Plastic material details for engineering plastics.pptx

III. Types of Plastics
O Tlierinoplastic Polymers
- soften and melt on heating and
solidify again on cooling
mostly used in packaging
) Tliei•iiiosettiiig Polymer
- a liquid or solid state at room
temperature; melted and hardened on
heating and converted into
infusible
state by some chemical change.

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
More Plastic Products
• In addition, plastics are often the principal
ingredient in other materials, such as
— Paints and varnishes
— Adhesives
— Various polymer matrix composites
• Many plastic shaping processes can be adapted to
produce
items made of rubbers and polymer matrix composites

Trends in Polymer
Processing
• Applications of plastics have increased at a much faster
rate than either metals Or ceramiCS during the last 50 years
— Many parts previously made OI metals are now
being made of plastics
— Plastic containers have been largely substituted fOr glass
bottles and jars
• Total volume of polymers (plastics and rubbers) nOw exceeds
that of metals
• 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

These plastics are made up of lines of long chain molecules (below)
with very few cross linkages. This allows them to soften when heated
so that they can be bent into different shapes, and to become stiff
and solid again when cooled. This process can be repeated many
times.
Thermoplastics
Chemical structure
remains
unchanged during
heating and
shaping
More important commercially,
comprising more than 70O
é of
total plastics tonnage

Plastic types: Thermoplastics
General propertie,i: low melting point, softer, flexible.
Typical uses: bottles, food wrappers, toys,
Examples:
Polyethylene: packaging, electrical insulation, milk and water bottles, packaging
film Polypropylene: carpet fibers, automotive bumpers, microwave containers,
prosthetics Polyvinyl chloride (PVC): electrical cables cover, credit cards, car
instrument panels Polystyrene: disposable spoons, forks, Styrofoam*M
Acrylics (PMMA: polymethyl methacrylate): paints, fake lur, plexiglass
Polyamide (nylon): textiles and fabrics, gears, bushing and washers,
bearings PET (polyethylene terephthalate): bottles for acidic foods like
juices, food trays
PTFE (polytetrafluoroethylene): non-stick coating, Gore-TexTM
(raincoats),
dental floss

These plastics are made up of molecules that are
heavily cross-linked (below). This results in a rigid
molecular structure. Although they soften when
heated the first time, and can therefore be shaped,
they then become permanently stiP and solid, and
cannot be reshaped.
Thermosets
Undergo a curing process during
heating and shaping,
causing a
permanent change (cross-linking)
in molecular structure
Once cured, they cannot be remelted

Plastic types: Thermo sets
General properties: more durable, harder, tough,
light.
Typical uses: automobile parts, construction materials.
Examples:
Unsaturated Polyesters: lacquers, varnishes, boat hulls, furniture
Epoxies and Resins: glues, coating of electrical circuits,
composites: fiberglass in helicopter blades,
boats,

SHAPING PROCESSES FOR PLASTICS
1. Compression Moulding
2. Transfer Moulding
3. Extrusion Moulding
4. Injection Moulding
5. Blow Moulding
6. Thermoforming
7. Calendering
8. IenPolymer Foam Processing and
Forming
9. Product Design Considerations

pressur
e
Compression
Mouiding
thermos
et plastic
granules
cured
mouldin
g

Plastics Processing: Compression Molding
Ompressionmolding
Upper movable
ld half
Charge
mold halt’
Ejector pin
Compression Molding

Plastic Processing : Compression
Molding
1. MEAGURSD P
O
W
DE
R
M0tD
O
P
E
N
S
COMPRESSION
MOLDIN
Molding materials:
— Phenolics, melamine,
urea-formaldehyde,
epoxies, urethanes, and
elastomers
2. MOLD CLOSSS Ufl DSR
HEA
T & PRE66UR5
FIIJAL PR0D8CT
• Typical compression-molded products:
— Electric plugs, sockets,
and housings; pot handles, and
dinnerware plates
• Simpler than injection molds
• No sprue and runner system in a compression mold
• Process itself generally limited to simpler part geometries due
to lower flow capabilities of TS materials
• Mold must be heated, usually by electric resistance, steam,
or hot oil circulation

Plastics Processing: Compression Molding

Transfer
Molding
TRANSFER MOLDING
Transfer Molding
H
eø
tc
æ
Tr
a
n
sf
er

Transfer
po t s
Transfer
ram
Pot Transfer
Molding
Charga
{preform)
Ejector
pin
Cavitie
s
(a)
I loided
part
Figure (a) Pot transfer molding: (1) charge is loaded
into pot, (2) softened polymer is pressed into mold
cavity and cured, and (3) part is ejected.

Plunger Transfer
Molding
Charge
(preform)
Cavities
Ejector
pins
(b)
Molded
part
Figure (b) plunger transfer molding: (1) charge
is loaded into pot, (2) softened polymer is
pressed into mold cavity and cured, and (3) part
is ejected.

thermoplast
ic
granules
hoppe
r
motor
and
hydraulic
pump Screw
conveyOr
Extrusion Mou i
of a
thermoplastic.
heate
r
di
e
cold water
in
water
out
extrude
d solid

Extrusion of polymers
Screw Heaters Barrel Nuzzle

Thermoplastic granules (right) are fed from a hopper
by a rotating screw through a heated cylinder.
The tapered shape of the screw (right) compacts the
plastic as it becomes plasticized. This part of the process is
similar to the heating and compacting stages in the
injection moulding process. The difference being that the
softened material is allowed to flow out through a die in
a continuous stream
( Extrusion moulding ) rather than be pumped
intermittently
in measured amounts into a mould.( Injection
moulding )
The die (right) which is fitted to the end of the
extruder barrel determines the
cross-section of the
extrusion.

Extrude
r
Hopper
Plastic pellets
Heaters
Polymer melt
Screw
Barre
l
Breaker
plate
Feed section
Figure : Components and features of a (single-screw) extruder for plastics
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 of a die
• 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
• Carried out as a continuous process; extrudate is then cut
into desired lengths
Extrudate

Extruded
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

Die End of Extruder : Extrusion Die for Solid Cross
Section
Extruder
barrel
Screen pack -
—
Melt flow
direction
Breaker plate
Converging die
entrance
Clamping
ring
Die opening
size
D/ for round)
Polymer melt
(a)
(b)
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.
• 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 ol screen pack:
— Filter out contaminants and hard lumps
— Build pressure in metering section
—
Extrudat
e

Extrusion Die for Coating
Wire
Direction of melt
flow
Screen
pack
Vertical extruder
barrel
Core
tube
Base wire in
Partial vacuum drawn
Side view cross-section of die for coating of electrical wire by
extrusion.
Polymer
melt
Figur
e
• 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 O 50 mls (10,000

Plastics Processing: Extrusion

Extrusion
This process can be compared to squeezing toothpaste from a tube. It is a
continuous process used to produce both solid and hollow products that have
a constant
cross-section. E.g. window frames, hose pipe, curtain track, garden trellis.
Process
The photo below shows a typical thermoplastic extruder.

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 410 to 30 sec, but cycles of one
minute or more are not uncommon
• Mold may contain multiple cavities, so multiple
moldings are produced each cycle
Injection Molding Machine
Two principal components:
Injection unit
Melts and delivers polymer
melt Operates much like an
extruder

Plastics Processing: Injection Molding
Cavity
(1)
(1) mold is
closed
Fresh polymer
melt for next
shot
(3)
(3) screw is
retracted.
Movabl
e
platen Nonreturn
valve
Polymer
melt
(2)
(2) melt is injected into
cavity.
(
#
)
(4) mold opens and part is
ejected.
Cycle of operation for injection

Plastics Processing : Injection Molding
Machine
Feed
hopper
Heater
s
Cylinder for screw-
ram
Motor and
gears for ccre
rotat on
Reciprocating
screw
Nozzle
Stationary piaten
Movable
platen
Mold
Tie rods
(4)
Clamping
unit
Figure : Diagram of an injection molding machine, reciprocating
screw type (some mechanical details are simplified).
mpi
cylinde
r
Hydrauli
c
cylinder

Process & machine schematics
Schematic of thØrrtłODlãstíc Injection molding
machine

New developments- Gas
assisted injection
molding
Gas finger
ing
Extent of
penetration

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
• 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

Mold
(open)
Extrusion Blow
Molding
— Extruder
barrel
(1)
Tube
die
Blow
pin
(2) (3)
Figure : Extrusion blow molding: (1) extrusion of parison; (2)
parison is pinched at the top and sealed at the bottom around
a metal blow pin as the two halves of the mold come together;
(3) the tube is inflated so that it takes the shape of the mold
cavity; and (4) mold is opened to remove the solidified part.

Extrusion Blow Molding
Step C
Step 4 Step B
Extrudv•r Jic
htulJ half
(1) excursion of parison
(3) the tube is inflated in that it takes & shape of be eiold
caiñ6
(4) mo A is opened to remove the ,olidified part
Air blo» pin
Step D

Extrusion Blow
Molding
Extrusion Blow Molding involves manufacture of parison by conventional
extrusion
method using a die similar to that used for extrusion pipes.
Extrusion Blow Molding is commonly used for mass production of plastic
bottles.
The production cycle consists of he following steps.”
L1.The parison is extruded vertically in downward direction between two mold
halves.
72. When the parison reaches the required length the two mold halves close
resulting in pinching the top of parison end and sealing the blow
pin in the bottom of the parison end.
E3.Parison is inflated by air blown through the blow pin, taking a shape
conforming
that of the mold cavity. The parison is then cut on the top.
4.The mold cools down, its halves open, and the final part is removed.

extr
uder
A t‹iI›e of plastic (|›arison) is
ext›ti‹Ie‹I
2
The neat l‹l is
close‹l
Air is blow‹i in to infl.›te the
paiison

extrude
r
Mould opens and the moulding is
removed

Extrusion Blow
Moulding
hot
thermoplastic
tube or
"parison"
tubula
r
die
tube
squeezed and
gripped by
the mould
extruded f a i r
blown in
thermoplastic
tube
takes the
shape
of the
mould
bottl
e

Injection
unit
Injection
mold
Injection Blow
Molding
Blowing
rod
Blow
mold (2
)
(3)
Blow-molded
part
(4)
Figure :Injection blow molding: (1) parison is injected
molded around a blowing rod; (2) injection mold is
opened and parison is transferred to a blow mold; (3)
soft polymer is inflated to conform to the blow mold;
and (4) blow mold is opened and blown product is
removed.

Injection Blow
Molding
1. In Injection Blow Molding method a parison is
produced
by injecting a polymer into a hot injection mold around
a
blow tube or core rod.
2.Then the blow tube together with the parison is
removed from the injection mold and transferred to a
blow mold.
Following operations are similar to those in the
extrusion
blowing molding.
3. Injection Blow Molding is more accurate and
controllable process as compared to the
Extrusion Blow Molding.
It allows producing more complicated products from a
wider range of polymer materials.

The photo (below) shows fluid containers used in hospitals, ready to be
trimmed. A wide variety of colours can be achieved by adding a colouring
agent to the mix of thermoplastic granules (about 1% by volume).

Calenderin
g
Feedstoc
k
Sheetstoc
k
Feedstock is passed through a series of rolls
to
reduce thickness to desired gage
• Expensive equipment, high production
rates
• Process is noted for good surface finish
and high gage accuracy
• Typical materials: rubber or rubbery
thermoplastics such as
plasticized PVC
• Products: PVC floor covering, shower
curtains, vinyl table cloths, pool liners,
and inflatable boats and toys
Calendering is a prOcess where a large amount of molten plastic is placed between
rollers and rolled to turn it into sheets. The rollers are hot and keep the plastic in its
semi - molten state allowing it to be rolled thinner and thinner as it passes through
rollers which are closer and closer together. When it is at the required thickness it is
rolled through cold rollers to enable it to go hard and is then wound into rolls.
Figure : A typical roll configuration in calendering

Embossin
g
Punc
h
Blan
k
Die
Section through
embossed piece

Welding of
Plastics
Rlastics welding is the process of joining two
pieces of Thermoplastics at heated state and
under a pressure as a fesult of cross linking of
their polymer molecules. The work pieces are
fused together with or without filler material. The
joint forms when the parts are cooled below the
Glass Transition Temperature (for amorphous
polymers) or below the melting temperature (for
crystalline polymers).
Thermo sets (thermosetting resins) in cured
condition cannot be welded, since cross-linking of
their molecules has completed.
Plastics welding processes.
Hot Gas Welding
Hot Plate
Welding
Ultrasonic
Welding Spin

Welding of
Plastics :
Friction ( Spin )
Welding
Friction (spin) welding
• Similar to friction weldyig of steels
• One part stationary, other rotates at
constant angular velocity under
pressure
Friction (spin)
welding
- W BE 00
- alfitty to weld beneath ¥:luld
suññoe
• DisadvanBoes
- ctrcucs so¥ds w follow sect<n
comPonene

Welding of Plastics :
Hot Plate
Welding
mo›oing Gxlure
,
Tootingsop
Parts are held and
aligned by holding
fixluree.
(c)
Pans are pressed
againsl platen to melt
edges.
Parts are compressed so
edges fuse together ae
plaeTic cools.
(a›
I hating platen is
inserted.
(d)
Heating platen is
withdrawn.
Holding fixtures open,
leaving bonded part in lower
fixture.

Welding of
Plastics :
Vibration welding
• Frlctlonal heat genefateo by relative
rnove‹uent between two parts
whicn are
• The movements are linear
oscillations.
• Procedure.
— vbration ur<Ier Pressue unB meIIñ›g
of the
— vtration
stopped;
Vibration
Welding
Vibration welding
- aNlity tO w0Id Otge C‹xq›tex ioints
- high production rntas
—Join dissimilar shaked matenals
—aerospace irzxistnes

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