Die-face pelletizer

With these die-faced pelletizers the extrudate is cut on or near the die face by high-speed knives. There are different designs used that include ... 

Dekker [3] studied the effect of various flight geometries on solids conveying performance. He proposed that many extrusion instabilities might be due to internal deformation of the solid bed. Internal deformation is more likely to occur when the internal coefficient of friction of the polymer particles is low. Spherical particles tend to have a lower internal coefficient of friction than non-spherical (e.g. cylindrical) particles and are, therefore, more susceptible to internal solid bed deformation. This may explain the often observed difference in extrusion behavior between strand pelletized and die-face pelletized material. 

Some pelletizers cut extruded strands cooled in a water bath these are called strand pelletizers. Dicers cut extruded sheet rather than strands. The pellets from a dicer have a uniform cubic or octahedral shape. Other pelletizers cut the material right at the die exit these are called die face pelletizers. These cutting systems can be dry die face pelletizers, water ring pelletizers, and imder water pelletizers. 

In the second system, the die face is immersed in a chilled water bath. A strong current of water carries the pellets away from the die as soon as they are cut by the blades. 

Production scrap containing Ecoflex /PLA compound should be handled separately from PE-LD scrap. It has been proven that scrap from the dryblend could be repelletized using an underwater granulator, where the die face is externally water cooled. The repelletized pellets could be used after drying of the water using a spin dryer and, if necessary, a vacuum dryer. Drying conditions have to be adapted to the Ecoflex /PLA compound being used. 

The polymer collected in the large-volume high-pressure separator is fed to a low-pressure separator for further degassing and is then pelletized in a melt-extruder by a die-face cutter. The pellets are dried and conveyed to silos. 

Before the actual sintering, the powder was pelleted in a small die. The pellets varied from 7/16 to 3/16 inch in diameter and were about 0.006 inch thick. After pelletization, the sample was placed between the faces of the anvils. A ferric oxide-coated pyrophylite ring was used to contain the sample. Pressure was increased in 10,000-atm. increments. About 2 minutes were allowed for the dissipation of the heat generated by each compression. The sample was maintained at the maximum pressure for about 10 minutes, after which pressure was reduced continuously until the press rams opened, permitting withdrawal of anvils. 

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). 

The hot-cut pelletizer has melt going through a multi-hole die plate. A multi-blade cutter slices the plastic in a dry atmosphere and hurls the pellets away from the die at a high speed. Usually the cutter is mounted above the die so that each blade passes separately across the die face and only one blade at a time contacts the die. Pellets are then air and/or water quenched, followed with drying if water is involved. Throughput is up to at least 15,000 lb/h (6810 kg/h). 

Downstream equipment generally involves a vacuum pump, for devolatilization, and pelletizing hardware. Strand pelletizers are located at the end of a water trough, while hot-face pelletizers cut the extrudate immediately upon die exit. When direct extrusion... 

In the rotating disk apparatus, the die is then inverted and screwed onto a customized shaft on a dissolution tester, which is then lowered into the dissolution medium until the face pellet is 3.8 cm from the bottom of the vessel. The shaft is then rotated in the same way as USP Apparatus 1 and 2. In the stationary disk apparatus, the die containing compressed pellet is placed face up into a flat-bottomed dissolution vessel prefilled with the appropriate volume of dissolution medium. The medium is stirred by means of a rotating paddle (e.g., USP Apparatus 2) positioned 6 mm above the pellet surface. 

Granulation and pelleting of plastics are realized in special machines. Granulation machines are extruders (mainly twin-screw types), kneaders, etc., equipped with a die-face cutter. The advantages of granules are the following ... 

The shape and size of the particulates may be a mixture of strand cut and die face cut pellets, with irregular granulated platelets of edge trim and skeletal scrap from thermoforming. Incorporation of up to 40% of the latter may be necessary to meet packaging thermoforming economics. 

Fig. 29. TWin-screw extrusion compounding line for the preparation of mineral-filled thermoplastics showing pre-mixing, compounding and pelletizing stages [155] (a) Filler/poly-mer/additive feedstock, (b) high-speed pre-mixer, (e) dosing screw, (d) twin-screw compounder, (e) vacuum devolatilization, (f) water-cooled die-face cutter, (g) start-up diverter, (h) de-watering chute, (i) pellet dryer, (k) bagging...

Pelletizers The extrudate from the compounding extruder is pelletized. These pellets are then used to feed customer polymer processing equipment to make final products. Pelletization can be accomplished through chopping strands or underwater die face cutting the extrudate [4], The technical considerations for strand-cut pelletizers are summarized in the next section. 

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