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Plastic Extrusion Machinery Consultants Straw Extrusion Technology
Plastics – what are they and how do they behave? Plastics are organic polymeric materials consisting of giant organic molecules. Plastic materials can be formed into shapes by one of a variety of processes, such as extrusion, moulding, casting or spinning. Modern plastics (or polymers) possess a number of extremely desirable characteristics; high strength to weight ratio, excellent thermal properties, electrical insulation, resistance to acids, alkalis and solvents, to name but a few. These polymers are made of a series of repeating units known as monomers. The structure and degree of polymerisation of a given polymer determine its characteristics.
Linear polymers (a single linear chain of monomers) and branched polymers (linear with side chains) are thermoplastic, that is they soften when heated. Cross-linked polymers (two or more chains joined by side chains) are thermosetting, that is, they harden when heated.
Extrusion Machinery Consultancy On Plastic
Examples of thermoplastics include:
- - High density polyethylene (HDPE) used in piping, automotive fuel tanks, bottles, toys.
- - Low density polyethylene (LDPE) used in plastic bags, cling film, flexible containers.
- - Polyethylene terephthalate (PET) used in bottles, carpets and food packaging.
- - Polypropylene (PP) used in food containers, battery cases, bottle crates, automotive parts and fibres.
- - Polystyrene (PS) used in dairy product containers, tape cassettes, cups and plates.
- - Polyvinyl chloride (PVC) used in window frames, flooring, bottles, packaging film, cable insulation, credit cards and medical products.
There are hundreds of types of thermoplastic polymer, and new variations are regularly being developed. In developing countries the number of plastics in common use, however, tends to be much lower. Thermosets make up the remaining 20% of plastics produced. They are hardened by curing and cannot be re-melted or re-moulded and are therefore difficult to recycle. They are sometimes ground and used as a filler material. They include: polyurethane (PU) – coatings, finishes, gears, diaphragms, cushions, mattresses and car seats; epoxy – adhesives, sports equipment, electrical and automotive equipment; phenolics – ovens, handles for cutlery, automotive parts and circuit boards (The World Resource Foundation). Nowadays, the raw materials for plastics come mainly from petrochemicals, although originally plastics were derived from cellulose, the basic material of all plant life.
Linear Polymer, Branched Polymer Cross-linked Polymer Recycling of Plastics. Practical Action 2 Why recycle plastics? Although there is also a rapid growth in plastics consumption in the developing world, plastics consumption per capita in developing countries is much lower than in the industrialized countries. These plastics are, however, often produced from expensive imported raw materials. There is a much wider scope for recycling in developing countries due to several factors:
- - Labour costs are lower.
- - In many countries there is an existing culture of reuse and recycling, with the associated system of collection, sorting, cleaning and reuse of ‘waste’ or used materials.
- - There is often an ‘informal sector’ which is ideally suited to taking on small-scale recycling activities. Such opportunities to earn a small income are rarely missed by members of the urban poor.
- - There are fewer laws to control the standards of recycled materials. (This is not to say that standards can be low – the consumer will always demand a certain level of quality).
- - Transportation costs are often lower, with hand or ox carts often being used.
- - Low cost raw materials give an edge in the competitive manufacturing world.
- - Innovative use of scrap machinery often leads to low entry costs for processing or manufacture.
Sources Of Waste Plastic
Sources of waste plastics Industrial waste (or primary waste) can often be obtained from the large plastics processing, manufacturing and packaging industries. Rejected or waste material usually has good characteristics for recycling and will be clean. Although the quantity of material available is sometimes small, the quantities tend to be growing as consumption, and therefore production, increases. Commercial waste is often available from workshops, craftsmen, shops, supermarkets and wholesalers. A lot of the plastics available from these sources will be PE, often contaminated. Agricultural waste can be obtained from farms and nursery gardens outside the urban areas. This is usually in the form of packaging (plastic containers or sheets) or construction materials (irrigation or hosepipe). Municipal waste can be collected from residential areas (domestic or household waste), streets, parks, collection depots and waste dumps. In Asian cities this type of waste is common and can either be collected from the streets or can be collected from households by arrangement with the householders
Manufacturing Techniques
Manufacturing Techniques:
- - Extrusion: The extrusion process used for manufacturing new products is similar to that outlined above for the process preceding pelletisation, except that the product is usually in the form of a continuous ‘tube’ of plastic such as piping or hose. The reclaimed plastic is forced along the heated tube by an Archimedes screw and the plastic polymer is shaped around a die. The die is designed to give the required dimensions to the product and can be interchanged.
- - Injection moulding: The first stage of this manufacturing process is identical to that of extrusion, but then the plastic polymer emerges through a nozzle into a split mould. The quantity of polymer being forced out is carefully controlled, usually by moving the screw forward in the heated barrel. A series of moulds would be used to allow continual production while cooling takes place. This type of production technique is used to produce moulded products such as plates, bowls, buckets, etc.
- - Blow moulding: Again the spiral screw forces the plasticised polymer through a die. A short piece of tube, or ‘parison’ is then enclosed between a split die -which is the final shape of the product and compressed air is used to expand the parison until it fills the mould and achieves its required shape. This manufacturing technique is used for manufacturing closed vessels such as bottles and other containers.
- - Film blowing: Film blowing is a process used to manufacture such items as garbage bags. It is a technically more complex process than the others described in this brief and requires high quality raw material input. The process involves blowing compressed air into a thin tube of polymer to expand it to the point where it becomes a thin film tube. One end can then be sealed and the bag or sack is formed. Sheet plastic can also be manufactured using a variation of the process described.
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