Q 6

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Q 6

• PLASTICS

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Short History of Plastics

• 1862 first synthetic plastic
• 1866 Celluloid
• 1891 Rayon
• 1907 Bakelite
• 1913 Cellophane
• 1926 PVC
• 1933 Polyethylene
• 1938 Teflon
• 1939 Nylon stockings
• 1957 velcro
• 1967 The Graduate

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• Any solid substance that can be moulded into a
  new shape under pressure is said to be plastic.
• When referring to plastics we talk about
  polymers, mainly because plastic is a state,
  condition, property.
• e.g. metals when heated to a certain temp are
  said to be plastic and certain polymers when
  moulded are no longer plastic after moulding.
• Plastics can be any colour and can be moulded
  into complex shapes.

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• Numerous items such as household equipment, toys,
  building materials, decorative objects,
  containers, packaging, aircraft parts, motor car
  parts, safety screens, etc are made form
  plastics.

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• Plastics can have a wide range of properties
  e.g. plastic grill on front of motor car must be
  decorative and tough where as a plastic drinking
  cup must be able to withstand temps up to 100
  degrees
• Plastics can be hard or soft, transparent or
  opaque, rigid flexible. They are poor conductors
  of heat and electricity. Plastics are suitable
  for insulation

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TYPES OF PLASTICS

• Plastics are divided into three main categories
  
• 1.) Natural plastics
• 2.) Modified Natural Plastics
• 3.) Synthetic Plastics

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Natural Plastics

• Decorative objects have been made as far back as
  the 1860s form natural plastics. Items such as
  spoons, combs, Knife and tool handles, horn cups
  etc.
• The most popular natural plastics are
• AMBER
• ANIMAL HORN
• SHELLAC
• NATURAL RUBBER

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Amber

• Amber is a resinous substance which oozes from
  pine trees and solidifies. It is fairly soft and
  can be carved into shape and polished.
• Pine trees that produce amber usually grow in the
  Baltic region

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Animal Horn

• Animal horn is composed completely of keratin.
• This is a fibrous protein which is found in
  the outer layers of the skin, finger nails, hair
  and animal horns.

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Natural Rubber

• Natural rubber is a cream coloured elastic
  material extracted from the rubber tree. It is
  more often referred to as a Natural polymer.

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Shellac

• Shellac is a resinous substance. It is produce by
  the parasitic insects which live on trees,
  generally in India. Shellac was once important as
  an adhesive and wood polish.

• Lact insect

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How Plastics are Classified
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Introduction

• We use plastics daily, anything from plastic
  garbage bags to compact discs to the cars we
  drive incorporate some type of polymer.
• Most common material used is production of
  polymers is petroleum. (oil)
• These materials contain the basic elements that
  are used in forming polymers...carbon, hydrogen,
  oxygen, nitrogen, chlorine, and fluorine.

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CARBON

• Considered to be the backbone of
  polymerization.
• A general model of a monomer is below

H            H               C 
C                 H          "X"
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Polymerization

• Polymers are macromolecules because smaller
  molecules are joined together to form polymer
  chains.
• This process is called polymerization.
• For a monomer to polymerize, it must be capable
  of forming at least two covalent bonds,  one on
  the front, one on the back.

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Network Structure

• Some of these monomers are poly functional,
  meaning that three or more bonds can be formed,
  resulting in a network.
•   This network structure determines the
  properties of the material.
•   There are two classifications of
  polymerization addition and condensation, with
  addition being the most common.
•   During the addition polymerization process
  atoms add onto one another to form the polymer
  chains.
•   No atomic change takes place, atoms simply
  connect to the "arms" of their neighbor. 

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Branched Polymers

• Structural branching can be achieved by causing
  a macromolecule to grow at several locations
  rather than just at the ends.
•   These polymers will not  pack together as
  tightly as linear polymers.
•   An example of a branched polymer is low density
  polyethylene however high-density polyethylene
  remains essentially linear.

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Thermoplastics

• A polymer formed through the addition process
  is called a thermoplastic material.
• Properties of thermoplastics in general are
  that they become soft when heated and harder when
  cooled.
•   These materials have no strong bonds between
  individual molecules and can be softened by heat
  and remolded.
•   Thermoplastics are recyclable materials.

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Cross Linking
Thermoplastics may have varying degrees of cross
linking ranging from none to heavy.
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ADDITION POLYMERISATION

• Adding together of large number of mers so that
  they form into long CHAIN-LIKE molecule
• Example Polymerisation of Ethylene
• Catalyst added to Ethylene molecule.

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• Weaker Bond is broken down, leaving Carbon atom
  free to link up with other carbon atoms
• Weak Bond is attacked by a Free Radical
• Element.

Chain stopper
Free radical

• Weak Bond contains two electrons,
• one which is attacked by radical other is left.

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• New molecule behaves like a radical and the
  process is repeated continuously.
• Several thousand molecules can be joined together
  this way in a single second until eventually a
  chain stopper is incorporated which terminates
  the process

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• The result is that the pot is now full of a large
  number of interwoven chain molecules, twisted
  around each other like spaghetti.
• Where these chains touch off each other Secondary
  Bonds are formed. Therefore the Secondary Bonds
  provide the three dimensional structure. These
  bonds are much weaker than Primary Bonds, and are
  susceptible to heat.

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CONDENSATION POLYMERISATION

• Used to produce Thermosetting Plastics
• It forms a Strong Primary Bond with cross-linking
  between chains
• Two monomers react chemically to form a new
  molecule with water eliminated as a by-product
• The polymer produced cannot be re-softened, has
  high tensile strength and high melting point

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Phenol Formaldehyde is an example
Phenol
Phenol
Formaldehyde
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Phenol Formaldehyde

• Each of the Phenol molecules give up a Hydrogen
  atom, and the Formaldehyde molecule gives up an
  Oxygen atom, and these join to form water.

WATER
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CO-POLYMERISATION

• This type of polymerisation involves adding
  together different types of Mers.
• This allows many different polymers to be
  manufactured
• Example Polyvinylchloride, this is a
  combination of PVC and POLYETHYLENE

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CO-POLYMERISATION
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THERMOPLASTICS THERMOSETTING
Chemical Bonding CovalentBonding Secondary Bonding Weak Van Der Walls Forces ii) Polymerisation Process Addition Polymerisation Chemical Bonding Covalent Bonding Primary Bonding Strong 3-D structure held together by strong rigid cross links ii) Polymerisation Process Condensation Polymerisation
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THERMOPLASTIC THERMOSETTING
iii) Internal Structure Linear Branched Chains iv) Properties Low Melting Point Allows for easy moulding Easily disrupted by heat Low tensile strength Branched structures have higher tensile strength than linear Ideal for recycling iii) Internal Structure Cross Linked Structure iv) Properties High Melting point High tensile strength Good thermal insulation Can withstand high temperatures without loosing rigidity Stiff and less flexible
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Thermosetting Plastics
Primary covalent bonding giving 3-D
structure. Internal structure is
cross-linked. They are rigid, strong,
inflexible and cannot be remoulded. High
melting point.
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Thermo Plastics

• Bonded by weak van-der-waals forces between
  adjacent chains.
• Internal structure can be linear or branched.
• They are soft,

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• Polyethylene terephthalate (PET) used in bottles,
  carpets and food packaging
• Polypropylene (PP) used in food containers,
  battery cases, bottle crates, automotive parts
  and fibres
• Polystyrene (PS) used in dairy product
  containers, tape cassettes, cups and plates
• Polyvinyl chloride (PVC) used in window frames,
  flooring, bottles, packaging film, cable
  insulation, credit cards and medical products.

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Compare
Thermoplastic
Thermoset
Elastomer (Rubber)
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Additives

• Catalysts plus the application of heat and
  pressure are used to accelerate reactions between
  substances.
• Other agents are added to achieve desired
  properties of the material.
• Some of those include plasticizers, fillers,
  reinforcing agents, and stabilizers.

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COMMON TERMINOLOGY

• LINEAR STRUCTURE this type of chain results
  from the process of Addition Polymerisation.
• The chain molecules are bonded together along
  their length by weak Vander walls Forces
• Bonds are so weak heat and high pressure can
  overcome them
• Polymer material with Linear structures have a
  low melting point and tensile stength. ie
  Elastomers

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• BRANCHED CHAINS this type of chain results from
  addition ploymerisation.
• Weak Van Der Walls forces bond the molecular
  chains together along their length
• Side Branching means each chain has greater
  surface area and More Van der Walls forces
• The cumulative effect of these Van der Wall
  forces gives a stiffer, stronger chain structure
  with a higher melting point. I.e. Polyethylene

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• CROSSLINKING - In condensation, a strong rigid
  3-D network is formed by primary covalent bonds
  between adjacent chains.
• These bond and are called crosslinks and give the
  polymer higher tensile strength, rigidity and
  resistance to heat.
• Thermosetting plastics are formed due to
  crosslinking.

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• FILLER These are materials that are added to
  polymers to increase their volume and strength
• GRP Glass reinforced plastic known as Fibre
  glass consists of a polymer mixed with glass
  fibres. This increases the tensile strength of
  the material
• PIGMENTS have the function of giving colour to
  the polymer

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• LUBRICANTS make a polymer easier to mould.
  Various types of waxes are used in small amounts
  for this purpose
• PLASTICISERS are added to polymers to improve
  their flexibility. They achieve this by altering
  the forces of attraction between molecules of the
  polymer
• NATURAL RUBBER have linear chain molecules
  which are folded in nature. They are both plastic
  and elastic

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NATURAL RUBBER

• This is the sap from the rubber tree. Its polymer
  chains are of a folded nature and are bonded by
  weak Van-der-Walls forces.
• Natural rubber is both plastic and elastic.

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SYNTHETIC RUBBER

• These are natural rubbers which are processed
  with sulphur. This is called VULCANISATION.
• Vulcanised rubber have folded chains, which are
  cross-linked. This causes the chains to be
  permanently bonded.
• They are more durable and less flexible than
  natural rubbers

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AMORRPHOUS POLYMER

• These structures are random and disorganised.
  They have a low tensile strength and melting
  point.
• Example - Thermoplastics

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CRYSTALLINE POLYMER

• This structure is arranged in a regular geometric
  pattern. This gives a strong rigid structure with
  high tensile strength

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VAN DER WALLS FORCES

• These are bonding forces between polymer chains
  as a result of addition polymerisation.
• They are weak secondary bonds and are easily
  broken by heat and pressure.

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ELASTOMERS

• A group of polymers consisting of linear chains
  that are coiled, entangled and are subject to
  minimal cross-linking.
• This irregular internal structure and bonding
  arrangement allows these materials to be very
  elastic at room temperature.

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• ELASTIC MEMORY This is the ability of a
  thermoplastic to return to its original shape
  when heated
• MONOMER - This is a single polymer unit. Polymers
  are made from many mers.
• CATALYST a substance that speeds up a chemical
  reaction. ie salt in boiling water
• STABILISER helps to prevent the degrading
  effects of heat, ultraviolet light on the polymer
• PROMOTERS are similar to catalysts in that they
  speed up a chemical reaction
• INHIBITOR prohibits a reaction from occur

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CO-POLYMER

• This is a polymer formed when two different mers
  are linked together in the same polymer chain.
• This new polymer may have a mixture of new
  improve properties, it is similar to alloying in
  metals.

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PARISON

• In BLOW MOULDING, a tube called a parison is
  extruded between the two halves of a split mould.
  The mould closes around the parison and air is
  blown into it at either end, forcing it out
  against the wall of the mould.The component is
  allowed to cool before being removed from the
  mould. Using this process, thermoplastic
  materials can be moulded into bottles and drums.

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EXAMPLE PARISON
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Reinforcing Agents

• Added to improve mechanical properties and
  increase strength, impact resistance, stiffness,
  and hardness.
• Various types of fibers are used as reinforcing
  agents.
• These fibers may include glass fibers, carbon
  fibers, or even metal fibers.

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Composites
Short Fibers
Long Fibers
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Plasticizers

• They lower the glass temperature.
•   Glass is quite brittle and thus the glass
  temperature is very important for changing the
  properties of some materials.

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Fillers

• Polymer filler materials
• These additives control the mechanical
  properties, such as material strength, of the
  polymer.
• They reduce the amount of expensive polymer used.
• Fillers such as chalk, wood flour and glass
  fibre can be used.

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Stabilizers

• Added to maintain the integrity of plastic
  during forming and service.
•   These agents may be as simple as carbon black
  to prevent degradation of strength through
  reaction to ultraviolet light.
•   Sometimes, lead compounds are added to increase
  or stabilize the material against weathering, if
  used in an outdoor product

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Extenders

• Organic materials such as oils or waxes.
•   Extenders are added to reduce the volume of
  plastic material needed per unit area required.

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Molecular Arrangement and Structural Properties

• Molecular arrangement determines the size and
  weight of the plastic molecule.
• As molecular size and weight increases, it
  affects several mechanical properties.
•   Among those most affected are strength,
  stiffness, and hardness.
•   Molecular arrangement also affects the
  viscosity of the plastic.  This property is very
  important for processes that involve molding.

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Two Types of Structure

• Two general types of structure that relate to
  properties of polymers.
• Crystalline and amorphous
•   Crystalline thermoplastics is much higher as
  temperature increases compared to amorphous

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Crystalline Structure

• The structure is arranged in a regular
  geometrical pattern.
• This gives a strong rigid 3-dimensional structure
  with high tensile strength and melting point, as
  in thermosetting plastics

Examples  Nylon, polypropylene,.
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Amorphous Structure

• Here the chain structure is random and
  disorganised.
• This structure has a lower tensile strength and
  melting point

Examples  Polystyrene, acrylic, polyvinyl
chloride.
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General Characteristics and Properties Electrical
resistance

• Primarily due to covalent bonding.
• Locks the electrons into relatively fixed
  positions. 
• Electrical conductivity increases with the
  hydroscopic materials.
• Some plastics absorb and hold water more
  easily.
• In this case ionic conduction may be present.

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General Characteristics and Properties Thermal
Conductivity

• Plastics have very low thermal properties and
  are relatively good insulators against heat and
  cold.
•   Since electrons are closely bound by covalent
  bonds, and there are larger spaces between atoms,
  heat transfer is inhibited.
•   Voids are often present and create insulating 
  "gas" pockets.
• Example Insulation in walls.

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Density

• Plastics exhibit very low densities unless
  heavier reinforcing materials are added.
•   Typically, commercial plastics range from about
  .3 to .75 pounds per cubic foot.
•   

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Corrosion Resistance

• Excellent tolerance to corrosion.
•   Large size of plastic molecules prevent
  transitions into solution.
•   Plastics are subject to a special type of
  corrosion called swelling.
•   This happens due to the large molecules
  allowing solvent agents (like grease and oil) to
  penetrate.
• Example outside furniture

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Manufacturing Processes and Materials

• Polymer Processing

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The following processes are used when working
with plastics
Injection Molding Compression Molding Transfer
Molding Rotational Molding Extrusion Blow
Molding     
Blown film extrusion Thermoforming Calendaring
Fibering Foaming Laminating
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Introduction

• Polymers can be made into various shapes and
  forms including resins, powders, granules, or
  sheets

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Injection Molding

• Plastic granules of the thermoplastic materials
  are fed into the hopper.
• A plunger forces the plastic along the machine
• barrel where they are melted by the heaters and
  compacted by the torpedo.
• The softened polymer is then forced into the
  mould where it solidifies.
• The mould is opened and the plastic product is
  ejected.

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APPLICATIONS

• GOLF TEES
• MILK CRATES

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Extrusion

• This process is used to produce items of uniform
  profile such as curtain rails and plumbing pipes.
• Plastic granules are fed from a hopper through a
  die by a rotating screw.
• The plastic is heated in the chamber before it
  enters the die and cooled by air jets or water as
  it leaves the die.

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Extrusion Cont.

• The extruded products can be cut into lengths or
  coiled.
• Polythene, PVC and nylon are commonly extruded.

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APPLICATIONS

• Products of of uniform profile such as curtain
  rails and plumbing pipes etc

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Making Fiber Optic Cable
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Compression Molding

• This process is suitable for thermosetting
  plastics. It uses a split mould formed to the
  shape of the object to be moulded.
• The combination of heat and pressure allows a
  measured amount of polymer to be shaped.
• The polymer can be in powder or slug form..

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Compression Molding

• As the mould closes, the application of heat
  triggers the chemical reaction of crosslinking
  and the object sets (curing).
• The mould is opened and the object is removed.
• These mouldings can have a high quality finish
  requiring only the removal of flash.

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Compression Molding a Composite
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APPLICATIONS

• ELECTICAL SOCKETS
• MICROWAVE DISHES

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Transfer Molding

• The powder is placed in a compartment above the
  mould where it is heated.
• A plunger forces the molten polymer into the
  mould cavity where the polymer takes the shape of
  the cavity and solidifies.
• It can be used to produce thermosetting objects
  with high definition such as plug tops.

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APPLICATIONS

• Car Doors
• Baths
• Boats

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Blow Molding

• An extruded thick-walled tube, called a parison,
  is placed in the mould.
• The mould closes and air is blown into the
  parison.
• The parison takes the shape of the mould.

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Blow Molding

• Compare to blowing up a balloon inside a bottle.
• After cooling, the split mold opens and the
  part is ejected.
• Economical method of producing plastic bottles,
  hollow toys, balls. 

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Blow Moulding
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APPLICATIONS

• PLASTIC BOTTLES
• PLASTIC CONTAINERS

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Calendaring

• Continuous lengths of sheets are produced in
  thermoplastics by calendaring.
• The material passes through a series of heated
  rollers to gradually produce the
• desired thickness of material.
• These sheets may then be cut to size or collected
  in a roll.

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APPLICATIONS

• PLASTIC FLOORING

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Foaming

• Plastics can also be produced as foams
• Polyurethane, polyethyelene, and polystyrene
  are some of the foaming type materials.
• Use of these materials is varied...from
  expanded polyethylenes and polystyrene
  (styrofoaming
• materials...such as drinking cups) to insulating
  foams or packing materials to structural foam
  used in construction.
•  

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Foaming

• Elastomer foams (foam rubber) are produced in a
  number of ways including molding and compressed
  extrusion.
•   Products include cushions, flotation devices,
  and many others. 
• Recent advances in foaming and reinforcing
  materials such as kevlar have allowed for
  innovative structural composite materials.

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Kevlar is a composite material used in bullet
proof vests
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Thermoforming

• Primarily for forming thermoplastic sheet stock
  into desired shapes.
• Most common name is vacuum forming.
•   Sheet of thermoplastic is placed over a mold
  and heated.
•   Mold cavity is evacuated of air causing the
  sheet to be pushed into or over the mold by
  atmospheric pressure.
•   Vacuum pressure and heat range varies with type
  and thickness of material being formed.
• Products include disposable packing trays,
  household storage bins,

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APPLICATIONS

• HOUSEHOLD STORAGE TRAYS
• WASTE DISPOSAL BINS

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Laminating

• Thin layers of materials bonded together.
• High strength plastics can be produced by layers
  of paper or cloth coated with resin being bonded
  together.
• Heat and pressure can be used.
•  
• Products produced include kitchen counter tops
  and reinforced insulating tubes.

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Laminated Plastic Kitchen Worktop
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Fiber Drawing

• Modification of extrusion and is used to
  produce synthetic fibers such as rayon, nylon,
  and polyester.
• Extrusion die has a small slit or has
  multi-orifices (called a spinneret).
• Small fibers are extruded through the die and
  then post die conditioned.
•   Conditioning processes may be chemical
  treatments to harden the fiber, dye or coloring
  the material, or mechanical treatments for
  crystallization.

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Fiber Drawing Cont.

• Oriented and crystallized plastic is strong in
  the longitudinal direction weakening the material
  in the lateral direction.
• Since process is for producing fibers, this is
  desirable if the product is made into fabric.

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Pultrusion
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Filament Winding