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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.