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The relative distribution of large,medium and small molecularchains in the polyolefin resin isimportant to its properties. Whenthe distribu-tion is made up ofchains close to the average length,the resin is said to have a “narrowmolecular weight distribution.”Polyolefins with “broad molecularweight distribution” are resins witha wider variety of chain lengths. Ingeneral, resins with narrowmolecular weight distributions havegood low-temperature impactstrength and low warpage. Resinswith broad molecular weightdistributions generally have greaterstress cracking resistance andgreater ease of processing (Figure4).copolymer ethylene ethyl acrylate(EEA) and vinyl acetate toproduce ethylene vinyl acetate(EVA).Ethylene is used as a comonomerwith propylene to producepolypropylene randomcopolymers. Polypropylene can bemade more impact resistant byproducing a high ethylenepropylene copolymer in a secondreactor forming a finely dispersedsecondary phase of ethylenepropylene rubber. Products madein this manner are commonlyreferred to as impact copolymers. Working closelywith moldersLyondellBasell offers a wide rangeof polyolefin resins for injectionmold-ing, including Alathon andPetrothene HDPE,Petrothene LDPE, LLDPE, and PP, UltratheneEVA copolymers and Flexathene TPOs. These resins are tailored tomeet the requirements of manyareas of application. LyondellBasell polyolefins technicalservice representatives are alsoavailable to assist injection moldersand end-users by providingguidance for tool and part designand the development of specialtyproducts to fulfill the requirementsof new, demanding applications.CopolymersThe comonomers most often usedwith LLDPE and HDPE are calledalpha olefins. They includebutene, hexene and octene. Othercomonomers used with ethyleneto make injection molding gradesare ethyl acrylate to make the NucleatorsClarifiersLubricantsPolyolefin resins with distinctly different properties can be made bycontrolling the four basic molecularproperties during resin productionand by the use of modifiers andadditives. Injection molders canwork closely with theirLyondellBasell polyolefins sales ortechnical service representative todetermine the resin that best meetstheir needs.The type of catalyst and thepolymerization process used toproduce a polyolefin determines itsmolecular weight distribution. Themolecular weight distribution(MWD) of PP resins can also bealtered during production by controlled rheology additives thatselec-tively fracture long PPmolecular chains. This results in anarrower molecular weightdistribution and a higher melt flowrate.Polyolefins made with one basictype of monomer are calledhomopolymers. There are,however, many polyolefins, calledcopolymers, that are made of twoor more monomers. Many injectionmolding grades of LLDPE, LDPE,HDPE and PP are made withcomonomers that are used toprovide specific propertyimprovements. Modifiers andadditivesNumerous chemical modifiers andadditives may be compounded withpolyolefin injection molding resins.In some grades, the chemicalmodifiers are added during resinmanufacture. Some of theseadditives include: AntioxidantsAcid scavengersProcess stabilizersAnti-static agentsMold release additivesUltraviolet (UV) light stabilizers5How polyolefinsare madeHigh-purity ethylene and propylenegases are the basic feedstocks formaking polyolefins (Figure 5).These gases can be petroleumrefinery by-products or they can beextracted from an ethane/propaneliquified gas mix coming throughpipelines from a gas field. Highefficiency in the ethane/propanecracking and purification results invery pure ethylene and propylene,which are critical in the productionof high quality polyolefins.

LyondellBasell can producepolyolefins by morepolymerization technologies andwith a greater range of catalyststhan any other supplier can. Twoof LyondellBasell’s plants arepictured in Figure 6.Low densitypolyethylene (LDPE)To make LDPE resins,LyondellBasell uses highpressure, high temperaturetubular and autoclavepolymerization reactors (Figures7 and 8). Ethylene is pumpedinto the reactors and combinedwith a catalyst or initiator tomake LDPE. The LDPE meltformed flows to a separatorwhere unused gas is removed,recovered, and recycled backinto the process. The LDPE isthen fed to an extruder forpelletization. Additives, ifrequired for specific applications,are incorporated at this point.High densitypolyethylene (HDPE)Figure 7. LDPE high temperature tubular process diagram6There are a number of basicprocesses used byLyondellBasell for mak-ingHDPE for injection moldingapplications —including thesolution process and the slurryprocess. In the multi-reactorslurry process used byLyondellBasell (Figure 9),ethylene and a comonomer (ifused), together with an inerthydrocarbon carrier, are pumpedinto reactors where they arecombined with a catalyst.However, in contrast to LDPEproduction, relatively lowpressures and temperatures areused to produce HDPE. Thegranular polymer leaves thereactor system in a liquid slurryand is separated and dried. It isthen conveyed to an extruderwhere additives are incorporatedprior to pelletizing.

Linear low densitypolyethylene (LLDPE)Figure 8. High temperature autoclave process diagramLyondellBasell uses a gas phaseprocess for making LLDPE (Figure11). This process is quite differentfrom the LDPE process, butsomewhat similar to the HDPEprocess. The major differencesfrom the LDPE process are thatrelatively low pressure and lowtemperature polymerization reactorsare used. Another difference is thatthe ethylene is copolymerized withbutene or hexene comonomers inthe reactor. Unlike HDPE, thepolymer exits the reactor in a drygranular form, which issubsequently compounded withadditives in an extruder.With changes in catalysts andoperating conditions, HDPE resinsalso can be produced in some ofthese LLDPE reactors.Figure 9. HDPE parallel reactors — slurry processPolypropyleneTo make PP, LyondellBasell uses avertical, stirred, fluidized-bed, gasphase process (Figure 12).LyondellBasell 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.LyondellBasell’s polyolefinproduction facilities are describedinShipping andhandling ofpolyolefin resinsLyondellBasell also utilizes a multireactor solution process for theproduction of HDPE (Figure 10). Inthis process, the HDPE formed isdissolved in the solvent carrier andthen precipitated in a downstreamprocess. An additional adsorptionstep results in a very clean productwith virtually no catalyst residues.Because both of these processesutilize multiple reactors,LyondellBasell has the capabilityof tailoring and optimizing themolecular weight distribution ofthe various product grades toprovide a unique range ofprocessability and physicalproperties.7It is of utmost importance to keeppolyolefin resins clean.LyondellBasell ships polyolefinresins to molders in hopper cars,hopper trucks, corrugated boxes,and 50-pound plastic bags. Strictquality control throughout resinmanufacture and subsequenthandling, right through delivery tothe molder, ensures the cleanlinessof the products.

Table 2. LYB polyolefinproduction facilitiesFigure 10. HDPE solution processBAYPORT, TXFigure 11. LLDPE fluidized bed processvirgin resin. In all cases, theproportion of regrind used shouldbe carefully controlled to assureconsistency of processing andpart performance.Material handlingLyondellBasell utilizes materialhandling systems and inspectionprocedures that are designed toprevent external contamination andproduct cross-contamination duringproduction, storage, loading andshipment.Since polyolefin resins are nonhygroscopic (do not absorb water)they do not require drying prior tobeing molded. However, undercertain conditions, condensationmay form on the pellet surfaces.When bulk containers aredelivered, the molder must useappropriate procedures forunloading the resin. Maintenanceof the in-plant material handlingsystem is also essential. Whenbags and boxes are used, specialcare is necessary in opening thecontainers, as well as coveringthem, as they are unloaded.Reground resin, whether used asa blend or as is, should also bestringently protected to keep itfree of contamination. Wheneverpossible, the regrind materialshould be used as it is generated.When this is not possible, thescrap should be collected in aclosed system and recycled withthe same precautions taken for8When cartons of resin aremoved from a cold warehouseenvironment to a warmmolding area or when transferringcold pellets from a silo to an indoorstoragesystem, the temperature of thematerial should be allowed toequilibrate, for up to eighthours to drive off any condensation before molding.

the pellets. Proper design of thetransfer lines is also critical interms of utilizing the optimum bendradii, blind tees, and proper angles.Consult your LyondellBaselltechnical service engineer forguidance in this area.Figure 12. PP dual reactors – gas-phase processHow to solve materialhandling problemsSince smooth piping is a leadingcontributor to angel hair andstreamers, one solution is toroughen the interior wall of thepiping. This causes the pellets totumble instead of sliding along thepipe, minimizing streamerformation. However, as the rapidlymoving polyolefin pellets contact anextremely rough surface, smallparticles may be broken off thepellets creating fines or dust.The best way to improve resinutilization is to eliminatecontaminants from transfersystems. If bulk handling systemsare not dedicated to one materialor are not adequately purged,there is always the possibility ofcontamination resulting fromremnants of materials previouslytransferred.Occasionally, clumps of “angelhair” or “streamers” mayaccumulate in a silo and plug theexit port. Contaminants of this typecan also cause plugging of transfersystem filters and/or problems thataffect the molding machine. All ofthese problems can result inmolding machine downtime,excessive scrap and the time andcosts of cleaning silos, transferlines and filters. Polyolefin dust,fines, streamers and angel haircontamination may be generatedduring the transfer of polymerthrough smoothbore piping. Thesetransfer systems also may containlong radius bends to convey theresin from a hopper car to the siloor holding bin. A polyolefin pelletconveyed through a transfer linetravels at a very high velocity. Asthe pellet contacts the smooth pipewall, it slides and friction isgenerated. The friction, in turn,creates sufficient heat to raise thetemperature of the pellet surface tothe resin’s softening point. As thishappens, a small amount of moltenpolyolefin is deposited on the pipewall and freezes almost instantly.Over time, this results in depositsdescribed as angel hair orstreamers.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 transferair temperature and velocity.Other good practices of materialhandling include control (cooling)of the transfer air temperature tominimize softening and melting of9Two pipe finishes, in particular,have proven to be effective inminimizing buildup and giving thelongest life in transfer systems.One is a sand-blasted finish of 600to 700 RMS roughness. This finishis probably the easiest to obtain.However, due to its sharp edges, itwill initially create dust and finesuntil the edges become rounded.The other finish is achieved withshot blasting using a #55 shot with55-60 Rockwell hardness toproduce a 900 RMS roughness.Variations of this finish arecommonly known as “hammerfinished” surfaces. The shotblasting allows deeper penetrationand increases hardness, which inturn leads to longer surface life.The rounded edges obtainedminimize the initial problemsencountered with dust andfines. They also reduce metalcontamination possibly associatedwith thesandblasted finish.Whenever a new transfersystem is installed or when aportion of an existing system isreplaced, the interior surfacesshould be treated by either

sand or shot blasting. Theinitial cost of having this doneis far outweighed by theprevention of future problems.Elimination of long-radiusbends where possible is alsoimportant as they are probablythe leading contributor tostreamer formation. When thistype of bend is used, it iscritical that the interior surfaceshould be either sand- or shotblasted.The use of self-cleaning,stainless steel “tees” in placeof long bends prevents theformation of streamers alongthe curvature of the bend,causing the resin to tumbleinstead of slide (Figure 13).However, there is a loss ofefficiency within the transfersystem when this method isused. Precautions should betaken to ensure that sufficientblower capacity is available toprevent clogging of the transferlines and maintain the requiredtransfer rate.Figure 13. Eliminate long-radiusbends where possible. The useof stainless steel "tees"prevents the formation ofstreamers along the curvatureof the bend.To extend the life of the transferpiping, it should be rotated 90 atperiodic intervals. Resin pelletstend to wear grooves in thebottom of the piping as they aretransferred which not onlycontributes to fines and streamerformation but also acceleratedwear due to non-uniform abrasion.Regardless of the type ofequipment used or the materialstransferred, a transfer systemshould be maintained and keptclean in the same manner as anyother piece of productionequipment. Periodic washing anddrying of silos and holding binsreduces the problem of fines anddust build-up due to staticcharges.Other steps to eliminatecontamination include: Inspect the entire transfersystem on a regular basisClean all filters in the transfersystem periodicallyEnsure that the suction line isnot lying on the ground duringstorage or when the system isstarted to prevent debris fromentering t

A Guide To Polyolefin Injection Molding Introduction Polyolefins are the most widely used plastics for injection molding. This manual, A Guide to Polyolefin Injection Molding, contains general information concerning materials, methods and equipment for producing high quality, injection