Plastics

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General Background
 Plastics are useful durable materials
 They have the disadvantage that they don't naturally
decompose
 This poses a major environmental problem
 It also poses a major concern for the future
 Plastics can be classified into two groups
 Thermosetting plastics
 Thermoplastics
Thermosetting plastics
 Is a plastic that has been heated to form it into shape
 When the plastic cools and sets it cannot be
resoftened again by reheating
 The chemical reaction that takes place when the plastic
is heated cannot be reversed
 Thermosetting plastics can only be set once
 Thermoplastics are used where an item must
withstand high temperatures
 Ashtrays and saucepan handles
 Polyurethane and polyester are thermosetting plastics
Thermoplastics
 \Can be softened and reshaped by re-heating again
and again
 There are many types of thermoplastics and each is
softened at a different temperature
 Some thermoplastics can withstand temperatures over
100°C
 Most will become soft at temperatures lower than 100°C
 Acrylic, nylon, polyvinyl chloride (PVC) and polythene
are thermoplastics
Properties of plastics
 The properties of plastic vary
 Some are hard while others are
soft
 Some are brittle while others
are flexible
 Some plastics are resistant to
high temperatures and
chemicals
 Most are good electrical
insulators
 They can be moulded quite
easily into complex shapes
 E.g. A jug kettle must resist high
temperatures, be water resistant
and not conduct electricity
Homework
Natural Plastic: Rubber
 Rubber is a natural plastic
substance
 Natural rubber comes from
the latex of rubber trees
 Latex is a milky fluid that is
drained from the trees and
then processed to from the
rubber we know
 The rubber tree is native of
the forests of the Amazon
Basin in Brazil but grows
wildly in south-east Asia
 Rubber was named when it
was first used to erase pencil
marks in the 18th century
Natural Plastic: Rubber
 Rubber is different from other materials because of its
elasticity
 It will often stretch up to 13 times its own length
 Synthetic rubbers were developed during World War 2
because ti was difficult to obtain rubber from southeast Asia
 A number of synthetic rubbers are now in use, such as
Noeprene (used in wetsuits)
 Other natural resins are used in making plastics
Recap
 What are the two group plastics are separated into?
 What is the difference between them?
 Name a material in each group.
 Give and advantage / disadvantage of plastics.
Learning Objective
 Understand how plastic affects the environment
 How do we work with plastics, cut, file, bend and drill.
Plastics and the Environment
 Plastic continue to have a major
environmental impact on our
environment
 Plastic litter our streets, parks,
fields and beaches, causing danger
to wildlife and nature
 Plastics do not decay or breakdown
as easily as other materials
 We must be careful how we
dispose of our plastics
 Burning plastic releases poisonous
chemicals into the air
 Plastics are being developed which
will breakdown and decay over a
time
 We must dispose properly of all the
plastics we use and recycle them
where possible
Working with plastics
 Plastic available in school and in shops are generally in
solid form (sheets, rolls, lengths, cylindrical or
tubular)
 With a simple number of tools and processes they can
be quite easily worked
 The can be cut using saws files or planes and their
edges can be sanded smooth
Cutting Plastic
 Thin rolls of plastic can be
cut using a sharp knife or
scissors
 The rule is necessary to
obtain a straight edge
 Sheets between 1mm and
10mm thick must be cut
with a saw
 A hacksaw or on a
bandsaw
 When sawing the blade
should be kept at a low
angle to prevent damage
Cutting Plastics
 Polystyrene can be cut on a
special heated wire cutter
(rather like a scroll saw)
 The wire is heated
electrically
 Then the polystyrene is
pressed against the wire
and the heat melts the
plastic giving a clean
cutting action
 This is very useful for
cutting shapes for model
making
Cutting Plastics
 Similarly holes can be cut out using a hot wire rod
 The rod is heated over a flame
 When the piece of metal gets cool it is simply re-heated
 When holding plastics in a vice they can get scratched
 These surfaces can be protected by placing smooth
pieces of wood in the vice
Planing
 Once the workpiece has been
cut or shaped with a saw the
edges must be finished with a
plane or file
 The work is held low in the
vice to secure the piece and
support it while it is being
worked on
 A block plane is most suitable
as its blade is set at a very low
angle, this allows for a slicing
cut
 The plane is held at an angle
to the piece
Filing
 A file can be used to
being a piece down to a
line
 This is done in two
stages
 Firstly the piece is cross
filed down to the line
 Then it is draw filled to
remove file marks

The edge can then be
sanded smooth and finally
polished
Drilling Plastics
 Plastics can be drilled to form holes and to take joints,
screws, nuts, bolts and other fittings
 The ordinary twist drill can be used
 This is the same type of drill bit used to drill holes in
wood or metal
 However the twist drill must be modified slightly to
cut plastic properly
Drilling Plastics
 The tip is re-ground to give a
lower angle to the tip
 This modification is given so
the bit won’t burst through
the thin sheet of plastic
before the cutting edge has
made contact with the face
 When drilling plastic the
speed of the drill bit must be
quite fast to prevent chipping
 However the piece should
drilled slowly (the feed rate)
to allow the drill to cut
properly
Drilling Plastics
 A waste piece of wood should be placed underneath to
prevent the bench or drill press becoming damaged
 Also the waste piece keeps the plastic firm while it is
being drilled
Bending
 Thermoplastics, particularly acrylic can be reheated
with no ill effect
 This allows it to be shaped into straight-forward or
complex patterns by simple but precise heating
 When the thermoplastic cools, it hardens to the
required shape
Bending
 The development of the shape is marked out on the
plastic sheet
 With the strip heated the plastic sheet is heated along
the bend line until the soft, allowing the piece to be
bent
 A mould is used to keep the bends more accurate
 With more complex shapes, an oven is used to heat
and soften the whole piece, so no uneven stresses
build up in the piece
Bending: Avoiding Injury or
mistakes
 Do not over heat the thermoplastic or the surface will
bubble
 Remember to allow for bend in the length of the piece
by adding the thickness of the sheet to the length
 Remember to wear protective gloves and goggles when
handling hot materials
Formers
 A former is the shape around which the plastic is
moulded
 Wood is an excellent material for making formers
 It does not cool the plastic before it has a chance to be
shaped
 Wood is very easy to mould into shape
 Formers and jigs ensure that shapes can be accurately
copied many times
Formers
 Allowances should be
made when bending
thermoplastic as they
tend to spring back
slightly into their
original shape on cooling
 https://www.youtube.co
m/watch?v=aqAtiawy81g
Line Bender/Strip heater
Vacuum Former
Glass reinforced plastic (GRP)
 Polyester resins can be strengthened
with elastic strands of glassfibre
 This material is formed into a
laminate which can then be
moulded
 Carbon fibre can be used to and is a
much stronger form of
reinforcement
 However it is more expensive
 Glass reinforced plastic is widely
used for boat hulls, canoes, car and
bus bodies, moulded seats, and even
tennis and squash racquets
 It is an extremely strong, tough,
durable and hard-wearing
material
Homework
Homework
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