What Are The Plastic Recycling Processes?
So, we are all used to putting certain plastic types into our recycling bin BUT what are the Plastic Recycling Processes? How is plastic recycled?
Enabling plastics to be collected for recycling is as important a part of the process as the physical recycling itself. On their own the plastics are often too light to be viable for economic collections.
Local authorities have an obligation to have your waste collected (and we pay for it through council taxes). But before plastics can continue on the recycling journey they must be prepared.
Baling – Compressing materials in a horizontal or vertical press to produce a bale held together by twine or wire.
Particularly useful for products like plastic film where if left uncompressed, takes up large volumes of space (mainly fresh air).
The compression rate achieved can be as much as 10:1 enabling you to transport 10 times as much plastic in the same space compared to if it was left un-baled.
Compacting – Similar to baling, a compactor compresses materials into a smaller space than they would otherwise take up. In plastic recycling compactors are usually used for Expanded Polystyrene (EPS) where the material is compressed and heated, compacting the EPS into a briquette. The heating helps to encourage the material to bind to itself to form the briquette and stop the material from expanding again afterwards.
What Are The Plastic Recycling Processes? – Continued … Preparation.
As in many areas of business and in life, preparation is the key to successful plastic recycling and producing a quality plastic raw material that can be used for manufacturing new plastic products.
Prior to any mechanical recycling process taking place we need to ensure that different plastic types are not mixed, metal parts, paper labels and excess dirt and dust are removed from the plastic scrap. The following preparation processes may be required:-
Sorting – Identifying components of differing polymer types to allow them to be processed separately and thus ensure the highest quality plastic raw material feedstocks using a variety of manual, mechanical, optical, density and chemical reaction techniques.
Sawing – Some plastic components (like long lengths of extruded pipe) are just too large to fit into the granulation and shredding equipment and so has to be cut down to more manageable sizes.
Stripping – Plastic is often only one component part of a product sent for recycling. Things like computers, televisions, wheeled bins, refridgerators etc. are often complicated assemblies which can include metal parts, different polymer types, plastic or paper labels, foam etc. These items all need to be stripped off and segregated prior to mechanical recycling.
Washing / Drying – Some waste plastics sent for recycling from post consumer or post industrial applications may have been sat around gathering dust for a long time or may contain residual contaminaton from its original life (ie. fizzy drink residues in a drinks bottle). This contamination can reduce the quality of the resultant plastic recyclate and therefore should be removed by washing and drying.
What Are The Plastic Recycling Processes? – Continued … Size Reduction
The final recycling process for most plastics is a heat based process where they plastic recyclate is melted down and either reformed into new product or compounded with additives and colourants and pelletised into a high quality raw material that can then be used to manufacture new products. Before these heat processes can be used it is usual to have to reduce the plastic waste down in size. This is generally done by one or both of the following processes:-
Shredding – Shredding is a quick and efficient way of reducing large unwieldy plastic components into a manageable size. The process involves a spinning rotor which rips the plastic apart. The resulting output is an irregular shaped, roughly cut – approx. 60mm (can be smaller) – product that is generally still too large to use into a heat based compounding or thermoforming process.
It is usual therefore to combine shredding and granulation to provide the best quality feedstocks in the shortest possible time.
Granulation – This process is similar to shredding, whereby spinning blades cut the plastic components into irregular shaped pieces usually 10-12mm in diameter (known as ‘reground’ or ‘regrind’).
Unlike shredding however, the use of grids in the grinding process allow us the plastic recycler to control the resulting size of the reground, which can then be used as a raw material for subsequent heat based processes.
What Are The Plastic Recycling Processes? – Continued … Blending
Blending is the process of creating a large batch of a homogenous mix of product from multiple smaller batches of the same plastic type. The “blend” may also include the addition of process additives and property enhancing additives (although in modern plastic recycling these enhancing additives are generally dosed in during the heat based processes along with colourants).
The most common type of blender is a fountain blender where plastic raw material is fed into the bottom of the blender, where a vertical rotating screw continually moves it to the top of the blender where it then fountains or sprays the regrind from the top of the screw. Foils on the inside of the blender (similar to those inside a concrete mixer) then help to disperse and mix the material. This process continues until a uniform mix of product is achieved.
The blending process is often used at various stages of the plastic recycling process to maximise the quality of the resulting raw material.
Although the main purpose for doing so is to assure product quality, each stage has its own specific benefit. The three stages of blending are as follows:-
Pre-Blending – plastic regrind is often transported in 1 tonne bags known as FIBC’s. If you look at these bags you can often see the plastic regrind through the bags and it looks like one of those gifts you buy from the beach with multiple layers of different coloured sand.
The plastic recycler has to be sure that each of the layers in a bag is of the same material type before mixing with other bags of plastics and potentially contaminating large quantities of valuable raw material.
By blending material from a single source which are supposed to be the same plastic type they can get a representative sample of product to check for contamination and to obtain the physical properties for that batch of feedstock.
Blending – allows the plastic recycler to create a formula for a required specification from a selection of plastics of the same type but possibly with different characteristics or from different sources.
Again the purpose of blending is to create a homogenous batch of plastic which will produce a consistent quality recycled plastic compound. Without this uniformity the characteristics of the resulting plastic compound may vary as it goes through the batch which will cause processing problems for the moulder, increasing production cycyles, increasing scrap rates and potentially reducing product quality.
Post blending – no matter how much attention the recycler gives to ensuring the consistency of their finished compound it is still possible that minor fluctuations throughout the compounding process will cause variations in the physical properties or colour of the resulting product.
Post-blending is a post production process where the finished compound can be mixed to obtain a more consistent final product.
What Are The Plastic Recycling Processes? – Continued … Compounding
Each stage of the plastic recycling process is designed to improve the quality of the resulting recycled plastic raw material and to thus increase its value and to improve the quality of the resulting new product.
It is possible to use regrind from the granulation process directly into a heat based process such as injection moulding or sheet extrusion and produce a new moulded product but use of regrind in this way generally has numerous processing and quality downsides and so is generally only used in lower grade applications.
The compounding process is a heat based recycling process where plastics and additives are metered into a hopper at one end of a barrel. The material is transported along the length of the barrel via an Archimedes screw and is melted by applying heat via external heater bands as well as the friction (shear heat) generated by the action of the screw(s) within the barrel of the compounding extruder.
As the plastic travels along the barrel, specially designed sections of the screw knead, mix and compound the plastic and special openings (vents) allow any volatiles (noxious gases) and moisture to be removed. Additives and fillers can be added via additional side-feeders.
At the end of the barrel, the melted plastic flows through a wire screen that filters out any residual dirt, grit or other contaminants, and is then forced through a die plate.
Other Melt Reclamation Processes
Some plastic products are difficult to recycle by following the grind, blend and melt routine on different pieces of equipment. Products such as plastic film, plastic bags, FIBC’s, corrugated plastic board (Corex), plastic based fabric etc. are too light to be processed in the usual way.
A melt reclamation plant linked to a shredder at the front end enable such materials to be recycled and like a standard compounding process additives, fillers and colours can be introduced to the polymer pellet produced and offer material to agreed repeatable specifications.
Find out more about plastics and plastic recycling at the British Plastics Federation.