1. SHAPING PROCESSES FOR PLASTICS Chapter 13- Part 1 - Properties of Polymer Melts - Extrusion
Manufacturing Processes, MET 1311
Dr Simin Nasseri
Southern Polytechnic State University
(© Fundamentals of Modern Manufacturing; Materials, Processes and Systems,
by M. P. Groover)

2. 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

3. 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

4. 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

5. 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

6. 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)

7. Newtonian and Non-Newtonian fluids
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.

8. Viscosity and Temperature
Viscosity decreases with temperature, thus the fluid becomes thinner at higher temperatures
Figure Viscosity as a function of temperature for selected polymers at a shear rate of 103 s-1.
Temperature Viscosity

9. Viscosity and Shear Rate
Viscosity of a polymer melt “usually” decreases with shear rate, thus the fluid becomes thinner at higher shear rates.
Figure Viscosity relationships for Newtonian fluid and typical polymer melt.
Shear rate Viscosity

10. Effect of various parameters on viscosity
Newtonian fluid
Non-Newtonian fluid
Temperature
Shear rate
no change or
Time

11. 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

12. Elastic versus Viscous Behavior
Viscous Material
All energy added is dissipated into heat.
Examples:
Silicon Oil
Water
Elastic Material
All energy added is stored in the material
Example:
Rubber
Simin Nasseri,
School of AMME, The University of Sydney

13. 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.

14. 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
Simin Nasseri,
School of AMME, The University of Sydney

15. 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.
Simin Nasseri,
School of AMME, The University of Sydney

16. Applications
Rheology 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).
Simin Nasseri,
School of AMME, The University of Sydney

17. School of AMME, The University of Sydney
Rheological Effects
Rod Climbing
Die Swell
Simin Nasseri,
School of AMME, The University of Sydney

18. Rheological Effects
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
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.

19. 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.

20. Extrusion 13.2

21. 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.
Extrudate

22. Feedstock is moved from hopper and preheated.
Extruder
Figure 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.

23. 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 Details of an extruder screw inside the barrel.
Pressure is largely determined by dc.

24. Two Main Components of an Extruder
Barrel
Screw
Die - not an extruder component
Special tool that must be fabricated for particular profile to be produced.

25. Die 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 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

26. 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

27. Extrusion of Solid Profiles
Regular shapes such as
Rounds
Squares
Irregular cross sections such as
Structural shapes
Door and window moldings
Automobile trim
House siding

28. Extrusion Die for Solid Cross Section (FYI)
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.

29. 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

30. Extrusion Die for Hollow Shapes (FYI)
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.

31. 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.

32. 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)

33. Extrusion Die for Coating Wire
Figure Side view cross‑section of die for coating of electrical wire by extrusion.

34. Test yourself!
What are the names of different sections along the extrusion barrel? 1 3 2
1- Feed Section
3- Metering Section
2- Compression Section

35. Test 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.