Plastics General Background Plastics are useful durable materials

Plastics

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

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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 18 th 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 1 mm and 10 mm 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 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 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=aq. Atiawy 81 g 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

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