Transcript 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
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 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 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
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