1 shaping processes for plastics chapter 13- part 1 - properties of polymer melts - extrusion...

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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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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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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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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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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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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.


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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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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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


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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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


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


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


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


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


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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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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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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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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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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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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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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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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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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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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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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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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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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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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1 shaping processes for plastics chapter 13- part 1 - properties of polymer melts - extrusion...

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