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New underwater pelletizing techniques under development at BKG
Pe l l e t i ze r s
New underwater pelletizing techniques under development at BKG Detailed analysis undertaken by German company BKG Bruckmann & Kreyenborg using photography shot at 18,000 frames per second has identified some of the mechanisms at work in an underwater pelletizing system. The molten polymer is cut into drops by the rapidly rotating blades of the pelletizer just as the polymer leaves the die holes and emerges into the process water. Due to the temperature difference between the molten drops and the process water, the polymer drops solidify. As a result of tendency of the polymer drop to optimize the ratio of volume to surface area, a spherical shape is obtained. The company cites the advantages of the underwater pelletizing process, for example in PET chip production, to include optimized chip shape. In addition, less cooling water is required because it can be warm for most materials. Less space is needed for the process and there are environmental advantages, due to the closed loop for cooling water. Underwater pelletizing also offers ease of operation, less capital investment and lower operating costs, says BKG. For the PET chip end-user, the advantages provided by the underwater pelletizing process include higher bulk density, reduced bridging, better extruder feeding and shortened melt time, and more uniform and consistent intrinsic viscosity of the chips. The two types of underwater pelletizer offered by BKG include the Type A 300 and A 2000, with throughput in the range 2-2000 kg/h and the set-up of the blades is by handwheel. Types AH 2000, AH 4000 and AH D 250 offer throughputs in the range 300-15000 kg/h and are fitted with a hydraulic system. A new compact pelletizer in-between these two types is under development. BKG adds that by cutting the liquid polymer strands after they leave the die plate, then no oxidation of the polymer takes place. The design of the die and cutter hub ensures that the blade is adjusted precisely to provide consistent pellet quality. In addition, blade adjustment ensures that wear is uniform across the whole blade. Die plates are evacuated during manufacturing to min-
imize heat conduction through the die body and concentrate the heat at the die holes. The improved heat transfer means that a reduced area for die/cooling water interface contact is required. Consequently 25 percent less cutting surface area is needed. As a result of these developments, BKG has successfully developed straight cut blades that are set at a 90 degree angle. The company says that the advantages include that instead of the 12 blades available in an angle design, it is possible to include 20 straight blades. This means that a higher throughput is possible on the same pelletizer. In addition, BKG says that the simple blade design means that no regrinding is required. This is not yet suitable for all materials, but has been tried with polyamide and polypropylene. The new design is particularly suitable for micro pellets, adds the company. An exact blade/die distance can be maintained through hydraulic adjustment of the cutter shaft. The advantages of this include no tails on the pellets, as well as no smearing polymer at the die. An underwater cut of even the lowest viscosity materials is possible, while blade and die plate life is extended. The pelletizer achieves this through a determination of the optimum hydraulic pressure to obtain the optimum blade/die distance. This also facilitates automatic sharpening of pelletizer blades and continuous cleaning of the die plate during the sharpening process.
Resharpening of the blades occurs after set time intervals through increasing the hydraulic pressure on the blades for a short period. A fine film can build up on the blade and can cause problems with resharpening. This is achieved automatically as the pressure required to break the film is determined. The hydraulic pressure is ramped up in short intervals, while the torque of the motor is continuously measured. When the torque of the motor rises, the blades are in contact with the die plate and the hydraulic pressure is set back to a defined sharpening pressure for a certain period of time. The hydraulic pressure is then set back to a minimum production pressure. BKG adds that the procedure provides good pellet quality and reduced dust. Bypass piping is used to control the water entry into the cutting chamber closely through bypass valves and a PLC program. This avoids freeze-off of the die holes, with the process water entering too early, or agglomeration of the pellets caused by the process water entering too late. Contact: BKG Bruckmann & Kreyenborg Granuliertechnik GmbH Tel: +49 2512 6501 0 Fax: +49 2512 6501 98 E-mail: [email protected] Website: www.bkg.de
The Master 300 underwater pelletizer from BKG.
39
Plastics Additives & Compounding November/December 2003
New underwater pelletizing techniques under development at BKG Detailed analysis undertaken by German company BKG Bruckmann & Kreyenborg using photography shot at 18,000 frames per second has identified some of the mechanisms at work in an underwater pelletizing system. The molten polymer is cut into drops by the rapidly rotating blades of the pelletizer just as the polymer leaves the die holes and emerges into the process water. Due to the temperature difference between the molten drops and the process water, the polymer drops solidify. As a result of tendency of the polymer drop to optimize the ratio of volume to surface area, a spherical shape is obtained. The company cites the advantages of the underwater pelletizing process, for example in PET chip production, to include optimized chip shape. In addition, less cooling water is required because it can be warm for most materials. Less space is needed for the process and there are environmental advantages, due to the closed loop for cooling water. Underwater pelletizing also offers ease of operation, less capital investment and lower operating costs, says BKG. For the PET chip end-user, the advantages provided by the underwater pelletizing process include higher bulk density, reduced bridging, better extruder feeding and shortened melt time, and more uniform and consistent intrinsic viscosity of the chips. The two types of underwater pelletizer offered by BKG include the Type A 300 and A 2000, with throughput in the range 2-2000 kg/h and the set-up of the blades is by handwheel. Types AH 2000, AH 4000 and AH D 250 offer throughputs in the range 300-15000 kg/h and are fitted with a hydraulic system. A new compact pelletizer in-between these two types is under development. BKG adds that by cutting the liquid polymer strands after they leave the die plate, then no oxidation of the polymer takes place. The design of the die and cutter hub ensures that the blade is adjusted precisely to provide consistent pellet quality. In addition, blade adjustment ensures that wear is uniform across the whole blade. Die plates are evacuated during manufacturing to min-
imize heat conduction through the die body and concentrate the heat at the die holes. The improved heat transfer means that a reduced area for die/cooling water interface contact is required. Consequently 25 percent less cutting surface area is needed. As a result of these developments, BKG has successfully developed straight cut blades that are set at a 90 degree angle. The company says that the advantages include that instead of the 12 blades available in an angle design, it is possible to include 20 straight blades. This means that a higher throughput is possible on the same pelletizer. In addition, BKG says that the simple blade design means that no regrinding is required. This is not yet suitable for all materials, but has been tried with polyamide and polypropylene. The new design is particularly suitable for micro pellets, adds the company. An exact blade/die distance can be maintained through hydraulic adjustment of the cutter shaft. The advantages of this include no tails on the pellets, as well as no smearing polymer at the die. An underwater cut of even the lowest viscosity materials is possible, while blade and die plate life is extended. The pelletizer achieves this through a determination of the optimum hydraulic pressure to obtain the optimum blade/die distance. This also facilitates automatic sharpening of pelletizer blades and continuous cleaning of the die plate during the sharpening process.
Resharpening of the blades occurs after set time intervals through increasing the hydraulic pressure on the blades for a short period. A fine film can build up on the blade and can cause problems with resharpening. This is achieved automatically as the pressure required to break the film is determined. The hydraulic pressure is ramped up in short intervals, while the torque of the motor is continuously measured. When the torque of the motor rises, the blades are in contact with the die plate and the hydraulic pressure is set back to a defined sharpening pressure for a certain period of time. The hydraulic pressure is then set back to a minimum production pressure. BKG adds that the procedure provides good pellet quality and reduced dust. Bypass piping is used to control the water entry into the cutting chamber closely through bypass valves and a PLC program. This avoids freeze-off of the die holes, with the process water entering too early, or agglomeration of the pellets caused by the process water entering too late. Contact: BKG Bruckmann & Kreyenborg Granuliertechnik GmbH Tel: +49 2512 6501 0 Fax: +49 2512 6501 98 E-mail: [email protected] Website: www.bkg.de
The Master 300 underwater pelletizer from BKG.
39
Plastics Additives & Compounding November/December 2003