Transcript No Slide Title

Rapid Prototyping
Plastics Processing
Polymer Chemistry
Thermoforming plastics
Thermosetting Plastics
Processing Methods
http://ameriplas.org/benefits/about_plastics/about_plastics.html
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Polymer Chemistry
H
| CH4
H-C-H methane
|
H -259
H H
| |
C2H6
H-C-C-H
| | ethane
-128
H H
H H H H
| | | |
H-C-C-C-C-H
| | | | CH
4 10
H H H H
butane
31
H H H
| | |
H-C-C-C-H
| | |
H H H C3H8
propane
-44
H H H H H H H H
| | | | | | | |
H-C-C-C-C-C-C-C-C-H
| | | | | | | | CH
8 18
H H H H H H H H
octane
258
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Polymer Chemistry
C10H22
decane
345  (boiling temp)
-22 (melting temp)
C15H32
pentadecane
518  (boiling temp)
50  (melting temp)
C20H42
eicosane
653 (boiling temp)
97  (melting temp)
H H
| |
-C-C| |
H H
n
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Ethenic Polymers
H H
| |
-C-C- Polypropylene
| |
PP
H CH3
H H
| | polyvinyl
-C-C- chloride
| |
PVC
H Cl
F F
| |
-C-C| |
F F
polytetrafluoroethylene
PTFE
H CH3
polymethyl
| |
-C-C- methacrylate
| |
PMMA
H COOCH3
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• Macromolecules
Thermoforming
Plastics
– Primary bonds:
covalent
– Secondary bonds:
Van der Waal
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Branched Polymers
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Benzene Rings
C
|
C
C
H
C
|
|
C = C
|
|
H
H
C
|
C
polystyrene
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Thermosets
Saturated
polyesters
Cross-linking
Giant
macromolecule
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Fabrication Processes
• Injection Molding
• Extrusions
• Thermoforming
– vacuum forming
– plug assist vacuum
forming (cups)
• Expansion Processes
– RIM
– foams
• Rotational Molding
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Simplified Injection Molder
http://www.a-mtool.com/html/injection_molding.html
http://www.aplastic.com/photo.htm
http://www.bwpi.com/f_products.htm
http://www.bruceplastics.com/pages/designkit.html
http://www.coteplastics.com/html/tools.htm
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• Mold halves
closed
• Clamping force
increased
• Reciprocating
Screw advanced
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• Clamp
remains
closed during
solidification
• New charge
brought
forward
• Clamp
opened, part
released from
mold
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Extrusion
Structural forms: tube, tracks, weather-stripping
different colors can be co-extruded
http://www.hunsinger.com/pictures.html
http://www.generalplastic.com/tour.html
http://www.duall-plastics.com/tubing.htm
http://www.synseal.com/
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Compression
Molding
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Air Tube
Split
Mold
Blow
Molding
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Reaction
Injection
Molding
(RIM)
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Rotational Molding
http://www.trendtooling.com/
http://www.rotoplastics.co.tt/frames.htm
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Design Advantages
• light, possible high
strength to weight:
isotropic vs anistropic
• chemical resistance
• resistance to shock and
vibration
• absorbs sound
• high abrasive and wear
resistance
• self lubricating
• Often easier to
fabricate (or foamed)
• integral color
• cost trends downward
(can fluctuate)
• low overall
comparative costs
• consolidation of parts
• recyclable
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Design Disadvantages
• Lower overall strength
• high thermal expansion
• more susceptible to
creep, cold flow, load
deformation
• low heat resistance to
distortion and
degradation
• less ductile
• flammable
• More subject to
embrittlement at low
temperature
• ultraviolet degradation
• absorption of moisture
• solvents
• softer
• higher cost than
competing metals
(volume)
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Classification of Polymers (6)
• Source: where did it come from?
– Natural: animals, vegetables, minerals: latex,
lignin, cellulose, casein, asphalt, rosin
– Synthetic: by far the most important source: oil
by fractional distilling: cracking towers. Also,
agricultural by-products: looking for
hydrocarbons.
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Light Penetration
• Opaque: light will not pass through
• Transparent: light will pass through, can be
seen through.
• Translucent: light passes, not sight
• Luminescent
– fluorescent: emits light when bombarded with
electrons
– phosphorescent: glows after exposure to energy
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Heat Reaction
• What happens when you turn up the heat?
– Thermoplastic: can be heated and reused
– Thermosets: Cannot
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Polymerization
• How the stuff was made: Time, Pressure,
Temperature
– Addition: A-A-A-A-A-A-A– Condensation polymerization:
A-A-A-A
B-B-B-B
C-C-C-C-
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Molecular Structure
• How does it all fit together
– linear
– branched
– crosslinked
• Strength properties
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Crystal Structure
• How is it organized?
– Amorphous: unorganized, very”plastic”, usually
low in strength
– Crystalline Structure: repeated, packed
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Categories of Plastics Industries
• Producers of Resins: chemical industry
• Conversion Processors: plastics industry
• Fabricators and Finishers: consumer
products industry
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Principles of Melt Processing
• Heat:
– TM for crystalline Polymers
– TG for amorphous
• Pressure: plastics are too viscous to flow
with gravity (in most industrial production
activities)
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Starting Materials
• Powders: Rotational Molding
• Pellets: injection molding, materials that
“flow” for ease of handling: (phenolics in
the materials science lab)
• Films and structural shapes
• Heated to eliminate moisture when recycled:
may be pre-heated for processing
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Volume Change
• Due to rearrangement of molecules and
establishment of secondary bonds.
• Shrinkage increases with:
– slower cooling rate, usually from higher
temperatures: moral: don’t overheat
– decreased injection pressure
– shortened injection time (increased rate)
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Distortion
• Occurs when uncoiled (heated) molecules
are not given enough time to cool slowly
enough to recoil before freezing (low mold
temp). This may be desirable, but it usually
not. Slow cooling causes shrinkage:
balanced trade off
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Molding
• Should be done at high shear strain rates:
injecting at high rate through small gates:
lower temps can be used, cycle times
reduced. The small gates prevents material
from backing out of the mold adding to
shrinkage
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Gates, Feed and Runners
• Critical variables also:
– knit lines
– effect on distortion
– too small a gate: premature freezing
• Computer simulations mow add in
fabrication variable establishment.
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Materials Added to Polymers
• Plasticizers: lubricants added to allow
molecules to move I relationship to one
another: lowers strength, increases
flexibility
• Fillers: like aggregate in concrete: synergetic
effect, lubrication, strength, dimensional
stability, fibers added for anisotropy and
isotropy
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Materials added to Polymers
• Blends, Alloys: mixing of plastics to achieve
required properties: unlimited permutations,
leads to new polymers: very proprietary.
• Interpenetrating Networks: we will discuss
composites later.
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Thermosets
• 15% of applications
• must be held in mold longer to allow
polymerization and crosslinking to occur
• some may be removed green and cured
• some start crosslinking with heating process:
cold mold-hot mold-cold mold makes for a
very long process
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Introduction to Composites
• General Description
• Strength Calculations
• Product Comparisons
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Introduction
• Composites are produced when two
materials are joined to give a combination of
properties that cannot be attained in the
original materials: synergy
• Three general categories: particulate, fiber,
and laminar.
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Particulate
Concrete
True particulate
composites
contain large
amounts of
coarse particles
that do not
effectively block
slip.
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Particulate Applications
• Carbon black in vulcanized rubber: Carbon
black consists of tiny carbon spheroids only
5nm to 500nm in diameter. The carbon
black improves the strength, stiffness,
hardness, wear resistance, and heat
resistance to the rubber. (Askeland)
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Fillers and Extenders
• Calcium carbonate, glass spheres, various
clays. Extenders stiffen the polymer,
increase hardness, wear resistance, creep
resistance, thermal conductivity. Cost,
strength and ductility usually decrease, and
weight is effected.
• For example: design a clay-filled
polyethylene having a minimum tensile
strength of 3000psi, and a minimum E of
80,000psi.
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Fiber Reinforced Composites
(FRC)
• Improved strength, fatigue resistance,
stiffness, strength/weight ration by
incorporating strong, stiff but brittle fibers
into softer, ductile matrix. The matrix
material transmits the force to the fibers,
which carry most of the applied force.
• Examples: straw in brick, re-bar, glass in
polymers, etc.
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The rule of mixtures applies, to
a point, for E. When applied stress
is very large, the matrix deforms
(shown below), taking it past the
proportional limit. The modulus
is now approximated as:
Ec = ff Ef
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Fiber length and diameter
• The strength of a composite improves as the
fiber aspect ratio of length/diameter
increases.
• Smaller diameters have less surface area,
reducing probabilities of crack and flaw
propagation under load.
• The ends of a fiber carry less load, so longer
fibers (less ends) are desirable.
• 80% fiber usually all that is possible.
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Orientation of Fibers
Long, continuous fibers with
unidirectional arrangement
produce anistropic properties
as seen to left
Short, randomly oriented
having a small aspect ratio
typical of fiberglass, are easily
introduced into the matrix and
give good isotropic properties
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Three Dimensional Weaves
Also: chopped, mats, roving
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Three Dimensional Weaves
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Fiber Properties
• The highest specific modulus is usually
found in materials with the lowest atomic
number, and covalent bonding: carbon,
boron.
• Whiskers are single crystal, large aspect
ratio. No imperfections, so crack
propagation low. Very expensive
• Most common are glass: S-Glass and Eglass
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Comparing Fibers
• Specific strength: sometimes called the
strength to weight ratio.
• Often the substantial cost associated with
composites is not justified by their strength,
but by their reduced weight: savings in fuel,
and assembly. (Each pound reduction in
weight saves 500 gallons of fuel per year on
an airplane.)
2.54cm = 1.0 in
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454g = 1.0 lb
®
Kevlar
2.54^3/454 =
0.036095
Specific Strength:
Specific E
Specific  y
modulus 
strength

Conversion factor: (0.036095) from g/cm3 to lb/in3
y = 525ksi
E = 18x106 psi
 = 1.44g/cm3
525,000
 101
.  106
144
. (0.036095)
18  106
 34.6  107
144
. (0.036095)
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Material
Comparisons
Note: the units:
lb
2
in  in
lb
3
in
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Note
Even wood
is lb per lb
“stronger”
than steel
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Matrix Properties
• Supports the fibers, and keeps them in
position. Protects them during fabrication,
and prevents crack propagation.
• Bonding is critical for load transfer.
• Fibers are sometimes coated, or sized to
improve initial bonding.
• coefficient of thermal expansion must be
similar or t will break composite apart.
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Bonding
Failure
Between fibers and
matrix, as well as
between layers.
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Good Bonding
Poor Bonding
Bit more difficult to discern. Close examination shows failure.
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Laminars
• Laminar composites include very thin
coatings, thicker protective surfaces,
claddings, bimetallics, laminates, etc.
• Many are designed to improve corrosion
resistance
• High strength, directional strength, superior
wear, improved appearance, and unusual
thermal characteristics
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Laminates
Layers of material joined by an organic adhesive
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Laminate applications
• Decorative: Formica® counters and veneers
• Safety glass: a sandwich of tempered glass
and polyvinyl butyral
• Insulation in motors, PCB’s
• complex components such as gears.
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Clad Metals
• US silver coinage: Cu - 80% Ni alloy
bonded to both sides of a Cu - 20% Ni alloy,
at a ratio of 1/6, 2/3, 1/6. The Ni coat
provides silver color, the Cu provides low
cost bulk.
• Alclad: pure aluminum surface mechanically
rolled to surface of alloy aluminum:
excellent corrosion on surface, with
improved mechanical properties of the alloy
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Bimetallic materials
• Temperature indicators and controllers take
advantage of different coefficients of
thermal expansion.
• Must have reversible, repeatable expansion
characteristics and a high modulus of
elasticity in order to do work.
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Sandwich Structures
• Thin layers of facing materials joined to
lightweight filler materials: such as polymer
foam
• With cardboard, neither filler or face have
good strength properties, but combined they
do (remember the moment of inertia)
• Also used as sound and vibration absorbing
materials