关 键 词：

蛋糕 注塑 模具设计 三维 PROE

资源描述：

喜欢这套资料就充值下载吧。。。资源目录里展示的都可在线预览哦。。。下载后都有，，请放心下载，，文件全都包含在内，图纸为CAD格式可编辑，【有疑问咨询QQ:414951605 或 1304139763】

内容简介：

A Guide toPolyol One Source. More Resourceful.efin Injection MoldingTa b le o f Co n t e n t sA Gu id e t o Po lyo le fin In je ct io n Mo ld in gIn t ro d u ct io n PackagingMo le cu la rst ru ct u re a n dco m p o sit io na ffe ct p ro p e rt ie sa n d p ro ce ssa b ilit y Sporting goods Toys and noveltiesPolyolefins are the most widelyused plastics for injection molding.This manual, A Guide to PolyolefinInjection Molding, contains generalinformation concerning materials,methods and equipment forproducing high quality, injectionmolded, polyolefin products atoptimum production rates.This manual contains extensiveinformation on the injection mold-ing of polyolefins; however, itmakes no specific recommendationsfor the processing of Equistar resinsfor specific applications. For moredetailed information please contactyour Equistar polyolefins sales ortechnical service representative.Four basic molecular propertiesaffect most of the resin characteris-tics essential to injection moldinghigh quality polyolefin parts. Thesemolecular properties are:Polyolefins that can be injectionmolded include: Chain branching Low density polyethylene (LDPE)Po lyo le fin s a red e rive d fro mp e t ro ch e m ica ls Crystallinity or density Average molecular weight Molecular weight distribution Linear low density polyethylene(LLDPE) High density polyethylene (HDPE) Ethylene copolymers, such asethylene vinyl acetate (EVA)The materials and processes used toproduce the polyolefins determinethese molecular properties.Polyolefins are plastic resins poly-merized from petroleum-basedgases. The two principal gases areethylene and propylene. Ethylene isthe principal raw material for mak-ing polyethylene (PE) and ethylenecopolymer resins; propylene is themain ingredient for making Polypropylene and propylenecopolymers (PP)The basic building blocks for thegases from which polyolefins arederived are hydrogen and carbonatoms. For polyethylene, theseatoms are combined to form theethylene monomer, C2H4. Thermoplastic olefins (TPO)In general, the advantages ofinjection molded polyolefins com-pared with other plastics are:polypropylene (PP) and propylenecopolymer resins. LightweightH|H| Outstanding chemical resistancePolyolefin resins are classified asthermoplastics, which means thatthey can be melted, solidified andmelted again. This contrasts withthermoset resins, such as phenolics,which, once solidified, can not bereprocessed.C = C Good toughness at lowertemperatures|H|H Excellent dielectric properties Non hygroscopicIn the polymerization process, thedouble bond connecting the carbonatoms is broken. Under the rightconditions, these bonds reform withother ethylene molecules to formlong molecular chains.The basic properties of polyolefinscan be modified with a broadrange of fillers, reinforcements andchemical modifiers. Furthermore,polyolefins are considered to berelatively easy to injection mold.Most polyolefin resins for injectionmolding are used in pellet form.The pellets are about 1/8 inch longand 1/8 inch in diameter and usual-ly somewhat translucent to white incolor. Many polyolefin resins con-tain additives, such as thermal stabi-lizers. They also can be compound-ed with colorants, flame retardants,blowing agents, fillers, reinforce-ments, and other functional addi-tives such as antistatic agents andlubricants.H H H H H H H H H H| C C C C C C C C C C Major application areas for poly-olefin injection molding are:|H H H H H H H H H H| AppliancesThe resulting product is polyethyl-ene resin. Automotive products Consumer products Furniture Housewares Industrial containers Materials handling equipment2 For polypropylene, the hydrogenand carbon atoms are combined toform the propylene monomer,CH CH:CH .occur which may adversely affectthe polymers properties. This oxida-tion or degradation may causecross-linking in polyethylenes andchain scission in polypropylenes.Figure 3. Linear polyethylenechain w ith short side branchesC32CC C C C C C C C C C C C C C C C CHH|CC|Polypropylene, on the other hand,can be described as being linear(no branching) or very highlybranched. Although the suspendedcarbon forms a short branch onevery repeat unit, it is also responsi-ble for the unique spiral and linearconfiguration of the polypropylenemolecule.H C C = C|H|H|Hnominal specific gravity range of0.895 to 0.905 g/cm3, which is thelowest for a commodity thermo-plastic and does not vary appreciablyfrom manufacturer to manufacturer.The third carbon atom forms a sidebranch which causes the backbonechain to take on a spiral shape.For polyethylene, the density andcrystallinity are directly related, thehigher the degree of crystallinity,the higher the resin density. Higherdensity, in turn, influences numer-ous properties. As density increases,heat softening point, resistance togas and moisture vapor permeationand stiffness increase. However,increased density generally resultsin a reduction of stress crackingresistance and low temperaturetoughness.H|H|H|H|H|H| C C C C C C |H HCH H HCH H HCH|De n sit y|H|H|HPolyolefins are semi-crystalline poly-mers which means they are com-posed of molecules which arearranged in a very orderly (crystalline)structure and molecules which arerandomly oriented (amorphous). Thismixture of crystalline and amorphousregions (Figure 2) is essential inproviding the desired properties toinjection molded parts. A totallyamorphous polyolefin would begrease-like and have poor physicalproperties. A totally crystalline poly-olefin would be very hard and brittle.Ethylene copolymers, such as ethyl-ene vinyl acetate (EVA), are madeby the polymerization of ethyleneunits with randomly distributedcomonomer groups, such as vinylacetate (VA).Ch a in b ra n ch in g LDPE resins have densities rang-ing from 0.910 to 0.930 gramsPolymer chains may be fairly linear,as in high density polyethylene, orhighly branched as in low densitypolyethylene. For every 100-ethyleneunits in the polyethylene molecularchain, there can be one to ten shortor long branches that radiate three-dimensionally (Figure 1). The degreeand type of branching are con-per cubic centimeter (g/cm LLDPE resins range from 0.915 to0.940 g/cm3)3HDPE resins have linear molecularchains with comparatively few sidechain branches. Therefore, thechains are packed more closelytogether (Figure 3). The result iscrystallinity up to 95 percent. LDPEresins generally have crystallinityfrom 60 percent to 75 percent.LLDPE resins have crystallinity from60 percent to 85 percent. PP resinsare highly crystalline, but they arenot very dense. PP resins have a HDPE resins range from 0.940to 0.960 g/cm3As can be seen, all natural poly-olefin resins, i.e, those without anyfillers or reinforcements, havedensities less than 1.00 g/cm3. Thislight weight is one of the keyadvantages for parts injection mold-ed from polyolefins. A generalguide to the effects of density onthe properties for various types ofpolyethylene resins is shown inTable 1.trolled by the process (reactor), cat-alyst, and/or any comonomers used.Chain branching affects many ofthe properties of polyethylenesincluding density, hardness, flexibili-ty and transparency, to name a few.Chain branches also become pointsin the molecular structure whereoxidation may occur. If excessivelyhigh temperatures are reachedduring processing, oxidation canFigure 2. Crystalline (A) and amor-phous (B) regions in polyolefinMo le cu la r w e ig h tAtoms of different elements, such ascarbon, hydrogen, etc., have differ-ent atomic weights. For carbon, theatomic weight is 12 and for hydro-gen it is one. Thus, the molecularweight of the ethylene unit is thesum of the weight of its six atoms(two carbon atoms x 12 + fourhydrogen x 1) or 28.Figure 1. Polyethylene chain w ithlong side branchesC CC CC C C C CCCC CCCCCCCC CCCC CCCC C C C C C C C C C C C C C C C CCCC C CCCCCCC C C C CCCC C C C3 Unlike simple compounds, likeethylene or propylene, every poly-olefin resin consists of a mixture oflarge and small chains, i.e., chainsof high and low molecular weights.The molecular weight of thepolymer chain generally is in thethousands and may go up to overone million. The average of theseis called, quite appropriately, theaverage molecular weight.MFR is the weight in grams of amelted resin that flows through astandard-sized orifice in 10 minutes(g/10 min). Melt flow rate isinversely related to the resinsaverage molecular weight: as theaverage molecular weight increases,MFR decreases and vice versa.injection molding resins are char-acterized as having medium, highor very high flow.For injection molding grades, theMFR (MI) values for polyethylenesare generally determined at 190C(374F) using a static load of2,160 g. MFR values for polypropy-lenes are determined at the sameload but at a higher temperature230C (446F). The MFR of otherthermoplastics may be determinedusing different combinations oftemperatures and static load. Forthis reason, the accurate predictionof the relative processability ofdifferent materials using MFR datais not possible.Melt viscosity, or the resistance ofa resin to flow, is an extremelyimportant property since it affectsthe flow of the molten polymerfilling a mold cavity. Polyolefinswith higher melt flow rates requirelower injection molding processingpressures, temperatures and shortermolding cycles (less time neededfor part cooling prior to ejectionfrom the mold). Resins with highviscosities and, therefore, lowermelt indices, require the oppositeconditions for injection molding.As average molecular weightincreases, resin toughness increases.The same holds true for tensilestrength and environmental stresscrack resistance (ESCR) crackingbrought on when molded parts aresubjected to stresses in the pres-ence of materials such as solvents,oils, detergents, etc. However, high-er molecular weight results in anincrease in melt viscosity andgreater resistance to flow makinginjection molding more difficult asthe average molecular weightincreases.Mo le cu la r w e ig h td ist rib u t io nDuring polymerization, a mixture ofmolecular chains of widely varyinglengths is produced. Some may beshort; others may be extremely longcontaining several thousandmonomer units.It should be remembered thatpressure influences flow properties.Two resins may have the same meltindex, but different high-pressureflow properties. Therefore, MFRor MI must be used in conjunctionwith other characteristics, suchas molecular weight distribution,to measure the flow and otherproperties of resins. Generally,Melt flow rate (MFR) is a simplemeasure of a polymers melt vis-cosity under standard conditionsof temperature and static load(pressure). For polyethylenes, it isoften referred to as melt index (MI).The relative distribution of large,medium and small molecular chainsin the polyolefin resin is importantto its properties. When the distribu-tion is made up of chains close tothe average length, the resin is saidto have a “narrow molecularTable 1. General guide to the effects of polyethylene physical propertieson properties and processingweight distribution.” Polyolefinswith “broad molecular weightdistribution” are resins with a widervariety of chain lengths. In general,resins with narrow molecularAS MELT INDEXINCREASESAS DENSITYINCREASESDurometer hardness (surface)Glossremains the sameimprovesincreasesimprovesweight distributions have good low-temperature impact strength andlow warpage. Resins with broadmolecular weight distributionsgenerally have greater stress crack-ing resistance and greater ease ofprocessing (Figure 4).Heat resistance (softening point)Stress crack resistanceMechanical flex liferemains the samedecreasesimprovesdecreasesdecreasesremains the sameincreasesdecreasesProcessability (less pressure to mold)Mold shrinkageimprovesdecreasesThe type of catalyst and theMolding speed (faster solidification) remains the sameincreasespolymerization process used toproduce a polyolefin determinesits molecular weight distribution.The molecular weight distribution(MWD) of PP resins can also bealtered during production by con-trolled rheology additives that selec-tively fracture long PP molecularPermeability resistanceStiffnessremains the sameremains the samedecreasesimprovesincreasesToughnessdecreasesdecreasesincreasesTransparencyWarpageremains the samedecreases4 chains. This results in a narrowermolecular weight distribution and ahigher melt flow rate.Mo d ifie rs a n da d d it ive smeet the requirements of manyareas of application.Polyolefin resins with distinctly dif-ferent properties can be made bycontrolling the four basic molecularproperties during resin productionand by the use of modifiers andadditives. Injection molders canwork closely with their Equistarpolyolefins sales or technical servicerepresentative to determine theresin which best meets their needs.Numerous chemical modifiers andadditives may be compounded withpolyolefin injection molding resins.In some grades, the chemical modi-fiers are added during resin manu-facture. Some of these additivesinclude:Co p o lym e rsPolyolefins made with one basictype of monomer are calledhomopolymers. There are, however,many polyolefins, called copoly-mers, that are made of two or moremonomers. Many injection moldinggrades of LLDPE, LDPE, HDPE andPP are made with comonomers thatare used to provide specific propertyimprovements. Antioxidants Acid scavengers Process stabilizers Anti-static agents Mold release additives Ultraviolet (UV) light stabilizers NucleatorsEquistar polyolefins technical servicerepresentatives are also available toassist injection molders and end-users by providing guidance for tooland part design and the develop-ment of specialty products to fulfillthe requirements of new, demand-ing applications.The comonomers most often usedwith LLDPE and HDPE are calledalpha olefins. They include butene,hexene and octene. Othercomonomers used with ethylene tomake injection molding grades areethyl acrylate to make the copoly-mer ethylene ethyl acrylate (EEA)and vinyl acetate to produce ethyl-ene vinyl acetate (EVA). Clarifiers LubricantsWo rkin g clo se lyw it h m o ld e rsHo w p o lyo le fin sa re m a d eEquistar offers a wide range ofpolyolefin resins for injection mold-High-purity ethylene and propylenegases are the basic feedstocks formaking polyolefins (Figure 5). Thesegases can be petroleum refinery by-products or they can be extractedfrom an ethane/propane liquifiedgas mix coming through pipelinesEthylene is used as a comonomerwith propylene to produceing, including AlathonAlathon LDPE, Petrotheneand LLDPE, Equistar PP, UltratheneEVA copolymers and FlexatheneTPOs. These resins are tailored toHDPE,polypropylene random copolymers.Polypropylene can be made moreimpact resistant by producing ahigh ethylene-propylene copolymerin a second reactor forming a finelydispersed secondary phase of ethyl-ene-propylene rubber. Productsmade in this manner are commonlyreferred to as impact copolymers.LDPEFigure 5. Olefin manufacturing processETHYLENECRACKERPURIFIEDPROPYLENETO PIPELINE ORPOLYMERIZATIONFigure 4. Schematic representationof molecular w eight distributionLPG, HYDROCARBONS,AND FUEL COMPONENTSSEPARATIONCOLUMNPROPYLENENarrow MolecularWeight Distribution654654Broad MolecularWeight DistributionPURIFICATION COLUMNS321321PURIFIEDETHYLENETO PIPELINE ORPOLYMERIZATIONFRACTIONATIONCOLUMNETHYLENE ANDPROPYLENEMIXEDFEEDSTOCKETHYLENEETHANE AND PROPANEFEED TO CRACKERMOLECULAR WEIGHT5 from a gas field. High efficiency inthe ethane/propane cracking andpurification results in very pureethylene and propylene, which arecritical in the production of highquality polyolefins.Figure 6. Left, polypropylene unit at Morris, Illinois plant. Right, HDPE unitat Matagorda, Texas plantEquistar can produce polyolefins bymore polymerization technologiesand with a greater range ofcatalysts than any other suppliercan. Two of Equistars plants arepictured in Figure 6.Lo w d e n sit yp o lye t h yle n e (LDPE)Figure 7. LDPE high temperature tubular process diagramTo make LDPE resins, Equistar useshigh pressure, high temperaturetubular and autoclave polymeriza-tion reactors (Figures 7 and 8).Ethylene is pumped into the reac-tors and combined with a catalystor initiator to make LDPE. The LDPEmelt formed flows to a separatorwhere unused gas is removed,recovered, and recycled back intothe process. The LDPE is then fed toan extruder for pelletization.FIRST STAGESECOND STAGECOMPRESSORCOMPRESSORHIGH PRESSURETUBULAR REACTORETHYLENEUNREACTED MONOMERTO RECOVERYPOLYETHYLENE MELTSECOND STAGESEPARATORFIRST STAGESEPARATORADDITIVESAdditives, if required for specificapplications, are incorporated atthis point.POLYETHYLENE MELTHig h d e n sit yp o lye t h yle n e (HDPE)HOT MELT EXTRUDERThere are a number of basicprocesses used by Equistar for mak-ing HDPE for injection moldingapplications including the solutionprocess and the slurry process. Inthe multi-reactor slurry process usedby Equistar (Figure 9), ethylene anda comonomer (if used), togetherwith an inert hydrocarbon carrier,are pumped into reactors wherethey are combined with a catalyst.However, in contrast to LDPE pro-duction, relatively low pressures andtemperatures are used to produceHDPE. The granular polymer leavesthe reactor system in a liquid slurryand is separated and dried. It isthen conveyed to an extruderFigure 8. LDPE high temperature autoclave process diagramFIRST STAGECOMPRESSORSECOND STAGECOMPRESSORHIGH PRESSUREAUTOCLAVEREACTORETHYLENEUNREACTED MONOMERTO RECOVERYPOLYETHYLENE MELTSECOND STAGESEPARATORFIRST STAGESEPARATORADDITIVESPOLYETHYLENE MELTwhere additives are incorporatedprior to pelletizing.Equistar also utilizes a multi-reactorsolution process for the productionHOT MELT EXTRUDER6 of HDPE (Figure 10). In this process,the HDPE formed is dissolved in thesolvent carrier and then precipitatedin a downstream process. An addi-tional adsorption step results in avery clean product with virtually nocatalyst residues.Sh ip p in g a n dh a n d lin gp o lyo le fin re sin sout resin manufacture and subse-quent handling, right throughdelivery to the molder, ensures thecleanliness of the products.When bulk containers are delivered,the molder must use appropriateprocedures for unloading the resin.Maintenance of the in-plant materi-al handling system also is essential.When bags and boxes are used,It is of utmost importance to keeppolyolefin resins clean. Equistarships polyolefin resins to molders inhopper cars, hopper trucks, corru-gated boxes, and 50-pound plasticbags. Strict quality control through-Because both of these processesutilize multiple reactors, Equistarhas the capability of tailoringand optimizing the molecularweight distribution of the variousproduct grades to provide a uniquerange of processability and physicalproperties.Figure 9. HDPE parallel reactors slurry processUNREACTED MONOMERS TO RECOVERYSTIRREDSEPARATIONVESSELREACTORVESSELSTIRREDREACTOR VESSELLin e a r lo w d e n sit yp o lye t h yle n e (LLDPE)ETHYLENEBUTENEETHYLENEBUTENEEquistar uses a gas phase processfor making LLDPE (Figure 11). Thisprocess is quite different from theLDPE process, but somewhat similarto the HDPE process. The majordifferences from the LDPE processare that relatively low pressure andlow temperature polymerizationreactors are used. Another differ-ence is that the ethylene is copoly-merized with butene or hexenecomonomers in the reactor. UnlikeHDPE, the polymer exits the reactorin a dry granular form, which issubsequently compounded withadditives in an extruder.CATALYSTCATALYSTPOWDERDRYERHOLD VESSELSPOWDER SLURRYPOWDERFEEDADDITIVESADDITIVE BLENDEREXTRUDERFigure 10. HDPE solution processWith changes in catalysts andoperating conditions, HDPE resinsalso can be produced in some ofthese LLDPE reactors.FIRST STAGE PARALLEL REACTORSADSORPTION UNIT(CATALYST REMOVAL)ETHYLENEOCTENEPo lyp ro p yle n eUNREACTEDCATALYSTSOLVENTTo make PP, Equistar uses both avertical, stirred liquid-slurry process(Figure 12) and a vertical, stirred,fluidized-bed, gas-phase process(Figure 13). Equistar was the firstpolypropylene supplier in the UnitedStates to use gas-phase technologyto produce PP. Impact copolymersare produced using two, fluidizedbed, gas phase reactors operatingin series.MONOMERSAND SOLVENTTO RECOVERYADDITIVESSECOND STAGEREACTORETHYLENESOLVENTTHREE STAGESEPARATOR SYSTEMEquistars polyolefin productionfacilities are described in Table 2.TUBULAR REACTORHOT MELT EXTRUDER7 special care is necessary in openingthe containers, as well as coveringthem, as they are unloaded.When cartons of resin are movedfrom a cold warehouse environmentto a warm molding area or whentransferring cold pellets from a siloto an indoor storage system, thetemperature of the material shouldbe allowed to equilibrate, for up toeight hours to drive off any conden-sation before molding.The best way to improve resin uti-lization is to eliminate contaminantsfrom transfer systems. If bulk han-dling systems are not dedicated toone material or are not adequatelypurged, there is always the possibili-ty of contamination resulting fromremnants of materials previouslytransferred.Reground resin, whether used as ablend or as is, should also be strin-gently protected to keep it free ofcontamination. Whenever possible,the regrind material should be usedas it is generated. When this is notpossible, the scrap should be col-lected in a closed system and recy-cled with the same precautionstaken for virgin resin. In all cases,the proportion of regrind usedshould be carefully controlled toassure consistency of processingand part performance.Figure 11. LLDPE fluidized bed processUNREACTED MONOMERS TO RECOVERYFLUIDIZEDBED REACTORMa t e ria l h a n d lin gEquistar utilizes material handlingsystems and inspection proceduresthat are designed to prevent exter-nal contamination and productcross-contamination during produc-tion, storage, loading and shipment.CATALYSTREACTOR POWDERADDITIVESBUTENE ORHEXENESince polyolefin resins are non-hygroscopic (do not absorb water)they do not require drying prior tobeing molded. However, undercertain conditions, condensationmay form on the pellet surfaces.POWDERFEEDADDITIVE BLENDERETHYLENEEXTRUDERTable 2. Equistar polyolefinproduction facilitiesFigure 12. PP slurry processBAYPORT, TXPolypropyleneLow Density PolyethyleneDILUENT AND UNREACTEDMONOMER TO RECOVERYCHOCOLATE BAYOU, TXHigh Density PolyethylenePROPYLENECATALYSTDILUENTCLINTON, IAWET REACTOR POWDERLow Density PolyethyleneHigh Density PolyethyleneSEPARATIONVESSELLAPORTE, TXLow Density PolyethyleneLinear Low Density PolyethylenePOWDER DRYERADDITIVESSTIRRED REACTORVESSELMATAGORDA, TXHigh Density PolyethylenePOWDERFEEDMORRIS, ILLow Density PolyethyleneLinear Low Density PolyethylenePolypropyleneADDITIVE BLENDERVICTORIA, TXHigh Density PolyethyleneEXTRUDER8 Occasionally, clumps of “angel hair”or “streamers” may accumulatein a silo and plug the exit port.Contaminants of this type can alsocause plugging of transfer systemfilters and/or problems that affectthe molding machine. All of theseproblems can result in moldingmachine downtime, excessive scrapand the time and costs of cleaningsilos, transfer lines and filters.Figure 13. PP dual reactors gas-phase processUNREACTED MONOMERSTO RECOVERYETHYLENEREACTOR POWDERADDITIVESSEPARATIONVESSELSTIRRED SECONDARYREACTOR VESSELPolyolefin dust, fines, streamers andangel-hair contamination may begenerated during the transfer ofpolymer through smoothboreADDITIVE BLENDERpiping. These transfer systems alsomay contain long radius bends toconvey the resin from a hopper carto the silo or holding bin. A poly-olefin pellet conveyed through atransfer line travels at a very highvelocity. As the pellet contacts thesmooth pipe wall, it slides andfriction is generated. The friction,in turn, creates sufficient heat toraise the temperature of the pelletsurface to the resins softeningpoint. As this happens, a smallamount of molten polyolefin isdeposited on the pipe wall andfreezes almost instantly. Over time,this results in deposits described asangel hair or streamers.PROPYLENECATALYSTPOWDER FEEDSTIRRED PRIMARYREACTOR VESSELEXTRUDERHo w t o so lve m a t e ria lh a n d lin g p ro b le m shardness, which in turn leads tolonger surface life.The rounded edges obtainedminimize the initial problemsencountered with dust and fines.They also reduce metal contamina-tion possibly associated with thesandblasted finish.Since smooth piping is a leadingcontributor to angel hair andstreamers, one solution is to rough-en the interior wall of the piping.This causes the pellets to tumbleinstead of sliding along the pipe,minimizing streamer formation.However, as the rapidly movingpolyolefin pellets contact anWhenever a new transfer system isinstalled or when a portion of anexisting system is replaced, theinterior surfaces should be treatedby either sand or shot blasting. Theinitial cost of having this done is faroutweighed by the prevention offuture problems.As the pellets meet the pipe wall,along the interior surface of a longradius bend, the deposits becomealmost continuous and streamersare formed. Eventually, the angelhair and streamers are dislodgedfrom the pipe wall and find theirway into the molding process, thestorage silo or the transfer filters.The amount of streamers formedincreases with increased transfer airtemperature and velocity.extremely rough surface, smallparticles may be broken off thepellets creating fines or dust.Two pipe finishes, in particular, haveproven to be effective in minimizingbuildup and giving the longest lifein transfer systems. One is a sand-blasted finish of 600 to 700 RMSroughness. This finish is probablythe easiest to obtain. However, dueto its sharp edges, it will initiallycreate dust and fines until theElimination of long-radius bendswhere possible is also important asthey are probably the leading contrib-utor to streamer formation. Whenthis type of bend is used, it is criticalthat the interior surface should beeither sand- or shot-blasted.Other good practices of materialhandling include control (cooling)of the transfer air temperature tominimize softening and melting ofthe pellets. Proper design of thetransfer lines is also critical in termsof utilizing the optimum bend radii,blind tees, and proper angles.Consult your Equistar technicalservice engineer for guidance inthis area.edges become rounded.The use of self-cleaning, stainlesssteel “tees” in place of long bendsprevents the formation of streamersalong the curvature of the bend,causing the resin to tumble insteadof slide (Figure 14). However, thereis a loss of efficiency within thetransfer system when this method isused. Precautions should be takenThe other finish is achieved withshot blasting using a #55 shotwith 55-60 Rockwell hardness toproduce a 900 RMS roughness.Variations of this finish are com-monly known as “hammer-finished”surfaces. The shot blasting allowsdeeper penetration and increases9 Figure 14. Eliminate long-radiusbends w here possible. The use ofstainless steel “tees” prevents theformation of streamers along thecurvature of the bend. Allow blowers to run for severalminutes after unloading toclear the lines and reduce thechance of cross-contaminationof product.transfers the finished blend to theindividual molding machines.Th e in je ct io nm o ld in g p ro ce ssInformation regarding transfer sys-tems and types of interior finishesavailable can be obtained frommost suppliers of materials handlingequipment or by consulting yourEquistar technical service engineer.Complete systems can be suppliedwhich, when properly maintained,efficiently convey contamination-free product.The injection molding processbegins with the gravity feeding ofpolyolefin pellets from a hopper intothe plasticating/injection unit of themolding machine. Heat and pressureare applied to the polyolefin resin,causing it to melt and flow. Themelt is injected under high pressureinto the mold. Pressure is main-tained on the material in the cavityuntil it cools and solidifies. Whenthe part temperatures have beenreduced sufficiently below the mate-rials distortion temperature, themold opens and the part is ejected.to ensure that sufficient blowercapacity is available to prevent clog-ging of the transfer lines and main-tain the required transfer rate.A general reference manual,“Handling and Storage of EquistarPolyolefins” is also available.To extend the lif