What Is Plastic Recycling Granulation?
Plastic recycling granulation is the recycling of waste plastic produced in human activities, and then through the plastic granulation process to process the recycled plastics into particles. Recycled plastic particles can be used for molding processing, the product performance is similar to the original product performance, which has high economic value.
What is the plastic recycling granulation line?
Waste plastic recycling generally involves the following materials: PE PP PA PS ABS PVC PET PC POM.
Plastic recycling granulation production process
Sorting → Washing / Crushing → Crushing / Washing → Drying → Feeding → Plasticizing → Pelletizing
To classify the miscellaneous waste plastic products according to the type of raw materials and the shape of the products.
2. Washing → Crushing → Drying
There are different processes for cleaning and crushing different waste plastic, which is roughly divided into three types:
Cleaning first and then crushing
Large waste plastic that is not seriously polluted and have a complex structure should be cleaned first and then crushed, such as car bumpers, instrument panels, turnover boxes, plates, etc. First soak in water with detergent, then rinse with clean water, remove and air dry. Large parts that cannot be put into the crusher hopper due to their big volume should be coarsely crushed and then finely crushed for feeding by extrusion granulation. In order to ensure the quality of the recycled pellets, they should be dried after fine crushing, and rotary dryers equipped with heating interlayers are often used, and superheated steam is introduced into the interlayer, which rotates while being heated, and the drying efficiency is high.
Rough washing → Crushing → Fine cleaning → Drying
For contaminated profiles, waste agricultural film, and packaging bags, rough washing should be carried out first to remove foreign matter such as sand, stones and metals to prevent it from damaging the crusher. Waste plastic products are roughly washed and centrifugally dewatered, and then sent to the crusher for crushing. Crush and then wash further to remove debris trapped inside. If the waste plastic contains oil, it can be soaked in an appropriate concentration of alkaline water or warm washing liquid, and then stirred to cause friction and collision between the waste plastic blocks (sheets), remove the dirt, dehydrate and dry after rinsing.
Before the waste plastic enters the cleaning equipment, it is crushed in a dry or wet crushing equipment, and after drying, it is blown into a storage silo, and then the crushed material is fed into the cleaning tank by the screw feeder. Two counter-rotating blade shafts slowly transport the material and rinse off the plastic by the vortex generated by the washing tank. The dirt sinks to the bottom of the tank and is removed at the bottom of the tank at specified intervals. After cleaning, the waste material floats up and is discharged by the screw conveyor, and most of the water is removed. The screw feeder feeds the crushed material into the drying system in quantity. 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%~2%. The cleaned material is sent to the storage silo and then sent to the extrusion granulator by this storage silo to produce granular material.
The bulk density of the material after the crushing of waste plastic products is small, especially the crushed material of waste film and fiber, in order to ensure that this material can be fed accurately and the melting zone and granulator head are adequately supplied, the design form of increasing the size of the feeding section can be adopted. When the bulk density of waste plastics is less than 200g/L, forced feeding is required, and no forced feeding device is required when it is greater than 200g/L. The larger dosing section allows for satisfactory recycling of materials that are not easy to convey, such as PP, PA and PET fiber waste. For PA and PET, the feeding section screw heating can be used to improve the conveying efficiency, groove the barrel of the PP feeding section, and fully cool the hopper seat, which will greatly improve the feeding and conveying performance. If the added material is film, wire and ribbon scraps, the feeding opening can be opened larger to facilitate dosing.
For plastic waste plastics, it is necessary to consider the fact that the recycled material is a mixture composed of different melt flow rates, different lubricant components, different fillers or different types of polymers, so the plastic refining of waste plastics should be sufficient to homogenize the various components in the material and the quality is uniform.
Generally, the granulation process of waste plastics is only recycled without filling and reinforcement with a single screw extruder. If the filling, strengthening and alloying modification process are also carried out in the process of granulation, the twin screw extruder with a good mixing effect is required. In terms of output, twin screw extruders are higher than single screw extruders.
Since recycled material is often processed with a certain proportion of new material, if the particle size difference is too large and the shape is irregular, it will cause uneven feeding of new and old materials, and eventually cause uneven product performance. Therefore, the recycled material is granulated by a water-cooled die surface, and the shape and size of the pellet obtained are the least different from the new material, and it is easiest to mix evenly with the new material.
Equipment composition of waste plastic extrusion granulation line
Cleaning tank, Mixer, Crusher, Centrifugal dehydrator, Dryer, Extruder, Pelletizer, Water-trough, Air dryer
Extruder – Plastic Pellet Making Machine
The selection of the right extruder for the whole process is the most important, it directly affects the production capacity and the quality level of recycled pellets. The choice of extruder depends mainly on what material the plastic needs to be recycled by the customer and what output is required per hour. For example, customers produce PBT, and ABS recyclables, raw materials are fine particles, and the required output is 150~250kg/h. It is recommended to choose our CHT-52B twin screw extruder with a length-diameter ratio of 40:1, which is granulated by a water-cooled strip pelletizing method and is cooled by a 4-meter sink. If there are higher output requirements, you can choose the CHT-65B twin screw extruder, the output of this model equipment can reach 300~400kg/h. However, if the customer produces PP recycling and granulation, the required output is 100~150kg/h, the raw material particles are large but the plasticization requirements are not high, the CSE-65 single screw extruder is the most suitable. This equipment is affordable and has the advantage of precise temperature control and low energy consumption for production.
Most polyolefins are regenerated without venting, while hygroscopic polymers such as PA and PET are required. Some uncleaned contaminants on waste plastics may also be volatile substances, which will produce gases during heating. The exhaust section should ensure that the molten material has a long residence time, high melt temperature, strong shear deformation and large melt surface area here, so that the gas in the melt can be fully removed.
The function of melt filtration is to filter out impurities in waste plastics. These impurities can significantly reduce the quality of the recycled material. Impurities will cause bubbles when blowing film, broken wire during spinning, nozzle blockage in injection molding, and eventually lead to product quality degradation or all unqualified.
The permissible level of contamination depends on the level and quality required for the final product. If recycled material is used to produce films, impurity particles should be less than 20 μm so that the film 30 μm thick can be produced without breaking bubbles. For injection molding, impurity sizes even greater than 100 μm are acceptable. Therefore, the selection of filter fineness must be adapted to the quality requirements or the use of secondary raw materials. Too coarse filtration is not good for quality, and too fine affects economic benefits. In addition to the low yield, the fine filter is changed frequently. The result is a decrease in productivity and an increase in energy consumption. Generally, 20 mesh and 60 mesh herringbone filter screens are used.