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SHAPING PROCESSES FOR PLASTICSChapter 13- Part 1- Properties of Polymer Melts- Extrusion
Manufacturing Processes, MET 1311Dr Simin Nasseri
Southern Polytechnic State University(© Fundamentals of Modern Manufacturing; Materials, Processes and Systems,
by M. P. Groover)
Manufacturing Processes, Prof Simin Nasseri
SHAPING PROCESSES FOR PLASTICS
Properties of Polymer Melts Extrusion Production of Sheet, Film, and Filaments Coating Processes Injection Molding Other Molding Processes Thermoforming Casting Polymer Foam Processing and Forming Product Design Considerations
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Manufacturing Processes, Prof Simin Nasseri
Plastic Products
Plastics can be shaped into a wide variety of products: Molded parts, Extruded sections, Films, Sheets,
Insulation coatings on electrical wires, Fibers for textiles
In addition, plastics are often the principal ingredient in other materials, such as Paints and varnishes, Adhesives, Various
polymer matrix composites
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Manufacturing Processes, Prof Simin Nasseri
Plastic Shaping Processes are Important
Almost unlimited variety of part geometries
Plastic molding is a net shape process
Further shaping is not needed
Less energy is required than for metals due to much lower processing temperatures
Handling of product is simplified during production because of lower temperatures
Painting or plating is usually not required
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Manufacturing Processes, Prof Simin Nasseri
Polymer Melts
To shape a thermoplastic polymer it must be heated so that it softens to the consistency of a liquid
In this form, it is called a polymer melt
Important properties of polymer melts: Viscosity Viscoelasticity
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Manufacturing Processes, Prof Simin Nasseri
Viscosity of Polymer Melts
Fluid property that relates shear stress to shear rate during flow.
Shear stress=viscosity times shear rate
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)
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Manufacturing Processes, Prof Simin Nasseri
Newtonian and Non-Newtonian fluids
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Newtonian Material: Viscosity changes by temperature, not by time or shear rate). Examples: Air, water, honey, glycerine, Kerosene.
Non-Newtonian Material: Viscosity changes by temperature, time and shear rate). Examples: Polymer melt, ketchup, custard, toothpaste, starch suspensions, paint, shampoo, cream, etc.
Manufacturing Processes, Prof Simin Nasseri
Viscosity and Temperature Viscosity decreases with temperature, thus the fluid becomes
thinner at higher temperatures
Figure 13.2 Viscosity as a function of temperature for selected polymers at a shear rate of 103 s-1.
Temperature Viscosity
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Manufacturing Processes, Prof Simin Nasseri
Viscosity and Shear Rate
Viscosity of a polymer melt “usually” decreases with shear rate, thus the fluid becomes thinner at higher shear rates.
Figure 13.1 Viscosity relationships for Newtonian fluid and typical polymer melt.
Shear rate Viscosity
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Manufacturing Processes, Prof Simin Nasseri
Effect of various parameters on viscosity
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Parameter Newtonian fluid Non-Newtonian fluid
Temperature
Shear rate no change or
Time no change or
Manufacturing Processes, Prof Simin Nasseri
Viscoelastic Behavior
Material property that determines the strain that the material experiences when subjected to combinations of stress and temperature over time
Combination of viscosity and elasticity
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Manufacturing Processes, Prof Simin Nasseri
Elastic versus Viscous Behavior
12Simin Nasseri,School of AMME, The University of Sydney
Elastic Material
All energy added is stored in the material
Example:
Rubber
Viscous Material
All energy added is dissipated into heat.
Examples:
Silicon Oil
Water
Manufacturing Processes, Prof Simin Nasseri
Viscoelasticity
Combination of viscosity and elasticity.
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.
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Manufacturing Processes, Prof Simin Nasseri 14Simin Nasseri,School of AMME, The University of Sydney
Viscoelastic Material
Viscoelastic Material:Exhibits both viscous and elastic material properties. Deformation of the material is dependant of the strain rate.
Examples:Paint, polymer melt, bread dough, soft tissue, shampoo, toothpaste, mayonnaise, etc.
Polymer pellets
Manufacturing Processes, Prof Simin NasseriSimin Nasseri,School of AMME, The University of Sydney
Rheology
The study of the deformation and flow associated with physical properties of fluids.
Rheometry is concerned with experimental characterization of non-Newtonian fluids in the construction of constitutive equations which are fundamental to analytical considerations of the more practical situations.
Manufacturing Processes, Prof Simin NasseriSimin Nasseri,School of AMME, The University of Sydney
ApplicationsRheology is important and its applications spanning a range of
disciplines. The specific application areas are:• Polymer Industries,• Surface Coatings Industries,• Food Industries,• Manufacturing & Industrial Applications (eg pumping, mixing,
extrusion, pouring and filling).
Manufacturing Processes, Prof Simin NasseriSimin Nasseri,
School of AMME, The University of Sydney
Die Swell
Rheological Effects
Rod Climbing
Manufacturing Processes, Prof Simin Nasseri
A problem in extrusion of polymers is die swell, in which the profile of extruded material grows in size, reflecting its tendency to return to its previously larger cross section in the extruder barrel immediately before being squeezed through the smaller die opening
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Shape memory: 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.
Rheological Effects
Manufacturing Processes, Prof Simin Nasseri
Rod Climbing In 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.
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Extrusion13.2
Manufacturing Processes, Prof Simin Nasseri
Extrusion
Compression 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.
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Extrudate
Manufacturing Processes, Prof Simin Nasseri
Extruder Figure 13.4 Components and features of a
(single‑screw) extruder for plastics and elastomers.
Feedstock is moved from hopper and preheated.
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.
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Manufacturing Processes, Prof Simin Nasseri
Extruder 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 13.5 Details of an extruder screw inside the barrel.
Pressure is largely determined by dc.
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Manufacturing Processes, Prof Simin Nasseri
Two Main Components of an Extruder
• Barrel• Screw
Die - not an extruder componentSpecial tool that must be fabricated for particular profile to be produced.
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Manufacturing Processes, Prof Simin Nasseri
Die End of Extruder
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 holes Functions of screen pack:
Filter out contaminants and hard lumps
Build pressure in metering section
Straighten flow of polymer melt and remove its "memory" of circular motion from screw
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Manufacturing Processes, Prof Simin Nasseri
Die Configurations and Extruded Products
The shape of the die orifice determines the cross‑sectional shape of the extrudate
Common die profiles and corresponding extruded
shapes: Solid profiles Hollow profiles, such as tubes Wire and cable coating Sheet and film Filaments
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Manufacturing Processes, Prof Simin Nasseri
Regular shapes such as Rounds Squares
Irregular cross sections such as Structural shapes Door and window moldings Automobile trim House siding
Extrusion of Solid Profiles
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Manufacturing Processes, Prof Simin Nasseri
Extrusion Die for Solid Cross Section (FYI)
Figure 13.8 (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.
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Manufacturing Processes, Prof Simin Nasseri
Hollow Profiles Examples: tubes, pipes, hoses, and other cross‑sections
containing holes Hollow profiles require mandrel to form the shape Mandrel held in place using a spider. Polymer melt flows
around legs supporting the mandrel to reunite into a monolithic tube wall
Mandrel often includes an air channel through which air is blown to maintain hollow form of extrudate during hardening
FYI: Extrusion 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
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Manufacturing Processes, Prof Simin Nasseri
Extrusion Die for Hollow Shapes (FYI)
Figure 13.10 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.
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Manufacturing Processes, Prof Simin Nasseri
Blow Extrusion Blow 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.
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Manufacturing Processes, Prof Simin Nasseri
Wire and Cable Coating
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
Wire provides rigidity during cooling - usually aided by passing coated wire through a water trough
Product is wound onto large spools at speeds up to 50 m/s (10,000 ft/min)
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Manufacturing Processes, Prof Simin Nasseri
Extrusion Die for Coating Wire
Figure 13.11 Side view cross‑section of die for coating of electrical wire by extrusion.
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Manufacturing Processes, Prof Simin Nasseri
Test yourself!
What are the names of different sections along the extrusion barrel?
1- Feed Section
3- Metering Section
2- Compression Section
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Manufacturing Processes, Prof Simin Nasseri
Test yourself! Could extrusion be used for the following products?1. The body of a food mixer. 2. Copper pipe for a central heating system. 3. The body of a pen.
1. The body of the food mixer has a complex 3D shape, so it certainly could not be extruded.
2. Copper pipe is ideal for manufacturing by extrusion, using a bridge die to extrude the hollow shape.
3. 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.
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