Plastics are polymers characterized by large molecules formed by joining smaller molecules. They are classified based on their industrial usage and temperature dependence into thermoplastics and thermosets. Thermoplastics soften when heated and harden when cooled in a reversible process, allowing recycling, while thermosets undergo an irreversible chemical reaction when heated. Common thermoplastics include polyethylene, polypropylene, PVC, and nylons, which are used in a variety of applications. Injection molding is a popular plastic processing method that uses heat and pressure to form molten plastic into molds.

2. Plastics or polymer
 Definition: A group of engineered materials
characterized by large molecules that are built up by
the joining of smaller molecules.
 They are natural or synthetics resins.

3. Properties of plastics
 Light weight
 Good resistance to corrosion
 Easy of fabrication into complex shapes
 Low electrical and thermal conductivity
 Good surface finish
 Good optical properties
 Good resistance to shock and vibration.

5. Classification – Polymers
 Classification based on their industrial usage:
(a) plastics and
(b) elastomers.
 Classification based on their temperature dependence:
(a) thermoplasts and
(b) thermosets

6. Thermoplasts
 Plastics which softens up on heating and hardens up
on cooling where the softening and hardening are
totally reversible processes.
 Hence thermoplasts can be recycled.
 They consist of linear molecular chains bonded
together by weak secondary bonds or by inter-winding.
 Cross-linking between molecular chains is absent in
theromplasts.
 E.g.: Acrylics, PVC, Nylons, Perspex glass, etc.

7. Thermoplastics
 Acrylonitrile-butadiene-styrene (ABS):
Characteristics: Outstanding strength and toughness,
resistance to heat distortion; good electrical properties;
flammable and soluble in some organic solvents.
Application: Refrigerator lining, lawn and garden
equipment, toys, highway safety devices.
 Acrylics (poly-methyl-methacrylate) PMMA
Characteristics: Outstanding light transmission and
resistance to weathering; only fair mechanical properties.
Application: Lenses, transparent aircraft enclosures,
drafting equipment, outdoor signs.
Contd…

8.  Fluorocarbons (PTFE or TFE,Teflon)
Characteristics: Chemically inert in almost all
environments, excellent electrical properties; low
coefficient of friction; may be used to 260ooC; relatively
weak and poor cold-flow properties.
Application: Anticorrosive seals, chemical pipes and
valves, bearings, anti adhesive coatings, high temperature
electronic parts.
 Polyamides (nylons)
Characteristics: Good mechanical strength, abrasion
resistance, and toughness; low coefficient of friction;
absorbs water and some other liquids.
Application: Bearings, gears, cams, bushings, handles,
and jacketing for wires and cables.
Contd…

9.  Polycarbonates
Characteristics: Dimensionally stable: low water
absorption; transparent; very good impact resistance
and ductility.
Application: Safety helmets, lenses light globes, base
for photographic film
 Polyethylene
Characteristics: Chemically resistant and electrically
insulating; tough and relatively low coefficient of
friction; low strength and poor resistance to
weathering.
Application: Flexible bottles, toys, tumblers, battery
parts, ice trays, film wrapping materials.
Contd…

10.  Polypropylene
Characteristics: Resistant to heat distortion; excellent
electrical properties and fatigue strength; chemically
inert; relatively inexpensive; poor resistance to UV
light.
Application: Sterilizable bottles, packaging film, TV
cabinets, luggage
 Polystyrene
Characteristics: Excellent electrical properties and
optical clarity; good thermal and dimensional stability;
relatively inexpensive
Application: Wall tile, battery cases, toys, indoor
lighting panels, appliance housings.
Contd…

11. Thermosets
 Plastics which are ‘set’ under the application of heat
and/or pressure.
 This process is not reversible, hence thermosets can
not be recycled.
 They consist of 3-D network structures based on
strong covalent bonds to form rigid solids. linear
molecular chains bonded together by weak secondary
bonds or by interwinding.
 Characterized by high modulus / rigidity /dimensional
stability when compared with thermoplasts.
 E.g.: Epoxies, Amino resins, some polyester resins, etc.
Contd…

12.  Thermosets are strengthened by reinforcements .
 Different reinforcements are in use according to the
necessity. Glass fibers are most commonly used to form
structural and molding plastic compounds.
 Two most important types of glass fibers are E (electrical)-
and S (high strength)- glasses.
 E-glass (lime-aluminium-borosilicate glass with zero or
low sodium and potassium levels) is often used for
continuous fibers.
 S-glass (65%SiO2, 25%Al2O3 and 10% MgO) has higher
strength-to-weight ratio and is more expansive thus
primary applications include military and aerospace
applications.
 Carbon fiber reinforced plastics are also often used in
aerospace applications. However they are very expansive.
Contd…

13.  The other classes of reinforcements include aramid (aromatic
polyamide) fibers.
 They are popularly known as Kevlar.

14. Examples – Thermo setting polymers
 Epoxies
Characteristics: Excellent combination of mechanical
properties and corrosion resistance; dimensionally stable;
good adhesion; relatively inexpensive; good electrical
properties.
Application: Electrical moldings, sinks, adhesives,
protective coatings, used with fiberglass laminates.
 Phenolics
Characteristics: Excellent thermal stability to over 150o C;
may be compounded with a large number of resins, fillers,
etc.; inexpensive.
Application: Motor housing, telephones, auto
distributors, electrical fixtures.
Contd…

15.  Polyester (PET or PETE)
Characteristics: One of the toughest of plastic films;
excellent fatigue and tear strength, and resistance to
humidity acids, greases, oils and solvents
Application: Magnetic recording tapes, clothing,
automotive tire cords, beverage containers.

16. Chemical
classification
Trade name characteristics Typical
application
Epoxy Araldite oxiron Good toughness.
Resistant to;acids.
alkalies and
solvents.
excellent
adhesion to metal,
glass and wood.
Adhesive and
coatings, tools
and
dies, filament
wound vessels,
laminates for
aircraft, patching
compound for
metal and
plastics.
Melamine-formal-
dehyde
Good for
application
requiring cycling
between wet and
dry conditions.
Hard and abrasion
resistant. Good
dielectric.
Tablc-ware, electric
insulation,
automotIve Ignition
parts, cutlery
handles,jars and
bowls.

17. Phenol-
formaldehy
de
Bakelite
Marblette Durez
Cataljn
Good dimensional
stability Excellent
insulating qualities.
Inert
to most solvents and
weak acids.
Good strength around
inserts.
Industrial electrical
parts.
automotive electrical
components,
paper impregnated
battery
separators.
Electrical insulation.
Phenol-
furfural
Durite Similar to
Phenolfonnaldehyde.
Electrical insulation.
Mechanical
parts. Housings and
containers.
Alkyd
(Modified
polyester)
Glyptal Duraplex
Beckosol Teglac
Rezly
Can be made flexible,
resilient or rigid. Can
resist acids but not
alkalies, with glass fibre
reinforcement resists salt
water and fungus growth.
Boats, Tanks, Trailer and
Tractor
components. Ducts,
shrouds.
Vaulting poles.

18. Additives to Polymers
 The properties of polymers can be further modified by
the addition of agents which are basically of two types.
 Those that enter the molecular structure are usually
called "additives", whereas those that form a clearly
defined second phase are called "fillers".
Contd…

19. 1. Plasticizers
 Plasticizers are liquids of high boiling point and low
molecular weight, which are added to improve the
plastic behaviour of the polymer.
 They are essentially oily in nature. Organic solvents,
resins and even water are used as plasticizers.

20. 2. Fillers
 A filler is used to economize on the quantity of polymer
required and/or to vary the properties to some extent, for
example, mechanical strength, electrical resistance etc.
 A filler, whose function is to increase mechanical strength, is
termed a "reinforcing filler".
 A filler is commonly fibrous in nature and is chemically inert
with respect to the polymer with which it is to be used.
 Common fillers are wood flour, cellulose, cotton flock, and
paper (for improving mechanical strength); mica and asbestos
(for heat resistance); talc (for acid resistance).Other filler
materials are : fabric, chipped-wood moulding compound,
wood veneer, textile or glass fibres.
 The commonly used "reinforcing filler agents" with plastics are :
fibres/filaments of glass, graphite or boron.

21. 3. Catalysts:
 These are usually added to promote faster and more
complete polymerization and as such they are also
called 'accelerators' and 'hardeners' e.g., ester is used
as a catalyst for Urea Formaldehyde.
4. Initiators:
 As the name indicates, the initiators are used to
initiate the reaction, that is, to allow polymerization to
begin. They stabilize the ends of the reaction sites of
the molecular chains. H2O2 is a common initiator.
5. Dyes and Pigments:
 These are added, in many cases, to impart a desired
colour to the material.

22. 6. Lubricants:
 Lubricants are added to the polymers for the following
purposes : to reduce friction during processing, to prevent
parts from sticking to mould walls, to prevent polymer
films from sticking to each other and to impart an elegant
finish to the final product. Commonly used lubricants
include : oils, soaps and waxes.
7. Flame retardants:
 Most plastics will ignite at sufficiently high temperatures.
The non-inflammability of the plastics can be enhanced
either by producing them from less inflammable raw
materials or by adding "flame retardants". The common
flame retardants are : compounds of chlorine, bromine and
phosphorous.

23. 8. Solvents:
 Solvents are useful for dissolving certain fillers or
plasticizers and help in manufacturing by allowing
processing in the fluid state, For example, alcohol is
added in cellulose nitrate plastics to dissolve Camphor.
However, subsequently, the solvents must be removed
by evaporation.
9. Stabilisers and anti-oxidants are added to retard
the degradation of polymers due to heat, light and
oxidation.
10. Elastomers are added to plastics to enhance their
elastic properties.
Note: Above, excepting fillers, all other materials used,
fall under the category of "Additives“.

24. Plastic Process
 Moulding
1. injection moulding
2. compression moulding
3. transfer moulding
4. blow moulding
Forming
1. extrusion
2. thermoforming
3. rotational moulding, slush moulding, casting
4. caledaring
5. spinning
Others : lamination, reinforcement and coating

25. Injection Moulding
 Hot runner moulding
 Gas injection moulding process
 Multi component injection moulding process
 Multi colour injection moulding process
 Reaction injection moulding process

26. Injection Moulding

27. Injection Moulding
 The polymer is melted and than forced into a mould.
 Thermoplastic pellets melted and melt injected under
high pressure (70 MPa) into a mold using a plunger
 Molten plastic takes the shape of the mold, cools,
solidifies, shrinks and is ejected.
 Molds usually made in two parts (internal and external
part).
 Use of injection molding machine mainly used for
thermoplastics (gears, cams, pistons, rollers, valves,
fan blades, rotors, washing machine agitators, knobs,
handles, camera cases, battery cases, sports helmets
etc…)
Contd…

28. Advantages
Moulding are produced in the finished
Moulded to the repeatable precision
Metal inserts, threads and holes can be
moulded
High out put rate can be achieved

29. Disadvantages
Capital cost of the injection moulding
machine can be high compared with
other
Mould costs can be high compared
with other

30. Hot runner moulding

31. Hot runner moulding
Video

32. Hot runner moulding
 In injection moulding process runners and sprue are
cools and solidifies along with the part, and then
ejected mould with the part.
 These are cut from the part during machining
operation.
 To avoid the need for these steps by the use of hot
runner mould
 Its incorporate electrical units to heat the runner so
that material in them remains in the molten state and
not ejected from the mould with the part

33. Advantages
 Trimming of the runner from the part is
eliminated
 Less material needs to be heated for each shot
 Material is not subjected to repeated heating
 Thin walled parts do not have to wait for the
thicker sprues and runners to solidify, and less
material needs to be heated for each cycle

34. Co-Injection moulding

35. Co-Injection moulding
video

36. Co-Injection moulding
 Co-Injection means that 2 or more different plastics
are ‘laminated’ together.
 These are same except for colour. When different
plastics are used, they must be melt at same
temperature and provide proper adhesion
 2 or more injection unit required, with each material
having its on injection unit.
 Types: multi component moulding(two colour
moulding), multi colour moulding(double shot
moulding), multiple shot injection moulding etc…

37. Multi component moulding

38. Multi component moulding
Video

39. Multi component moulding
 Two colour moulding using two injection unit and one
nozzle.
 Outer polymer with colour is injected first then the
core polymer is injected through hot melt core of the
first polymer
 Suitable for thicker walled parts
 First unit inject outer polymer with proper colour. It
fills in the mould partially. As soon as melt gets in
contact with mould it form a solid skin at the mould
contact surfaces but the central core of the polymer
remains fluid.
 The proportion of first injection and second injection
is decided by trial

40. Multi colour moulding

41. Multi colour moulding
Video

42. Multi colour moulding
 Also called double shot moulding
 It is used for making two colour moulded parts by
means of successive moulding operations
 First moulding the basic case then moulding with the
next shot in to the first moulded part.
 These steps can be accomplished using two separate
machines .

43. Gas injection moulding

44. Gas injection moulding
Video

45. Gas injection moulding

46. Gas injection moulding
 Similar to two component moulding
 First outer material injected practically and then insert
gas(nitrogen) is injected as core instead of another
polymer
 It has 2 phase
1. filling phase
2. holding phase

47. Gas injection moulding
 Filling phase
 when melt flow through the mould, solid skin takes place
on the core and surface.
Middle section remain hot and fluid
The gas as to penetrate the hot fluid middle section for its
progress.
 Holding phase
Gas transforms uniform pressure through out the mould
There for lower avg pressure required during hold on a
phase

48. Gas injection moulding
 Holding phase(conti…)
Hold on pressure is through gas hence no stress on the
mould. That is no stress cracking

49. Advantages
 It significantly reduces volume shrinkage which causes
the sink marks in injection moulding
 Variable wall thickness part design
 Lighter weight
 No moulded-in stress in part
 Least deformation
 Lower clamp force required
 Simplified and coast efficient mould
 More uniform shrinkage and less warpage

50. Disadvantages
 Additional coast for gas pressure unit
 Additional coast for special nozzle
 More difficulties during start up process

51. Reaction injection moulding

52. Reaction injection moulding
Video

53. Reaction injection moulding
 It involves high pressure mixing of two or more
reactive liquid components and injection of the
mixture into a closed mould at low pressure.
 Relatively low coast
 Reinforced RIM also produce
 It consist of two metering and heads. It has a mixing
head. Where the two components mix dynamically
 Inside the mixing head, the two streams collide with
each other under high pressure.
 Then stream injected into the mould

54. Advantages
 RIM is the logical process to consider for moulding
large and thick part
 Less capital coast
 Low energy requirement
 Low pressure moulding process

55. Compression moulding

56. Compression moulding
Video

57. Compression moulding
 The mould is held between the heated plates of a
hydraulic press
 Moulding compound(powder or granule form) is
placed in the mould
 Mould close and press
 The moulding compound softens and flows to shape as
the mould temperature high enough
 For thermo plastic material is held under pressure for
specified period
 For thermosetting plastic material is held under
pressure and temperature for specified period

58. Compression moulding

59. Compression moulding
assignment 1
 Types of compression mould
1. flash moulds
2. Semi positive moulds
3. Positive moulds
4. Landed positive mould

60. Advantages
 Both thermo plastic and thermosetting materials may
be moulded
 Wastage of material is low. No runner or sprue
 With material having fibre reinforcement, this
method gives products of maximum impact strength
 For larger parts (>1.5kg) this method recommended as
equipment cost of other method is very high
 Lowest equipment cost

61. Disadvantages
 Not suitable for complicated shape
 Less surface finish
 Articles which require very close dimensional
tolerances are not suitable for compression moulding

62. Comparison
Compression moulding Injection moulding
 Both material use, preferably
thermoset material
 Post mould operation
required
 Wastage of material is less
 Complicated shapes cannot
be mould
 Reinforcement is possible
 Recommended for larger
products
 Low production rate
 Both material use, preferably
thermoplastic material
 Produced in the finished
form. So no post operation
 Wastage of material is high
 Complicated shapes can be
mould
 Reinforcement is not possible
 Not recommended for larger
products
 High production rate

63. Transfer moulding process

64. Transfer moulding process
video

65. Transfer moulding process
 Usually used for thermosetting plastics
 Transfer moulding is very similar to compression moulding
and is developed to avoid the disadvantages found in that
process.
 In this method, thermosetting charge is heated and
compressed in a separate chamber and then injected into
the closed mould where it is allowed to cool and solidify.
 Transfer moulding is capable of moulding part shapes that
are more intricate than compression moulding but not as
intricate as injection moulding
 Types: 1. pot transfer mould
2. plunger transfer mould

66. Pot transfer moulding

67. Pot transfer moulding
 Chamber is loaded with moulding material then heat
it in the chamber and melts
 The melted material forced with the help of a plunger
through the heated sprue into the runner, gate and
finally, the heated cavity.
 The ejector pin then push the moulded articles out.
 The cull with sprue go along with the plunger which
sub sequentially cut-off.
 Wastage of material and operation time are high as
compared to plunger transfer moulding.

68. Plunger transfer moulding

69. Plunger transfer moulding
 Chamber is loaded with moulding material then heat
it in the chamber and melts
 The melted material forced with the help of a plunger
through the heated runner and then to the heated
cavity.
 The ejector pin then push the moulded articles out.
 There is no Sprue in plunger transfer moulding.
 The cull and runner go along with the mould which
sub sequentially cut-off. But they are small in size.
 Wastage of material and operation time are low as
compared to pot transfer moulding.

70. Comparison
Compression moulding Transfer moulding
 Breathing is required to
remove gas and cure time
 Cure time ranges to 30-
300sec
 Mouldable size is limited by
the capacity of the press.
 Incorporation of insert is
difficult
 Tolerance level is fair
 shrinkage is minimum
 venting is required to
remove gas and cure time
 Cure time ranges to 45-90 sec
 Mouldable size is limited by
the geometry of the parts
 Incorporation of insert is
easy. Complicated parts can
also be accommodated.
 Close tolerance are possible
 Shrinkage is high

71. Blow moulding
 Blow moulding is the process of inflating a hot,
hollow, thermoplastic preform or a plastic
parison(plastic tube) inside a closed mould so that its
shape conforms to that of the mould cavity.
 Typical parts made are bottles, toys, air ducts of
automobiles, chemical and gasoline tanks, and a
number of households goods.
 Material used: PE,PP,PVC
 Types: 1. Extrusion blow moulding(75%)
2. Injection blow moulding
3. Stretch blow moulding

72. Blow moulding
video

73. Extrusion blow moulding
 Two stages
1. Production of parison
2. processing of the parison

74. Extrusion blow moulding(EBM)

75. Extrusion blow moulding
video

76. Injection blow moulding

77. Injection blow moulding
video

78. Injection blow moulding
 It is a scrap free process used for the manufacture of
plastic bottles, containers etc..
 First stage-Injects hot melt through the nozzle of an
injection moulding machine into one or more
preformed shaped cavities(test tube shaped)
 An exact amount of melt is injected around the core
pins.
 second stage- air is introduced via the core pin and the
plastic blows out.
 Ejection can be done by combination of striper plate
and air or robots

79. Stretch blow moulding
 It involves stretching a parison length wise
mechanically and radially through the blowing
process.
 Stretching must done after the parison is solidified
 Types: 1. extrusion stretch blow moulding
2. injection stretch blow moulding

80. Stretch blow moulding
video

81. Extrusion stretch blow moulding

82. Extrusion stretch blow moulding
 The parison mould is smaller than the stretch mould.
 The parison is champed in the perform mould and
blown to shape in the usual way.
 Then the blow pin stretching the parison to the
bottom of the mould and allows compressed air to
enter and blow the perform to shape.
 Eject by using stretch plate

83. Injection stretch blow moulding

84. Injection stretch blow moulding
 Similar to extrusion stretch mould but the only
difference is that the parison is injection moulded
 First stage-Inection moulding of the parison
 Second stage-Parison is conditioned to an accurate,
consistent temperature
 Third stage- conditioned parison is placed in a
container, stretched and blowed.

85. Thermoforming

86. Thermoforming
video

87. Thermoforming
 In this process, a thermoplastic sheet can be formed into a
three- dimensional shape by the application of heat and
differential pressures.
 First, the plastic sheet is clamped to a frame and uniformly
heated to make it soft and flowable.
 Then a differential pressure (either vacuum or pressure or
both) is applied to make the sheet conform to the shape of
a mould or die positioned below the frame.
 It is possible to use most of the thermoplastic materials.
The starting material is a plastic sheet of uniform
thickness.
 It is a relatively simple process and is used for making such
parts as covers, displays, blister packaging, trays, drinking
cups and food packaging.
Contd…

88. Pultrusion

89. Pultrusion
video

90. Pultrusion
 It is a process used for making parts of reinforced
plastics that have constant cross sections of any length
 Such sections are similar in shape to profile extrusions,
but contains reinforcing fibres.
 Usually fibres are guided through liquid resin to form
reinforcement. This reinforcement are then guided
together and performed into a approximate profile.
 This perform is then pulled through a heated die of
exact profile desired.

91. Extrusion

92. Extrusion
video 1
video 2

93. Extrusion
 Long plastic products with uniform cross sections are
readily produced by the extrusion process.
 Thermoplastic pellets & powders are fed through a hopper
into the barrel chamber of a screw extruder. A rotating
screw propels the material through a preheating section,
where it is heated, homogenized, and compressed, and
then forces it through a heated die and onto a conveyor
belt.
 As the plastic passes onto the belt, it is cooled by jets of air
or sprays of water which harden it sufficiently to preserve
its newly imparted shape.
 It continues to cool as it passes along the belt and is then
either cut into lengths or coiled.
Contd…

94. Advantages
 Trimming of the runner from the part is
eliminated
 Less material needs to be heated for each shot
 Material is not subjected to repeated heating
 Thin walled parts do not have to wait for the
thicker sprues and runners to solidify, and less
material needs to be heated for each cycle
 The process is continuous and provides a cheap
and rapid method of moulding.
 Common production shapes include a wide variety
of solid forms, as well as tubes, pipes, and even
coated wires and cables.

95. Extrude screw

96. Extrude screw
video

97. Extrude screw
 It has 3 different zones
 Feed zone- preheat the plastic and convey it into the
sub sequent zone. Screw depth constant in this zone.
 Compression- this has decreasing channel depth. It
expels air tapped between the original granules.
 Metering zone- constant screw depth but less than
feed zone. Main function is homogenize the melt and
quality.

98. Extrude screw types

99. Extrusion(Types)
 Pipe extrusion
 Blow film extrusion
 Cast film extrusion

100. Pipe extrusion

101. Pipe extrusion
 A typical pipeline consist of a single or a twin-screw
extruder, a die, equipment for inside and outside
calibration, a cooling tank, a wall thickness measuring
device, haul off and a windup unit for self supporting
pipe units.
 A small diameter tube(less than 10mm) is usually
made with a free extrusion process. Large diameter
pipes are usually made with pipe extrusion.
 Which is done by the help of calibrating unit and
pipe sizing device just below the die.
 Calibrator solidifies the plastic and transfer the stress
acting on the product.

102. Pipe sizing

103. Pipe sizing
 Sizing sleeve method- pipe is drawn through a water
cooled metal sleeve and held to the sleeve by air
pressure inside the pipe. Alternatively, the pipe may be
sized internally by mandrel
 Sizing plate method- pipe takes its size by passing
through a series of metal sizing plates
 Vacuum through method- pipe is fed through along
closed water filed through. Metal sizing rings gives
desired diameter. A vacuum drawn over the water in
the through to reduce the external pressure and
expands the pipe against the sizing rings

104. Blow film extrusion

105. Blow film extrusion
video 1
video 2

106. Blow film extrusion
 Employed to produce wide and tubular films.
 Consist of extruder with a blowing head.
 A blown film line is quite different from a flat film is
extruded vertically upwards.
 Air is introduced inside the tube; as a result the tube
expands to a bubble with a diameter larger than the
diameter of the die.
 Blow up ratio- ratio of the bubble diameter to the
die diameter

107. advantages
 It can produce not only tubular products but also flat
films.
 Film has uniform strength in both machine and
transverse direction
 No edge film is required
 Used for bag making

108. Cast film extrusion

109. Cast film extrusion
video

110. Cast film extrusion
 This film are often cast on a roll rather than extruded
into a roll stack
 Main components are extruder, the film die, the chill
roll unit, thickness gauging system, the surface
treatment unit and winder
 The film is extruded downward on the chill. Initial
contact between the film and chill roll is established
by the use of an air knife
 The air knife produce a thin steam of high velocity air
across the width of the chillroll; the air steam pushes
the film against the roll surface.

111. Co extrusion
 Fig ?
 Video?

112. Co extrusion
 It is a commonly used technique to combine two or
more plastic passing through a single extrusion die.
 Types: 1. Feed block co extrusion sheet die
2. Multi manifold co extrusion sheet die

113. Rotational moulding

114. Rotational moulding
video1
video2

115. Rotational moulding
 It is the only operaion in plastic industry by which
relatively stress free, one piece, hollow items of uniform
wall thickness can be produced in a single operation.
 Four steps: 1. loading 2. heating 3. cooling 4. unloading

116. The End