MOULDED IN STRESS模塑的应力

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1、MOULDEDINSTRESS模塑的应力AccordingtoRAPRATechnology,whocarriedoutpost-mortemsonthousandsofindustrialplasticparts,foundthatenvironmentalstresscrackingwasthemajorcauseofplasticpartfailures.Well,Stresscrackinginserviceisduetothemoulded-in(residual)stressinthemoulding.Therearethreepossibilitiesofin

2、ducingmoulded-instressinaplasticpart.Thereare:Whenmeltisoverpacked(whilemoulding)itcancausemoulded-instress.Thiscanbeavoidedbyapplyingfollowuppressurejustenoughtoavoidsinkmarks.Evenunbalancedmeltflowinthemouldcancauseoverpackinsomeregions.Itisnotpossibletoavoidoverpackingwithunbalancedmelt

3、flowwhileprocessing.Thisproblemshouldhavebeenconsideredwhiledesigningthepartitself.Un-equalfreezingofmeltthroughoutthepart,canalsocausemoulded-instress.Coolingsystem,whichdoesnotprovideuniformmouldsurfacetemperaturecanberesponsibleforthiscause.Thisistheresponsibilityofmoulddesign.MOULDABIL

4、ITYItisassumedthateveryshapecanbemouldableinplastics.Itispartlynotcorrect.Forperfectmouldingyoushouldhave95to100%balancedmeltflow.Itmeansthatmeltfrontindifferentdirectionshouldreachtheboundaryofthemouldatthesameinstant.Otherwise,youmayhavesomeregionfilledearlierandwhilefillingunfilledregio

5、nthepreviouslyfilledregiongetsoverpacked..Tosolvethisproblemflowbalancingtechniqueshouldbeusedtomodifythepartdesign.Bythistechnique,flowleaderorflowdeflectorsareusedtobalancetheflowtoavoidmoulded-instress.Moulded-instressalsocausesunequalshrinkageinthemouldingresultingindimensionalproblem.

6、Toavoidmoulded-instressduetoun-uniformfreezing,wemustensurethatmeltflowvelocityinthemouldisuniformorconstantthroughoutthepartduringfillingphase.STRESSCRACKINGStresscrackingcanbeclassifiedintoEnvironmentstresscrackingandSolventstresscracking.Itcanoccurinalmostanyplastic.However,itismorecomm

7、onwithamorphousthermoplastics.Thesolventactuallyreplacesthepolymeratthesurfaceofthepart.Thebondbetweenpolymerandthesolventmoleculeisweakerthantheoriginalbondbetweenthepolymerchainsanddoesnotcontributetothestrengthofthematerial.Inotherwords,thesolventweakensthe