Manufacturing2.810 JohnLewandowski

InjectionMoldingQuizReviewSheet

OverviewInjectionmoldingisoneoftherichesttopicsinthecoursebecauseithasthemostinterestingformulas/theory,hasgreatdiagrams,andtherearelotsofexampleparts.Iwouldsuggesttakingseveralplastictoysandpartsanddrawingthemoldforthem.

• Findthegate• Findthepartingline• Findtheejectorpinmarks• Drawthecoreandcavity• Notethedraftangle• Showthematerialfrontlines• Measurethethickness• Derivethecoolingtime

Youhavetoknowthediagramsfromslides8and9fromthelecture.Thosearesocriticaltotheprocess.Actually,Iwouldencourageyoutodrawaprocessingwindowforeverytypeofprocessthatwediscussed.Forheattransfer,seetheheattransferreviewsheetwhereallofthistypeofinformationisaggregatedtogether.KeyFormulasThermalDiffusivityThethermaldiffusivityisarelationshipbetweenthedensity,thermalconductivity,andtheheatcapacitance.Youshouldbepreparedtouseeitherofthemandrearrangetheminthefuture.

𝛼 =𝑘𝜌𝑐&

Thisistheexactsolutionforthecoolingtemperature.Youwillneverneedtousethistosolvesomething,howeveryoushouldknowhowthisrelatestothesimplerequation.

𝑡())* =ℎ,

𝜋,𝛼 𝑙𝑛4𝜋

𝑇23*4 − 𝑇67**/2)*9𝑇3:3(4;)< − 𝑇67**/2)*9

Thehardestpartaboutthisformulaisunderstandingwhatthesubscriptsmeanonthetemperatures.Whenyourealizethatthemelttemperatureforplasticsisroughly10xtheejectiontemperature(roomtemperature),theneverythingreducestothesimpleequation.

𝑡())* =(ℎ/2),

𝛼

Manufacturing2.810 JohnLewandowski

FluidDynamicsClampingForceThefluidflowcanbemodeledbetweentwoplates.

𝑑𝑃𝑑𝑥 = 𝜇

𝑑,𝑈𝑑𝑦,

∆𝑃 =12𝜇𝑄𝐿𝑤ℎK

andweknowthatQ=vol/time=whL/timesowecanreduce

∆𝑃 =12𝜇𝑡L;**

𝐿ℎ

,

𝐹(*72& =𝜇𝑡L;**

𝑤𝐿K

ℎ,

andwecouldsubstituteinarea=whifneeded

ViscosityViscosityisresistancetoshear.Theformulaiseasywhenyourememberthat.Itisrelatingthesheartothatvelocityprofile/distribution.

Manufacturing2.810 JohnLewandowski

𝜏 = 𝜇𝜕𝑈𝜕𝑦

ShearthinningKeepingtheshearstressthesame,asshearrateincreases,viscosity(slopeoftheabovediagram)decreasesmeaningthevelocityisfasterateachrespectivepoint.Thisoccursinlowmolecularmassandsmallmolecules(plastics).KeyDimensionlessNumbersReynoldsnumberTheReynoldsnumbercomparestheinertialforcesinthesystemtotheviscousforces.Itisbasicallysayingiftheflowisflowingsmoothandconstant(lownumber,laminar)versusflowingwithvorticesandchaotically(highnumber,turbulent).Thinkaboutwhatthismeansformanufacturing.Onthelectureslidewediscussthedifferencebetweeninjectionmoldinganddiecasting.Thekeyhereistheviscositydifferencesbetweenmoltenplastic(10^3)andmoltenmetal(10^-3).ThatsixorderofmagnitudedifferenceiswhatcausesthedifferenceinReynoldsnumberandalsowhyplasticflowslaminarandmetalisalwaysatriskofbeingturbulentdependingonthegeometryofthesystem.Laminarflowcreatesnicelookingparts,whereasturbulentflowcreatespartswithvoidsanddefects.BrinkmannumberThistimewearecomparingtherateofviscousheating(temperaturerise)torateofconduction(temperatureloss).Therefore,itistellingusifwearegoingtoburnthematerialorfreezetooquickly.Goingbacktotheprocessingwindow,thetopandbottomboundariesaredictatedbytheBrinkmannumber!PecletnumberFinally,wearelookingattheleftandrightsideofthatprocessingwindow,comparingifweareflowingfastenoughcomparedtotheheattransferrateoutofthesystem.Thisisjustlookingattherelationshipbetweenthosetworates!Theflowrateistheinverseoftheamountoftimethatittakestogoasetlength.

𝑣 = Q4RSTU

so𝑡L*)6 =QVand W

4RSTU= V

Q

Theheattransferrateistheinverseoftheamountoftimethatittakestocool.

𝑡())* =(ℎ/2),

𝛼 𝑠𝑜1

𝑡())*=

𝛼(ℎ/2),

Thenyouarejustmakingaratioofthoseratesandsolving.ThePecletnumberfallsoutbecausetherestoftheequationisjustacoefficientandgeometricalrelationshipbetweenheightand

Manufacturing2.810 JohnLewandowski

distance.SimilartotheReynoldsnumber,themostimportantpartofthisishowitrelatestothedifferencebetweenmoltenplasticandmoltenmetal.Becauseofthatthermaldiffusivityterminthecoolingtime,sincemoltenmetalcoolssoquickly,itisverysusceptibletoshortshots.Thatmeansyouneedthickrunnerssoyoucangetmorematerialinbeforeitcoolsandalsohavethecenterofthematerialfartherawayfromthecoolerwalls.DesignRules

• Gateshouldbeontheendforlaminarflow• Needdraftangletoremovepart• Musthaveanevenwallthickness• Avoidsharpcorners• Noundercuts

DefectsOnslide24,whydoesincreasingtemperaturecausetheshrinkagetoincrease?It’sbecausealargerdifferencebetweenthemelttemperatureandtheejectiontemperaturemeanstherewillbealongertimetocool(it’snotinthatsimplified𝑡())*expressionbecauseitisalreadyreducedtogetthebulkparametermodel.Andthereasonwhytherearedifferencematerialsisbecauseeachofthosehasdifferentmeltingtemperaturesandphysicalpropertiesthataffecttheabovedimensionlessnumbers.Animportantgraphtounderstandisthisisspecificvolumeversustemperature.

Noticehowthedifferentpressurecurveshelppaintapictureforusofwhatisgoingon.Tracewhatthematerialfeels.ItgoesfrompointAtopointB.Higherprocessingpressureandhighertemperaturetoloweratmosphericpressureandlowertemperature.ThedistancebetweenAandBistheshrinkageassociatedwiththepart.Meanwhile,ifyouincreasethepressure,the

Manufacturing2.810 JohnLewandowski

distancebetweenAandBwoulddecrease(lessshrinkage).Likewise,ifyouusedahighermelttemperature(differentmaterial),theamountofshrinkagewouldincrease.Youcanimaginethatshrinkagedependsontheorientationofthepolymermoleculesandthinkaboutwhichdirectionwouldbemorepronetoshrinkage.KeyPagesintheBookTBDKeyReadingsBoothroydetal,"DesignRulesforInjectionMolding”Thesetwosectionsaremostimportant8.3TheMoldingCycle8.7.2CoolingTime