Plastics

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

Homework

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

Homework

Homework