Processing of polymer and plastics
Department Of Mechanical Engineering
National Institute Of Technology, Durgapur
Raghvendra Kumar P andey
Under the supervision of
Prof . A.B. Puri
Prof. J. Dey
Classification on polymers
Polymers synthesis and types of
Mechanical behavior of polymers
Types o polymer forming
Characteristics and typical application
The word polymer is originated from Greek word meros which means unit.
Monomer = single unit and polymer =many units joined together
Polymers are basically organic compounds but few are inorganic too.
Characteristics of polymers :- Low temperature stability
- Low hardness
- Low mechanical strength
- High elongation under application of stress
- Low thermal and electrical conductivities
- High sensitivity of properties to their morphology
• 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.
• Thermoplasts have the property of increasing plasticity, i.e., increasing ability to deform plastically with increasing
• E.g.: Acrylics, PVC, Nylons, Perspex glass, etc.
Thermosetting plastics:• 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 inter-winding.
• Characterized by high modulus / rigidity / dimensional stability when compared with Thermoplasts.
• E.g.: Epoxies, Amino resins, some polyester resins, etc.
• These polymers are known for their high elongations, which are reversible upon release of applied loads.
• They consist of coil-like molecular chains, which straighten up on application of load.
• Characterized by low modulus / rigidity / strength, but high toughness.
• E.g.: natural and synthetic rubber
• Processing of polymers primarily limits to synthesis followed by forming.
• Polymers are synthesized by process known as polymerization.
• Polymerization is process in which multi-functional monomers are attached to form linear/3-D macro molecular
• Involves single kind of monomer
• Three stages i.e. initiation (e.g., benzoyl peroxide) , propagation and termination
• As molecular chain grows longer, reaction rate decreases. However the growth process is terminated either by the
combination or disproportionation process.
• E.g., polyethylene
• Involves more than one monomer
• Also known as step growth polymerization
• Unpredicted chemical formula
• Reaction time is longer as compare to addition polymerization
• E.g., Formation of a polyester from Ethylene glycol and Adipic acid
Formation of a polyester from Ethylene glycol and Adipic acid:-
Degree of polymerization
• Extant of polymerization is characterized in terms of ‘degree of polymerization’.
• It is defined as number of mer units in molecular chain or ratio of average molecular weight of polymer
to molecular weight of repeat unit.
• Average molecular weight is defined in two ways: Weight average molecular weight (based on weight
fraction) and Number average molecular weight (based on the number fraction).
• Number average molecular weight is always smaller than the weight average molecular weight.
Mechanical behavior of polymers
1) Parameters like temperature, strain rate and morphology of polymers has strong influence on
2) Mechanical property changes dramatically with temperature, going from glass like brittle at very low
temperature to rubber like behavior at high temperature.
3) Due to cross linkage, recoverable deformation up to high strain also observed in
4) Highly crystalline polymers behaves like brittle whereas amorphous polymers exhibit plastic
5) Tensile modules and strength is very lower than those of metals but elongation can be up to 1000% in
6) Unlike metals, plastics typically possesses glass transition temperature, exhibits prolonged creep, and
tend to degrade ( due to heat) rather than corrode.
7) Plastics impregnated with organic can be subjected to bacterial infestation.
Thermoplasts usually formed above their glass transition temperature.
Thermosets are formed in two stages – making liquid polymer and then moulding it.
Melting of pallets
Injection into the mould.
Crystallization temperature polymer ∝ molecular weight
Glass transition temperature range 0.5 to 0.75 of absolute melting temperature.
Polymer melting and TG depends upon following factors:- chain stiffness (e.g., single vs. double bonds)
- size, shape of side groups
- size of molecule
- side branches
1) Place the appropriate
amount of heated
polymer (say pallet) in
between the male and
female part of mould
2) Punch presses the molten
pallets causing the
viscous plastic flow to
3) If flash present, it is
1. Palletized material is fed
with use of hopper into a
cylinder where material
melts due to heating
2. Molten metal is impelled
through nozzle into the
3. Edges are trimmed if
characteristic of this
mounding process is
cycle time is very less.
i.e. rate of production is
1. This technique is adapted
to produce continuous
length with constant
2. Thermoplastic material is
forced through a die
3. One of the most famous
products of extrusion
moulding is the optical
1. Using this manufacturing
process hollow parts are
made like bottle or sphere
2. Air is blown into a thin
walled plastic cylinder called
the parison. The parison is
formed by melted plastic
material being pushed
through an extruder.
3. When the parison reaches a
certain length, the two
halves of the mould close
around the parison sealing it
at the bottom.
4. Compressed air is then used
to inflate the parison to form
the shape of the cavity inside
1. The molten resin is
transferred by pressure from
a melt pot into a mould
which is at a temperature
above the melt point of the
2. Durable and dimensionally
3. Minimal release of airborne
emissions during production
4. Uniform thickness of parts
5. Prevents corrode or rust
Characteristics and typical application
• Outstanding toughness and
• Resistance to heat distortion
• Soluble in some organic
• Refrigerator lining
• Highway safety devices
• Outstanding light transmission
• Resistance to weathering
• Drafting equipment's
• Transparent aircraft enclosures
3. Polyester (PET or PETE)
• One of the toughest plastic film
• Excellent fatigue and tear
• Resistance to grease, acids and
Magnetic recording tapes
Automotive tire chords
• Good thermal and dimensional
• Relatively inexpensive
• Electrical resistance
• Wall tile
• Battery cases
• Indoor lighting panels
• Chemically resistance and
• Tough and low coefficient of
• Poor resistance to weathering
• Good adhesion
• Corrosion resistance
• Protecting coating
• With fiberglass laminates
• Excellent thermal stability over
150 degree C
• Motor housing
"Better things for better living... through chemistry“
Polymers are being used now smartly in electrical and electronic, packaging, automobile and
construction areas. This sector has potential to increase exponentially through proper planning
Any shaped product is now possible to manufacture through various moulding processes to
The global market for engineering plastics in terms of revenue was estimated to be worth $45.2
billion in 2011 and is expected to reach $76.8 billion by 2017.