injection molding quiz review sheet - mit -...
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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