Unit 6 part c plastic processing

PPP
PROCESSING OF PLASTICS
CH.PAVAN KUMAR

PLASTICS : -
 Plastic is a materialconsistingof anyof awide range
of syntheticorsemi-syntheticorganicthatare malleableand
can be moulded into solid objects of diverse shapes.
 Plasticsare typicallyorganicpolymersof high molecularmass,
but theyoftencontainothersubstances.
 Theyare usually synthetic, mostcommonlyderived
from petrochemicals.
 Theworld's first fullysyntheticplasticwas Bakelite, invented in
NewYork in 1907 by Leo Baekelandwho coined the term
'plastics‘.

TYPES OF PLASTIC : -
1. THERMOSETTING PLASTIC: -
The molecules of thermosetting plastics areheavily
cross-linked. They form a rigidmolecular
structure.
Although they softenwhen heated the first time,
which allows them to be shaped they become
permanently stiff and solid and cannot be
reshaped.
 Ex. Polyester resin and ureaformaldehyde
Cross-linked molecules
Contd.

2. THERMOPLASTIC: -
 Thermoplastics can be heated and reshaped
becauseof theways in which the moleculesare
joined together.
 The molecules of thermoplastics are in linesor
long chains with very fewentanglements.
 When heat is applied the molecules moveapart,
which increases the distance between them,
causing them to become untangled. This allows
them to become softwhen heated so that theycan
be bent into all sortsof shapes.
 Ex. Polyvinyl chlorides(PVC), Nylons, etc.
Contd.
individual monomer molecule

PROCESSING OF PLASTICS: -
1. COMPRESSION MOULDING
2. TRANSFER MOULDING
3. INJECTION MOULDING
4.EXTRUSION MOULDING
5. BLOW MOULDING
6.CALENDARING
7. THERMOFORMING
8.ROTATIONAL MOULDING
9.LAMINATING

1. COMPRESSION MOULDING : -
Compression moulding is the forming process.
Compression moulding process is followed by two step first one preheating
and pressurizing.
Measured charge of compound placed between halves of splitmold
Pressure is applied so thatcompound flows to fill mold cavity
 Compression moulding is generally used for thermosettingplastic.
 Example:- urea formaldehyde and phenolformaldehyde.

PARAMETERS : -
Thequantityof charge ( moulding material ) put into the mould.
 Pressure of the mouldingprocess:-
Range of pressure 2000-3000 psi ( 13.8-20.7 mpa).
Mould temperature:-
Temperaturerange 300ºf to 375ºf ( 149⁰C- 191⁰C )
Cure timevariables:-
The period required to harden thermosetting material topartial and complete
polymerization iscalled cure time. Cure times typicallyrange from 1 - 5 min

2. TRANSFER MOLDINGS : -
 It is similar to compressionmoulding.
 Thermosetpolymerscan be formed whena presetamountof material is placed in a
separate cavity andheated.
 A plunger movesthe material into the shaped mould with high pressure.

3. INJECTION MOULDING : -
It is a manufacturing technique for making parts from
thermoplastic and thermoset materials.
Injection moulding is a highly automated productionprocess
for producing large quantities of identicalitems.
A measured amountof molten thermoplastic is driven bya
ram pasta heating system into the mould.
The mould is split toallow finished object to be removed after
cooling.
Example : - Polystyrene, Nylon, Polypropylene, PVC,
Acrylonitrile Butadiene Styrene

PROCESS : -
contains a rotatingscrew.
material.
barrel.
 Plastic powderare fed froma hopper into a hollow steel barrel which
 The barrel is surrounded bya jacket of heaterswhich melt the plastic
 The screw is forced back as the melted plastic collects at the end of the
 Once a sufficient charge of melted plastic has accumulated a hydraulic
ram forces the screw forward injecting the thermoplastic througha sprue
into the mouldcavity.
 This one is capableof exerting forces of up to 250 tonnes.
 Pressure is kept on the mould until the plastic has cooled sufficiently for
the mould to beopened and thecomponentejected.

Unit 6 part c plastic processing

4. EXTRUSION MOULDING: -
 Continuous process used to produce both solid and hollow products that havea
constantcross-section.
 Ex: - Window frames, hosepipe, curtain track, garden trellis.
 Thermoplasticgranulesare fed from a hopper by a rotating screw through a
heated cylinder.
 The tapered screw compacts the plasticas it becomeselasticised. Thediewhich
is fitted to the end of theextruder barrel determinesthecross-section of the
extrusion.
 Thicker cross-sections are extruded more slowly as more time is required for the
initial heating and subsequent cooling of the larger quantities of material which
are involved.
 As the extrusion leaves the die it is cooled by passing through a cooling trough
containing cold water.

5. BLOW MOULDING:-
 Blow Moulding is an automated process that is used extensively to make
bottlesand other lightweight, hollow parts from thermoplastic materials.
 A hollow length of plastic, called a parison, is extruded down betweenthe
two halves of the mould.
 The mould closes
 Compressed air is blown into the inside of the parison which inflates it,
pushing the soft plastic hard against the cold surfacesof the mould.
 The mould is then opened the moulding ejected and the waste (called
flash) is trimmed off with aknife.
 Example : - highdensity polytheneand low densitypolytheneareused.

Blow Moulding Process
Extrusion
Blow
Moulding
Injection
Blow
Moulding
Stretch Blow
Moulding

6. CALENDARING : -
 It is a intermediateprocess where theextruded plasticsectionsare reduced to
sheet.
 Only used forthermoplastics.
 Involves rollingouta massof premixed plastics material between largerollersto
form acontinuousand accurately sized film.
 The process beginswith the ingredients being blended and fluxed in a mixing mill
at approx. 100°c.
 Niprollerscontrol the thicknessof the sheet materialcan be graduallyreduced in
thickness.
 Rollsof semi-rigid PVCwhich will be used to manufacturetransparentA4 folder
'pockets'.

Example: -
1. Flexible PVC :- Rainwear, Shower curtains, Tapes,etc.
2. Rigid PVC: - Credit cards, Lamination

7. THERMOFORMING: -
 Process used to shape thermoplasticsheet intodiscrete parts.
 Basic principles: -
i. Heata thermoplasticsheetuntil it softens.
ii. Forcethe hotand pliable materialagainst thecontours of a mold by using
eithermechanical,airorvacuum pressure
iii.Held against mold and allow to cool, and plastics retains its shape
 Thiscan bedone by: -vacuumand pressure forming.
 Lower pressures are required tothermoform.

8. ROTATIONAL MOULDING : -
 Rotational moulding isa process used forproducing hollow plasticproducts.Such
as road cones and storage tanksup to 3m³ capacity.
 A measuredweight of thermoplastic isplaced insideacold mould.
 Heated toa temperature of 230-400 cwhilst being rotated around bothvertical
and horizontalaxes.
 The mould moves intoacooling area orchamberwhere it iscooled by airor
water jets.
 The hollow mouldingcan be removed assoonas it iscool enoughto hold its
shape.

The rotational molding
(roto-molding or roto-
casting) process. Trash
cans, buckets, and plastic
footballs can be made by
this process.
90% of rotational
mouldings are madefrom
polyethylene

9. LAMINATING: -
 Layers of materialsare bonded with a resin intoastrong solid structure, often
with heat andpressure.
 This can be done in followingways: -
 Gravurecoating
 Wet bond laminating
 Dry bond laminating
 Solvent less laminating
 Extrusion laminating
 Example: -Foil, Snack food bag, Saran- coated nylon, Metallized propylene, etc.

Substrate A
Extrusion die
Substrate B
Strip-off roll
Nip roll
Extrudate
Pressure
roll
Three ply
laminate
Extrusion laminating Dry bond laminating
Gravurecoating

Unit 6 part c plastic processing

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Unit 6 part c plastic processing