Starting where the polymer manufacturing process stops, before the plastics are sold, consumed and recycled, their use requires the plastic resins to be converted into finished or semi-finished products. And while the operations change according to the nature of the products, we can still list several of the main methods used by industrial companies.
METHODS FOR USING THERMOPLASTICS
1/ INJECTION MOULDING
In this operation, the heated material, compressed and softened in an injection press, is injected into the mould that gives it its final shape.
For this, the resins are poured into a tank (or hopper), softened with heat and then pressure-conveyed by a rotating screw (also known as a plastification screw) to the closed, cooled mould, the shape of which it will adopt as it cools. On completion of these stages, the part is ejected.
Injection blow-moulding produces hollow bodies (bottles, flasks) by combining, as the name suggests, injection and blowing techniques. Preformed in an injection press, parts are expanded and pressed against the walls of a mould, using a jet of compressed air. The mould is then cooled, allowing the final hollow body to be extracted. Commonly used, injection moulding (also known as plastic injection) is used to produce parts of varying complexity in long runs, for example, for the automotive industry. The main plastic materials used in injection are polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC), all recovered and recycled by Paprec Plastics (and more specifically by France Plastiques Recyclage for plastic bottle recycling) in plastics recycling factories.
For the internal recycling of industrial plastics (machine purges, cores, manufacturing offcuts), grinders positioned next to injection presses recover and crush the plastic waste produced by the machines and can also re-introduce it back into the production process. We should point out that grinding is also one of the many techniques used by the plastics recycling industry for recycling this type of waste.
2/ EXPANSION MOULDING
During this plastic processing operation, used mainly to produce expanded polystyrene parts, thermoplastic resin beads are placed in a boiler, where, under the effect of steam, they can reach up to thirty times their initial size (to achieve this, expansion aids such as pentane are added). After drying, these pre-expanded beads will be placed in a closed mould where they will be homogenised and finally expanded under the effect of a new injection of steam.
Plastic extrusion consists of compressing the softened, homogenised material (in a heated cylinder with rotating screws similar to in an injection press) and pushing it through a die (a rigid tool that has at least one hole in it) thus giving it the desired shape. Then, the material, expelled from the die in a continuous process is then cooled and cut to the desired length.
Called “extruders” or “extrusion machines”, they produce solid, hollow or particularly long parts (tubes, pipes, door and window profiles, cables, ducts, etc.) at high speeds. A variant of this method, extrusion blowing, also produces hollow bodies but by cutting a parison (an extruded plastic tube drilled at one end to allow compressed air or gas to pass through) and placing it in a two-part mould, to give it the desired shape. A blower wand then injects sufficient compressed air or gas to press the material against the walls of the tube.
Another variant, extrusion-inflation, which consists of injecting compressed air directly into the heated material at the exit from the extruder, thus creating a vertical bubble, which will be flattened after cooling. When recycling plastic materials internally (when plastic waste is not recovered immediately by a plastic recycling company or collected after consumption) parts are also treated, according to their size, by suitable grinders or shredders.
We should also note that extrusion techniques are used by waste recycling companies to recycle agricultural and industrial plastic.
Calendering operations produce sheets, leaves or films of various thicknesses by compressing the resin using rotating cylinders with a greater or lesser gap between them (and at varying speeds), called rolling mills. Once transformed into a film or sheet by the heated rollers and then cooled and stretched to the desired dimensions, the thermoplastic material is rolled around an industrial reel. This process is also used to manufacture flat wide PVC products and coatings for upholstery and leather goods.
When coating products, the liquid material or plastic solution is spread onto a substrate (paper, fabric or aluminium) and then sent to a drying oven. This transformation process is used to manufacture PVC floor and wall coverings, various PET films and flexible industrial packaging, among other products.
6/ ROTARY MOULDING
Processing plastic using rotary moulding comprises heating a thermoplastic powder in a closed rotating metal mould that can be taken apart, transforming the machinable plastic material by centrifuging it in the mould, cooling it using cold air and/or water and finally ejecting the solidified part from the mould.
This is how some hollow objects, such as canoes, wind-surfing boards, toys, tanks, vessels and various furniture items, are manufactured. We should also note that parts transformed by rotary moulding are mainly made from recyclable medium or low-density polyethylene. Where materials are recycled internally, plastic waste is treated in three stages: grinding, screening and finally the re-introduction of the recycled plastic into the rotary moulding process.
This process (a second transformation) consists in heating reels of precompressed plastic material, softening the sheets and forming them into a shape using a metal mould. Thin items, such as miscellaneous packaging and yoghurt pots are often manufactured in this way.
It should be pointed out that used thermo-formed products are often found in post-consumption waste and are therefore recovered by plastic recycling companies. Where plastic films are recycled internally, the recycled material is ground, extruded and re-introduced into the production chain as granules.
METHODS FOR USING THERMOSETTING PLASTICS
The use of these materials is special, since parts made of thermosetting plastic become insoluble and infusible during transformation. Industrial manufacturers mainly use cast moulding (liquids poured into a heated mould), compression moulding (powder similarly compressed in a heated mould) or injection moulding (as for thermoplastics; a moving screw or piston injects the material into a closed heated mould and it takes the shape of the mould).
It should be noted that, following some processing operations, semi-finished products (such as plastic sheets or leaves) may be thermoformed (see above), welded or glued and/or undergo various factory operations (sawing, turning, milling, drilling, planing, etc.), to attain their final form.
The plastic recycling cycle is a step-by-step process. Discover the different steps :
- Collecting plastics
- Making plastic : extracting raw material
- Making polymers and plastic
- Plastic material classifications
- Recycling PVC : The cycle
- Recycling Platics Bottles - The cycle
- Regeneration, micronisation and grinding plastics
- Selling plastic materials
- Sorting plastic waste
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