mechanical engineering chapter 13 easy way to learn. shaping and processes for plastic.

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M AT E R I A L S , P R O C E S S E S A N D S Y S T E M S ,
B Y M . P. G R O O V E R )
SHAPING
PROCESSES FOR
PLASTICS
CHAPTER 13
- PROPERTIES OF POLYMER MELTS
- EXTRUSION
-INJECTION MOLDING
-BLOW MOLDING

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
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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
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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
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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
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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.
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 

Ketchup, a highly
viscoelastic fluid (like
the polymer melt)

7. 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

8. 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. 8
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.
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Shear rate Viscosity
Figure 13.1 Viscosity relationships for
Newtonian fluid and typical
polymer melt.

10. 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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11. 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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12. VISCOELASTIC MATERIAL
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Viscoelastic Material:
Exhibits both viscous and elastic
material properties. Deformation of the
material is dependent on the strain rate.
Examples:
Paint, polymer melt, bread dough, soft
tissue, shampoo, toothpaste,
mayonnaise, etc.
Polymer pellets

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EXTRUSION
13.2

14. 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

15. EXTRUDER
Figure 13.4
Components
and features of
a (single-screw)
extruder for
plastics and
elastomers.
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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.

16. 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 after 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.

17. RHEOLOGICAL EFFECTS
• Die swell can be most
easily measured for a
circular cross section by
means of the swell
ratio, defined as
𝑟𝑥 =
𝐷𝑥
𝐷𝑑

18. 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.
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Figure 13.5 Details of an extruder
screw inside the barrel.
Pressure is largely determined by
dc.

19. MAIN COMPONENTS
OF AN EXTRUDER
• Barrel
• Screw
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Die - not an extruder component
Special tool that must be fabricated for
particular profile to be produced.

20. 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
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21. EXTRUSION DIE FOR SOLID CROSS SECTION
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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22. 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
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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.

23. EXTRUSION DIE FOR
HOLLOW SHAPES
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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24. 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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25. EXTRUSION DIE FOR
COATING WIRE
Figure 13.11 Side view
cross-section of die for coating of
electrical wire by extrusion. 25

26. EXTRUSION DIE FOR
COATING WIRE
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INJECTION MOLDING
13.6

30. INJECTION MOLDING
 Polymer is heated to a highly
plastic state and forced to
flow under high pressure into
a mold cavity where it
solidifies and the molding is
then removed from cavity.
 Produces discrete
components almost always
to net shape.
 Typical cycle time 10 to 30
sec, but cycles of one minute
or more are not uncommon.
Similar to hot-chamber die casting.
• Pellets fed into heated chamber.
• Melt forced into split die chamber.
• 10,000 to 50,000 psi.
• Mold is water cooled.
• Part is ejected. 30

31. INJECTION MOLDED PARTS
 Complex and intricate shapes are possible
 Shape limitations:
• Capability to fabricate a mold whose cavity is the
same geometry as part
• Shape must allow for part removal from mold
 Part size from  50 g (2 oz) up to  25 kg (more
than 50 lb), e.g., automobile bumpers
 Injection molding is economical only for large
production quantities due to high cost of mold
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32. POLYMERS FOR INJECTION MOLDING
 Injection molding is the most widely used molding process for
thermoplastics.
 Some thermosets and elastomers are injection molded
• Modifications in equipment and operating parameters must be made to
avoid premature cross-linking of these materials before injection.
Parts manufactured by
injection molding
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33. Injection Molding Machine
Figure 13.21 Diagram of an injection molding machine, reciprocating
screw type (some mechanical details are simplified).
- Melts and delivers
polymer melt
- Operates much like
an extruder Two principal components
- Opens and
closes mold
each injection
cycle
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34. INJECTION MOLDING PROCESS

35. INJECTION MOLDING MACHINES
• Injection molding machines differ in both injection unit
and clamping unit
• Name of injection molding machine is based on the
type of injection unit used
• Reciprocating-screw injection molding machine
• Plunger-type injection molding machine
• Several clamping designs
• Mechanical (toggle)
• Hydraulic
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36. INJECTION MOLDING MACHINES
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37. SHRINKAGE
• Reduction in linear size during cooling from molding to room
temperature.
• Polymers have high thermal expansion coefficients, so significant
shrinkage occurs during solidification and cooling in mold.
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38. SHRINKAGE FACTORS
Fillers in the plastic tend to reduce shrinkage
Injection pressure – higher pressures force more
material into mold cavity to reduce shrinkage
Compaction time - similar effect – longer time forces
more material into cavity to reduce shrinkage
Molding temperature - higher temperatures lower
polymer melt viscosity, allowing more material to be
packed into mold to reduce shrinkage
Shrinkage decreases if the following quantities increase:
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39. REACTION INJECTION MOLDING (RIM)
• A processing technique for the
formation of polymer parts by
direct polymerization in the
mold through a mixing
activated reaction.
• Two highly reactive liquid
ingredients are mixed and
immediately injected into a
mold cavity where chemical
reactions leading to
solidification occur
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40. REACTION INJECTION MOLDING (RIM)
• RIM was developed
with polyurethane to
produce large
automotive parts such
as bumpers and
fenders.
• RIM polyurethane
parts possess a foam
internal structure
surrounded by a dense
outer skin.
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42. COMPRESSION MOLDING
• A widely used molding process for thermosetting plastics.
• Also used for rubber tires and polymer matrix composite parts.
• Molding compound available in several forms: powders or pellets,
or liquid.
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43. BLOW MOLDING
• Molding process in which air pressure is used to inflate soft
plastic into a mold cavity
• Important for making one-piece hollow plastic parts with
thin walls, such as bottles
• Because these items are used for consumer beverages in
mass markets, production is typically organized for very
high quantities
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44. BLOW MOLDING
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45. MATERIALS AND PRODUCTS IN BLOW
MOLDING
• Blow molding is limited to thermoplastics
• Materials: high density polyethylene, polypropylene
(PP), polyvinylchloride (PVC), and polyethylene
terephthalate (PET)
Products: disposable containers
for beverages and other liquid
consumer goods, large shipping
drums (55 gallon) for liquids
and powders, large storage
tanks (2000 gallon), gasoline
tanks, toys, and hulls for sail
boards and small boats
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46. BLOW MOLDING PROCESS
• Accomplished in two steps:
1. Fabrication of a starting
tube, called a parison
2. Inflation of the tube to
desired final shape
• Forming the parison is accomplished
by either
• Extrusion or
• Injection molding
parison
Final
product
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