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MIT2.810Fall2016 Homework4
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MIT2.810ManufacturingProcessesandSystemsHomework4
InjectionMoldingandHeatTransfer
Revised6October2016Problem1.Waxcasting.
(a) Consider the open (aluminum)mold casting of a wax candle as shown in Figure 1.AssumethatL>>D,D=2cm,themoldisataconstanttemperatureof30°Candthewaxispouredat100°C.Estimatethetimetocoolthewaxto37°C.Notethatwaxhassimilarthermalproperties aspolymers.Discusswhat the consequenceswouldbe if L andDwereofthesameorder.
Note: Youwillneed to refer to thechartof transient temperaturedistribution ina cylinder(fromLienhard’sAHeatTransferTextbook,4thed.,Figure5.8).
(a) AssumingL=D,drawthesolidifiedshapeofthecandle.Explainhowthisshapewouldevolve. If thiswereglass,would therebeanypossibilityof cracking?Where?What isimportantaboutthedifferencebetweenglassandwaxforthisproblem?
Figure1:OpenMoldforCandleCasting
MIT2.810Fall2016 Homework4
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Problem2.Injectionmolding.Considertheinjectionmoldingofthetapeholder(weight=0.5oz,wallthickness=1.6mm)showninFigures2and3.Pleaseanswerthefollowingquestionsaboutthemanufactureofthispart:
(a) Estimatethecoolingtime.(b) Indicate any special features thatwould result inmoving parts for themold.Were
ejectorpinsused?(c) Indicatethemoldfillingsequenceandlocationsofthegateandweldlines(ifany).(d) Specify roughly the sizeof the injectionmoldingmachineneeded tomold4of these
partsatatimeusinga4-cavitymold.
Figure2:TapeDispenser
MIT2.810Fall2016 Homework4
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Figure3:TapeDispenserDimensions
Problem3.Designforinjectionmolding.TwoconceptualdesignsforthetoolingforthepartshowninFigure3(aftereliminatingthe0.25in.ridgeatthebase)areshownindrawings(A)and(B).
(a) Whatwouldbethedifferencebetweenthesetwoapproaches?(b) DoanyfeaturesonthesedrawingsviolateDFMrules?(c) Drawinthelocationofthepartimmediatelyafteropeningthemold.(d) Pleaseshowwhereyouwouldputastripperplateand/orejectorpinsonthesemolds.(e) Considertwodifferentgatingsituations.Ononedrawthegateandspruelocationif
theyaretoremainconnectedtothepartondieopening.(f) Inanother,showthegateandspruelocationiftheyaretoseparatefromtheparton
dieopening.
MIT2.810Fall2016 Homework4
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Figure3.Injectionmoldedpart(alldimensionsininches).
MIT2.810Fall2016 Homework4
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Problem4.Diecastingversusinjectionmolding.Onenoticeabledifferencebetweendiecastingtoolingandinjectionmoldingtoolingisthesizeoftherunnersystem.Figure4showsamulti-cavitydiefordiecasting.Notethattherunnersystem is quite large and changing in size. In comparison, the runner system for injectionmoldingisusuallyofsmallerandconstantdiameter.Canyouexplainwhythesearedifferent?Isthereafeatureofthediecastingdiethatismissinginthispicture?
Figure4.Multiple-cavitytoolfordiecasting.Notetherunnersizecomparedtopartsize.Problem5.Coolingtime.Showthatequation8.5inBoothroydetal.reducestoourresult
𝑡! ≅ (ℎ/2)!
𝛼