Pelletizing your compound: what are your options?

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  • Plastics Additives & Compounding January/February 2006

    42

    Pelletizing your compound:what are your options?

    Dwayne Free of FG Group, Inc.,Consultant to GalaIndustries, Inc.weighs up the relative merits of differentmethods of pelletizing plastic compounds and makes a casefor considering an underwater cutting system.

    Numerous historical and currenttechniques have been used to producepolymers in small, easily handled forms.While many forming techniques have beendeveloped, the most common productforms include pastilles, chips and pellets.Pastilles are produced by a rotating drumproducing droplets, which are deposited ona continuous, cooled belt conveyor andscraped loose when sufficiently cooled tobe solid. Chips are produced by continuousextrusion of a strip or ribbon andcontinuously cut, or 'diced' producingsmall rectangular chips of product. Pelletsare produced by extrusion of polymerthrough round die holes, and the materialcontinuously cut into small cylindrical orspherical pellets. Today, pelletization of

    polymers is the prevailing approach toproduction of plastic compounds.

    Pelletization techniquesWhile several pelletization techniques exist,two are widely used for the pelletization ofpolymer compounds. The first of these,strand pelletization, is the production ofpellets by die extrusion of strands and thecutting of solidified strands into smallcylindrical pellets. The second, moreversatile approach is the die face cutting ofpolymer extrudate in a water-filled box.This technique is called underwaterpelletization. Small to large systems areavailable in both forms, and can be usedwith a wide range of upstream polymermachinery, including gear pumps, single-screw and twin-screw extruders.In strand pelletization, polymer is extrudedthrough a series of round die holes,typically arranged in horizontal rows on adie block, forming parallel strands. Thesestrands are continuously pulled orconveyed through a water cooling tank,where the strands solidify. Upon exit fromthe cooling trough the strands are strippedof residual water using high velocity air orsometimes high vacuum around thestrands to avoid aspiration of the water intothe work environment. After drying, thestrands are pulled into a continuousrotating cutter called a strand pelletizer,which cuts the polymer into shortcylindrical pellets. The pellets then fallthrough the discharge side of the strandpelletizer and are screened to remove finesand/or oversized product prior topackaging.

    Underwater pelletization typically uses adie plate with round holes arranged insingle or multiple circular rows around theouter portion of the die plate. Concentricwith the die centre is an opposing motordriven cutter assembly. Both the die plateand cutter assembly are designed to bejoined together, forming a closed box, orcutting chamber. Upon start-up of thepelletization process, water is circulatedthrough the cutting chamber, providingboth the means of cooling and transportingthe pellet. As polymer extrudes through thedie holes, a series of rotating blades cuts themolten polymer, which floats free andsolidifies as it is carried away in the slurry ofcirculating water. These pellets form intospherical or egg-shaped pellets. Thewater/pellet slurry is circulated to adewatering screen and the pellets typicallydried in a circulating or centrifugal dryer.

    ComparisonStrand and underwater techniques areavailable in systems of up to 11,340 kg/hr(25,000 lb/hr), covering the practical rangemost producers require in polymer pelletproduction. Within the range of typical productionrequirements, differences in capital cost ofequipment are not significant inproduction-sized systems. The basic cost ofequipment is somewhat lower with strandpelletizing equipment, while productivityis higher with underwater systems. Sincethese differences are measured inpercentages, not multiples, the impact ofcapital cost on product unit cost isgenerally not significant.

    ISSN1464-391X/06 2006 Elsevier Ltd.All rights reserved.

  • P e l l e t i z i n g

    Plastics Additives & Compounding January/February 2006

    43

    The general arrangement of a strandpelletizing system consists of the followingequipment: 1) The pelletizing die block attached to

    upstream equipment.2) A water cooling trough or tank of

    sufficient length (up to 7.3 m/24 feet) toallow the strand to quench and solidify, anozzle or directed air flow assembly thatimpinges air on the strands at such anangle as to strip away any residual waterfrom the strands as they emerge from thecooling tank.

    3) A cutter assembly consisting of a rotorassembly equipped with replaceable knifeblades and a fixed bed knife that togethercut the strands into small pellets.

    4) A vibratory multi-screen device thatconveys the cut pellets, and separatesoversized and fines from the productprior to discharge to product packaging.

    The overall layout of this equipment islinear, with overall equipment widthgenerally two or three feet, and significantequipment length of up to 10.7 m (35 feet)due to tank size, and the sequentialarrangement of tank, air knife, pelletizerand screener.Underwater pelletizing equipment requiresless overall space and has more flexibility interms of the equipment arrangement. Thegeneral arrangement of an underwaterpelletizing system consists of a 1) Die plate assembly, which is coupled to 2) The pellet cutter, 3) The water box assembly with water box

    piping and flow control valves, 4) A de-watering screen and water tank

    system which recycles the cooling water,5) A centrifugal dryer which rotates and

    uses an air blower to dry the underwatercut pellets, and

    6) Underwater pelletizing/drying systemsutilize residual latent pellet heat for bothdrying and potential crystallization.

    The die plate-pelletizer-water box assemblyis generally less than 0.6 m (two feet) inwidth and 1.8 m (six feet) or less in overalllength. The slurry line to the dryer andsupply water line from the water tanksystem can be routed to allow theequipment installation to fit in a range ofavailable space in the production plant. Theoverall layout of the equipment can bedescribed as more of a production cell,

    rectangular in footprint versus linear forstrand pelletizing equipment.Start-up requires one person for moststrand or underwater pelletization systems.Start-up is accomplished with one-buttonstart-up of an underwater cutter, whilestrand systems require manual handling ofstrands through the cooling trough, waterstripper and fed into the strand pelletizer.Start-up is generally possible at a higherpercentage of the production rate withunderwater pelletizers, while start-up rate isoften reduced to ensure problem-freestring-up of the strands on a strandingsystem.Both strand and underwater pelletizingallow the production of a range of pelletsizes. Underwater cutting of polymersallows the use of significantly smallerdiameter die holes, and shorter effectivecut of the extrudate as it emerges from thedie hole. This allows the production ofpellets as small as 0.2-0.3 mm indiameter.Operator safety is an overriding priorityin any production setting, and differencesdo exist between strand and underwaterpelletization. Underwater pelletizersoperate at reduced noise levels comparedwith strand pelletizers, presenting lessnoise hazard to operators. Additionally,the cutter system is enclosed duringoperation, precluding operator exposureto the cutting assembly. No manualfeeding of strands is required, thusavoiding operator exposure to hazard

    during start-up. The closed cuttingchamber also provides additionalprotection for operators by preventingbuild-up and degradation of polymerresidue on the die face - a source of fumesand smoke in strand die pelletization.

    ConclusionOf the wide variety of pelletizingtechniques that have been developed,strand-cut and underwater-cut pelletizingequipment are the prevalent systems usedglobally by polymer producers andcompounders. Of these two techniques,underwater pelletization providessomewhat greater flexibility in pelletsizing, equipment general arrangement,and affords greater operator protection. Contact:Gala Industries, Inc.181 Pauley Street, Eagle RockVA 24085, USATel: +1 540 884 2589Website: www.gala-industries.com

    The Gala Model 12 A5-PAC.

    Pelletizing your compound: what are your options?Pelletization techniquesComparisonConclusion