Underwater pelletizers

Modeling the Effect of Polymer Rheology on the Performance of Underwater Pelletizers... 

One of the common problems associated with underwater pelletizers is the tendency of the die holes to freeze off. This results in nonuniform polymer melt flow, increased pressure drop, and irregular extrudate shape. A detailed engineering analysis of pelletizers is performed which accounts for the complex interaction between the fluid mechanics and heat transfer processes in a single die hole. The pelletizer model is solved numerically to obtain velocity, temperature, and pressure profiles. Effect of operating conditions, and polymer rheology on die performance is evaluated and discussed. 

Pellets produced by each system have characteristics shapes. Pellets created with strand pelletization are typically short cylinders with well defined edges since they were cut when in the solid state. Underwater pelletization yields pellets that are more rounded. 

Heat Transfer in Underwater Pelletizing In underwater pelletizing, the melt strands are extruded directly in a water bath and chopped by a rotating, highspeed knife into short-length cylinders called pellets. Consider an LDPE extrudate at 200°C, chopped into pellets of L = D = 0.4 cm in a bath kept at 10°C. (a) Formulate the complete heat-transfer problem, (b) Estimate the time required to cool the center of the pellet to 70°C by assuming that pellet surface temperature equals the temperature of the water. 

Fig. 12.28 Melt-fractured pellets extruded at 8 kg/h per die hole (3.2 mm in diameter) from a conventional underwater pelletizing die (left) and smooth pellets extruded under the same conditions at 24 kg/h per die hole from the same underwater pelletizing die when locally heated to high temperature (right) (57).

Diverter valve Gear pump Screen changer Underwater pelletizer Water pre-seperator Dryer... 

The downstream discharge parts correspond exactly to the established components specified for the required throughputs, including throttle start-up valve, screen changer, and underwater pelletizer. 

Very popular are the wet-cut underwater pelletizer. The die face is submerged in a water housing and the pellets are water quenched followed with a drying cycle. Throughput rates are at least up to 50,000 lb/h (22,700 kg/h). Smaller units are economical to operate as low as 500 lb/h (227 kg/h). 

For polymer pellets strand pelletizer is versatile but capacity is limited by the strand handling capacity. Usually < 200 strands per machine. Water ring pelletizer not for water sensitive polymers or high melt flow polymers and tend to produce nonspherical pellets. Underwater pelletizer good for PE and PP produces spherical pellets not for nylon, PET or melts that solidify rapidly. 

Drop the batch on a heated roll mill, cut into a continuous strip and convey on an air-cooled belt to a dicer or feed to an extmder attached to an underwater pelletizer or a strand die, conveyer belt and a dicer. If mbber level is high, then the pellets may stick together. In such instances one can dust the pellets with a small amount of dusting agent. One can also use a two roll stuffer to feed the molten blend to the single-screw extruder. 

Removal of water from hygroscopic polymers (e.g., ABS, PMMA, PA, PC, SAN, CA, PU, PPO, polysulfone) all polymer particles can have surface moisture left from underwater pelletizing or surface condensation from storage at varying temperatures and relative humidity. 

The Banbury mixer is ideally suited to the making of thermoplastic rubbers. Polystyrene/SBR blends can be mixed in 3-4 min and TPEs requiring higher temperatures in 5-6 min. The requirement is to have a mixer with high rotor speeds, e.g. 120 rpm for an 80-litre mixer, discharging into an extruder with an underwater pelletizing head. 

The TPO formulations were prepared using a Haake twin screw, counter rotating, 25-mm extruder, equipped with underwater pelletization. All of the ASTM test specimens were prepared by injection molding with an 85 ton, Cincinnati Milacron injection molding machine. 

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