Plastics

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Plastics

• Design and Manufacture

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Plastics the basics

• Natural plastics fossilised tree resin and
  latex
• Form of rubber
• Synthetic plastics Chemically manufactured from
  carbon based materials such as crude oil, coal
  and gas
• Basic chemicals used in creating plastics are
  from coal and crude oil.
• Obtained by using heat and chemical catalysts

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Plastic properties

• Ideal for mass production of quality products
• Light in weight
• Resistance to corrosion
• Low thermal conductivity
• Translucent, transparent and opaque
• Electrically resistant
• Easily formed
• Recyclable

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Forms plastic comes in

• Profiled sheets, rods, tubes and bars
• Moulding compounds
• Thin layers of films and sheets
• Foam
• Casting compounds such as ingots
• Paint, varnish and lacquer used for finishing
• Filaments and fibres
• Composites that contain reinforcing material

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Thermosetting plastics

• Formed by chemical reactions that leave them in a
  relatively fixed state.
• They cannot be softened or reformed when
  reheated.
• Can withstand high temperature, excess heat will
  cause material to decompose.
• Used when products needs resistance to extremes
  in temperature, electrical current, chemicals and
  wear.

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Thermosetting plastics

• Some examples of thermosetting plastics are
• Epoxy Resin ER(Araldite) good adhesive, low
  shrinkage, strong when reinforced. Used in
  bonding, laminating, surface coating, printed
  circuit boards
• Polyester Resin PR(Orel Beetle) stiff, hard,
  brittle, resilient as laminated GPR, formed
  without heat or pressure. Used in panels for car
  bodies and boat hulls, casting and embedding
• Melamine formaldehyde MF(Formica Melaware)
  waterproof, tasteless, odourless, mark and
  scratch resistant. Used in worktops, tableware,
  buttons, electrical insulation.
• Urea Formaldehyde UF(Aerolyte Cascamite) good
  adhesive, stiff, hard, brittle, good electrical
  insulator. Used in electrical fittings, paper
  and textile coating, wood adhesive.
• Phenol Formaldehyde PF (phenol methanal) Hard,
  brittle plastic with dark colour, glossy finish.
  Resists heat. Used in parts for domestic
  appliances, bottles tops, kettle/iron/saucepan
  handles.

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Thermoplastics

• Softens when heated, hardens when cooled down
• Can be repeated indefinitely
• When soft they are easily formed under pressure.
• PLASTIC MEMORY this is the property of the
  thermoplastic returning to its original shape
  when softened after being shaped.

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thermoplastic

• Polyethene HDPE High density, tough, resists
  chemicals, feels waxy. Used for rigid objects
  buckets, bowls, sterilised containers.
• Polyethene LDPE Low density, soft and pliable,
  electrical insulator. Used for flexible objects
  bags, bottles, cable, sheathing, toys.
• Polypropene PP rigid, light, good chemical
  resistance, resistance to fatigue, bending. Used
  for crates, seats, string, rope, medical
  equipment, hinges, kitchenware, film.
• Polystyrene PS light, buoyant, stiff,
  water/weather resistant. Used for packaging,
  containers, insulation and toys.
• Polyvinyl chloride UPVC rigid, abrasive
  resistance, water/weather resistant. Used for
  piped, gutters, bottles, roofing, window frames.
• Poly-methacrylate PMMA (Acrylic/Perspex) Stiff,
  hard, clear, durable, scratches easily, easily
  machined and polished. Used for signs, lighting,
  reflectors/lenses cases, jewellery.
• Nylon Hard material, good resistance to wear
  and tear. Solid nylon has low friction qualities
  and high melting point. Used in curtain rail
  fittings, combs, clothes and gear wheels.

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elastomers

• Particular branch of thermoplastics
• Has elastic properties
• Similar to rubber in how it reacts to pressure
  and their texture
• Withstands large amounts of deformation
• Can be stretched several times to their length.
• Uses
• Waterproof seals
• Diaphragms for mechanical purposes
• Flexible handles
• Sportswear
• Foam padding

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Composite materials

• Consists of two or more substances
• Combines to produce properties that no single
  substance can achieve.
• One component forms the matrix (base) while the
  other provides reinforcement
• Properties are controlled by size and
  distribution of reinforcing substance.

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Example of composite material

• Using carbon fibres embedded in Resin
• Carbon-fibre reinforced plastic CFRP
• Properties are
• Low in density
• Light in wieght
• High tensile strength
• Better corrosion resistance and fatigue
  performance than most metal alloys
• Used in aerospace industry and car frameworks.

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Plastic processing

• Plastics part 2

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Injection moulding

• Allows large quantities of plastic components to
  be made quickly
• Most important industrial process in mass
  production of plastic goods.
• Costs of machining original moulds are very high
  therefore necessary to sell large numbers of
  products to recover costs.
• There are a few features which help identify this
  process
• Sprue marks
• Draw angles
• Mould split lines
• Ejection pin marks
• Webs used for strength
• Variation of finishing techniques from high
  quality shine to texture

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Injection Moulding

• Granules of plastic powder are poured or fed into
  a hopper which stores it until it is needed.
• 2. A heater heats up the tube and when it reaches
  a high temperature a screw thread starts turning.
  
• 3. A motor turns a thread which pushes the
  granules along the heater section which melts
  then into a liquid.
• 4. The liquid is forced into a mould where it
  cools into the shape (in this case a sphere).
• 5. The mould then opens and the sphere is removed.

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Injection moulding (uses)

• High volume, automated industrial manufacturing
  process.
• Golf tees
• Spoons
• Wash basins
• Buckets
• Product casings

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extrusion

• Used for products with long uniform cross
  sections.
• Variety of metals and thermo-plastics are suited
  to the extrusion process.
• Plastic is easier as it requires less force
• Polythene, PVC, polypropylene
• Lead, copper, brass, bronze, aluminium, steel
• Features that help identify this process
• smooth-walled long sections
• Line texturisation

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Extrusion
A machine is used to extrude materials which is
very similar to the injection moulding. A motor
turns a thread which feeds granules of plastic
through a heater. The granules melt into a
liquid which is forced through a die, forming a
long 'tube like' shape. The extrusion is then
cooled and forms a solid shape. The shape of the
die determines the shape of the tube.
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Extrusion (Uses)

• Curtain rails
• Drainpipes
• Electric cable sheathing
• Fluorescent light covers
• Hose pipes

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Blow moulding

• Several variations to the process, producing
  various sizes
• Finish is not as high a standard as injection
  moulding
• Good mould allows
• Uniform thickness
• Complex shape
• Good quality of production
• Fast process
• Little waste
• Features that allow for identification
• Mould separation lines
• Flash lines
• Any shape of bottle or container
• Transparent, opaque or coloured

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Blow moulding
The process is similar to injection moulding and
extrusion. 1. The plastic is fed in granular
form into a 'hopper' that stores it. 2. A large
thread is turned by a motor which feeds the
granules through a heated section. 3. In this
heated section the granules melt and become a
liquid and the liquid is fed into a mould. 4.
Air is forced into the mould which forces the
plastic to the sides, giving the shape of the
bottle. 5.  The mould is then cooled and is
removed.
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Blow moulding (uses)

• Variety of mass produced bottles and liquid
  containers, using variety of polymers and
  laminates.

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Compression moulding

• Used for shaping thermosetting plastics
• These are strong and brittle but have poorer
  impact resistance than thermoplastics
• Features that identify compression moulding
• Walls of uniform thickness 3-6mm
• Draft 1 minimum
• Flashes on edges
• Quality finish on female mould
• Plastic used can resist temperature increases.

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Mould through compression
The stages are as follows 1. The mould is
heated. 2. A 'slug' or piece of the plastic is
placed into the mould and warms up. 3. The
hydraulic press begins to move down when the
plastic has reached the correct temperature.. 4.
As the upper and lower mould meet, the plastic is
compressed into the shape of the mould. 5. The
upper mould moves upwards and the plastic piece
(in this case a dish) is removed.
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Compression moulding (uses)

• Automotive distributor caps (cars)
• Camera cases
• Electrical wall sockets
• Handles
• Door knobs
• Light switches

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Vacuum forming

• Widely used in industrial processes to mould
  plastic.
• No need for expensive moulds or dies
• Moulds generally used from wood/ steel/ aluminum
• Process is over in a few minutes
• Thermoplastics are suited for this process
• Features that identify this process
• Thin sheets normally used
• Patterns and textures are transformed from the
  mould to the product surface
• Venting holes cause pips on surface
• Tapers are pronounced
• Thinning on side surfaces

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Vacuum forming
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Vacuum forming (uses)

• Packaging items that have complex shapes
• Trays
• Dishes
• Margarine tubs
• Toys
• Light panels

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Rotational moulding

• Produces closed and seam free components made
  from plastic polymers
• Products produced are hollow, stress free and
  come in a wide range of shapes and sizes.
• Moulds made from steel
• copper coated for better surface detail
• Aluminium for better heat transfer
• Electroformed nickel for best surface detail.
• Moulds rotates several axes with speed dependent
  on production requirements.
• Large moulds have air vents to avoid distorting
• Suited to small production runs used for
  prototypes

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Rotational moulding (process)

• Measured quantity of plastic power is added to
  mould
• Mould is closed and rotated slowly around 2/3
  axes. Allows distribution over hot surface
• Plastic melts on contact and builds up even
  coating on inside surface of mould
• Mould surface is cooled, plastic retains its
  shape
• Mould opens and product is removed.

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Rotational moulding (uses)

• Plastic toys
• Play equipment
• Road markers
• Buoys
• Large tanks (10,000litres)

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Other processes

• Calendering Thermoplastics are squeezed between
  hot rollers forming thin sheets or film
• Casting useful for thermosetting plastics.
  Molten plastic poured into mould
• Bending thermoplastics heated along a line using
  strip heater, folded to desired angle. Bending
  jig sometimes used to hold in place.
• Fabrication joining together using a variation
  of fixings and adhesives
• Coating powered thermoplastic melts on surface
  of heated product.(dip coating)
• Forming Layers o glass fibre matting and
  polyester resin formed over a mould. Resin cures
  it hardens giving rigidity to fibre/resin matrix.
  Thermoplastic formed by heating whole sheet till
  soft then pressed between two formers.