Design Process for Plastic

7/24/2019 Design Process for Plastic

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Design Process for PlasticAfter the concept design & development of

mechanism. We must ascertain &thoroughly evaluate the part & materialrequirement which in uence both the partdesign & material selection. Whileevaluating these requirements considermore than just intended, end-use conditions

& load. lastics are subjected to more harshconditions during manufacturing & shippingthan actual use.

!"#echanical loading-$arefully evaluate all types ofmechanical loading including short-termstatic loads, impacts, and vibrational orcyclic loads that could lead to fatigue.Ascertain long-term loads that couldcause creep or stress rela%ation. $learlyidentify impact requirements .

a" When designing parts made ofplastics, be sure to consider not onlythe magnitude of mechanical loadsbut also theirtype and duration. #ore so than formost materials, plastics can e%hibitdramatically di erent behaviordepending on whether the loading is


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instantaneous, long term, orvibratory in nature. 'emperature andother environmentalconditions can also dramaticallya ect the mechanical performance of the plastic material.

b" $arefully evaluate all of thestructural loads the part must endurethroughout its entire life cycle.

c" #echanical properties are timeandtemperature dependent. lasticstypically e%hibit nonlinearmechanical behaviour( androcessing and ow orientation cangreatly a ect properties

d" )ti ness *lastics e%hibit much less strengthand sti ness. +ncreasing wallthic ness maycompensate for the lower sti ness ofplastic resins. +n practice, however,the molding process limits wallthic ness to appro%imately . / inchin solid, injection molded part. 0enerally, good part designsincorporate sti ening features and


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use part geometry to help achieverequired sti ness and strength.

e" 1iscoelasticity *lastics e%hibit viscoelasticbehaviours under load2 they showboth plastic and elastic deformation .1iscoelasticity causes most plasticsto lose sti ness and strength as thetemperature increases. As a plasticpart is e%posed to highertemperatures, lastic partsalso e%hibit creep , the increase indeformation over time in parts undercontinuous load or stress, as well asstress relaxation , the reduction instress over time in a part underconstant strain or deformation. 'oaccount for this behaviour, designersshould use datathat re ect the correct temperature,load, and duration to which the partwill be e%posed.

f" #olding 3actor * 'he injection-molding processintroducesstresses and orientations that a ectthe mechanical performance ofplastic parts .


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'he high molding stresses in anactual partmay reduce certain mechanicalproperties,such as the amount of applied stress.+n glass-4lled resins, 4ber orientationalso a ects mechanical performance2fatigue strength for a given 4ber-4lled resin is often many timesgreater when the 4bers are alignedlengthwise, rather thanperpendicular to the fatigue load.)tress-strain performance in thedirection of 4ber orientation can alsodi er greatly from the performancein the directionperpendicular to the 4bers.

e" 5ong 'erm mechanical roperties*

'ime and temperature a ect thelongterm mechanical properties ofplastics because they a ect polymer-chain mobility .

i. $reep roperties *6ver time, parts subjected to aconstantload often distort beyond theirinitial


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deformation( they creep. 5ong-termcreep data helps designersestimate.Another popular form for creepdata,the isochronous stress-straincurve ,plots tensile stress versus strainat giventime increments

ii. )tress 7ela%ation)tress rela%ation, the stressreductionthat occurs in parts subjected toconstantstrain over time, is an importantdesignconcern for parts that will besubjectedto long-term de ection. 8ecauseof stress rela%ation, press 4ts,spring 4ngers, and other partfeatures subject to constantstrain can show a reducedretention or de ection force overtime.


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9ou can derive stress-rela%ationinformation from isochronoustressstrain curves by noting thechange instress corresponding toa given strain on the di erenttime curves.

iii. 3atigue roperties#olded plastic parts e%posed tocyclicloading often fail at substantiallylowerstress and strain levels thanparts understatic loading, a phenomenonnownas fatigue . Applications thate%poseparts to heavy vibrations orrepeatedde ections : such as snowplowheadlight housings, one-piecesalad tongs, and high-use snap-latch closures : need plasticswith good fatigue characteristics.3atigue properties are sensitiveto many factors including notche ects, environmental factors,stress concentrators, loading


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frequency, and temperature.)urface te%ture, surface 4nish,and whether the part is platedalso a ect fatigue performance.+n contrast to metals, plasticshave a high degree of inherentdamping and relatively lowthermal conductivity. 'herefore,vibration frequencies as low as! ;< can cause heat generationin plastic parts. 'his can lead tothermal failure if the energycannot be properly dissipated byother means, such as convection.

'ypically 4llet radii of . !/ to. = inch provide a goodcompromise between fatigueperformance and partmoldability.

iv. 3iber orientation can also a ectfatigueperformance. 3atigue strength foragiven 4ber-4lled resin can bemanytimes greater when the 4bers arealigned lengthwise in thedirection of


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term loads, use a creep orapparent modulus derived fromisochronous stress-strain curves.A time dependent property,creep modulus is the calculatedstress divided by thecorresponding strain value readfrom the isochronous stress-strain curve for the desired timespan .

vi. )tress & strain limits *)tress limits are best determinedfromisochronous stress-strain curvesshowingeither crase a safety factor of atleast. : higher values arenecessary incritical application.


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" 'emperature#any material properties in plastics :impact strength, modulus, tensilestrength, and creep resistance to name afew : vary with temperature. $onsiderthe full range of end-use temperatures,as well as temperatures to which thepart will be e%posed duringmanufacturing, 4nishing, and shipping.7emember that impact resistancegenerally diminishes at lowertemperatures .

="$hemicals *lastic parts encounter a wide variety ofchemicals both during manufacturingand in the end-use environment,including mold releases, cutting oils,degreasers, lubricants, cleaningsolvents, printing dyes, paints,adhesives, coo ing greases, andautomotive uids. #a e sure that thesechemicals are compatible with yourselected material and 4nal part.

@" lectrical erformance/"Weather 7esistance *


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'emperature, moisture, and >1 sune%posure a ect plastic parts? propertiesand appearance. 'he end-use of aproduct determines the type of weatherresistance required

B"7adiation *A variety of arti4cial sources : suchas uorescent lights, high-intensity

discharge lamps, and gammasterili


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We ta e as much as low sinited )tates Gepartment of Agriculture F>)GA" for plastics in meatand poultry equipment, and Hational)anitation 3oundation 'esting5aboratory, +nc. FH)3" for plastics infood-processing and potable-waterapplications. Always chec forcompliance and approval fromappropriate agencies

I" 5ife %pectancy *#any functional parts need to meetcertain life-cycle e%pectations. 5ifee%pectancy may involve a time duration: as in years of outdoor e%posure :time at a speci4c set of conditions :such as hours in boiling water : or


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disassembled or if assembly will beautomated. 5ist li ely or proposedassembly methods2 screws, welds,adhesives, snap-latches, etc. Hotematingmaterials and potential problem areassuch as attachments to materials withdi erent values of coeKcient of linearthermal e%pansion.At each stage loo for opportunities tosimplify and improve the assembly.$onsider consolidatingparts, reducing fastener and assemblysteps, improving automation, andselecting other assembly methods. Welldesigned parts include features to easeassembly and assure correct positioningand orientation. +n addition to cost andquality concerns, the optimi


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J )elect good-quality screwswithshaft-to-head-diameter ratiosandhead styles suited toautomatic feedin assembly equipment(J Avoid handling loosewashers : usescrews with washers aK%edunderthe head(J >se self-tapping screws toavoid asecondary tapping step(J >se metal threaded insertsfor screw connectionssubjected to frequentdisassembly.

e" 7etention 3eature - $omponentscan nest between ribs orslide into molded-in retainers forassembly without hardware Fsee4gure @-!@".+n some products, halves of theassemblycan captivate components without


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additional attachment Fsee 4gure@-!/".

'his joining method permitseKcientassembly and simpli4esdismantlingfor repairs or recycling.

f" Alignment 3eature * 'o help inassembly, consider designing yourpart with alignment features.arts must assemble easily andeKciently, despite minormisalignments. arts with sharpleading edges can snag or catchduring assembly, requiring moretime and e ort. $hamfers addedto either or both leading edgesquic ly align mating features,reducing the positioning accuracyneeded for assembly . ;ousing orenclosure sidewalls can bowduring molding or de ect underloading,resulting in poor alignment alongmatingedges. 'he stepped-edge designsupports the wall in just onedirection. Adding a


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protruding rib to support theinsidesurface loc s the walls in twodirectionsand provides better alignment

g"6rientation 3eature *Adding orienting features tomoldedparts can simplify assembly,reducecosts, and prevent assemblyerrors.When possible, incorporatefeatures thatprevent assembly unlesscomponentsare oriented correctly. 6therwise,clearlyindicate correct orientation on themating parts Fsee 4gure @- ".)ymmetrysimpli4es assembly. 6ften partsneedonly minor modi4cations toincreasesymmetry and allow orientation inmore


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than one direction Fsee 4gure @-!".

f"'65 7AH$ ) *Avoid specifying arbitrarily tight

tolerances to components and theassembly process, as it can addneedlessly to costs(J Accommodate part and processvariability in your design(J +nclude design features such asslotted holes, alignment features,andangled lead-ins to lessen the needfortight tolerances(

J 'a e advantage of the abilityof theinjection-molding process tomoldsmall features with e%cellentrepeatability( andJ Avoid tight tolerances on longdimensions and on featuresprone towarpage or distortion

0ive the tightest tolerances tothe part,feature, or process that adds the


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leastcost to the entire process. +t maybemore economical to loosen thetolerance on the plasticcomponent and tighten thetolerance on the assemblyprocedure or matingcomponents. $onsider all thesources of variability andoptimi


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sti ness. $rown round the surface to formslightly dome shape that add considerablesti ness with very little addition of material.

" 3or Hon-cosmetic part we usecorrugations to add sti ness to the part &distributed the load. 'he height & spacing of the corrugated adjusted to achieve thedesired sti ness.

=" 5ong unsupported edges, such as thoseside walls of the bo% shaped part e%hibitlower sti ness. 'hey also tend to wrapduring moulding. Adding curvature to theside walls increases sti ness & reduceswarpage. We can add sti ening pro4les for


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unsupported edges to increase theirstrength.

@" 'o ma%imise the sti ness of part, designnon-appearance bottom half of theassembly with hollow towers, centre walls orribs that support to the underside of theupper half.

Wall Thickness

/" As )ti ness is proportional to thethic ness cube, relatively small increase inthic ness reduce de ection greatly. A /Mincrease in thic ness doubles the sti ness.

8ut increase in thic ness also adds to partweight, cycle time & material cost. Alsomolding related issues such as shrin agestress, pac ing diKculties & cycle timestypically set the practical thic ness limit wellbelow . / inches for solid thermoplastics.

#ore typically, wall thic ness ranges from. B to .!B inches. )o that the optimumthic ness is always balance betweenopposing tendencies such as strength vsweight reduction or durability vs cost.


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B" Also consider the moldability while

selecting the wall thic ness of your part.3low length the distance from the gate tothe last area 4lled must be withinacceptable limits for plastic resin chosen.%cessively thin walls may develop highmolding stresses, cosmetic problems &

4lling problems. $onversely overly thicwalls can e%tend cycle times & createpac ing problems.

D" Avoid design with thin areas surroundedby the thic perimeter section as they proneto gas entrapment problems.


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E" #aintain uniform wall thic ness as muchas possible. +f wall thic ness variationrequired thic ness transition should besmooth & gradual. of transition


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!!" 7ib also add sti ness selectively inspeci4c areas & directions. Also it require tosti en the structural elements such ashinges, attachment parts.

! " roper rib design involves 4ve mainissues2 thic ness, height, location, quantity,and moldability .

!=" #any factors go into determining the

appropriate rib thickness . 'hic ribs oftencause sin and cosmetic problems on theopposite surface of the wall to which theyare attached. 1ery thin ribs can be diKcult to4ll because of ow hesitation.3low enteringthe thin ribs hesitates and free


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thumb limit rib height to appro%imatelythree times the rib-base thic ness .

15) Bosses8osses 4nd use in many part designs aspoints for attachment and assembly. 'hemost common variety consists ofcylindrical projections with holesdesigned to receive screws, threaded


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inserts, or other types of fasteninghardware

16) As a rule of thumb, the outsidediameter of bosses should remain within. to .@ times the outside diameter ofthe screw or insert.

17) 'o reduce stress concentration andpotential brea age, bosses should have ablended radius, rather than a sharp edge.

18) 3or most applications, a . !/- inchblend F4llet" radius provides a goodcompromise between strength andappearance.

1 ) Avoid bosses that merge intosidewalls because they can form thicsections that lead to sin . +nstead, positionthe bosses away from the sidewall, and ifneeded, use connecting ribs for support

!") Hormally, the boss hole shoulde%tend to the base-wall level, even if thefull depth is not needed for assembly.)hallower holes can leave thic sections,resulting in sin or voids. Geeper holesreduce the base wall thic ness, leading to4lling problems, nitlines, or surfaceblemishes. 'he goal is to maintain auniform thic ness in the attachmentwall Fsee 4gure -!E".


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!1) #usset#ussets are rib-li e features that addsupport to structures such as bosses,ribs,and walls with ribs, limit gusset thic nessto onehalf to two-thirds thethic ness of the walls to which they areattached

" 'he location of gussets in the mold steelgenerally prevents practical direct venting.Avoid designing gussets that could trapgasses and cause 4lling andpac ing problems. Adjust the shape orthic ness to push gasses out of thegussets and to areas that are more easilyvented.


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=" Sharp Corners *Avoid sharp corners in your design.)harp inside corners concentrate stressesfrom mechanical loading, substantiallyreducing mechanical performance. +t isbest not to design parts with sharpcorners. )harp corners act as notches,which concentrate stress and reduce thepart?s impact strength. A corner radius, asshown in 3igure B, will increase thestrength of the corner and improve mold4lling. 'he radius should be in the range of /M to D/M of wall thic ness( / M issuggeste A radius-to-thickness ratio ofappro%imately .!/ provides a goodcompromise between performance andappearance for most applications


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subjected to light to moderate impactloads.

@"+n addition to reducing mechanicalperformance, sharp corners can causehigh, locali


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elements can be diKcult to redesign.)ometimes, the part can e% enough tostrip from the mold during ejection,depending upon the undercut?s depthand shape and the resin?s e%ibility.>ndercuts can only be stripped if theyare located away from sti ening featuressuch as corners and ribs. +n addition, thepart must have room to e% and deform.

!) Slides and Cores#ost undercuts cannot strip from themold, needing an additional mechanismin the mold to move certain componentsprior to ejection Fsee $hapter D". 'hetypes of mechanisms include slides,

') $lever part design or minor designconcessions often can eliminate comple%mechanisms for undercuts. 1ariousdesign solutions for this problem areillustrated in 4gures - I through -=! .

$ol"e" %n threa"s

1) 'he molding process accommodatesthread forming directly in a part,

avoiding the e%pense of secondary,


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thread-cutting steps. 'he cost andcomple%ity of the tooling usuallydetermines the feasibility of moldingthreads. Always comparethis cost to the cost of alternativeattachment options, such as self-tappingscrews.

!) asily molded in both mold halves,external threads centered on the moldparting line add little to the moldingcost. 'ypically, threads that do not lie onthe parting line require slides or sideactions that could add to molding costs.All threads molded in two halves areprone to parting line ash or mismatch.

') #ost of the mechanisms for moldinginternal threads : such as collapsibleand unscrewing cores : signi4cantlyincrease the mold?s cost and comple%ity .

() J >se the ma%imum allowable radiusat the thread?s crest and root(J )top threads short of the end to avoidma ing thin, feathered threads that caneasily cross-thread Fsee 4gure -=/"(J 5imit thread pitch to no more than =threads per inch for ease of molding andprotection fromcross threading( and


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J Avoid tapered threads unless you canprovide a positive stop that limits hoopstresses to safe limitsfor the material.

etterin'1) 'he molding process adapts easily

for molding-in logos, labels,warnings,diagrams, and instructions,saving the e%pense of stic -on orpainted labels, and enhancing

recyclability!) J 5imit the depth or height of

lettering into or out of the part surfaceto appro%imately . ! inch( andJ Angle or round the side walls of theletter


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Tolerance!) 'o improve your ability to maintain

speci4ed tolerances in productionJ >se low-shrin age materials in partswith tight tolerances(J Avoid tight tolerances in dimensionsa ected by the alignment of the moldhalves or moving mold componentssuch as slides(J Gesign parts and assemblies toavoid tight tolerances in areas proneto warpage or distortion( andJ Adjust the mold to producedimensions in the middle of tolerancerange at optimum processingconditions for the material. 'o avoid unnecessary molding costs,

specify tight tolerances only whenneeded. 0enerally, the si


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range to features that are diKcult tocontrol.7eserve tight tolerances forfeaturesthat can accommodate themreasonably

Desi'n (or % pact

!" +f your part will be e%posed to impactstrains, address energy management

issues early in the design process,including2J )tress concentration(J nergy dissipation( andJ #aterial impact properties .

" Avoid stress concentrations. While

this is an important goal in gooddesign practice, it becomes ofparamount importance in impactapplications. An impact causes a highenergy wave that passes through thepart and interacts with its geometry.

Gesign features such assharp corners, notches, holes, andsteps in thic ness can focus thisenergy, initiating fracture. As cornersor notches become sharper, the part?simpact performance will diminish


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=" 7ound inside corners and notches toreduce stress concentrations

@" 5oo for potential problems fromsources other than part design, suchaspost-molding operations. #achining,for instance, can leave deepscratches,microcrac s and internal stressesleadingto stress concentrations

/" osition gates and nit lines in areasthat will not be subjected to highimpact forces. 'he area around gatesgenerally has higher levels of molded-in stress. +n addition, improper gateremoval can leave rough edges andnotches. Pnit lines typically e%hibitlower strengththan other areas and can concentratestresses along the 4ne 1-notch thatforms the visible nit lines

B" 6ften a better strategy is to designthe part to e%, so it can absorb anddistribute the impact energy. +n someinstances, this can involve reducingthic ness and removing orredistributing ribs to accommodate


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controlled e%ure. $onsider thefollowing rules of thumbto improve impact performance2J +f using multiple ribs, space themunevenly or orient them to preventresonance ampli4cation from theimpact energy(J Avoid bo%y shapes that concentrateimpact forces on rigid edges andcorners( andJ >se rounded shapes to spreadimpactforces over larger areas .

D" )elect a material with good impactperformance throughout the part?swor ing-temperature range(J Address all temperatures and impactloads including those found in themanufacturing process and shipping(J $onsider notch sensitivity of thematerial in applications withunavoidable notches and stressconcentrators( andJ $hec ow orientation : especiallyin 4ber-4lled materials : and thedi erence between ow and cross owmechanical properties .


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Design Process for Plastic

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