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

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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

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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.

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Figure3.Injectionmoldedpart(alldimensionsininches).

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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)!

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