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Plastic processing.ppt

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									Plastic processing
              introduction
• Those methods used to convert plastics materials
  in the form of pellets, granules, powders, sheets,
  fluids, or preforms into formed shapes or parts.
  The plastics materials may contain a variety of
  additives which influence the properties as well as
  the process ability of the plastics. After forming,
  the part may be subjected to a variety of ancillary
  operations such as welding, adhesive bonding,
  machining, and surface decorating (painting,
  metalizing).
                     plastics
• Plastics(polymers) are formed when small organic
  molecules are stitched together to form a long chain. This
  process is called polymerization.
• Those organic molecules suitable for polymerization are
  called monomers.
• A monomer is the single building block used in creating the
  polymer.
• It is necessary for a monomer to be at least
  bifunctional(capable of forming two covalent bond) to
  polymerize.
• Some monomers are polyfunctional, capable of forming
  three or more bonds. These can form three-dimensional
  arrangements of chain called a network.
              Polymerization
polymerization is generally of two types:
• Addition polymerization
• Condensation polymerization
• Addition polymerization:
• It consists of breaking the double carbon bond (C=C) in
   bifunctional polymers so that the chain can be formed.
• Addition polymerization to produce thermoplastic materials.
Some plastic materials made by AP-
I. Poly- styrene(CH2=CH-C6H5)
II. Poly -vinyl chloride(CH2=CHCl)
III. Ploy- propylene(CH3CH=CH2)
IV. Poly-methyle methacrylate(CH2=CC3O2H4)
V. Poly-vinyl acetate(CH3COOCH=CH2)
VI. Poly-ethylene(CH2=CH2)
                        Conti…..
• Condensation polymerization:
• In this polymerization, two different organic molecules react to
   form a plastic molecules. The reaction generally results in the
   separation of a small molecules such as H2O as a by-product.
• Typical plastic materials formed by CP:
I. Poly-epoxy(araldite)
II. Poly-phenol formaldehyde(Bakelite)
III. Polyamide(Nylon)
IV. Poly-urathane
                       Additives
A no. of additives are normally used with plastic materials to modify
  their behaviour, improve properties or reduce the overall cost
  thereby increasing the range of application of plastic.
Examples:
• Plasticizers: these are mixed with plastic to improve their flow
  characteristics & decrease their brittleness.
• Fillers: these are inexpensive materials that are added to plastic to
  reduce their cost.
• Flame retardants: these are added to plastic to reduce the
  flammability of plastic by preventing oxygen reaction & charring.
• Reinforcing agent: these are the materials that are specifically
  added to plastic materials to raise their mechanical properties.
• Stabilizers: are used to stabilize the properties of the plastic
  throughout its useful life
• Colourants: are used to give colour to the plastics.
              Plastic materials
                                                  Polymeric materials



                                  plastics                              elastomers



                  thermoplastic                      thermosetting




 Commodity             Engineering             Commodity                  Engineering
  plastics               plastics                plastic                    plastics

Polyethylene           polyamides                 phenolics                 silicones
Polypropylene         acetals                unsaturated polyester          urethanes
Polystyrene,PVC       cellulosics                 ureas                     melamines
                       Conti….
• Thermoplastics: these are the plastic that can be softened &
  melted by heat and can then be formed into the required shape
  when they are hot.
• These materials can be melted a number of times. that means it
  is possible to recycle thermoplastic.
• Frequent remelting is avoided in industry since some chemical
  degradation occurs during remelting.
• Thermoplastic materials tend to consist of long polymer chains
  with little breadth, akin to a two-dimensional structure.
• The fabrication processes used for thermoplastics such as blow
  moulding,& injection moulding etc.
                      Conti……
• Thermosetting materials: these are the plastics that can be
  melted once they are solidified.
• The raw materials for thermosetting materials are usually
  called resins. They are mixed and placed in the mould, and
  heated and compressed during which process the materials
  achieve the strength and hardness.
• polymerization occurs by strong network bonds (cross-linking)
  with the application of heat, pressure and/or time.
• The manufacturing process(compression moulding, transfer
  moulding) used are more expensive compared to those of
  thermoplastic materials.
• These materials are characterized by a three-dimensional
  networks of molecules. These materials can not be recycled.,
  when heated these materials burn and char.
             Extrusion of plastic
1.   It is the process of confining the
     material in a closed cavity and then
     allowing it to flow from only one
     opening so that the metal will take
     the shape of the opening. The
     operation is identical to the
     squeezing of toothpaste out of a
     toothpaste tube.
2.   Extrusion can be used to process
     most thermoplastics.
3.   It is possible to combine a variety of
     resins to gain special physical,
     biological or chemical properties.
4.   The plastic that is generally in the
     form of pellets or granules is fed
     into the extruding machine through
     a hopper.
                        Conti….
There are no. of plastic extrusion processes used in the industry:
• Solid extrusion: this produces a part with a uniform cross
  section.
• Hollow extrusion: this process is used to extrude part that
  have hollow cross-sections such as pipes and tubes.
• Co-extrusion: this is the process of extruding two or more
  materials through a single die with two or more orifices
  arranged so that the extruded from the multiple openings
  merge & weld together into a laminar structure before chilling.
• Advantages:
• practically any cross-section can be easily produced.
• Overall cost of the part produced by extrusion is low.
• The tooling costs are relatively low.
• The equipment used is simple and relatively inexpensive.
                 Injection moulding
•   Injection moulding is similar to pressure
    die casting.
•   In this process, plastic material in a highly
    softened state is forced to flow at high
    pressure through a nozzle into the mould
    cavity, the plastic solidifies in the die and
    then is ejected by opening the die.
•   Injection moulding pressure usually
    ranges from 70MPa to 200MPa.
•   Injection moulding is the most widely
    used plastic processing method. It can be
    used to produce a wide variety of
    products.
•   Complex shape may be produced & size
    may ranges from very small (50gm) to
    very large(25kg).
•   Most polymer may be injection moulded,
    including thermoplastic, fibre-reinforced
    thermoplastics, thermosetting plastic &
    elastomers.
Injection moulds
    Reaction injection moulding

• Reaction injection moulding is the
  process      used     for     moulding
  thermosetting materials such as
  polyurethane and epoxy, which exist
  in liquid form before they polymerize.
• The process was originally developed
  to mould very large automobile parts
  such as bumpers, interiors trim panels
  & spoilers.
• Because of the low pressure used, the
  cost of the mould is much less
  compared to the conventional injection
  moulding.
      Liquid injection moulding
• Liquid injection moulding is used for
  moulding silicone products. In this
  process, pumping systems deliver a two
  part-liquid silicone (catalyst & cross
  linker) directly into a mixer for
  homogenization.
• The mix is then injected directly into
  heated mould cavities in as little as3 to
  10 sec using a relatively low pressure.
• Moulding & vulcanization (curing)
  occur inside the mould cavities within
  10 to 90 sec due to the high mould
  temp.
           Co-injection moulding
• This is the process that uses two
  materials to mould a part. the two
  materials have different quality with
  one being hard that form the skin,
  while the softer one forms the core
  through the injection moulding
  process.
• It requires two injection units for the
  two different plastic being used.
• The skin materials is injected first
  into the mould cavity, & is
  immediately followed by a core
  material.
                    Blow moulding
• Blow moulding is the process of
  inflating a hot ,hollow thermoplastic
  preform or parison inside a closed
  mould so that its shape conforms to
  that of the mould cavity.
• A wide variety of hollow parts,
  including plastic bottles, can be
  produced from many different
  thermoplastic materials using this
  process.
• Typical parts made are bottles, toys,
  air ducts for automobile, chemical &
  gasoline tanks, & a no. of household
  goods.
       Extrusion blow moulding
•   An extrusion blow-moulding machine
    consists of an extruder similar to that
    used with plastic extrusion, which
    softens the plastic & forms it into a
    tube (called a parison or preform)
    through a conventional type die & a
    split body mould.
Steps involved in this process:
• The die closed around the parison,
    sealing both end.
• A blow pin is inserted into the parison
    to inflate it, causing it to expand &
    confirm the shape of the mould cavity.
• The mould is cooled & once the part
    has solidified, the mould opens & the
    part is removed.
It is usually to make items of weight
    greater than 350g such as containers
    for food, laundry or waste.
       Injection blow moulding
• Injection blow moulding is a two-
  stage process with the parison being
  produced in the first stage which
  needs to be transferred to the blow
  mould. Thus the first operation is
  identical to the injection moulding.

• The air is injected into the plastic at
  a pressure between .5 to 1 MPa.

• Injection blow moulding is used to
  achieve     very   accurate     wall
  thickness, high quality neck finish,
  wide mouth openings & to process
  polymers that can not be extruded.
• The usual applications include
  pharmaceutical, cosmetic, single-
  serving liquor bottles that weight
  less than 350g
        Stretch blow moulding
• Stretch blow moulding is best
  known       for     producing    PET
  (polyethylene terephthalate) bottles
  commonly used for water, juice & a
  variety of other products.
• It produces a part with the required
  properties for the work material by
  producing       desirable   molecular
  orientation.
• In this process, a preform or parison
  is elongated mechanically in the
  mould & then expanded radially in
  the blowing process.
• Stretch blow moulding is only used
  for difficult-to-blow crystalline &
  crystalizable polymers such as
  polypropylene & PET.
               thermoforming

• In this process, a thermoplastic sheet can be formed into a 3-D shape
  by the application of heat & differential pressures.

• First, the plastic sheet is clamped to a frame & uniformly heated to
  make it soft & 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.

• It is used for making such parts as covers, displays, blister
  packaging, trays, drinking cup & food packaging.
      Vacuum thermoforming
• Vacuum         forming      is
  thermoforming process that
  forms thermoplastic sheets
  into 3-D shapes through the
  application of heat & vacuum.

• During this process, plastic
  material is heated (170-220
  deg C) until it becomes
  pliable, & then is placed over
  a mould of the requisite shape
  & drawn in by a vacuum until
  takes on the desired shape.

• Typical value for the vacuum
  developed by the vacuum
  pump should be about 35 torr.
    Pressure thermoforming
• Pressure thermoforming is
  an      improvement     over
  vacuum forming, in that it
  utilizes both vacuum and
  compressed air to forced the
  plastic sheet against the
  mould.
• Vacuum pulls on one side of
  the sheet & compressed air
  pushes on the other.
• Reasonably high pressure
  approaching 3.5 MPa are
  used for forming the
  requisite shapes.
                Drape forming
• A thermoforming operation
  with a positive mould (male
  mould). This is termed drape
  forming.

• In the drape forming, the
  sheet is framed & heated, it is
  mechanically stretched, and a
  pressure differential is then
  applied to form the sheet over
  a positive mould.
          Compression moulding
• Compression moulding is the oldest
  plastic-processing method.
• A compression mould is made of
  two halves with one each being
  connected to the platens of the press.

• The mould is electrically heated to
  maintain the required temp.

• The pressure maintained on the
  materials is of the order of 14 to 40
  MPa of moulding area.

• The most widely used plastic is
  phenol formaldehyde’ commonly
  known as ‘Bakelite’
            Transfer moulding
• 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       &
  compressed in a separate
  chamber & then injected into
  the closed mould where it is
  allowed to cool and solidify.
  Plastic production design
• The designer should first determine the most desirable
   moulding process that is to be used for the manufacture. Then
   the designer should design the part taking the best aspects of
   that process into account.
A few of the general guidelines for designing the plastic parts are
   given:
• Wall thickness: walls of plastic parts should be maintained as
   uniform as practically possible.
• Draft: the part needs to be released & ejected from the mould.
   For this purpose, a draft or taper needs to be provided on all
   surfaces that are normal to the parting plane.
• Radii and fillets: it is necessary to provide generous radii &
   fillets from the strength point of view as well as for the flow of
   plastic melt through the mould cavity.
                      Conti…….
• Holes:
The holes sizes & their length need to be considered properly in
  order to provide the necessary rigidity to the core pins.
• Bosses:
The function of the bosses is for locating, mounting & assembly.
• Ribs:
Ribs are often the practical way to increase the stiffness of large
  surfaces. They also help as feeders to allow the material flow
  for isolated bosses.rib thickness should be smaller than the
  wall thickness

								
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