Unit 2: Plastics

Index 1. What Are Plastics? 8. Elastomers. 2. What Are Plastics Made Of? 9. Textile Fibers. 3. Obtaining Plastics. 10. Fabrication of Plastics: Techniques. 4. Properties Of Plastics. 11. Polymerization & Recycling. 5. Types of Plastics. 6. Thermoplastics. 7. Thermosettings. 12.Work With Plastics In The Workshop.

What Are Plastics? The term plastic is used to designate some synthetic materials obtained from fossil fuels, which, in general, are flexible, resistant, light and insulators. The plastics are also called polymers and they are mainly compound by oxigen, hydrogen, carbon, nitrogen and sulfur. Plastics are malleable and ductile, and they have a great plasticity which gives them their name.

What Are Plastics Made Of? The fossil fuels (oil, coal and natural gas) are formed in a period of millions of years, by the accumulation of the remains of living beings under the Earth's crust. Polymers are obtained from the multiple addition of the molecules from the fossil fuels.

Obtaining Plastics 1.Obtainig the raw materials. 2.Synthesizing the monomers. 3.Linking the monomers to obtain the polymers (polymerization).

Obtaining Plastics: Polymerization In this process a monomer is introduced in a machine called reactor with a solvent and a catalyst, at a specific pressure and temperature. During the chemical reaction, the monomers are linked forming macro-molecules, the polymers.

Obtaining Plastics 4. Making the pellets. 5. Adding the polymers some additives to modify their properties. 6. Heating them to get the form desired. (conformation).

Properties Of Plastics (I) 1. Plastics are very bad conductors of electricity and heat. They're insulators. 2. They are combustible i.e. They can burn easily. 3. They can be combined with other materials and sustances, for example pigments. 4. Plastics are non-toxic and non-corrosive.

Properties Of Plastics (II) 5. Plastics have a high mechanical strength in relation to its extreme lightness. 6. They have, in general, a great elasticity. 7. They can be moulded when you heat them, so they have plasticity. 8. Plastics are versatile materials and can be processed easily. 9. They are relatively cheap

Types of Plastics The plastics can be clasified in three main groups: Thermoplastics. Thermosettings. Elastomers.

Thermoplastics Thermoplastics can be heated, moulded and shaped various ways, lots of times. They are softer and less resistant than thermosettings. Thermoplastics are, for example, PET (water bottles), PVC (pipes) & Polyethene (plastic bags).

Thermosettings They are harder and resist higher temperatures than thermoplastics. They're not recyclable heating them again. Thermosetting plastics are, for example, Amines (boat hulls), Polyester Resins (ski) & Phenols (cookware handles).

Elastomers Elastomers are characterized by their elasticity. They can't be recycled. They don't resist high temperatures. Elastomers are, for example, Synthetic Rubber (ballons) & Neoprene (wetsuits).

Textile Fibers Textile fibers are threads which can twisted, stained and weaved to make fabrics. They are divided into two groups: 1 Synthetic (or man-made) Fibers: they are obtain by a chemestry reaction. F.e.: Naylon, Polyester, Elastane, 2 Natural Fibers: they are obtained from animals, like the Silk and the Wool, or vegetables, like Cotton or Flax.

Fabrication of Plastics: Techniques Extrusion: It is used to produce a wide range of different forms of plastic, such as tubes, sheets and films, structural parts, etc. Plastic pellets are placed in a feed hopper which feeds into the system. A turning screw pushes the plastic into the barrel where heaters increase the temperature and a melted polymer is obtained. The melted plastic is forced through a shaping die. Depending on the particular shape of this element, a continuous shape is formed and pulled out of the extrusion machine.

Fabrication of Plastics: Techniques Injection: Plastics pellets flows, from the feed hopper onto a turning screw. It is converted into a melted plastic by the action of heaters situated along the barrel. The screw moves the molten plastic forward, forcing the plastic through a gate into the cooled mould. The mould is opened once the plastic has solidified and the piece is pushed from the mould. After we get the manufactured piece, the mould is closed and the process begins again.

Fabrication of Plastics: Techniques Compression molding: This is a similar process to extrusion. In compression moulding, plastics pellets, sometimes called moulding powder, are placed in the feed hopper and pushed to the gate by the action of the turning screw. It is heated and compressed while it passes through the barrel. After the gate, the molten charge is quickly transferred to a press where it is moulded while still hot. The part is removed after sufficient cooling.

Fabrication of Plastics: Techniques Blow molding: Plastic grocery bags, bottles and similar items are made using this processing. As in compression processing, plastic pellets are melted and the plastic is forced through a gate into the blow pin camera. The plastic substance is expanded and cooled by being made to flow around a massive air bubble. After a few seconds, the mould is opened and the manufactured product is ready.

Fabrication of Plastics: Techniques Vacuum forming: Vacuum forming works by removing air, thereby creating a partial vacuum underneath a soft and flexible thermoplastic sheet and allowing atmospheric pressure to push the plastic down onto a mould. The vacuum forming process may start with a blow that stretches the plastic.

Fabrication of Plastics: Techniques Laminated: in this process the pellets go througth a series of heated roller, making a sheet. Spinning: in this tecnique the pellets are heated above a kind of colander. Then it is pushed to make threads. When they are falling they must be dried. Spinning is used to make synthetic textile fibers.

Work With Plastics In The Workshop You must follow these eight steps: 1. Marking. Before cutting we need to mark the plastics with a soft pencil or a permanent marker. 2. Holding. To hold plastics we use the jack. 3. Cutting. - if the sheet are thin and soft we use scissors. -if they're thick we use a metal bow saw. -if they're expanded (plastics) we use a heated metal wire.

Work With Plastics In The Workshop 2 4. Drilling. To make holes we use an electric drill. 5. Smoothing down. To sand them down we use a sandpaper. 6. Folding. To do this we use piece of wood and heat the plastic to bend it. 7. Finishing. To get a good finished we can use a paint compresson or paint the plastic after give it a glue layer. 8. Joining. To join plastics we use different glues depending on the type of plastic.

Recycling plastics We can recycle plastics by heating them again when they have no longer use. But we can only recycle thermoplastics. To recycle thermosettings we need to use another process more complex.

The End