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A Guide to

Polyolefin Blow Molding

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Polyolefins for Blow Molding

Polyolefins are the most widely usedplastic for blow molding. This book, AGuide to Polyolefin Blow Molding,contains general information

concerning materials, methods andequipment for producing high qualitypolyolefin blow molded products atoptimum production rates.

Blow-Moldable Polyolefins and

Applications

Polyolefins that can be blowmolded include:

z Low density polyethylene (LDPE)z Linear low density polyethylene

(LLDPE)z Medium density polyethylene

(MDPE)z High density polyethylene (HDPE)z Ethylene copolymers, such as

ethylene vinyl acetate (EVA)z Polypropylene and propylene

copolymers (PP)

In general, the advantages ofpolyolefin blow molding resins are goodprocessability, light weight, goodtoughness, outstanding chemicalresistance and relatively low costcompared to other plastics.Furthermore, the basic properties ofpolyolefins can be modified to cover a

broad range of end use properties.Polyolefins can also be coextruded withvarious other polymers, includingethylene vinyl alcohol (EVOH), nylonand tie-layers, to produce multilayercontainers with improved barrierproperties.

Major application areas forpolyolefin in blow molded productsinclude:

z Packaging for such products asmilk and other foods, cleaningfluids, medicines, cosmetics andpersonal care products

z Automotive items, such as gastanks, oil bottles and windshieldfluid containers, air ducts and seatbacks

z Consumer products, including toys,housewares and sporting goods

z Objects for materials handling,including 55-gallon drums andchemical carboys

z Industrial products, such asbusiness machine fluid containers,

z Bellows-shaped shields anddoublewall instrument and toolcarrying cases.

This book contains extensiveinformation on polyolefin blow molding;however, it makes no specificrecommendations for the processing ofEquistar Chemicals resins for specific

applications. For more detailedinformation, please contact yourEquistar polyolefins salesrepresentative.

Other Products from Equistar

Equistar Chemicalsoffers anextensive range of polyolefin resins,plus polyolefin-based tie-layer resinsnot only for blow molding, but also for:

z Injection Moldingz Film Extrusionz

Extrusion Coatingz Sheet and Profile Extrusionz Wire and Cable Coatingz Rotational Molding and Powder

coatingz Blending and Compoundingz Flame Retardant Applicationsz Pipe

Equistar also produces ethylalcohol, ethyl ether, ethylene glycol andethylene oxide. Information on all theseproducts also can be obtained fromyour Equistar sales representative.

Polyolefins are

Thermoplastics Derived from

Petrochemicals

Polyolefins are plastic resinspolymerized from petroleum-basedgases. The two principal gases areethylene and propylene: Ethylene is theprincipal raw material for makingpolyethylene and ethylene copolymerresins; propylene is the main ingredientfor making polypropylene and

propylene copolymer resins.Polyolefin resins are classified as

thermoplastics, which means that theycan be melted, solidified and meltedagain. This contrasts with thermosetresins which, once molded, can not bereprocessed.

Polyolefin resins for blow moldinggenerally are produced in pellet form.The pellets are about one-eighth inchlong and one-eighth inch in diameterand are usually somewhat translucent

H H

C = C

H H

Figure 1. Ethylene monomer

molecular structure

H H H H H H H H H H

C C C C C C C C C C

H H H H H H H H H H

Figure 2. Polyethylene molecular

chain.

Figure 3. Polypropylene molecular

chain.

H H

H HCH H H H HCH

C C C C C C

H H H HCH H H

H

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and water-white in color. Polyolefinresins sometimes will contain additives,such as thermal stabilizers. They canalso be compounded with colorants,antistats, UV stabilizers, etc. to meetspecific end-use needs.

Effects of Molecular Structure

The uses for polyolefin resins are

primarily determined by three basicproperties. These are:

z Average Molecular Weightz Molecular Weight Distributionz Crystallinity, or Density

These properties are essentiallyfixed by the catalyst technology andmanufacturing process used to producea specific grade of resin.The basic building blocks for the gasesfrom which polyolefins are derived arehydrogen and carbon atoms. For

polyethylene, these atoms arecombined to form the ethylenemonomer, CA, i.e., two carbon atomsand four hydrogen atoms (see Fig. 1).In the polymerization process, thedouble bond connecting the carbonatoms is broken. Under the rightconditions, these bonds reform withother ethylene molecules to form longmolecular chains (Fig. 2). The resultingproduct is polyethylene resin.

For polypropylene, the hydrogenand carbon atoms are combined toform the propylene monomer,

CH3CH:CH2, which has three carbonatoms and six hydrogen atoms (Fig. 3).The third carbon atom remains pendantand spirals regularly around thebackbone chains.

Ethylene copolymers, such asethylene-vinyl acetate (EVA) are madeby the polymerization of ethylene unitswith randomly distributed comonomergroups, such as vinyl acetate (VA).

The polymerization of monomerscreates a mixture of molecular chainsof varying lengths. Some are short,others enormously long, containing

several hundred thousand monomerunits. For polyethylene, the ethylenechains have numerous side branches.For every 1,000 ethylene units in themolecular chain, there are about one toten short or long branches. Thebranches radiate three-dimensionally(Fig. 4).

Branching affects many polymerproperties, including density, hardness,flexibility and transparency. Chainbranches also become points in the

molecular network where oxidation mayoccur. In some processing techniqueswhere high temperatures are reached,oxidation can adversely affect thepolymers properties.

Density

Polyolefin blow molded productshave a mixture of crystalline and

amorphous areas. Molecular chains incrystalline areas are arrangedsomewhat parallel to each other. Inamorphous areas, they are randomlyarranged. This mixture of crystallineand amorphous regions (Fig. 5) isnecessary for the proper balance ofproperties in good blow moldedproducts. A totally amorphous polyolefinwould be grease-like and have poorphysical properties; a totally crystallinepolymer would be very hard and brittle.HDPE resins have molecular chainswith comparatively few side branches.

Therefore, the molecular chains arepacked more closely together. Theresult is crystallinity up to 95%. LDPEresins generally are 60 to 75%crystalline, while LLDPE resinscrystallinity levels range from 60 to85%. PP resins are highly crystalline,but they are not very dense. Forpolyethylene, the higher the crystallinity,the higher the resin density. Density is aprimary indicator of the level ofcrystallinity in a polyethylene.Polyolefin blow molding resins have thefollowing density ranges:

LLDPE resins have densities

ranging from 0.910 to 0.940 gramsper cubic centimeter (g/CM3).

LDPE resins range from 0.916 to

0.925 (g/CM3)

MDPE resins range from 0.926 to

0.940 (g/CM3)

HDPE resins range from 0.941 to

0.965 (g/CM3)

PP resins range from 0.890 to

0.905 (g/CM3)

EVA copolymers densities arefunctions of the proportion of vinylacetate incorporated into the resin; asVA increases, density increases.

The density of polyethylene resinsinfluences numerous properties (Table1). With increasing density, someproperties, such as stiffness, areenhanced, while others, such asenvironmental stress crack resistanceand low temperature toughness, arereduced toughness.

Figure 4. Polyethylene chain with

side branches.

Figure 5. Crystalline (A) and

amorphous (B) regions in

polyolefin.

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Molecular Weight

Atoms of different elements, suchas carbon, hydrogen, etc., havedifferent atomic weights. For carbon,the atomic weight is 12, and forhydrogen, it is 1. Thus, the molecularweight of the ethylene unit is the sum ofits six atoms (2 carbon +4 hydrogen) or28. The molecular weight of an

individual polymer molecule is the sumtotal of teh atomic weights of allcombined ethylene units.

Every polyolefin resin consists of amixture of large and small chains, i.e.,chains of high and low molecularweights. The molecular weight of thepolymer chain generally is in thethousands. The average of these iscalled, quite appropriately, the averagemolecular weight. As averagemolecular weight increases, resintoughness increases. The same holdstrue for telongational properties and

environmental stress crack resistance(cracking brought on when a moldedpart is subjected to stresses in thepresence of solvents, oils, detergents,etc.).

Melt Index

Melt Index (for polyethylene)andMelt Flow Rates (for Polypropylene) arean indirect, simple measurement ofthe polymers average molecularweight. Polyethylene resin melt indexis expressed in terms of the weight offlow through a standard orifice in a ten-minute time period. This property istested under standard conditions oftemperature and pressure. Melt index(MI) is inversely related to the resinsaverage molecular weight: as averagemolecular weight increases, MIdecreases. Generally, a polyolefin resinwith high molecular weight has a lowMI, and vice versa.

Melt index is an extremelyimportant property since it describesboth the flow of the molten polymer andmany of the polymers end-useproperties. The resins flow (or output inpounds/hour through an extruder)increases with increasing MI.Polyolefins with lower MI levels cansometimes require higher extrusiontemperatures to make them flow easier.Conversely, most physial properties ofthe solid polymer are enhanced withlower melt index.

Pressure can also influence flowproperties. Two resins may have thesame MI, but different high-pressureflow properties. MI data (Table 2) musttherefore be used in conjunction with

Table 1: General Guide to the Effects of Polyolefin Physical

Properties on Resin Mechanical Properties and

Processing.

Melt Index Density

Increases Increases

Chemical Resistance Stays the Same Increases

Clarity Increases Decreases

Elongation at Break Decreases Decreases

Flexibility Stays the Same Decreases

Gloss Improves Stays the Same

Heat Resistance Decreases Increases

(softening point)

Impermeabil ity to Gases/L iquids Stays the Same Increases

Low Temperature Flexibiity Decreases Decreases

Melt Viscosity Decreases Stays the Same

Mechanical Flex Life Decreases Decreases

Stress Crack Resistance Decreases Decreases

Tensile Strength at Break Decreases Increases

Figure 6. The importance of good shear sensit ivityhigh

viscosity in the parison, low viscosity through the die.

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other properties, such as molecularweight distribution, to fully characterizethe processability of a resin.

Shear Sensitivi ty

Melt index and density are twoextremely useful numbers fordescribing a polyethylene resin; a thirdis shear sensitivity. The viscosity of

melted HDPE appears to changedepending on how rapidly it isundergoing shear, i.e, how fast it isbeing pushed through the extruder ordie. Under moderate rates of shear,HDPE is moderately viscous. When it isforced at high speed through a narrowdie gap, the HDPE melt is sheared at afast rate; at this point it appears to havea low viscosity. Then, when it hangsfrom the die as a blow molding parison,under practically no shear, the sameHDPE appears to be very viscous.

All HDPE resins show varying

degrees of shear sensitivity; in general,the faster the shear rate, the lower theapparent viscosity. HDPE resins withhigh shear sensitivity require lesspower and heat input to yield fasterextrusion rates (Fig. 6).

Molecular Weight Distribution

The relative distribution of large,medium and small molecular chains inthe polyolefin resin is important to itsproperties and especially to shearsensitivity. When the distribution ismade up of chains close to the averagelength, the resin is said to have a

narrow molecular weight distribution(Fig. 7). Broad molecular weightdistribution polyolefins are those resinswith a wider range of chain lengths. Ingeneral, resins with narrow molecularweight distributions will have greaterstress crack resistance and betteroptical properties than resins withbroad molecular weight distributions.Broader molecular weight distributionresins exhibit greater shear sensitivity,providing certain processingadvantages in the blowmoldingprocess.

Comonomers

Polyolefins made with one basictype of monomer are calledhomopolymers. Many polyolefins,called copolymers, consist of two ormore monomers, each of which iscalled a comonomer. Many blowmolding grades of LLDPE, LDPE,HDPE and PP incorporate varyingtypes and amounts of comonomers,which provide specific propertyalterations.

The comonomers most often usedwith LLDPE and HDPE are called alphaolefins, and include butene, hexeneand octene. Another comonomer usedwith ethylene to make blow moldinggrades is vinyl acetate (VA), yieldingethylene vinyl acetate (EVA). Theincorporation of small amounts of VAwith polyethylene results in a resin

which extrudes similarly topolyethylene, but also provides tougherblow molded products with lessstiffness and potentially greater clarity.A wide range of properties is possibledepending upon the amount of VAincorporated and the synthesisconditions used to make the modifiedresins.

Ethylene is the primary comonomerused with PP. PP random copolymershave scattered ethylene groupsconnected to the propylene backboneof the polymer. Such resins are

distinguished by their superior clarity.PP impact copolymers have propylenebackbones containing series ofethylene groups. These propylenecopolymers are known for their mediumto high impact strength at and belowroom temperature.

Modifiers, Addit ives and Tie-Layers

Numerous additives are commonlycompounded with polyolefin blowmolding resins (Table 3). In somegrades thermal stabilizers, antistaticagents and nucleating agents are

added during resin manufacture.Tie-layers are polyolefin-basedresins that are used to bond one type ofnormally incompatible polymer withanother during coextrusion. Tie-layersare specifically designed for use withsuch barrier materials as ethylene vinylalcohol (EVOH), nylon (PA), polyester(PET) and polyvinylidene chloride(PVdC).

Blow Molding Resins

Avai lable from EquistarEquistar offers a wide range of

polyolefin resins for blow molding,including PETROTHENE, LLDPE,LDPE, HDPE and PP, ALATHONHDPE, FLEXATHENETPO, andULTRATHENEEVA copolymers.These resins can be tailored to meetthe requirements of many applications.Some typical specialty grades are:

Table 2: Polyolefin General Purpose

Blow Molding Resins/ Melt Indices

Resin Melt Index Ranges*

g/10 min.

LLDPE

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High Clarity Polypropylene

Injection Blow Molding HDPE High Stiffness HDPE

Excellent ESCR HDPE

Soft, Flexible LDPE

Injection Blow Molding PP

High Clarity EVA

In summary, polyolefin resins withdistinctly different properties can bemade by varying the three basicmolecular properties of density,molecular weight and melt viscosity andby adding comonomers and additives.Processors can work closely with their

Equistar polyolefins salesrepresentatives to determine whichpolyethylene, polypropylene or EVAbest meets their needs.

Shipping and Handling

Polyolefin Blow Molding

Resins

It is of utmost importance to keeppolyolefin resins clean. Contaminatedresins can produce poor products.Polyolefin resins are shipped toprocessors by Equistar in rail cars,hopper trucks, 1000- and 1500-poundpolyethylene-lined corrugated boxesand 50-pound plastic bags. Strictquality control throughout resinmanufacture and subsequent handling,right through delivery to the customer,ensures the cleanliness of the products.

When bulk containers aredelivered, the processor must use

clean, efficient procedures for

unloading the resin. Maintenance of thein-plant materials handling system alsois essential. When bags and boxes areused, special care must be taken inopening the containers, as well ascovering them as they are unloadedand used.

Stringent precautions must also betaken when using reground resin,whether as a blend or alone, to keep itfree of contamination. Wheneverpossible, the regrind should be used asit is generated. When this is notpossible, the scrap should be collected

in a closed system and recycled withthe same precautions taken for virginresin.

How Polyolefin Resins are

Made

High-purity ethylene and propylenegases are the basic feedstocks formaking polyolefins (Fig. 8). Thesegases can be a petroleum refinery

by-product or they can be extractedfrom ethane/propane liquefied gascoming through pipelines from gasfields. High efficiency in the ethane/propane cracking and purificationprocess results in very pure ethyleneand propylene (Fig. 9) that Equistarthen uses to produce its polyolefins.Equistar can produce polyolefins bymore polymerization technologies andwith a greater range of catalysts thanmost any other supplier.

LDPE

To make LDPE resins (Fig. 10),Equistar uses high pressure, hightemperature polymerization reactors.Ethylene gas, pumped into thereactors, is polymerized in thepresence of a catalyst under controlledpressure and temperature conditions.The molten LDPE product flows to aseparator where unused gas isremoved and recycled. Next, the LDPEgoes to a compounding extruder whereadditives are incorporated prior topelletizing.

HDPEThere are two basic processes for

making HDPE (Fig. 11): the morecommon particle form slurry processand the gas phase process. The flow ofthese processes is quite similar to theLDPE process except that relatively lowpressure, low temperature reactors areused.

LLDPE

Equistar also uses a low pressure,gas phase process for making LLDPE(Fig. 12). LLDPE processes are quitedifferent from the LDPE process, but

similar to the HDPE process. The majordifference from the LDPE process isthat low pressure, low temperaturepolymerization reactors are used.Another difference is that the ethyleneis copolymerized with butene or hexenecomonomers in the reactor. A finaldistinction is that the polymer exits thereactor as granules that are thencompounded with additives in anextruder and pelletized. HDPE resinsalso can be made in these reactors.

Figure 8. Diagram of the po lyethylene process from natuaral gas to resin pellets.

EthyleneEthane

Natural Gas

Pipeline

Extraction

Unit

Fractionation

Unit

Cracking and

Purification

Further

Purification

Polyethylene

Propane ButaneLiquified Petroleum

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Figure 9. Left, Polypropylene unit at Equistars Morris, IL, plant; Right, HDPE unit atEquistar s Matagorda, TX, plant.

Figure 10. Process diagram for LDPE product ion in an autoclave reactor.

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PP

To manufacture PP, Equistar uses avertical, stirred fluidized-bed,gas-phase reactor (Fig.13). Equistarwas the first polypropylene supplier inthe U.S. to use gas-phasepolypropylene technology. This processis more energy efficient and produces amore uniform product than other

polypropylene processes.

Materials Conditioning and

Handling

At its resin manufacturing plants,Equistar has extensive systems, suchas filters, cyclones, elutriators, etc. toprevent resin contamination duringproduction, storage, loading andshipping. Since polyolefin resins are

non-hygroscopic (they absorb virtuallyno water), they do not require dryingprior to being molded. However,precautions should be taken to ensurethe cleanliness of the polyolefin pelletsas they are handled at the processorsfacilities.

One of the best ways to improvepolyolefin resin utilization is to eliminatecontaminants from transfer systems.Whenever a polyolefin resin istransferred by a current of air,contamination is possible. This isbecause dust, fines, streamers and

angel hair can be generated duringthe movement of the resin fromshipping containers to storage silos toextruder hoppers.

These contaminants can clog filtersin the transfer system or in themechanisms connected directly to theblow molding machines. The result isstarvation of the extruder. Occasionally,large clumps of angel hair or streamerscan accumulate in a storage silo andclog the exit port. All of these problemscan lead to extruder downtime,excessive scrap and lost work as timeand money are spent cleaning silos,transfer lines and filters instead of blowmolding end products.

Many transfer systems consist ofsmooth bore piping and include in theirstructures long radius bends forconveying the resin from a hopper carto the silo or holding bin. A polyolefinpellet conveyed through a transfer linetravels at a very high velocity and as itcomes in contact with the smooth pipewall, it slides and slows down due tofriction. The friction, in turn, creates

Figure 11. Process diagram for HDPE production in a particle-

form reactor.

Figure 12. Process diagram for LLDPE production in a gas-phase

reactor.

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sufficient heat to raise the pelletssurface temperature to the resinsmelting point. As this happens, a smalldeposit of molten polyolefin is left onthe pipe wall. This deposit freezesalmost instantly.

Eventually, the deposits aredislodged from the pipe wall and findtheir way into the extrusion process, the

storage silo or the transfer filters. Thedislodged angel hair and streamersmay have different melt indices anddensities than the polyolefin beingmolded at the time. They contaminatethe polyolefin melt and can causeproblems in the end product. Theamount of angel hair and streamersformed increases with increases inconveying air temperature and velocityand is greater with smooth bore piping.

Materials Handling Equipment

Design

Since smooth piping is a leadingcontributor to angel hair and streamers,the logical solution is to roughen theinterior wall of the piping. Thisroughness causes the pellets to tumbleinstead of slide along the pipe andstreamer formation is minimized.However, as a rapidly moving polyolefinpellet comes in contact with anextremely rough surface, small particlesare broken off, and fines or dust arecreated.Two pipe wall finishes, in particular,have proved to be the best performers

and give the longest life. One is a sandblasted finish of 600 to 700 RMSroughness. This finish is probably theeasiest to obtain; however, due to itssharp edges, it will create dust andfines until the edges become rounded.The other finish is achieved with shotblasting with a #55 shot with 55-60Rockwell hardness toproduce a 900RMS roughness. Variations of this finishare commonly known as hammerfinished surfaces. The shot blastingallows deeper penetration andincreases hardness, which in turn leadsto longer surface life. The roundededges obtained minimize the initialproblems encountered with dust andfines. They also reduce the potential forcontamination associated with thesandblasted finish.

Whenever a new transfer system isinstalled or when a portion of anexisting system is replaced, the interiorsurfaces should be treated either bysand or, shot blasting. The initial cost isfar outweighed by the prevention offuture problems.

Figure 13. Process for polypropylene production.

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Eliminating long radius bendswhere possible also is important (Fig.14). Long radius bends are probablythe leading contributor to streamerformation. When this type of bend isused, the interior wall should either besand or shot blasted. Usingself-cleaning stainless steel tees inplace of long bends prevents the

formation of streamers along thecurvature of the bend (Fig. 14A)because the resin tumbles instead ofsliding. However, some loss ofefficiency will occur in the transfersystem when this method is used.

These transfer lines should berotated 90 degrees periodically tominimize the grooves resin pellets wearin the bottom of the piping as they aretransferred. The grooves contribute tofines and streamer formation.

Sufficient blower capacity shouldbe available to prevent the transfer

lines from clogging and to maintain therequired transfer rate.

Regardless of the type ofequipment used or the materialtransferred, the transfer system shouldbe maintained and kept clean. Silosand holding bins should be washedperiodically to reduce the problem offines and dust build-up from staticcharges. Other steps to take toeliminate contamination include:

Cleaning all filters in the transfer

system regularly;

Ensuring that suction lines are not

lying on the ground when thesystem is started; this will preventdebris or gravel from entering thesystem;

Placing air filters over hopper car

hatches and bottom valves duringunloading to prevent debris ormoisture from contaminatingmaterial;

First purging lines with air and then

with a small amount of product priorto filling storage silos or bins;

And running blowers several

minutes after unloading to cleanlines and reduce the chance ofcross-contamination.

Additional information regardingtransfer systems and available interiorfinishes can be obtained from mostsuppliers of material handlingequipment.

Figure 16. The five steps in the blow molding process.

Melt Resin

Extrude Parison or Mold Preform

Transfer Parison/Preform to Blow Mold

Blow Mold Part

Eject Part

Figure 15. Gravimetric blending units accurately measure

components in polyolefin extrusion.

Courtesy of HydReclaim Corp.

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Blending With Colorants and

Additives

Colorants and additives areblended with natural polyolefins duringextrusion to enhance the performanceand/or asthetics of the finished part.

Accurate and complete blending isessential for consistent quality in theend product. On-the-machine blending

units consist of multiple hoppers whichare fed different resin compoundingredients (Fig. 15). Colorant oradditive concentrates, regrind and baseresin are combined using eithervolumetric or weightloss feeding(gravimetric) techniques. The latterusually more accurate. Microprocessorcontrols determine and monitor theamounts of materials fed into a mixingchamber. Recipes can be stored in thecontrol unit for instant recall. Centralblending units also can be used whenmuch higher throughputs are needed

than are possible with on-the machineblenders. Transfer to the extruder isdone by a central vacuum loadingsystem.

The Main Blow Molding

Methods

There are three basic blow moldingtechniques: continuous extrusion,intermittent extrusion and injection blow

molding. There are numerous variationsof each. However, all the blow moldingprocesses consist of five successivestages (Fig. 16):

1 . Melting the resinThis is done using a continuousextruder, a reciprocating screwextruder or a reciprocating screwinjection unit.

2. Forming the parison or preformIn continuous extrusion andreciprocating screw extrusion, aparison is formed by forcing the

plastic melt through a die. Theparison is a hot tube, usually with acircular, but sometimes anelliptical or even rectangular crosssection. In continuous extrusion,the parison is continuouslyextruded.In the reciprocating extrusiontechnique, parisons areintermittently formed to set lengths.In injection blow molding, the meltis injected into a mold cavity to

form a test tube- like preform.Multiple preforms are usuallymolded in a single injection cycle.

3. Transferring the parison orpreform

In continuous extrusion, an openmold moves to the parison, closeson the parison, cuts a section off,and then reverses its path, allowing

the extruder to continue extruding aparison for the next cycle. Inreciprocating extrusion, the moldhalves generally are positioneddirectly around the parisonextrusion area and they close ontothe parison.In injection blow molding, there aretwo basic methods for transferringthe preforms:a. directly to the blowing station

(most common withpolyolefins).

b. indirectly -the preforms cool,

are ejected and then stored ininventory for subsequentforming in a separate blowmolding unit.In the direct method, the moldindexes to a second blowingposition. In the indirectmethod, the cooled preformsare loaded onto a conveyorthat first transports themthrough a heating oven andthen to a blow molding station(common with PETprocessing).

4. Inflating the parison or preformAll of the various blow moldingtechniques use the same basicprocess for forming the hollow,blow molded part: high pressure airinjected into the hot parison orpreform forces it out against theinside surfaces of a mold cavity.The high pressure air enters theparison or preform held in theclosed mold through a blow pin.

5. Ejecting the finished b lowmolded container or other article

After the part has been blown in the

mold cavity and cooled sufficientlyfor handling without distortion, themold opens and the object isejected. With injection blowmolding, the ejected item is totallyfinished, i.e., it requires nosubsequent machining to removeany scrap. With continuousextrusion and reciprocatingextrusion blow molding, the ejectedpart usually is transferred to atrimming station where scrap from

the bottom and top of the partcalled flash is cut off. Someextrusion blow molding machinestrim parts directly in the mold.

Blow Molding Equipment

Bothcontinuous extrusion blowmolding, the most widely usedtechnique, and intermittent blow

molding systems include a hopper,extruder, die head and blow handlingunit. The primary differences betweenthese two systems concern theoperation of the extruder screw whether it continually moves or stopsbetween parisons and the design ofthe blow molding unit. This section will

review the basic equipment commonto both systems and then deal withthe differences.Hopper

On top of the hopper, an automaticloader periodically feeds the resin intothe hopper, which, in turn, passes italong to the extruder. Some set upsmay have hopper blenders which feedand proportion resin pellets, regrind,color concentrate and/or additiveconcentrate to the extruder.Two basic types of automatic hopperfeeding systems are common:

1 . Volumetric feeders that refill thehopper on a schedule based on theextrusion systems output.

2. Gravimetric feeders, also calledloss-in-weight feeders, that directly

feed resin into the extruder feedthroat. These feeders measure theweight of material fed to theextruder from a special weighhopper and determine the rate atwhich it is being consumed.

Gravimetric feeders ensurethat the amount of resin in the feedsection of the screw is always thesame; with volumetric feeders,resin pellets tend to compact in thescrew when the hopper is full. Acomputer compares the actualmaterial consumption rate against

set points, performs statisticalanalysis and makes adjustments asnecessary to maintain specifiedoutput. When a deviation isdetected, the control systemcorrects this by changing the screwspeed.

Hopper throats may bewater-cooled to prevent resin pelletsfrom sticking together and bridgingover. During shut down, if an extruder

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is run neutral, i.e., hot, the screwshould be kept turning until all resin hasmoved out of the screw flights visiblefrom the hopper. Otherwise, resin maymelt and bridge near the hopper andchoke off output.

Extruder

The extruder consists of a barrelthat encloses a screw, heaters on theoutside of the barrel, thermocouples to

measure melt temperature and,sometines, a screen pack throughwhich the melt is forced as it flows intothe die head (Fig. 17).

Barrels

Extruder barrels are lined withwear-resistant metal liners, made frommaterials such as tungsten carbide.With such liners, these barrels can lastfor years under continuous use. Mostbarrels have smooth bores, but thoseused for processing high-molecularweight (HMW) HDPE and PP can have

a grooved feed zone for greaterthroughput efficiency (Fig. 18). Mostcommon extruder barrels have heatersalong their entire length. In the specificcase of grooved feed-zone extruders,the feed zones are water cooled.

Heaters

Heat to melt the resin on its waythrough the barrel is supplied internallyby frictional forces caused by themixing and compressing action of the

screw; and externally by heaters. Theadapter and die head also are heatedto prevent heat loss from the melt.Band heaters are the most commontype of electric heater used. Theyrespond rapidly, are easy to adjust andrequire minimal maintenance. Asshown in Fig. 17, the heater bands aredistributed along the barrels length inzones; generally there are three or fourindependent zones, each with its own

heating control. About 25 to 45 watt/in.(4 to 7 watt/cm) of barrel surface givesadequate heat.

Electric resistance heating isusually provided on extruders, althoughinduction heating is occasionally used.Each of the independent electricalheating zones is regulated by acontroller, such as an indicatormounted on the temperature controlboard. A drop in one zone may indicatea defective heater; a rise may point to ahot spot due, for instance, to therubbing of a damaged screw on the

barrel lining.It the heating capacity of the

extruder is inadequate, heaters will stayon most of the time. To determine ifand where heaters are not working,some extruders are equipped with aheat -failure safety alarm system. Thealarm warns of heat loss or blown fusessomewhere in the extruder or die.However, heater failures occurringduring operation may not be detectedsince internal frictional heat may be

Figure 17. Cross section of an extruder

sufficient to maintain operatingtemperatures. When an extruder hasbeen shut down, the heat controllersshould be checked before starting anew production run.

Blowers

Blowers, located along the barrellength, can be activated to decreasebarrel temperatures. The blowers alsopermit rapid barrel cooling when the

extruder is shut down. Some blowmolding equipment may also comeequipped with water-cooled orheat-transfer fluid zones for improvedheat transfer.

Thermocouples

Thermocouples, inserted deep intothe barrel wall in the various heatingzones and in some cases even into themelt, monitor processing temperatures(Fig. 17). Thermocouples signalelectronic temperature controllerswhich in turn, regulate heater bands

and cooling devices. Regularmaintenance checks should be made tomake sure thermocouples are tightlyseated in the barrel wall.

Screw

A motor-driven screw rotates withinthe hardened liner of the barrel. Screwstypically run between 50 to 100revolutions per minute (rpm). Mostscrews have three sections, withchannel depth becoming shallower ineach progressive section (Fig. 19).

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Some screws have an additional mixingsection. As the screw rotates, the screwflights force resin in the screw channelforward through the sections where it isheated, melted, thoroughly mixed andcompressed (Table 4).

A long, properly designed screwresults in better melting and mixing ofthe resin than a short one. Long screws

also yield better appearance and wallthickness uniformity in the parison, andan increased production rate.Furthermore, melt temperature is easierto control. The screw should be at least20 times as long as its diameter, andshould have a compression ratio of 3.1to 4:1, depending on the resin used.For example, because of its highermelting point, HDPE generallyprocessed better using a highercompression ratio than LDPE.

For polyolefin blow molding, ascrew with a feed section of six or

seven turns, a compression (ortransition) section of four or five turnsand a metering section of five to eightturns has been found most efficient.Such a screw provides a steady meltflow and increases output at moderatespeeds. At high compression, the meltis mixed well and internally generatedheat is high. Any air carried forwardwith the melt is pushed back and outthrough the hopper. Otherwise, this aircan cause bubbles and, if temperaturesare high enough, oxidization of theresin in the barrel. Screws for polyolefin

blow molding extruders are practicallyalways run neutral, that is, without thecooling that can be achieved bycirculating water through an axial boreabout halfway through the length of thescrew.

Breaker Plate

After travelling the length of thescrew, the melt passes through ascreen pack, supporting breaker plateand the adapter on its way to the die.The round breaker plate (Fig. 17, Table5), is located between the end of the

barrel and the head adapter, usuallyfitting into both, but sometimes only intothe adapter. This prevents leakage. Thethick breaker plate is enclosed by asturdy ring and is pierced by a largenumber of equally spaced holesmeasuring one-sixteenth to one-eighthin. (1.6 to 3.2 mm) in diameter.

Blow molding operators shouldalways keep a spare breaker plateavailable in case the one in use isdamaged or is exchanged along with

Table 4: Functions of the Four Sections o f a Polyethylene Extrusion Screw

Sections Channel Depth Functions

Feed Deep Cool resin pellets are movedforward into hotter barrel zones.

Compression Decreasing Resin is compressed, meltedand mixed. Air carried along slipsback to the feed section.

Metering Shallow Sufficient back pressure iscreated to make the melt homo-geneous (uniform), make itstemperature uniform.

Mixing Thorough blending of meltbefore feeding it into the die

system.

Figure 18. A cross section of the extruder barrel.

Grooved Feeding Zone

Cooling ZoneHopper

Courtesy ofBattenfeld Fischer Blow Molding Machines, Inc.

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the screen pack. Having a cleanbreaker plate ready may eliminatedowntime, because the spare can beplaced in service while the usedbreaker plate is burned out andcleaned.

Screen Pack

The screen pack, located in thebreaker plate (Fig. 17), consists of a

number of stainless steel screens. Themesh number associated with a screenrepresents the number of openingspresent per inch of screen. Therefore,higher mesh number screens are finer(have smaller openings) than lowermesh number screens. The screenpack acts as a filter for foreign matterthat may be inadvertently mixed in theresin. A screen pack can help increaseback pressure in the barrel, particularlyif an extruder (or pressure) valve is notused for this purpose. This can be abenefit in terms of improved melt

homogeneity and color dispersion. Onthe negative side, a screen pack (orswitching to a heavier screen pack),can result in higher resin temperatures.

There is no typical screen pack;different packs are best suited fordifferent jobs. A 20-80-20 mesh screenpack is sometimes recommended forblow molding, meaning a finer 80-meshscreen is sandwiched between twocoarser 80-mesh screens. Fine-meshed (dense) screens should

Table 5: Functions of a Breaker Plate

Support the screen pack

Develop back pressure

Reinforce screen packs action

Al ign the barrel and adapater

Straighten out the spiral flow of the

melt caused by the screw

Serve as a seal between the barrel

and the adapter (which a loose

breaker plate can not do)

Figure 19. Typical extruder screw for HDPE

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generally be located between coarserscreens as this offers better physicalsupport. The temperature of the meltmay be raised slightly by using a muchheavier screen pack (more or finerscreens, or both) which, by increasingthe back pressure, may also generateadditional frictional heat.

Always use stainless-steel

screens, never copper screens,although they are less expensive.Copper is too soft for such a highpressure application; moreover, it mayoxidize and contaminate the resin. Ahinged or swing-gate collar may beused to attach the adapter tightly to thebarrel head and to facilitate screenpack changes. Screen packs do needto be replaced on a periodic basis.

Automatic Screen Changers

Automaticscreen changers, whichhave either a continuous screen band

or rotary units that index when exposedsections become clogged, can be usedwith continuous extrusion blow moldingmachines. The indexing occurs withoutinterrupting the melt flow. Theseeliminate downtime necessary formanual screen changes.

Pressure valves

Pressure valves permit a constantcheck on internal pressures; thesehave a significant effect on melttemperatures. Two types of pressurevalves are common:

1. The internal pressure valve is amovable screw that can beadjusted forward or backward toincrease or decrease pressure.Moving thescrew varies the size ofthe opening between the end of thescrew and the breaker plate andadapter.

2. External pressure valves use a pinarrangement that varies the size ofthe opening at the extrudersadapter, thereby varying pressure.At the end of the extruder barrel,

between the melt screen and theparison die head, there is anadapter (Fig. 17) that constrictsand guides the resin melt flow in astreamlined path with a minimumof resin hangup.

Melt Pumps

Melt pumps,also called gearpumps, can be attached between theend of the extruder and the die head togreatly increase melt quality. Forcontinuous extrusion multilayer blow

molding, these units can deliver astable, surge-free melt output andprovide excellent layer uniformity.These benefits are particularlyimportant for thin barrier layers andadhesive tie-layers found in coextrusionblow molding.

Die Head Unit

The functions of the die head unit(Fig. 20) are:

to form the melt into its final shape,

the parison

to maintain the melt at a constant

temperature

to meter out the melt at a constant

pressure and rate to form a parisonwith a desired wall thickness.

The die head unit opens to theoutside through an accurately ma-chined tube-shaped orifice, through

which the melt is pressed in a steadyflow. The die head should be designedwithout sharp corners or edges that theresin must pass over. Sharp corners oredges tend to bleed off heat and cancause heat streaks in the finishedparison. With most die head designs,the resin must flow around a mandreland reknit on the other side to form thetubular parison. Die heads should beconstructed so that the length is great

enough to allow the resin to remarryafter forming the parison tube. If the diehead length is inadequate, a noticeableweld line will show on the parison,which may remain as a defect on thefinished product. Die heads are usuallydesigned so that dies can be ex-changed easily.

Heaters keep the adapter and diehead at the required temperature. Theyare controlled in the same way asbarrel heaters. However, the adapter

Figure 20. Typical Die Head

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and die head heaters maintain melttemperature and should not be used toincrease it. Melt temperature at the diehead, where the parison forms, rangesfrom 290 to 32F (145 to 165C) forLDPE; 350 to 400F (175 to 205C) forHDPE; and 375 to 425F (1900 to220C) for PP (PP homopolymers haveslightly higher, melting temperatures

than random copolymers). Resins withlower melt indices require melttemperatures at the higher end of theseranges than higher melt index resins.

Generally, LDPE resins for blowmolding have melt indices below 2 g/10min.; HDPE, below 1.0 g/10 min.; andPP, melt flow rates below 2 g/10 min. Ifresin melt temperatures are too low, theparison will be cloudy or dull inappearance and may also show signs ofmelt fracturing or aligatoring. A parisonof this type will not be hot enough toyield good weld lines at the pinch-off

areas. On the other hand, if the melttemperature is too high, the parison willlook clear, have glossy areas and mayemit smoke. The hot parison will bestringy and tend to stretch easily,resulting in thin areas. Pinch-offproblems may also occur, as well asextremely thin weld lines.

In a crosshead die unit, the meltflow is almost always turned 90 degrees(in a right angle) and diverted from thehorizontal to the vertical and extrudeddownward. Some types of extrusionblow molding machines do not require a

crosshead die because the extruderitself is positioned to extrude the parisontube vertically downward. In these moreunusual cases, there is no need to turnthe melt flow in the die head to obtain aseamless tube. At the die face, the meltshould flow straight down to form aparison with uniform wall thickness. Thedie and mandrel in the lower section ofthe die head can typically be changedfor individual jobs. Usually, dies are oneof two types: converging or diverging(Fig.21).

Dies and mandrels for parison

extrusion normally have a land. This isa flat section machined at the end of thedie and mandrel. The land is theworking area of the die and mandrelwhere the volume of the resin beingextruded is constant. Some blowmolders recommend a die land length10 to 40 times as long as the dieopening or clearance, that is, the widthof the die slot. Other molders prefer veryshort die lands. A long die landincreases back pressure on the melt in

Figure 21. Typical Converging and Diverging Dies

the extruder head and improvesmixing. Longer die lands are beneficialwhen the total head length does notallow the resin to properly reweld itselfafter flowing around the core.

A pressure ring above the die andmandrel is often used to erase orsmooth out flow lines in the melt beforeit is extruded as a parison (Fig. 20). Ashort die land, in conjunction with highextrusion speed, may result in outersurface roughness of the parison andthe blown part. This is known as meltfracture. Finding the most favorabledie land length for a particular job is amatter of experience and trial runs. Itmay be necessary to modify the coreand the die body in the land area, or ifthe end product permits a change inwall thickness, to modify the die gapopening.

Parison Die Designs

There are various parison diedesigns. The two kinds generally used

with continuous extrusion blow moldingare center-fed and side-fed.

In a center-fed die, the melt flowsvertically downward around the core,also called the pin (Fig. 22). Center-feddies have an advantage in that the meltdistribution around the core is ratheruniform. The melt flows down towardthe tip of the torpedo-shaped core for acertain distance after having beenturned by 90 degrees in the die head,resulting in straight flow all around thecore. The core is supported inside thedie head unit by either a breaker plateor a spider (Fig. 23).

Generally, the breaker-plate type ispreferable because it will keep the meltflow more uniform. However, both typesof support may yield flow and knit linesin the blown part, especially when lowermelt index resins are used. Flow lineswill result in blow molded items withpoor appearance and impaired strengthalong these lines.

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In side-fed dies, the melt flow hitsthe core, or flow pin, sideways (Fig.24). To obtain uniform melt flow aroundthe side-fed core and out of the die, agroove is frequently machined into thecore.

Multiple dies

In extrusion systems with multipledies, the melt from the extruder passesthrough a manifold into a series ofparison-forming dies. The amount ofmaterial reaching each die is controlled

by choke values in the manifold or, ifthe die is equipped with a parisonprogrammer, by weight control settings(Fig. 25).

Parison Programmer

While being formed, the parison issubject to the distorting downward pullof gravity. Unless extrusion is very fast,the parison thins out at the top andthickens toward the bottom instead ofmaintaining uniform wall thickness. Thisparison sag, also called draw-down andneck-down, results in excessive wall

thickness of the blown item near thebottom of the mold. Sometimes, thismay be preferred, as excess resin oftenstrengthens the shoulder, neck orbottom of a container. But in mostinstances, sag is to be avoided.

Deviations in wall thickness can beovercome by gradually increasing wallthickness during extrusion. This is doneby means of a movable mandrel. Themotion of the mandrel in the die iscontrolled by a servo valve assembly

called a parison programmer. Theparison programmer raises or lowersthe mandrel while the parison is beingformed (Fig. 26). The mandrelsposition is recorded and fed back to thecontroller via a LVDT.

As a result, resin is extruded toform a parison with a controlledvariability in its wall thickness (centersection in Fig. 27), resulting in an endproduct with more uniform wallthickness. Parison programming also isused if the blown end product is not

symmetrical and thus, requires anuneven resin distribution. Parisonprogrammers (Fig. 28) can have 100 ormore profile points, thus permittingexcellent control of the parison crosssection.

Cut-Off Devices

A parison cut-off device, frequentlymounted under the die face, severs theparison from the constantly extrudedtube when the parison reaches therequired length and the blow moldhalves have closed around it. The

cuffing device, as well as the parisonlength are controlled by a sensor, suchas a limit switch photoelectric cell oreven a laser beam. There are varioustypes of cutting devices, including stabknives (for parisons up to three inchesin diameter), hot wire knives (requiredwith PP parisons as well as thin-walledparisons), impact knives and coldparison knives (the most common typeof cuffing device).

Figure 22. Schematic of center-

fed die.

Figure 23. Left: perforated

(breaker plate); Right: spider

(cross-piece type of core support

in a blow molding die.

Figure 24. Schematic drawing of

a side-fed die with a grooved

core.

Figure 25. Multiple-dies, multiple-

stationary-mold equipment.

Figure 26. Parison programming system.

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CoextrusionMultilayer blow molded containersare produced by using multipleextruders, one for each type of resin processed, and special coextrusion dieheads and parison dies (Fig. 29).Coextruded containers can provide:

Excellent odor, moisture, and/or

oxygen barrier characteristics,such as needed for many foodproducts

Specially colored outer surfaces,

such as needed with fluorescents

and metallics Greater environmental stress crack

resistance, such as HDPE bleachbottles with an inner lower densityPE layer and coextruded blowmolded gasoline tanks.

Another growing use of coextrusionis the production of multilayer blowmolded containers with a middle layerof post consumer resin, i.e., recycledresin from discarded HDPE and PPcontainers, sandwiched between twovirgin resin layers.

The Blow Molding Unit for

Continuous Extrusion Blow Molding

Continuous extrusion systems areavailable with various types of blowmolding clamp designs, which differeach other mainly in the way the moldsare mounted and presented to the dieto close around the parison (Fig. 30).The two basic designs are:

Figure 27. A programmed parison designed to fit a particular mold

configuration.

Figure 28. Control unit for a

parison programmer.

Courtesy of Krupp Kautex.

Figure 29. Multiple extruders and special die heads are needed for

coextruded mul tiple-layer bottles.

Courtesy of Graham Engineering Corporation

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Figure 30. A rotary blow mold ing machine

Courtesy of Cincinnati Milacron

Figure 31. Continuous extrusion blow molding machine with a vertical wheel.

Courtesy of Wilmington

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molds that move on a rotating

wheel or table

molds that are shuttled to the

parison die.

The mold halves, mounted onplatens, are held together during theblowing step by presses that are

actuated either by a mechanical togglemechanism or hydraulic cylinders. Lowclamping pressures, compared withthose used with injection moldingmachines, are used with extrusion blowmolding. On some blow moldingpresses, mold closing is carried out intwo steps, first at high speed, with lowerpressure and then slower to full close,with higher pressure. This sequenceprotects the mold from damage byanything that might have fallen betweenthe halves. It also enhances andincreases operator safety.

Vertical Wheel Mold Carrier

Continuous extrusion blow moldingmachines with vertical wheel moldcarriers (Fig. 31) offer greaterproductivity compared to continuousextrusion shuttle or fixed moldreciprocating equipment. While thewheel continuously turns, one mold at atime passes around the die to receiveand close around the parison. In somevertical wheel machines, the moldhalves are connected by a hinge; inother designs they are separate.

Vertical wheel machines are availablein which the parison is extrudeddownward or, in others, upward. In bothcases, the wheel moves faster than theparison, and as a result, the parison isstretched. The control of the parison inthese cases is excellent. The ejecteditems may be connected by sections ofthe parison. This requires trimmingsteps at both ends of the molded itemhowever, creating a substantial amountof scrap that must be granulated andreused. Vertical rotary wheels maycarry 20 or more molds. Machines alsohave been built with two cavities permold, yielding 28 to 36 parts perrevolution. Cycle times yielding 150parts per minute can be achieved.

Horizontal Wheel Mold Carrier

Continuous extrusion blow moldingmachines also are available with themolds mounted on a horizontalturntable (Fig. 32), typically with 4 to 8mold stations. Their productivity isgenerally less than the vertical wheel

Figure 32. Multiple-station turntable.

Figure 33. Continuous extrusion b low mold ing machine, with

a shuttle system.

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machines which carry more moldstations. The operation of thecontinuously moving turntable isessentially the same as that of therotary wheel. Indexing turntables andwheels also are available.

Horizontal turntables may produceless scrap than vertical wheels becausethe parison is cut from the die face by a

cut-off knife. Consequently, the blownpieces are not connected to each otherand the tail flash can be kept short.Automatic ejection can also be easierthan from a mold on a rotary wheel,without the need for knockout pins.The advantages of higher output areobvious. However, the higher outputmachines are more expensive initially,as are dual cavity molds.

Shuttle Mold Carrier

In continuous extrusion blowmolding machines with a shuttle mold

clamp unit, one or more parisons areextruded and the molds move to the diehead horizontally from side-to-side ordiagonally up and down from both sidesof the die head (Fig. 33 and 34).

Open molds, carried along on waybars, alternately move up to closearound and pinch the parison. Theparison is then separated from theextruding tube by a cutting deviceattached to the die or the extruder.Next, the closed mold swings backdown to the clamping station. Blowingcan be done by means of a blowing pin

in the bottom of the mold or, as is morecommon, by injecting the air through ablowing pin that enters the top of themold. In the latter case, finishing workis considerably reduced because noreaming is required. However, flashmay occur on the outside of the part.After the molds open and the blownpart is ejected, the mold rises again toreceive another parison.

Production rates are high with thistype of equipment. One extruder headmay form multiple parisons simulta-neously to feed multiple mold cavities.

Dies are of the crosshead type. Or,to increase production, several headson one extruder may shape severalparisons simultaneously. In this case, avalved manifold die is required tocompensate for pressure and tempera-ture differences in the melt flow or tofeed two sets of molds alternately.

In equipment where the moldmoves sideways, a second mold closesover the continuously forming parisonthe moment the first mold moves to theright or left for blowing. Compared with

Figure 34. Continuous extrus ion blow moding machine with a

shuttle mold carrier.

Courtesy of Uniloy Blow Molding Systems, a division of J ohnson Controls

Figure 35. Extrusion stretch blow molding.

Courtesy of Battenfeld Fischer Blow Molding Machines, Inc.

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the swinging-mold procedure, thesliding-mold method either may permithigher production rates or allow for alonger cooling period, which may beadvantageous to obtain certain endproduct properties.

Hydraulic systems generally areused for clamping and moving themolds, the platens on which they are

mounted and other heavy parts of themolding equipment, such as rams usedin accumulator heads.

Extrusion Stretch Blow Molding

Extrusion stretch blow molding is avariation of extrusion blow molding withshuffle molds (Fig. 35) that producestransparent containers having high sidewall stiffness. Initially, the parison isblown to form a preform, which is thentransferred, while still hot, to a secondlarger and longer mold cavity. Here, along blow pin is inserted into the hot

preform to stretch it to the bottom of themold cavity. Air is then injected throughthe pin to form the container. Withcertain types of materials this stretchingresults in high transparency and thinside walls with good stiffness. Impactproperties are also enhanced.

Blowing air can be injecteddownward through the core via ablowing needle inserted sidewaysthrough the mold wall, or from belowthrough a blowing pin moved up intothe end of the parison that will becomethe neck of a bottle. Sometimes,

different blowing devices are used incombination. To obtain rapid inflation ofthe hollow piece, the volume of blowingair should be as large as possible. Theopening through which the air entersthe mold must be large enough tohandle this air volume. The thinner thewall thickness and the lower the meltand mold temperatures, the faster theblowing rate should be and the higherthe blowing pressure, up to about 150lb./in. (10. 5 kg/c m).

Compressed air injected into a coldmold may lose some pressure because

a cooling gas contracts. High blowingpressure requires high clamp pressureto keep the mold tightly closed duringthe blowing step.

Moisture in the blowing air mayresult in marks on the blown part.Blowing air can be dried by a heatexchanger, which cools thecompressed air, or by traps andseparators in the air lines. Blow pinsalso can serve as in-mold finishingunits for blow molded containers.

Figure 36. Schematic of in termittent parison extrusion into

a stationary mold. Extruder operation is in terrupted after

the parison has reached the desired length and before the

mold halves have closed around i t (left). Operation is

resumed only at the end of the ejection s tep (right).

Figure 37. Continuous extrusion, accumulator head.

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Various types of blow pins for in-moldneck finishing are available (seeIn-mold Trimming pg. 27).

Intermittent Blow Molding Systems

There are three basic types ofintermittent blow molding systems:

1. Intermittent extrusion blow molding

2. Reciprocating Extrusion BlowMolding

3. Injection Blow Molding

Intermittent Extrusion Blow Molding

Intermittent extruder operation witha stationary mold is the simplestpossible blow molding arrangement.The stationary mold is located directlybelow the crosshead die that extrudesthe parison between the mold halves.There may be several molds if morethan one parison is extrudedsimultaneously. Air is blown into the

mold usually through the core in the dieor through a mandrel (blow stick)inserted from below.

In intermittent extrusion blowmolding (Fig. 36), the extruder screwoperation is halted when the parisonhas reached the desired length. Themold halves close around the parison,the finished part is blown, cooled andejected. The next parison is then isextruded. Because the extruderoperation is stopped for the greater partof every blow molding cycle, thisprocess is less efficient than those in

which the extruder operatescontinuously and the cycle times of thevarious moldings overlap.

Accumulators fo r Making Large Parts

Intermittent extrusion blow moldingmachines can be equipped with anaccumulator, a reservoir for gatheringand holding a substantial volume ofwell plasticized melt. A ram in theaccumulator forces the melt at highspeed into and through the extrusiondie to form the parison tube (Fig. 37).The accumulator unit usually is located

between the adapter, or transitionblock, and the die. Most accumulatorsare vertical units, but horizontal typesalso are available.

In the setup illustrated in Fig. 38,the melt enters the accumulator unitfrom the side, filling the annularchamber from two different inlet points,each 180 degrees away from the other(see points A-A in both diagrams). Thisproduces an inner and an outer ring ofmelt in the chamber (B-B). The result is

Figure 39. Schematic cross section of a typical screw

injection mold ing unit showing the screw in the re-

tracted (A) and the forward (B) position.

Figure 38. Example of an accumulator used for blow

molding large parts.

Courtesy of Hartig Plastics Machinery

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that the seam formed by the material inthe inner ring is directly opposite theseam formed by the material in the

outer ring. When the material from theinner and outer rings comes togetherfurther downstream in the flow process(C-C), there is no weld or seam line thatgoes all the way through the parison atany one point, thus ensuring wallintegrity.

An accumulator offers theseadvantages:

1 . It holds ready a large volume ofmelt for large items requiring verylong molding (cooling) cycles, up toseveral minutes in length.

2. It permits high production rates.3. It enables fast extrusion of large

parisons (short parison formationtime) and consequently, shortsuspension time for the parison andthus, very little sag.

4. It results in good parison walluniformity.

5. It enables uniform shot sizes and,thus, a minimum of waste.

6. It decreases idle mold time to aminimum.

An accumulator may hold up to300 lb. (135 kg) of resin. Oneaccumulator may cover several molds

or each mold may have its ownaccumulator. In the latter case, whilethe extruder operates continuously, themolds may be fed alternately; whilesome will receive their parisons, othersare in the cooling stage, etc.

In accumulator type systems, backpressure can be increased by a needlevalve in the hydraulic system. Thisvalve (restriction) controls the hydraulicoil flow from the accumulator so thatmore pressure is required from thecompaction of the resin to force the ramback and the oil from the cylinder

reservoir into the tank reservoir. Thesame principle holds true forreciprocating screw machines. (Seenext section.)

Manifold die heads, generallywithout sequence valves, are also usedin machines equipped with anaccumulator. In such equipment,extrusion through all the dies takesplace simultaneously, so there is novalve-controlled alternate flow required.For best results in die heads that

produce intermittent resin flow, the areain the heads should be great enough toproduce a complete parison for the next

shot. This permits the resin to becomeheat stabilized and produce a moreuniform parison.

Reciprocating Screw Extrusion Blow

Molding

In reciprocating screw extrusionblow molding (Fig. 39), parisons areintermittently produced in separateinjection cycles similar to those of aninjection molding machine. However,the parisons are formed using anextrusion die head rather than a closedmold. The blow molding unit is located

directly under the die head. When aparison of the desired length hasdropped, the clamp closes on it and thecontainer is formed.

The reciprocating screw extrusionblow molding system is widely used toproduce small-mouthed polyethylenemilk bottles and juice containers (Fig40), as well as various wide-mouthedcontainers. The machines are simplerin construction than continuousextrusion blow molding machines, since

Figure 40. Reciprocating screw extrusion b low mold ing sys tem.

Courtesy of Uniloy Blow Molding Systems, a division of J ohnson Controls

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the clamp unit is stationary and does

not require any mold shuttlingmechanism. Further, tooling costs arelower (only one set is needed), andhandleware and off -set neckcontainers can be easily formed. Thefast parison formation that occurs in theinjection cycle permits resins withhigher melt indices to be used sincemelt strength requirements are lessstringent than those needed forcontinuous extrusion blow molding.

In reciprocating screw extrusionblow molding, polyolefin pellets are firstfed into the hopper of the plasticating

unit (see Fig. 39).The screw, in anelectrically heated barrel, rotates andretracts as the resin is pushed forward.As the resin pellets move forward in thescrew channels, they melt by thecombined action of screw and barrelshear forces and heaters located alongthe barrel. Gradually a melt forms; it ismixed, becomes more fluid, andreaches the viscosity required forinjection into the die head. While thepellets are melted in the barrel, thegradually decreasing depth of thescrew channels squeezes the air out of

the softened mass; the air flows backalong the clearance between the screwflights and the barrel and out throughthe hopper. The melt then flows overthe screw tip into a chamber at the frontof the barrel. The build-up of the meltforces the screw backwards.

When the desired amount of melt(shot size) is reached in the chamber,the screw pushes forward and forcesthe melt, at a high speed, through the

nozzle and out through the die head.

The screw then begins to turn againand retracts within the barrel to preparethe next shot. The speed of the screw isin balance with the parison size and thecontainer blow time. Reciprocatingscrew extrusion blow molding machinesare available with shot capacities up to2,500 grams (HDPE) and throughputrates up to 750 lbs./hr.

Generally, reciprocating screwextrusion blow molding machines havedie heads with as many as 10 parisondies. Parison programming also can beincorporated. Container necks can be

finished during the blow molding cyclein a process called prefinishing.In ram down finishing (Fig. 41), theblow pin is inserted into the mold afterthe mold closes on the parison. Theblow pin moves downward to compressthe plastic in the neck area and formthe neck finish. This method is usedwhen neck strength and reliability arerequired.

In pull-up finishing (Fig. 42), theneck is finished when the blow pin isinserted just before the mold closes onthe parison. At the end of the blow

cycle, but before mold opening, theblow pin moves upward to shear theinside diameter of the neck opening.This method often is used forlightweight, single use containers.Reciprocating screw extrusion blowmolding machines can also beequipped with a cooling conveyor topermit containers to cool more beforebeing trimmed. The additional coolingstep increases productivity.

Molds

The blow mold may have a numberof parts, including various inserts, but itusually consists of two halves (Fig. 43).When closed, these halves will formone or more cavities, which willpinch-off and enclose one or moreparisons for blowing. The two moldhalves are usually alike and must havebuilt-in channels for cooling water. Setsof guide pins and bushings or sideplates in both mold halves ensurecavity alignment and proper moldclosing.

Frequently, the head of the mold is

equipped with a cut-off device (shearsteel) level with the upper end of theprotruding mandrel which cuts theparison. A blowing pin in the mold mayhave the additional function of shapingand finishing the neck interior, referredto as calibrating.

Pinch-off

Pinch-off edges generally areprovided at both ends of the moldhalves. The pinch-off sections (Fig. 44)do not cut off the parison tail. Theirprotruding edges, about 0.005 to 0.015inches (0.1 to 0.5mm) wide, squeezethe two sides of the parison together,creating an airtight closure. Theseedges are called the pinch-off lands.Much depends on the construction ofthe pinch-off insert. The total angleoutward from the pinch-off lands shouldbe acute, up to 15 degrees. Thepinch-off should not form a groove,which would weaken the bottom of theblown item along the seam (Fig. 45). Astraight line pinch-off makes it easier toremove the tail piece.

Figure 41. Ram down finishing.

Courtesy of Uniloy Blow Molding Systems, adivision of J ohnson Controls

Figure 42. Pull-up finishing.

Courtesy of Uniloy Blow Molding Systems, adivision of J ohnson Controls

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A high quality pinch-off on a thick-walled parison is more difficult to obtainthan on a thin-walled parison. Onemethod of obtaining more uniformwelds at the pinch-off is to build damsinto the mold halves at the pinch-offareas. These dams force some of themolten resin back into the mold cavitiesto produce strong, even weld lines. The

pinch-off area on finished containersshould not contain heavy globs or foldsat the ends, since they may causecontainer failure. Also, these heavydeposits of resin retain heat. When thecontainers are ejected from the mold,these areas cool and shrink causinghigh stresses to be formed in theadjacent areas.

Mold Materials

Because of the comparatively lowclamping and blowing pressures used,the blow mold need not be made of a

high tensile strength material, althoughmolds for very long production runssometimes are made of steel. Blowmolds are most often made fromaluminum, cast aluminum, castberyllium-copper alloy and cast zincalloys, such as Kirksite. Durability andheat transfer rates are key factors inthe decision about mold materials.

High-grade aluminum alloy moldsare the easiest to machine. On theother hand, aluminum, one of thesoftest of these mold materials, is mosteasily damaged and worn, Cast

aluminum, which has a lower thermalconductivity than cut aluminum, andberyllium-copper molds may be slightlyporous. This porous surface may affectthe appearance of the blown part. Theremedy is to coat the inside of the moldhalves with a sealer, but this may affectthe heat transfer between the resinblown against the mold and the moldwalls.

Steel molds are heavier, moreexpensive and more difficult to machinethan those made of non-ferrous alloys.The higher weight results in more

set-up time. Further, the heatconductivity of steel is less than that ofthe three nonferrous mold materials.This results in a slower cooling rate, acorrespondingly longer cooling cycleand consequently, a lower productionrate for steel molds. However, sincenonferrous metals lack the strength andhardness required for maintaining agood pinch-off, these areas might bemade with steel or beryllium-copperinserts.

Figure 44. Shape of a p inch-off with insert as recommended by

some molders.

Figure 45. Good weld (left) and poor weld (right).

Figure 43. Blow molds

Courtesy of Uniloy Blow Molding Systems, a division of J ohnson Controls

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Fast heat transfer of the moldmaterial is of utmost importancebecause the cooling step is roughly 40to 80% of the entire blowing cycle andcontrols its length, Considering onlytheir heat transfer rate, the principalblowing mold materials follow eachother in this order of efficiency:beryllium-copper, Kirksite, aluminum

and steel.Occasionally, several different

alloys are used in the same mold toobtain desired strength and specialcooling conditions. However, as thesemold materials have different heattransfer rates, a blow mold, with theexception of the steel or beryllium-copper pinch-off inserts, should bemade of only one material. Differentmaterials, with consequently differentheat conductivity at various points ofthe mold, result in non-uniform cooling.This, in turn, might result in warpage

and stress in the finished piece.

Mold surface

High quality mold cavity finish andundamaged interior surfaces areessential in polyolefin blow molding toprevent surface imperfections in theend product. If high gloss is desired,the mold cavity should be sandblastedwith 100 grit flint sand and havevacuum assists for the removal ofentrapped air. If other end productfinishes are desired, the mold cavityshould be finished accordingly. Even an

excellent machining of the mold cavitycannot prevent the occurrence ofparting lines, especially if the blownitem has a very thin wall.

Venting

In highly polished molds, air maybe trapped between the mold walls andthe hot, still soft piece, marring itssurface. This will happen especiallywhen thick-walled, large objects areblown. In such cases, the mold must bevented by sandblasting, resulting in amatte surface; by grooves in the

separation lines (called face venting);or, in extreme cases, by valves in themold (called pin venting).

With venting of the mold partinglines, generally about one-half of theparting line periphery is vented to adepth of 0.002 to 0.004 in. (0.05,to 0.1mm, Fig. 46). In venting difficult areas,such as handles or thread inserts,holes 0.008 to 0.010 in. (0.2 to 0.25mm) in diameter are usually drilled sothat they vent to the atmosphere.

Figure 46. Mold Venting

Figure 47. Schematic of a blowing mold half, with the cooling

water channels indicated.

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Particular care must be taken whendrilling these holes so that the moldcooling channels are not pierced.

Cooling

An increase, or decrease, of about10F (5C) in melt temperature canextend, or shorten, the cooling cycle byas much as one second. Efficient mold

collingn is essential to achieve optimalcycle times. Mold halves have built-inchannels for cooling water (Fig. 47).Adequate cooling is needed to solidifythe part immediately after the parisonhas been blown out against the moldwalls. If locally available water forcooling has a high mineral content, aclosed system for circulating purifiedwater may be needed. The watershould be cold; if necessary, it shouldbe chilled by a heat exchanger to 40 to70F (40 to 20C). However, lowtemperatures below the dew point may

cause water condensation on moldwalls, a situation to be avoided as itcan lead to orange peel surfaceimperfections on the molded item.

Sometimes, copper tubing waterchannels are cast into the mold.However, to create the most useful flow,water channels are machined into thematerial cast into the mold halves.Well-placed channels ensure thecooling water comes as close to themold cavity surface as possible. Largermolds may be equipped with three ormore independent cooling zones.

Generally, in the top or base areasof a blown container, greater masses ofresin are required. Such areas, as wellas thicker wall sections, often requireadditional cooling. Otherwise, thesesections are still very hot when thepiece is ejected, resulting innon-uniform shrinkage and warpage.

Occasionally, the blow pin also isvented, allowing air to circulate insidethe blown part and speeding up itscooling. Cold carbon dioxide (CO

2) from

dry ice, introduced through the blowpin, can significantly reduce cycle time

for HDPE containers. Also, air coolingoccasionally is directed at the pinchedtail of the parison sticking out of themold bottom, additionally cooling thisthicker area.

Postcooling Station

A postcooling station, i.e., after theblown part has been ejected from themold, can significantly reduce the blowmolding cycle time and yield higherproductivity, particularly for parts with

thick walls. Postcooling units includeconveyor lines with chilled air directedagainst the warm blown parts or withchilled metal holding fixtures in whichthe blown parts are inserted.

In-Mold Trimming

Generally, in continuous extrusionshuttle blow molding, containers with

and without handles are trimmed in themold, after the product has beensufficiently cooled (Fig. 48). Specialblow pins can trim the neck and thebottom and the mold can have a cuttingtool for removing the tail. Also, thecalibration of the neck area generally isdone in-mold. Parts made oncontinuous extrusion wheels aretrimmed in a post-molding operationand the neck or finished areas are notcalibrated.

Wide mouth containers also can betrimmed in the mold. The container is

blown with a dome on top of the neck. Aknife trimmer, which is part of the moldassembly, cuts the dome off to producethe wide mouth container (Fig. 49).

In-Mold Decorating/Labeling

Labels can be inserted into moldcavities, on one or both sides, beforethe container is blow molded. Thein-mold labeling system consists of amagazine containing the labels, atransfer device for placing individuallabels against the mold cavity surface,and a vacuum unit for holding the

labels in place. Labels are designed sothat there is no distortion on curvedsurfaces. The in-mold labels have heatactivated adhesives that ensureexcellent bonding to the containerwithout wrinkles and bubbles.

Ejection System

Blow molded products can beejected forward between the moldhalves or downward, provided the pressis built in such a way that a free fallfrom between the open mold halves ispossible. Many machines have an

automatic ejector, or stripper assembly.Ejector, or knock-out, pins or platespush the piece out with a rapid motion,preferably hitting it on the flash area sothat the piece itself is not distorted. Ifthe piece is not too large, it can beblown forward out of the mold by an airjet from behind. Very large pieces aresometimes manually removed from themold.

Container dimensions, in general,are controlled by the mold cavity.

However, removing molded parts fromthe mold while they are still too hot orhave hot areas because of thicksections or poor heat transfer, will resultin warpage and/or excessive shrinkage.If the finish is too hot when thecontainer is removed from the mold, itmay shrink and become undersized orovalized. Periodically feel the

containers coming from the mold for hotspots and overall temperature. At notime should any area, except the flash,be too hot to touch. Furthermore, thetemperature of the entire object shouldbe relatively the same for best results.

Trimming Equipment

Once the flash is cooled enough forproper removal, the container istransferred to one of several differenttypes of trimmers. Rotary trimmers arenormally used for non-handle

Figure 48. Containers are

finished in the machine and

oriented to stand-up posit ion for

take-away conveyers.

Courtesy of Uniloy Blow Molding Systems, adivision of J ohnson Controls

Figure 49. A knife trimmer

cuts the dome off wide-mouthed, blow molded

containers.

Courtesy of Uniloy Blow Molding Systems, adivision of J ohnson Controls

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containers that are produced bycaptured parisons and need only facingof the neck and removal of the tail.However, as pre-finished neck systemsbecome more common, rotary trimmersare less necessary.

For flashed containers withhandles, the bed-type trimmer is mostcommonly used. This type of trimmer

utilizes cups or buckets that index thecontainer to the trimming station. Herea lower movable nest picks thecontainer from the bucket and moves itupward. The container contacts thefixed upper nest where the flash isbroken off and removedfor recycling.The lower nest now moves down andreturns the container to the bucket. Thebucket then indexes, and the entirecycle is repeated.

Scrap Recovery

Efficient recovery of trim scrap and

rejected containers is essential to aneconomic production operation. For thisreason, container blow moldingsystems generally have in-line scraprecovery equipment. Scrap from thetail, neck area, handle finishing, etc. aredirectly conveyed to a granulator andthen back to the extruders hopperblending unit. The percentage ofregrind varies with the design of theblown part. With coextruded blowmoldings, regrind is usually buried in aninner layer or blended with the outerlayer. One hundred percent regrind can

be used in blow molded layers, but it isimportant to maintain a consistentblend of whatever proportion of regrindis chosen. Otherwise, extruder feedproblems can result.

Leak Detector

After containers have beendeflashed, they may have irregularitiesin the areas that have been trimmed. Totest for possible leakers, a leak testercan be used (Fig. 50). As containersmove along a conveyor line, an airplunger is lowered into the neck of eachcontainer to form a tight seal. The airpressure in the container is measured.If the pressure remains constant, thecontainer is accepted. If not, thecontainer is rejected.

Take-Off Equipment

Blown polyolefin pieces often areejected into a chute that guides themonto a conveyor or they free fall ontoa conveyor or into a box. For largerparts or where parts are placed in

secondary finishing systems, specialfixtures are designed to reach into themold area and grab parts (Fig. 51).Large pieces also can be removedmanually.

Pieces dropped or placed onto aconveyor must be sufficiently cool sothey do not stick together or getdistorted. Frequently, the tail, since it

is bulkier and warmer than othersections of the part, tends to stick toother parts on the conveyor. Therefore,it is essential that the piece stay in themold long enough for the tail andnearby areas to cool and solidify. Tominimize the hazards of sticking anddistortion, compartmentalized conveyorsystems, which prevent ejected piecesfrom falling on top of one another, areadvantageous. On some conveyors,each blown item may be gripped andautomatically centered in a holdingfixture, to be deflashed and reamed.

Decorating

The principal decorating methodsused for blow molded items are silkscreen printing, foil stamping, decaltransfer and labelling. Plastic labels,such as printed PP film, permit the blowmolded containers to be more easilyrecycled, i.e., there is no need toseparate paper labels from the regrind.Generally it is preferable, and lesscostly, to treat and print polyolefinpieces without delay after blowing andfinishing. But first, the surfaces of blown

polyolefin products must be oxidized byflame treatment (Fig. 52) or electronictreatment (Fig. 53). Good adhesion fordecoration can also be achieved withsurface primers instead of oxidation.

Running the Extrusion Blow

Molding Equipment

This section discusses the start-up,shut-down and operation of acontinuous extrusion blow molding

machine. It is followed by a section onhow the continuous extrusion blowmolding process can be optimized.

Implementing optimum proceduresis important in minimizing materialdegradation which causes black specksand/or gel contamination upon start-up.During the extrusion process, polyolefinresins are subjected to heat which,when combined with oxygen, can causea build-up of oxidized material on the

Figure 50. Testing bot tles

for leaks using an air

pressure system.

Courtesy of Bekum America Corp.

Figure 51. Take off equipment on

a blow molding machine

Courtesy of Uniloy Blow Molding Systems,a division of J ohnson Controls

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internal surfaces of the extruder andhead tooling. Resins are oftenstabilized with antioxidants to helpprevent this oxidation, but even the bestadditive package can not protect thematerial if it is abused by very hightemperatures and/or long residencetimes. The lowest useable temperatureprofile and optimum shut-down and

start-up procedures will minimizematerial degradation. Improperprocedures or operating conditions cancause large quantities of degradedmaterial to be generated. If this occurs,various forms of purging may helpreduce the amount of built-up char, butmost often the equipment must bedismantled and thoroughly cleaned.The optimum start-up and shut-downprocedures are best found throughexperimentation. Because blow moldingequipment varies, the followingguidelines are only general

suggestions. If you have any specificquestions or need any additionalinformation, please contact an EquistarTechnical Service Representative.

Suggested Start-up Procedure

I. Heating the extruder may beaccomplished by a number ofmethods. Option A and B are twowhich can be used. If polymerdegradation is a problem, option Ais suggested.A. 1. Turn on power to die head

heater bands. If applicable,

turn on die head preheaters.Set die head heater setpoints to 275F (135C).

2. After die head temperatureshave reached the set points,turn on power to extruderheaters. Set heater set pointsto 275F(135C).

3. After extruder temperatureshave reached the set points,raise all set points to 325F(163C).

4. After die head and extrudertemperatures have reached

set points, proceed to step II.B.1. Turn on power to all extruder

and die heater bands. Ifapplicable, turn on die headpreheaters. Set temperatureset points at 325F (163C).

2. After die head and extrudertemperatures have reachedthe set points, allow one halfhour for machine heat tostabilize.

3. Proceed with step II.

Figure 52. Flame treating.

Courtesy of Society of the Plastics Industry, Inc.

Courtesy of Society of the Plastics Industry, Inc.

Figure 53. Electronic corona-discharge treatment.

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II. After the temperatures havereached the set points:A. Turn off die head preheaters.B. Start hydraulic system.C. Start extruding polymer at

minimum screw speed.CAUTION: Head pressureand extruder motor amper-age should be watched

closely and should notexceed the machinemanufacturers recom-mended maximums.

D. Set temperatures at desiredoperating levels.

III. After temperatures have onceagain reached set points:A. Turn on chiller for mold

cooling.B. Gradually increase extruder

speed to proper operatinglevel. CAUTION: Headpressure and extruder motor

amperage should be watchedclosely and should not exceedthe machine manufacturersrecommended maximums.

C. Open valve which suppliespressurized air to blowmolding machine. Air pressureshould be regulated at 80 psito 120 psi while preblow airshould be 5 psi to 15 psi formost typical blow molding

operations.

IV. Set timers and die programmers tothe established settings.

V. Start blow molder on automaticcycle for continuous operation. (SeeTable 7 for Suggested Operating

Condi