General Background Plastics are useful durable materials They have the disadvantage that they don't...
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Transcript of General Background Plastics are useful durable materials They have the disadvantage that they don't...
General BackgroundPlastics are useful durable materialsThey have the disadvantage that they don't
naturally decomposeThis poses a major environmental problemIt also poses a major concern for the future
Plastics can be classified into two groupsThermosetting plasticsThermoplastics
Thermosetting plasticsIs a plastic that has been heated to form it into
shapeWhen the plastic cools and sets it cannot be
resoftened again by reheatingThe chemical reaction that takes place when the
plastic is heated cannot be reversedThermosetting 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 againThere are many types of thermoplastics and
each is softened at a different temperatureSome thermoplastics can withstand
temperatures over 100°CMost will become soft at temperatures lower
than 100°CAcrylic, nylon, polyvinyl chloride (PVC) and
polythene are thermoplastics
Properties of plasticsThe 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
Natural Plastic: RubberRubber is a natural plastic
substanceNatural rubber comes from
the latex of rubber treesLatex is a milky fluid that is
drained from the trees and then processed to from the rubber we knowThe 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: RubberRubber is different from other materials
because of its elasticityIt will often stretch up to 13 times its own
lengthSynthetic rubbers were developed during
World War 2 because ti was difficult to obtain rubber from south-east Asia
A number of synthetic rubbers are now in use, such as Noeprene (used in wetsuits) Other natural resins are used in making plastics
RecapWhat 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 ObjectiveUnderstand 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 plasticsPlastic 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 PlasticThin 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
bandsawWhen sawing the blade
should be kept at a low angle to prevent damage
Cutting PlasticsPolystyrene can be cut on
a special heated wire cutter (rather like a scroll saw)The wire is heated
electricallyThen the polystyrene is
pressed against the wire and the heat melts the plastic giving a clean cutting actionThis is very useful for
cutting shapes for model making
Cutting PlasticsSimilarly holes can be cut out using a hot
wire rodThe rod is heated over a flameWhen the piece of metal gets cool it is simply
re-heatedWhen holding plastics in a vice they can get
scratchedThese surfaces can be protected by placing
smooth pieces of wood in the vice
PlaningOnce 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
FilingA file can be used to
being a piece down to a line
This is done in two stagesFirstly 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 PlasticsPlastics can be drilled to form holes and to
take joints, screws, nuts, bolts and other fittings
The ordinary twist drill can be usedThis is the same type of drill bit used to drill
holes in wood or metalHowever the twist drill must be modified
slightly to cut plastic properly
Drilling PlasticsThe tip is re-ground to give
a lower angle to the tipThis 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 PlasticsA 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
BendingThermoplastics, particularly acrylic can be
reheated with no ill effectThis 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
BendingThe development of the shape is marked out
on the plastic sheetWith the strip heated the plastic sheet is
heated along the bend line until the soft, allowing the piece to be bentA mould is used to keep the bends more
accurateWith 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 mistakesDo not over heat the thermoplastic or the
surface will bubbleRemember 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
FormersA former is the shape around which the
plastic is mouldedWood is an excellent material for making
formersIt does not cool the plastic before it has a
chance to be shapedWood is very easy to mould into shape
Formers and jigs ensure that shapes can be accurately copied many times
FormersAllowances should
be made when bending thermoplastic as they tend to spring back slightly into their original shape on cooling
https://www.youtube.com/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