellets is extracted from melt, and material is compressed.Metering section - melt is homogenized and sufficient pressure developed to pump it through die opening.Figure Details of an extruder screw inside the barrel.Pressure is largely determined by dc.
22Die End of Extruder Functions of screen pack: Progress of polymer melt through barrel leads ultimately to the die zone.Before reaching die, the melt passes through a screen pack - series of wire meshes supported by a stiff plate containing small axial holesFunctions of screen pack:Filter out contaminants and hard lumpsBuild pressure in metering sectionStraighten flow of polymer melt and remove its "memory" of circular motion from screw
23Die Configurations and Extruded Products The shape of the die orifice determines the cross‑sectional shape of the extrudateCommon die profiles and corresponding extruded shapes:Solid profilesHollow profiles, such as tubesWire and cable coatingSheet and filmFilaments
24Extrusion of Solid Profiles Regular shapes such asRoundsSquaresIrregular cross sections such asStructural shapesDoor and window moldingsAutomobile trimHouse siding
25Extrusion Die for Solid Cross Section Figure (a) Side view cross‑section of an extrusion die for solid regular shapes, such as round stock; (b) front view of die, with profile of extrudate. Die swell is evident in both views.
26Hollow ProfilesExamples: tubes, pipes, hoses, and other cross‑sections containing holesHollow profiles require mandrel to form the shapeMandrel held in place using a spiderPolymer melt flows around legs supporting the mandrel to reunite into a monolithic tube wallMandrel often includes an air channel through which air is blown to maintain hollow form of extrudate during hardeningExtrusion bridge die making a hollow section product. Note that in the picture the die has been split to show the material passing through it. In reality, the die and the ring fit together, with a gap for the extruded material to flow through.Openlearn, UK
27Extrusion Die for Hollow Shapes Figure Side view cross‑section of extrusion die for shaping hollow cross‑sections such as tubes and pipes; Section A‑A is a front view cross‑section showing how the mandrel is held in place; Section B‑B shows the tubular cross‑section just prior to exiting the die; die swell causes an enlargement of the diameter.
28Blow ExtrusionBlow extrusion, in which molten extrudate is forced past a tubing mandrel, expanded into a balloon shape by a stream of air, drawn upward by rollers, and pinched into a collapsed sheet to be cut into a number of products.
29Wire and Cable CoatingPolymer melt is applied to bare wire as it is pulled at high speed through a dieA slight vacuum is drawn between wire and polymer to promote adhesion of coatingWire provides rigidity during cooling - usually aided by passing coated wire through a water troughProduct is wound onto large spools at speeds up to 50 m/s (10,000 ft/min)
30Extrusion Die for Coating Wire Figure Side view cross‑section of die for coating of electrical wire by extrusion.
31Test yourself!What are the names of different sections along the extrusion barrel?1321- Feed Section3- Metering Section2- Compression Section
32Test yourself! Could extrusion be used for the following products? The body of a food mixer.Copper pipe for a central heating system.The body of a pen.The body of the food mixer has a complex 3D shape, so it certainly could not be extruded.Copper pipe is ideal for manufacturing by extrusion, using a bridge die to extrude the hollow shape.The ink tube in the pen has probably been extruded. Some pens’ bodies vary in diameter along their length and are typically closed off at one end. This would suggest that the pen body is not extruded. For pieces that have more complex shapes, like caps, ends, and mechanical components, injection molding is used.