The waste plastic granulation process is the process of metering, bonding, plasticizing and granulating polymer resin and various waste plastics and additives into granular plastics, which are semi-finished products in the plastic molding processing industry.
With the development of industry, all kinds of plastic products have existed in everyone's daily life. And with it, plastic waste is also increasing day by day. Various plastic packaging, plastic containers, plastic toys and stationery, plastic shoes, plastic household appliance shells, vehicle bumpers, plastic pipes, industrial waste, and other waste plastic products can be seen everywhere. caused great environmental pollution. The recycling of waste plastics is conducive to environmental protection and saves resources. They can be recycled and pelletized, or by modification, and then pelletized, which can be used again to produce plastic products.
Characteristics of Waste Plastics:
Waste plastics can be divided into three types according to the occasions where they are generated:
- Corner waste generated in the production process: This kind of waste is relatively clean, less polluted and contains impurities, such as substandard films and trimmings in film production, leading parts, or substandard products in pipe and profile production, injection Unfilled parts in production, etc.
- Used plastic waste with a single material system: such as dismantled pipes, doors, and windows, packaging materials, or other waste plastic products that have been strictly sorted by resin type.
- Mixed waste plastics that are difficult to distinguish or cannot be separated at all: such as multi-layer co-extruded composite films, coated plastic products, composite products of plastics and other materials, etc.
Different types of waste plastics have different characteristics. In terms of impurity content, the impurity content of corner waste in factory production is less than 0.1%, and the impurity content of scraps and other used products that have been stacked for a certain period is 0.1%. ~0.5%. The impurity content of composite waste plastics mixed with aluminum, cloth, and paper is often greater than 10%.
Used waste plastic products, depending on the conditions of use, will contain ultraviolet radiation, the effects of thermal and oxygen aging, and the effects of pollutants. Different shapes of waste plastics have very different bulk densities after crushing. The bulk density of crushed materials of films, sheets, and flat yarns is small, which is a problem that must be considered in the feeding process of waste plastic recycling and granulation.
What is the Purpose of Plastic Granulator Equipment?
The waste plastic granulator is mechanical equipment that produces plastic granules by extruding, cooling, and dicing polyethylene or pp polypropylene. The granules produced are widely used and can be used for film blowing, wire drawing, blowing barrel products, low investment, low cost, and simple operation.
Technological process: Select material → Pulverize → Clean → Extrude → Cooling → Dicing → Packaging.
Pretreatment of Waste Plastics:
Waste plastics from waste packaging, such as packaging bags, shopping bags, bottles, cans, boxes, and waste agricultural films, are pretreated before pelletizing. The process of pretreatment mainly includes classification, cleaning, crushing, and drying.
The classification work is to classify the various waste plastic products according to the type of raw materials and the shape of the products. Sorting according to the type of raw materials requires operators to have skilled knowledge in identifying plastic varieties. The purpose of sorting is to avoid the incompatibility of recycled materials and poor performance caused by the mixing of different types of polymers. Classification by product shape is to facilitate the smooth progress of the crushing process of waste plastics because the crushing equipment used for films, flat yarns, and their fabrics and the crushing equipment for some thick-walled and hard products often cannot replace each other.
The Cleaning Process and Crushing Process Before Granulation:
- Cleaning first and then crushing process large waste plastic products with less serious pollution and uncomplicated structure should be cleaned first and then crushed, such as automobile bumpers, instrument panels, turnover boxes, plates, etc.
First soak in detergent-laden water, then rinse with clean water, remove, and air dry. Larger parts that cannot be put into the hopper of the crusher due to their large size should be roughly crushed and then finely crushed for feeding into the extrusion granulator. To ensure the quality of the regenerated pellets, they should be dried after being finely crushed. A rotary dryer with a heating interlayer is often used. Superheated steam is introduced into the interlayer, and the drying efficiency is high.
- Rough washing - crushing - fine washing - drying process
For the polluted profiled materials, waste agricultural film, and packaging bags, rough washing should be carried out first to remove foreign objects such as sand, stones, and metals to prevent them from damaging the crusher. After rough washing, the waste plastic products are centrifuged and dehydrated, and then sent to the crusher for crushing. After crushing, it is further washed to remove the debris trapped in it. If the waste plastic contains oil stains, it can be soaked in alkaline water or warm washing liquid with an appropriate concentration, and then by stirring, friction and collision between waste plastic sheets can be generated to remove the dirt, dehydrated and dried after rinsing.
- Mechanized cleaning
Before the waste plastic enters the cleaning equipment, it is crushed in a dry or wet crushing equipment. After drying, it is blown into a storage bin, and then the crushed material is quantitatively input into the cleaning tank by the screw feeder. Two counter-rotating paddle shafts slowly transport the material through the wash tank, creating a vortex that rinses away the dirt from the plastic. Dirt sinks to the bottom of the cleaning tank and is removed at the bottom of the tank at regular intervals. The cleaned waste floats up and is discharged by the screw conveyor. Most of the water is removed. The screw feeder feeds the crushed material quantitatively into the drying system. The drying system consists of a rotary dryer and a hot air dryer. The residual moisture of the material output from the drying system accounts for 1% to 2%. The cleaned material is sent to the storage bin and then sent to the extrusion granulator from the storage bin to form pellets.
Extrusion Granulation Process of Waste Plastics
Scrap plastics differ in properties from virgin resins due to the thermal and shear history they undergo during the molding process. And it has experienced the effects of heat, oxygen, light, climate, and various media during use. Therefore, the mechanical properties of recycled materials, including tensile strength and impact properties, are lower than those of the original resin, and cracks cause changes in surface structure and appearance quality. It is also not as good as before; the color is yellow and the transparency is reduced.
The performance changes of various materials are different. The change in polyolefin material is relatively small. The reduction in relative molecular weight due to processing, especially multiple processing, can be compensated by the cross-linking reaction so that the processability can be kept constant to a certain extent.
The properties of waste plastics can be improved by blending new materials or adding specific stabilizers and additives, such as adding antioxidants and heat stabilizers, which can reduce the adverse effects of heat and oxygen during the granulation process of waste plastics. In some mixed waste plastics, a compatibilizer can also be added appropriately, and filling and modification can also be carried out in the recycling and granulation of waste plastics. The obtained recycled materials are used for injection products, which can effectively shorten the molding cycle and improve the quality of the products. Rigidity, increase heat distortion temperature and reduce shrinkage. Lubricants improve the flow of the melt. In the recycling of some engineering plastics, filling, reinforcement, and alloying can also be carried out. For some easily hygroscopic materials, such as PA, PET, etc., during processing, moisture will cause degradation, reducing the relative molecular weight, melt viscosity, and physical properties. Before processing, the moisture in waste plastics should be removed and fully dried to ensure the quality of recycled materials.
Precautions for the Operation of Plastic Recycling Granulation Equipment:
- Turn on the power of the recycled plastic granulator, in turn, set the temperature, and pay attention to whether the heating is normal.
- Clean the hopper and other units, the inlet and outlet pipes, and check the oil level and lubrication status of each gear oil, engine oil, and high-temperature butter. The gear oil of the new machine should be replaced at least once every six months, and the active parts should be added at least once a week.
- When the temperature of the recycled plastic granulator is approaching the set temperature, prepare the material and start the main motor in turn. The temperature of the extruder must reach the set temperature, and the feeding device can be turned on to pour the cleaning material or the residue in the raw material cleaning screw until the new material is extruded, then pause the extrusion, and quickly install the filter plate and die head in place.
- Close the die head, and turn on the main screw motor of the granulator after closing the die head, the operator is away from the die head currently to prevent the die head from clogging and bursting and hurting people, until the die head extrudes the silk material, then close the cutting Grain cover and connect the cutting knife holder and lock.
- After the above operations of the recycled plastic granulator are in place, turn on all fans in sequence > vibrating screen > granulator > main engine operation. Adjust the pelletizer during normal granulation, and adjust the granule size by frequency control. Before the screw is discharged, the pelletizer must be continuously rotated. If it is a water-cooled spray pelletizing, the pelletizing cover should be watered first, and finally, the main machine should be turned on for formal production and processing.