Most plastics are made from oil. Plastics are man-made materials. They are a useful invention because they are waterproof, easy to shape and tough. They have taken the place of traditional materials like wood and metal in many products. It is bad to waste plastic, as it is made from non-renewable resources and takes a very long time to decay.
A bit of Chemistry
Plastics are made of Polymers which are obtained form monomers ( low-molecular-weight ) by polymerization reactions, in which large numbers of monomer molecules are linked together.
In the image on the left a monomer ethene ( C2H4) is used to produce a Polimer ( poliethene).
Ethene or Ethylene –because it contains a carbon-carbon double bond, ethylene is called an unsaturated hydrocarbon.
The process by which we get a polymer is called polymerization. This process has three stages: In the first stage, the monomer is split into two identical parts, each with an unpaired electron ( or free electron ). Now we have a free radical ( A molecule with an unpaired electron ). The free radical ( second line in the image ), forms a new bond with a ” neighbor atom ” This process repeats over and over again to form large chains containing thousands of carbon atoms.
There are two main types:
thermoplastic and thermosetting.
EXAMPLES OF PLASTICS
Examples of plastic include PVC, acrylic, polystyrene, expanded polystyrene (aeroboard), man-made rubber, polythene (or polyethylene, or PET) and nylon. See photos
PVC Memorabilia Case, Acrylic Expanded Polystyrene
Bottles made of pet Nylon Man-made Rubber Sole
CHARACTERISTICS OF PLASTICS
• They are relatively cheap.
• Most plastics are synthetics and are made from oil.
• Plastics are good insulators which makes them safe to use for casing electrical equipment and for the covering on wires.
• They are waterproof.
• They are light.
• Most plastics can be made in different colours or can be clear.
• Plastics are non-corrosive and non-toxic
• They wash well
• They are not biodegradable and cannot be easily recycled.
Thermoplastics can be heated, moulded and shaped various ways, lots of times. You have probably shaped them in the vacuum former or strip heater in your Technology Room.
Each time a thermoplastic is heated, it tries to return to the shape it first was, usually a flat sheet. This is called plastic memory.
Thermosetting plastics are generally strong and resistant to heat, but they melt the first time they are heated to a high enough temperature and harden (set) permanently when cooled. They can never be melted or reshaped again. They are used in situations where resistance to heat is important, e.g. on kitchen work surfaces, good-quality plastic cups, saucepan handles and plug casings.
USE OF PLASTICS APPROPRIATE TO THE PRODUCT
Expanded polystyrene (aeroboard) is used as white foam-like packaging to prevent damage to fragile goods in transit. As it is a good insulator, it is also used as a hot drinks container. It keeps the heat in the drink and away from your hands,
Acrylic, is ideal for signs outside shops, cassette boxes and aeroplane windows, but is too hard for plastic bottles. It does not look as good as glass and is not suitable for things that should not be scratched.
Polythene (or polyethylene, or PET) is ideal for use in plastic bottles.
Rubber is an insulator, which makes it safe for use with outdoor electrical appliances. Even if the rubber gets wet, it will not conduct electricity.
Thermoplastics are usually softer than thermosetting plastics and usually melt at lower temperatures, so are not as suitable for casings on electrical equipment.
Fabrication of Plastics
Using this processing it is possible to produce a wide range of different forms of plastic, such as tubes, sheets and films, structural parts, etc. 1º Step: Plastic pellets are placed in a feed hopper which feeds into the system. 2º Step. A turning screw pushes the plastic into the barrel where heaters increase the temperature and a melted polymer is obtained. 3º Step. 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. 4º Steep. Solidification by cooling
Plastics pellets flows, due to gravity, 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 by automatic ejector pins. After we get the manufactured piece, the mould is closed and clamped and the process begins again.
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.
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.
Vacuum forming is a popular deforming process. 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. Figure 5 shows a typical mould. The vacuum forming process may start with a ‘blow’ that stretches the plastic or it may be started by raising the mould, on the plate, to create a draping form. On some machines a combination of these processes is used. The aim is always to create a high definition outcome without any excessive thinning having taken place.
Adhesives and assembly in plastics.
There are many adhesives (glues) available that have been developed over recent years. Some adhesives, such as wood glues, are for specific materials, whilst others, like epoxy resin, will bond dissimilar materials. Adhesives may be rigid or they may be flexible. Some will allow time for repositioning and adjustment whilst others bond instantly on contact. You must check instructions and if you are not certain then try a test join. It is important to select the correct adhesive to suit the application and to ensure that the correct preparation is carried out and that clamping or curing time instructions are followed. Adhesives will not normally bond to greasy, dusty or wet surfaces. Adhesive bonds often fail because the surface has been handled without care and a layer of natural oil from your skin is left deposited on the surface.
Types of Adhesives
1 2 3
4 5 6
1. PVA glue (polyvinyl acetate) is the most popular wood glue. It is white in colour and comes ready mixed. PVA glue is strong and does not stain; excess glue can be wiped off with a damp cloth. It requires only Iight clamping and sets within 2—3 hours depending upon temperature.
2. Tensol cements are a range of solvent-based adhesives for joining thermoplastics. It is important to have the correct adhesive — the most popular for acrylic is Tensol 12. The size of the surface gluing area will affect the strength of the joint.
3. Epoxy resin is a two-part adhesive for unlike materials. It will bond glass, ceramic, wood, metal and hard plastics. Mixing the two parts, resin and hardener, triggers a chemical reaction that begins the setting process. A popular brand is Araldite.
4. Contact adhesive, such as Evostik, is used for fixing plastic laminates (melamine) and other sheet and strip materials. Surfaces are coated and left until “touch dry” Correct positioning is essential as bonding is immediate.
5. Hot—melt glue comes in glue stick form and is used in conjunction with a glue gun. It is popular for modelling and temporary work. It tends to be messy, is not very strong and gives a poor quality finish.
6. Super glue is an adhesive that joins plastics very quickly. Great care must be taken because one of its component ( cyanoacrylate ) is a bit dangerous.
Tota la informació anterior està extreta de la següent web i ha estat realitzada per Juan Carlos Parejo Reja.