RECYCLING LINES | TECHNOLOGY
recycling machine, and using auto-feeding systems, can solve many material blockage problems. Periodic maintenance on the machine components, regularly changing filter meshes, and unclogging the die head are also easy and common preventions. Using force feeders for hopper-feeding recycling machines can help prevent material bridging.
4. Overheating Overheating the plastic waste in the integrated pre- conditioning equipment (such as the cutter compactor and shredder) or overcooking the plastic melt in the extruder often causes the degradation of the recycled pellets. This reduces their reusability and weakens the quality of final plastic products. Solution: Setting the right temperature on the
particular sections of the recycling machines based on the materials, calibrating heating and cooling elements efficiently, and using reliable components which do not cause temperature imbalance, can all significantly prevent material overheating prob- lems. Prevention of material blockages also reduces local clogging and overheating problems.
5. Sticky plastic pellets There might be several reasons that make recycled pellets stick to each other. Higher extruder tem- perature, poor pellet cooling, and material charac- teristics are the most common reasons. Solution: Setting the right extruder temperature
parameters based on your material, and ensuring the cooling water temperature (air temperature on air-cooled recycling machines) is at a suitable level can solve most of the sticky plastic pellets problem. Using a centrifuge dryer can also prevent pellets sticking to each other while also further drying them. Using a chiller, especially for hot climates, is also suggested.
6. Hollow plastic pellets Hollow plastic pellets are not preferred for plastic
IMAGE: POLYSTAR
production. They often cause inefficient production and poor final product quality because of their low density, reduced strength, inconsistent size, and shape. Poor degassing, poor plasticisation, the tem- perature, and cooling water circulation speed can be the common reasons for hollow plastic pellets. Solution: Vacuum degassing system options based on the condition of the material are an effective solution to remove gas from the materials which causes hollow pellets. Adjusting the right and stable temperature on particular zones of the recycling extruder and die head provides smooth plasticisation, and prevents quick temperature changes. Adjusting the right cooling water temperature and speed (not too cold and not too fast) also prevents recycled pellets forming a hollow shape.
7. Non-uniform pellets or chunks Uniformity in the final recycled pellet shape and size is one of the most important points determin- ing the quality of recycled plastic. Many factors including instability in feeding, material flow, and especially a poor pellet cutting system are the common reasons for non-uniform pellets output. Pelletising inefficiency is the most common reason for plastic chunk creation. Solution: Stabilising material feeding, ensuring smooth melt flow with the right temperature adjustment, preventing clogging, and using an auto speed and pressure-controlled pellet cutting system, will solve most of the non-uniform pellet output problems. Polystar’s water ring pelletiser automatically adjusts pellet cutting speed based on the speed of the melt coming out from the die head. The system also ensures the pelletising knives are always positioned on the die head synchronously with the die head pressure. These result in cutting uniform size and shaped pellets all the time. Dull pelletising knives or their dis- position might cause plastic chunk creation; replac- ing the knives with original spare parts and maintain- ing the right position can solve this problem. When investing in a plastics recycling machine,
recyclers and plastics producers are recommended to consider the machine’s performance, user- friendliness, and ease of maintenance all together. A stable plastics recycling machine must work in harmony across all its sections and equipment.
www.plasticsrecyclingworld.com October 2023 | PLASTICS RECYCLING WORLD 31
Above: Using auto-feeding systems can solve many material blockage problems
Left: Efficient degassing leads to good pellet quality
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