Die & Mould
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CAE DS – Injection Casting Design Drafts Tampere University of Technology – Sanna Nykänen Draft angles are almost always necessary for any vertical wall of an injection moulded product. Draft angles will ease the part’s ejection from a mould. If the parts are produced using a normal cavity and core configuration, usually the cavity is motionless and the core is attached to the moving plate. Usually the part will stay with the core when opening the mould. This is a consequence from two things: − Material shrinks and causes a contact pressure and frictional forces be‐ tween the core and the part Drafts ‐ 1 − The part will be pushed away from the cavity wall because of wall thick‐ ness material shrinkage. Draft angles are usually added to the cavity side of the mould because this way they assist the part’s release from the cavity when the mould opens. Cavity draft angles have many advantages. They reduce the effects of undercuts, eliminate sliding friction and facilitate air movement which compensates the vac‐ uum effect when opening the mould. Draft angles typically range from a fraction to several degrees. Draft angles are depended on depth of draw, material rigidity and shrinkage, surface lubricity, and mould surface roughness. Figure 1. Cavity draft angles ease the part’s ejection from the mould .Based on Malloy: Plastic part design for injection molding, page 89. It is also possible to manufacture parts with no draft angle at all, but some kind of special mould action is needed to pull the cavity away from the part when the mould opens. For example a split cavity is one possibility. When the mould has been opened and the part ejected from the cavity, it must be then ejected from the core. It is very characteristic for plastic materials to grip tightly onto the core. Therefore forces needed to eject the part can be very signifi‐ cant. The highest ejection forces are the initial breakaway forces which are influenced by material shrinkage and modulus, coefficient of friction, surface roughness, contact area and draft angle. Draft angles from 0.25 ° to 2° are com‐ monly used for cores. Reason for using the draft angles in cores is the same as it is with cavity drafts. If the draft angles are larger, they will improve the part’s ejection even more but they also have a remarkable effect on the product’s shape. Core draft angles reduce the initial ejection forces and this way simplifies ejection system design. Cavity draft angle “Zero‐draft” Core drafts
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this document is a guideline during manufacturing of moulds in order to give draft angle for different materials
Transcript of Die & Mould
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CAEDSInjectionCastingDesign
DraftsTampereUniversityofTechnologySannaNyknen
Draft angles are almost always necessary for any vertical wall of an injectionmouldedproduct.Draft angleswill ease theparts ejection from amould. If thepartsareproducedusinganormalcavityandcoreconfiguration,usuallythecavityismotionlessand thecore isattached to themovingplate. Usually thepartwillstaywiththecorewhenopeningthemould.Thisisaconsequencefromtwothings:
Materialshrinksandcausesacontactpressureandfrictionalforcesbetweenthecoreandthepart
Drafts1
Thepartwillbepushedawayfromthecavitywallbecauseofwallthicknessmaterialshrinkage.Draftanglesareusuallyaddedtothecavitysideofthemouldbecausethiswaytheyassistthepartsreleasefromthecavitywhenthemouldopens.
Cavitydraftangleshavemanyadvantages.They reduce theeffectsofundercuts,eliminatesliding frictionand facilitateairmovementwhichcompensates thevacuumeffectwhenopeningthemould.Draftanglestypicallyrangefromafractiontoseveraldegrees.Draftanglesaredependedondepthofdraw,materialrigidityandshrinkage,surfacelubricity,andmouldsurfaceroughness.
Figure1. Cavity draft angleseasethepartsejectionfromthemould.BasedonMalloy:Plasticpartdesignforinjectionmolding,page89.
Itisalsopossibletomanufacturepartswithnodraftangleatall,butsomekindofspecialmould action is needed to pull the cavity away from the partwhen themouldopens.Forexampleasplitcavityisonepossibility.
Whenthemouldhasbeenopenedandthepartejectedfromthecavity,itmustbethen ejected from the core. It is very characteristic for plastic materials to griptightlyontothecore.Thereforeforcesneededtoejectthepartcanbeverysignificant. The highest ejection forces are the initial breakaway forces which areinfluenced by material shrinkage and modulus, coefficient of friction, surfaceroughness, contact area anddraft angle.Draft angles from 0.25 to 2 are commonlyusedforcores.Reasonforusingthedraftanglesincoresisthesameasitiswithcavitydrafts.Ifthedraftanglesarelarger,theywillimprovethepartsejectionevenmorebuttheyalsohavearemarkableeffectontheproductsshape.Coredraftangles reduce the initial ejection forces and this way simplifies ejection systemdesign.
CavitydraftangleZerodraft
Coredrafts
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CAEDSInjectionCastingDesign
Drafts2
Figure2. Core drafts areneeded for parts ejection from thecore after the mould has opened.Based on Malloy: Plastic partdesign for injectionmolding, page89.
Draftanglesresultpartslooseningfromthecoresurfaceaftertheinitialbreakawayandtheejectionforceswilldropoffquicklyafterbreakaway.Thissuddenlylossofcontactareamakesaireasier to flowcores surroundingarea tocompensatevacuumeffectsandcan leadtopartsdamage. Airvalvesandventingpinsareusedforvery largeanddeepdrawparts tobreak thevacuumandblow thepartawayfromthecoreafterinitialbreakaway.Veryoftenthecoredraftanglesaresameascavitydraft angles. It is recommended to use paralleldraft because it results inpartsuniformwallthickness.
Figure3. External draft .Based on Rosato:Injectionmoldinghandbook,page602.
Abovepictureisagoodexampleofanapplicationofadraftangle.Whenasmalldraftangleisused,theoutsidesurface(mouldsurfaceproducingpart)willrequireahighdirectionalfinishbecauseofmakingpartsremovalfromthemouldeasier.
Figure4. Internal draft. Based on Rosato:Injectionmoldinghandbook,page602.
Abovepicturehasaseparatinginsidewallthatshouldbeperpendiculartothebase.Inthiscasethedraftwouldbeonthelowside.Thiswayadditionalmaterialusageis low, thevoidsclose to thebaseareavoidedandcycle times increasewontbehappening.Alsointhiscasetheverticalmouldingsurfaceswillneedamuchbettersurfacefinish.
Example1.
Example2.
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Figure5. Using of multiple drafts for easing partremoval .Based onRosato: Injectionmolding handbook, page603.
Figure6. Using paralleldrafts with heavy walls. . Based onRosato: Injection molding handbook,page603.
Whenapplyingadraft,onedifficultyisthecreationofheavywalls.Problemcanbesolvedbyusingparalleldraftsonwalls.
Draftanglesfordifferentplastics.Thesevalueshowevervarydependingonmanypreviouslymentionedreasons.
Example3.Example4.
Draftanglesfordifferentplastics
Plastictype Draftangle
Polycarbonate(PC) 12
Polystyrene(PS) >0.5
Polyarylsulfone(PSU) 12
Polyarylethersulfone(PES) 12
Liquidcrystalpolymers(LCP) >0.5
Polybutyleneterephthalate(PBT) 11.5
Polyethyleneterephthalate(PET) 11.5
Acrylonitrile/Polybutadiene/Styrene(ABS) >0.5
Polymethylmethacrylate(PMMA) >12
Polyethylene(PE) >0.70.8
Polypropylene(PP) >0.7
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Draftangleshaveaquitebig influenceon theplasticproductsshape.Every feature, likebosses,ribsandholesmustbedrafted. Forexampleribsshouldhaveadraftangleonbothsidesofthecavity.Thepositivepointaboutthesedraftsisthattheyeasetheejection. Howevertheribwillbecomehardertofillanditwillhavelessastiffeningeffect.Thereforeuseddraftangleshouldbeacompromisebetweenimprovedejectionwithoutcausingaremarkablelossofstiffness.
Iftheinjectionmouldedpartisproducedusingtexturedsurface,itneedsadditionaldrafttoeasetheejection.Cavities,thathavearandomtexture,needabout1to1.5draftanglepersideforevery0.025mmoftexturedepth.Somedirectionalpattern,likewoodgrain runningperpendicular to thedirectionof thedraw,mayusuallyneedgreaterdraftangles.Surfacetexturesareusuallyappliedtothecavitysideofthetool,outsidesurfaceofthemoulding.Insidetextureswhichformonatexturedcore are very difficult to eject becausematerial shrinks tightly around the core.Evengreaterdraftanglesareneededifthesekindoftexturedcoresareused.
Undercut isadraft to thewrongdirectione.g. to thedirection,whichpreventsthe casting ejection. Undercut can also be a hole or recession, which is at leastpartiallyparallel to themouldparting surface.Undercuts should be avoided oninjectionmouldedparts.Usingundercutson injectionmouldedpart isquitedifficult.Partswherematerials flexibilitymakespartejectionpossibleareexceptions.Undercutsraisethecostofthemould,approximatelyfrom15to30%.
Therearesomebasicruleswhendesigningundercuts.Theprotrudingdepthoftheundercutshouldbe2/3ofthewallthicknessorless.Alsothemouldsedge,whichisagainstthepartwhenthepartisejectedfromthemould,shouldberadiused.Itprevents shearing action. The part ejected should be hot enough to allow easystretchingandreturntoitsoriginalshapeafteritisremovedfromthemould.
Basically it is not possible to pull partswith external undercuts from the cavitywhenthemouldopens.Inthatcaseitisnecessarytousesideactionsorsplitcavities.Partsejectionfromthecoreispossiblewithsomedegreeofinternalundercut.At thisphaseof theprocess, thecavityhasbeenwithdrawnand thepartcandeformoutwardswhiletheinternalundercutridesoverthecore.Thesuitableamountofundercut isdependedonundercutdesign,materialpropertiesonejection temperature and tolerances required for dimension. Materials like thermoplasticelastomerscanbeinjectionmouldedwithverylargeundercuts.Ontheotherhandrigid,glassypolymerscanhaveaverysmallamountofundercut.
Insomecases internalundercutsarecut into thecore toensure that thepartwillstayonthecorewhenopeningthemould.Partsthathaveinternalundercutscanbemanufacturedtotightertolerances.Thesizeandtheshapeoftheundercutarenotlimitedbytheductilityoftheplasticmaterialwhenspecialtoolactionsareusedtorelease theundercut.Thesekindsofactionsareunscrewingmechanisms, removableinserts,liftersandcollapsiblecores.
Draftanglesandproductfeatures
Undercuts
Basicdesignrules
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CAEDSInjectionCastingDesign
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Partwontbewithdrawnbyusingnaturallyejectionfromthemould.Thereforeitisnecessary tomakemould parts,whichmake undercut areas,moving sideward.Thishassomeeffectsonpartsappearance:
Theboundariesbetweenmouldpartsbecomevisibleontheplasticpart. Therewillbeflashesbetweenthemouldparts. Theheatregulationofthemouldbecomesdifficultbecauseseparatecool
ingcircuitsarenecessaryformovingparts.Necessarilyitisnotalwaysevenpossible.
References
Jrveletal.,Ruiskuvalu,Plastdata,Tampere,2000.
Rosatoetal.,InjectionMoldingHandbook,3rded.,Kluwer,2000.
Malloy,PlasticPartDesignforInjectionMolding,HanserPublishers,1994.
Drafts References
