Pelletizing die

The flow of polymer melt through the pelletizing die is quite complex. This is mainly because the individual... 

Left, Jessica Weinland fills a KBr pellet die with a KBr-polyethylene oxide sample. Right, Ms. Weinland uses a laboratory press to create the pellet. Notice the pellet die positioned in the press in the photo on the right... 

Fig. 12.27 Surface morphological features of mLLDPE (ExxonMobil Exceed 350D60) extrudates obtained at 160 °C with a tungsten carbide die D — 0.767 and L — 25.5 mm just above and in the sharkskin melt fracture flow-rate region. [Reprinted by permission from C. G. Gogos, B. Qian, D. B. Todd, and T. R. Veariel, Melt Flow Instability Studies of Metallocene Catalyzed LLDPE in Pelletizing Dies, SPE ANTEC Tech. Papers, 48, 112-116 (2002).]...

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).

C. G. Gogos, B. Qian, D. B. Todd, and T. R. Veariel, Melt Flow Instability Studies of Metallocene Catalyzed LLDPE in Pelletizing Dies, SPE ANTEC Tech. Papers, 48, 112-116 (2002). 

The diameter of a pelleting die may be maintained to almost any tolerance specified. Similarly, the distance between the top and bottom punches of an automatic pelleting machine, or the punch-to-heel distance in a stop pressing tool, can be held to any desired tolerance. Thus, the dimensional variations are essentially the variations in expansion of the material, during and after ejection from the die. The immediate expansion upon ejection for a typical expl used for pressed pellets is about 0.3%. Feiiet-to-peilet variations are usually less than 0.1% but the expansion continues... 

Lower moisture tends to produce higher pellet densities, probably as a result of increased friction through the die and thus greater compaction of the feed. Higher moisture reduces friction, absorbs some of the heat of compaction, and results in poorly bound pellets. Die hole inlet taper appears to be the major control of pellet density a more severe taper preferred for denser pellets. 

Densification is accomplished by a 300 HP California Pellet Mill Model 7162-3. The pellets are formed by extrusion through nominally 3/8 inch diameter radial holes in a cylindrical die. Pressure is exerted on a "pad" of the material to be densified by rolls which have fixed shafts. The pellet die rotates about the pressure rolls. Pellet length is controlled by a "cut-off" knife positioned to clip the pellets as they exit the rotating die. The exiting material stream from the pellet mill is mechanically lifted to a pellet cooler, which discharges to a screen to remove under-size pellet pieces and fines. The fines and pieces are recycled to the pellet mill infeed system. Finished pellets are conveyed by high pressure air system to tank storage. 

Figure 330. (a) Typical concave pelleting die rings. (Courtesy of Sprout-Bauer, Muncy, Pa., USA.) (b) Flat dies and roller assemblies. (Courtesy of Amandus Kahl Nachf., Reinbek/Hamburg, FRG)... 

Cycolac BrandABS Pellet Dying, Technical Publication SR-601A, GE Plastics, 1989. 

A complete range of metastable cerium-zirconium mixed metal oxide powders (CexZr(i.x)Oy, 0 < X < 1) were prepared through a similar hydroxide precipitation technique reported by Hori, et al. [11]. Cerium (IV) ammonium nitrate and zirconium oxynitrate precursors are completely dissolved in de-ionized water with mild heat and precipitated through the addition of excess ammonium hydroxide (-100 vol%). The ceria-zirconia is thoroughly washed with excess distilled water and allowed to evaporate to dryness overnight. The ceria-zirconia system is calcined in atmosphere for 1 hour at 773 K and subsequently milled into a fine powder. The model ceria-zirconia catalysts are prepared from the ground cerium-zirconium oxide powders using a 13 mm diameter pellet die and hydraulic press. 

Figure 4.17 Schematic drawing of a typical IR pellet die showing the arrangement of the major components. (Reprinted from Aikens et al. by permission from Waveland Press, Inc. Long Grove, IL, 1984. All rights reserved.)...

Water ring pelletizer die-swell allowance is critical. 

Consider now an experiment which demonstrates molecular adhesion simply and unequivocally. Take a sample of very fine, pure solid powder, say 4 g of 0.2 pm diameter grains of titanium dioxide. This powder can be poured into a hard steel pelleting die, as shown in Fig. 2.9(a). Coulombic forces can be prevented by the presence of a radioactive source to ionise the air and leak away any stray electrons. 

Figure 6 Example of alkali-halide disk-making apparatus (A) photograph of a manually operated hydraulic press for preparing alkali-halide disks and (B) photograph of a range of evacuable pellet dies. (Reproduced by kind permission of Specac Ltd., Orpington, Kent, UK.)...

Figure 4.17 Schematic drawing of a typical IR pellet die showing the arrangement of the major components.

Melt extruder n. A short extruder, typically of constant channel depth and lead throughout, designed to receive a molten feed and raise its pressure for extrusion through a die, such as a pelletizing die. 

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