BASF Snap Fit Design Guide

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Tooppiicc PPaarrtt

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Introduction. . . . ntroduction. . . I. . . Introducntroductitionon

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Snap-Fip-Fit Dt Desesiign Agn Apppplliicacatitionsons. . . I. . . I

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Types ypes of Snap-Fiof Snap-Fitsts . . . I. . . III

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Snap-Fip-Fit Beam t Beam DDesesiign Usign Using Classng Classiicacal l Beam Beam TTheoheoryry. . . I. IIIII

Improved Cantilever Snap-Fit Design. . . IV Improved Cantilever Snap-Fit Design. . . IV

“ “ U“

U“&&““LL““ShaShapeped Sd Snapnapss . . . V. . . V

General D

General Desesiign Guign Guidedelliinesnes . . . V. . . VII

Engl

Engliissh/Mh/Metrietric Coc Conversinversion Chaon Chartrt . . . I. . . Insnsiide de Back CoverBack Cover

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BASF Plastics is a fully integrated, global supplier of BASF Plastics is a fully integrated, global supplier of eng

engiineeneeriring resng resiins ns ““ffrom prodrom production of uction of ffeeeedsdstoctocks to thks to thee comp

compoundoundiing, mang, manufnufacacture and ture and didistristributibution of hundredon of hundredss of

of resin grresin gradadeses.. BA

BASF iSF is s commcommiitted tted to cto contiontinuous nuous producproduct det develvelopmeopment tont to sustain rapid growth in the nylon resin market. In our sustain rapid growth in the nylon resin market. In our

Plastics Technology Laboratory, a highly experienced staff Plastics Technology Laboratory, a highly experienced staff of research and development engineers continues to of research and development engineers continues to de

devvelelop op new resnew resiins to furns to further extend ther extend the hothe horirizons zons of of product

product perfperformance.ormance.

BASF offers high-quality engineering resins, including: BASF offers high-quality engineering resins, including:

“ “ Ultramid Ultramid®® (nylon 6 and 6/6) (nylon 6 and 6/6) Nypel Nypel®® (

(a a poposst-it-indndusustritrial nylal nylon on 6)6) “

“ Petra Petra““®®

(

(pospost-const-consumer recyclumer recycled ed PETPET)) “

“ “ “

Ultradur

Ultradur®®““_{PBT T}_{PBT T}_hermo_hermoplas_plasti_ti_c_{c Po}_Po_l_l_ymer_ymer Ultraform

Ultraform®®_“_“

Acetal (POM) Acetal (POM) Ultrason

Ultrason®®““_{High Temp Polymers}_{High Temp Polymers} T

Theshese rese resiins ns ffrom BArom BASF, coupSF, couplled ed wiwith the cth the compompany’any’ss conc

concepept-through-commt-through-commerciercialaliizatization eon expertixpertisese, ca, cann comb

combiine to hene to hellp map make poke possssiiblble the e the mosmost efft effiicicient, coent, costst-eff

effecectitive snave snap-p-ffiit ft for yor your prodour product. uct. Our technOur techniicacal l ssuppupport iort iss read

ready y to heto hellp you wip you with all th all your needyour needss. . AAnd for morend for more i

informatinformation, you con, you can an alwayalways s vivissiit our wet our web sb siite ate att www.plasticsportal.com.

www.plasticsportal.com.

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This manual will guide you through the

basics of snap-fit design, including: types

of snap-fit designs and their applications;

how to calculate the strength of the unit and

amount of force needed for assembly; and the

three common causes of failure in snap-fits

and how to overcome them.

Introduction

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Why ususe se snanap-fip-fitsts? ? TThihis cs chaphapter witer willl l gigive yve you a thou a thumbumbnailnail s

sketch of the beneketch of the beneffiits of snapts of snap-f-fiits ats and the mnd the materiaterialals uss useded to make them.

to make them.

Snap-fits are the simplest, quickest and most Snap-fits are the simplest, quickest and most cost-eff

effecectitivve methoe method of assd of assembemblliing two parts. ng two parts. WWhen dehen designedsigned properly, parts with snap-fits can be assembled and

properly, parts with snap-fits can be assembled and disassembled numerous times without any adverse effect disassembled numerous times without any adverse effect on the as

on the assesemblmblyy. . Snap-fiSnap-fits are alts are also the moso the mostst en

envivironmeronmentantalllly fy fririendendlly fy form oorm of assf assemembly bly bebecacaususee of their ease of disassembly, making components of of their ease of disassembly, making components of different materials easy to recycle.

different materials easy to recycle. A

Allthough though snasnap-fip-fits ts can can be be dedesigned signed wiwith math many materiny materialals,s, the ideal material is thermoplastic because of its high the ideal material is thermoplastic because of its high flexibility and its ability to be easily and inexpensively flexibility and its ability to be easily and inexpensively mol

moldeded id into complex nto complex geogeometrimetrieses. . Other Other adadvantagesvantages include its relatively high elongation, low coefficient of include its relatively high elongation, low coefficient of f

fririction, action, and nd ssuffuffiicient scient strength trength and and ririgidigidity to mety to meet et thethe requi

requirements rements of most aof most apppplliicacatitionsons..

The designer should be aware that the assembly may have The designer should be aware that the assembly may have some “

some “plplay“ay“due due to tolerance to tolerance stastack-up ck-up of the two of the two matimatingng parts.

parts. Some sSome snap-fnap-fiits can alts can also iso increase the concrease the costst

of an injection molding tool due to the need for slides in the of an injection molding tool due to the need for slides in the mold.

mold. AAn experin experiencenced ded desesiignegner r cacan ofn often eliten elimiminanate thete the ne

need ed ffor sor slliidedes s by adby adding a sding a sllot in the waot in the walll l dirdirecectltly belowy below the undercut or by pl

the undercut or by placiacing the sng the snaps naps on the eon the edge dge of tof thehe part, so they face outward (see Figure I-1).

part, so they face outward (see Figure I-1).

REQUIRES SLIDE IN MOLD REQUIRES SLIDE IN MOLD

UNDERCUT UNDERCUT NO SLIDE REQUIRED NO SLIDE REQUIRED SLOT SLOT NO SLIDE REQUIRED, NO SLIDE REQUIRED, MOLD LESS COMPLEX MOLD LESS COMPLEX

Fi Figugure re II-1-1 I-1 I-1

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S N A P

-S N A P -F I T D E S I G N AF I T D E S I G N APPPPL IL IC A T IC A T IO N SO N S

I-2

Concluding points:

Concluding points: SnaSnap-fip-fits ts sosollve the pve the problroblem oem of f creating an i

creating an inexpenexpensivnsive ce compomponent thaonent that can t can be be quiquicklcklyy and e

and easasiilly y jjoioined with ned with ananotheother r piecepiece. . TThermohermoplasplastiticscs are the

are the iidedeal material material fal for snor snapap-f-fiits bts becaecaususe thee they have they have the f

fllexiexibilbiliity and ty and resresiilliienence ce necnecesesssary to ary to allallow for numow for numerouerouss as

assesemblmbly and y and didisasassssemblembly operatiy operations.ons.

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Thihis s chachaptepter provir providedes s an oan overvverviiew of the dew of the diifffferenerent types t types of of ca

cantintillever snaever snap-fip-fits ts and and gigives aves an in idedea oa of wf when thehen they are usey are used.d. M

Mosost et engingineeneeriring mateng materirial appal applliicacatitions ons wiwith sth snap-fnap-fiits ts ususe e thethe ca

cantintillever dever desesiign (segn (see Fie Figure gure IIII-1) and-1) and, thus, thus, this ma, this manuanual l wiwillll f

focus ocus on that don that desesiign. gn. TThe che cylyliindrindricacal l dedessiign cgn can ban bee em

employed wheployed when an an unfin unfilllled ed thermothermoplasplastitic mc mateateririal wial withth hi

highegher elongar elongatition wilon will l be be usused ed ((a typical apa typical appliplicacatition is aon is ann as

aspipiririn bon bottlttle/ce/cap ap asassesemblymbly))..

Y Y

CANTILEVER CANTILEVER

“U” SHAPED CANTILEVER “U” SHAPED CANTILEVER

“L” SHAPED CANTILEVER “L” SHAPED CANTILEVER

Fi

Figugure re III-1I-1

When designing a cantilever snap, it is not unusual for the When designing a cantilever snap, it is not unusual for the de

designer to gsigner to go o through sthrough several ieveral iteratiterations (ons (chachanginging ng llength,ength, thi

thicknesckness, s, dedeffllecectition don diimensmensiionsons, etc, etc.) .) to deto design a ssign a snapnap-f-fiitt with a lower allowable strain for a given material.

with a lower allowable strain for a given material. Other types of snap-fits which can be used are the “ Other types of snap-fits which can be used are the “UU““ or “

or “LL““sshahapeped cd cantantiillever sever snapnaps s ((ssee ee PaPart V rt V ffor moor more dre detaetaiill)).. T

Theshese ae are usre used ed when the swhen the straitrain of tn of the she straitraight caght cantintilleverever sna

snap cp cannot bannot be de desesiigned gned bebellow the aow the allllowabowablle se straitrain fn for theor the given material.

given material.

Concluding points:

Concluding points: MMosost apt applplicaicatitions ons cacan en emplmploy aoy a ca

cantintillever type sever type snapnap-fi-fit it in the dn the desesiign. gn. IIn apn appliplicacatitions ons wiwithth ti

tight paght packaging requickaging requirementsrements, the “, the “U“or “U“or “LL““sshaphaped sed snap manap mayy be required.

be required.

Automotive oil filter snaps

Cordless screw driver housing, cantilever snap-fit

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III-1 III-1 OVERHANG DEPTH OVERHANG DEPTH ENTRANC ENTRANCE SIE SIDEDE RETRACTION SIDE RETRACTION SIDE A

A dedesign engineer’sign engineer’s s jjob ob iis s to fito find a nd a baballancance be betweenetween integrity of the assembly and strength of the cantilever integrity of the assembly and strength of the cantilever be

beam. am. WWhihille a ce a cantiantillever beaever beam wim with a deth a deep ep overhangoverhang ca

can make the un make the uninit set secure, icure, it alt also so puts puts more smore straitrain on then on the be

beam duriam during assng assembemblly y and diand disassassesemblmbly. y. TThihis chaps chapterter expl

explaiains ns how thihow this s baballancance is ae is achichieved.eved.

A typical snap-fit assembly consists of a cantilever beam A typical snap-fit assembly consists of a cantilever beam wi

with ath an overhang n overhang at the at the end end of the bof the beaeam (see m (see FiFigure IIgure IIII-1).-1). T

The dhe depepth of th of the overhang dethe overhang deffiines nes the amthe amount of ount of deflection during assembly.

deflection during assembly.

Fri

Frictctiion Coon Coeffiefficiencientt µ = tanµ = tan ββ Mati

Mating ng ForceForce = W= W W W = P = P ttaann((αα++ ββ)) µ + tan µ + tanαα W W = = P P ——————————————— — 1– 1–µ taµ tannαα Fi

Figugure re IIIIII-2-2 Fi

Figugure re IIIIII-1-1

T

The ohe overhang typiverhang typicacalllly has y has a ga gentlentle ramp e ramp on the on the entranceentrance side and a s

side and a sharpeharper r angle on the rangle on the retraction etraction side. side. TThe she smalmalll angl

angle ae at the entrance st the entrance siide de ((αα) () (see see FiFigugure re IIIIII-2-2) helps ) helps toto reduc

reduce the e the asassesembly mbly effeffort, whiort, whille the e the ssharp aharp anglngle ae at thet the retracti

retraction son siide de ((αα) makes ) makes ““ disasdisasssembemblly vy very difery difffiicult orcult or i

imposmpossiblsible de depependendiing on ng on the intthe intendended ed ffunctiunction. Both theon. Both the assembly and disassembly force can be optimized by assembly and disassembly force can be optimized by mod

modiiffyyiing the ng the angles angles mentimentioned oned ababove.ove.

The main design consideration of a snap-fit is integrity The main design consideration of a snap-fit is integrity of

of the asthe asssembemblly y and and strestrength of ngth of the bethe beam. am. TThe ihe integntegririty ty of of the a

the assssembemblly iy is s cocontrolntrollled ed by the sby the stitiffffnesness s ((k) k) of the beof the beamam and

and the athe amount omount of f dedeffllecectition reqon requiuired for assred for assembemblly ory or di

disasassssembemblly. y. RiRigigididity can be ty can be iincreasncreased ed eieither by usither by using ang a hi

highegher modr modululus us mamateriterial (al (E) E) or by ior by increncreasasiing thng the ce crosrosss s

secectitionaonal l mommoment of ient of inertia (nertia (II) ) of of the bthe beaeam. m. TThe phe produroduct of ct of thes

these e two ptwo paramarameteeters rs ((EIEI) w) wiilll detel determirmine ne the the totatotal rl riigidigidity of ty of a g a giivven ben beaeam lm length.ength. α α'' _α_α R R W W P P W W P P R R FRICTION CONE FRICTION CONE α α α α ++ ββ } } β β MA MATITING FORNG FORCECE

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S N A P

S N A P- F I T D E S I G N U S I N G C L A S S- F I T D E S I G N U S I N G C L A S SI C AI C AL B E A M TL B E A M TH E O R YH E O R Y

III-2

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The integrihe integrity of ty of the athe assssemblembly can y can alalso so be be iimproved bmproved byy i

increasncreasiing the overhang depng the overhang depth. th. AAs a ress a resulult, the beamt, the beam has

has to deto deffllecect ft further and, therefore, requiurther and, therefore, requires res a greaa greaterter eff

effort to cort to clleaear the or the overhang verhang ffrom the rom the iinterlnterlocockiking hng hooook.k. However, as the beam deflection increases, the beam However, as the beam deflection increases, the beam s

strestress as allsso io increncreasaseses. . TThihis wils will l resresulult it in a fain a faillure iure if f the bthe beaeamm stres

stress s iis s ababove the yiove the yield seld strength of the matetrength of the materirialal..

Thus, the deflection must be optimized with respect to the Thus, the deflection must be optimized with respect to the yi

yield seld strength otrength or strain of tr strain of the he matemateririal. al. TThihis s iis s acachihieved eved byby opti

optimimizizing the ng the beabeam sm sectiection geon geometry tometry to eo ensure thansure that thet the des

desiired dred defleflectiection con can ban be reae reached ched wiwithout exceethout exceediding theng the strength or strain limit of the material.

strength or strain limit of the material.

The assembly and disassembly force will increase with The assembly and disassembly force will increase with bo

both sth stitiffffnesness s ((k) k) and and mamaxiximum mum dedeffllecectition oon of the bf the beaeam (Ym (Y)).. The force (P) required to deflect the beam is proportional The force (P) required to deflect the beam is proportional to the prod

to the product of tuct of the two fhe two facactors:tors: P= kY P= kY T

The she stitiffffnesness s valvalue (kue (k) ) dedepepends nds on bon beaeam gem geometry asometry as s

shohown wn iin n FiFigugure re IIIIII-3-3..

Stress or strain induced by the deflection (Y) is also shown Stress or strain induced by the deflection (Y) is also shown i

in Fin Figure gure IIIIII-3. -3. TThe che calculalculateated sd strestress s or sor straitrain valn value sue shouldhould be less than the yield strength or the yield strain of the be less than the yield strength or the yield strain of the material in order to prevent failure.

material in order to prevent failure.

When selecting the flexural modulus of elasticity (E) When selecting the flexural modulus of elasticity (E) f

for hygror hygrososcopcopiic mc materiaterials, als, ii.e., nyl.e., nylon, con, care sare shoulhould d be be taken.taken. I

In the n the dry as dry as molmoldeded d stastate (Dte (DAAM)M), the , the dadatastasheeheet valt value ue maymay be

be usused ed to cto calalculculate ate stifstifffnesness, des, deffllecectition on or retentior retention on fforceorce of

of ssnap nap dedessiign. gn. Under normaUnder normal l 50% relati50% relative humive humiditydity cond

condiititionsons, however, t, however, the phe physihysicacal l propepropertirties es dedecreascreasee and

and, therefor, therefore, the e, the stifstifffnesness s and and retentiretention foron force ce reducreducee whi

whille the de the defleflecectition ion increasncreaseses. . Both scBoth scenaenaririos os shshoulould bd bee checked.

checked.

Where: Where: E =

E = FlFlexuraexural l MMododulusulus P = P = FForceorce Y= Deflection Y= Deflection b = Width of Beam b = Width of Beam Fi

Figugure re IIIIII-3-3

b b b b tt P P L L L L tt 22 P P tt L L tt b b 44 P P I

I UnifUniform orm Cross Cross SSectection,ion, Fixed End to Free End Fixed End to Free End Stiffness:

Stiffness: Strain: Strain:

II

II UnifUniform Worm Width, Hidth, Height eight TapersTapers to t/

to t/ 2 at Free E2 at Free Endnd Stiffness:

Stiffness: Strain: Strain:

III Uniform H

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-S N A P -F I T D E S I G N U S I N G C L AF I T D E S I G N U S I N G C L AS SS S I C AI C AL B E A M TL B E A M TH E O R YH E O R Y

III-3

Concluding points:

Concluding points: In a typical snap-fit, the strengthIn a typical snap-fit, the strength of a beam is dependent on its geometry and maximum of a beam is dependent on its geometry and maximum de

deffllecectition durion during asng assesemblmbly. y. TThe fhe force to asorce to assesemble andmble and disassemble snap-fit assemblies is highly dependent on disassemble snap-fit assemblies is highly dependent on the overhang entrance and retraction angles.

the overhang entrance and retraction angles.

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IV-1

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The he cacantintillever beever beam am fformulormulas as usused ed iin cn conventionventionalonal s

snapnap-f-fiit dest desiign undgn undereserestitimate the mate the amouamount of nt of strain atstrain at the b

the beaeam/walm/wall l iinterfnterfacace be bececausause thee they do not iy do not inclnclude ude thethe de

defformaormatition ion in the wall n the wall iitstselfelf. . IInsnsteatead, thed, they y asasssume thume the walle wall to

to be be cocompmplleteetelly riy rigid wigid with the th the dedeffllecectition ocon occurricurring ng only ionly inn the be

the beam. am. TThihis as assssumptiumption maon may y be be vvalaliid when the ratid when the ratio of o of beam length to thickness is greater than about 10:1.

beam length to thickness is greater than about 10:1. Howev

However, to ober, to obtaitain a n a more amore accccurate purate prediredictiction of totalon of total allowable deflection and strain for short beams, a allowable deflection and strain for short beams, a magnification factor should be applied

magnification factor should be applied to the

to the coconventinventionaonal l fformula. ormula. TThihis wils will l enaenable greable greaterter flexibility in the design while taking full advantage of flexibility in the design while taking full advantage of the

the sstraitrain-cn-carryarryiing ng cacapapabilbiliity of the mty of the mateateririal.al. BA

BASF PlSF Plasastitics cs has has dedevvelelopeoped a d a method method ffor esor estitimatimatingng these deflection magnification factors for

these deflection magnification factors for vari

various ous snsnapap-f-fiit bt beaeam/walm/wall confl confiiguratigurations ons as as shoshownwn i

in Fin Figure gure IIVV-1. -1. TThe he resresulults ts of thiof this s tectechnihniquque, whiche, which have been verified both by finite element analysis and have been verified both by finite element analysis and ac

actual part testingtual part testing11_{, are shown graphically in Figure IV-1.}_{, are shown graphically in Figure IV-1.} Fi

Figure gure IIVV-2 s-2 shows hows ssiimimillar resar resulults ts ffor beor beamams s of of tapered

tapered cross cross sesectiction (on (beabeam thim thickness ckness decdecreasireasingng by 1/2 at the tip).

by 1/2 at the tip).

Snap-Fit Design Examples 1 &2 illustrate this procedure Snap-Fit Design Examples 1 &2 illustrate this procedure f

for deor dessiigning sgning snapnap-fi-fitsts, i, incncllududiing cng calculalculating the ating the mamaxiximummum strain developed during assembly and predicting the strain developed during assembly and predicting the snap-i

in force ren force requirquireded..

1

1_Chul_Chul_{S. Lee,}_{S. Lee,}_A_A_l_l_{an Dubin and El}_{an Dubin and El}_me_me_{r D}_{r D}_{. Jo}_{. Jo}_nes_nes_{, “}_{, “}_{Short Cantilever Beam}_{Short Cantilever Beam}

Defl

Deflecectition Analyon Analysis Applisis Applied ed to Thermopto Thermopllasastitic Sc Snapnap-Fi-Fit Dest Desiign,“gn,“1981987 S7 SPEPE

ANTEC, held in Los Angeles, California, U.S.A.

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IV-2 IV-2 D D E E F F L L E E C C T T I I O O N N M M A A G G N N I I F F I I C C A A T T I I O O N N F F A A C C T T O O R R Q Q ASPECT RATIO, L/t ASPECT RATIO, L/t 8. 8. 00 7. 7. 00 6. 6. 00 5. 5. 00 4. 4. 00 3. 3. 00 2. 2. 00 1. 1. 00 0. 0. 00 0 0 ..00 11 ..00 22..00 33..00 44..00 55..00 6..06 0 77 ..00 88..00 9 ..09 0 11 00..00 11 11..00 ON A BLOCK ON A BLOCK (SOLID WALL) (SOLID WALL) 11 ON A PLATE ON A PLATE (OR THIN WALL) (OR THIN WALL)

22 44

55 33

Uni

Uniform Beamform Beam, Q , Q FactFactoror Fi

Figugure re IIVV-1-1

I M P R O V E D C A N T I L E V E R S N A P - F I T D E S I G N

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IV-3 IV-3 I M P R O V E D C A N T I L E V E R S N A P - F I T D E S I G N I M P R O V E D C A N T I L E V E R S N A P - F I T D E S I G N 8 8 .0.0 7 7 .0.0 6 6 .0.0 5 5 .0.0 4 4 .0.0 3 3 .0.0 2 2 .0.0 1 1 .0.0 0 0 .0.0 0 0..00 11..00 22 ..00 33 ..00 44 ..00 55 ..00 6 ..06 0 77..00 88..00 9..09 0 11 00 ..00 1111 ..00 2T 2T 5T 5T D D E E F F L L E E C C T T I I O O N N M M A A G G N N I I F F I I C C A A T T I I O O N N F F A A C C T T O O R R Q Q ASPECT RATIO, L/t ASPECT RATIO, L/t 2T 2T 5T 5T tt_/2_/2 tt T

Tapapered Beaered Beam, Q m, Q FactorFactor Fi

Figugure re IVIV-2-2

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IV-4 IV-4 M MAATTEERRIIAALL UUNNFFIILLLLEEDD 3300% % GGLLAASSSS P PEEII 99..88%%(2)(2) P PCC 44%%(1)(1)_{- 9.2%}_{- 9.2%}(2)(2) Acetal Acetal 77%%(1)(1) _2.0%_2.0% Nyl Nylon on 66(4)(4) _8%_8%(5)(5) _2.1%_2.1%(1)(1) PBT 8.8% PBT 8.8%(2)(2) _2.0%_2.0% P PCC//PPEETT 55..88%%(2)(2) A ABBSS 66% % - - 77%%(3)(3) P PEETT 11..55%%(1)(1) M MAATTEERRIIAALL µµ P PEEII 00..2200--00..2255 PC PC 0.25 0.25 - - 0.0.4040 A Acceettaall 00..220 0 - - 00..3355 Ny Nyllon on 6 6 0.10.17 7 - - 0.0.4040 P PBBTT 00..3355--00..4400 P PCC//PPEETT 00..440 0 - - 00..5500 A ABBSS 00..5500--00..6600 P PEETT 00..1188--00..2255 Table IV-I Table IV-I NOTES NOTES_:_: (1) (1)

70% of tensile yield strain value 70% of tensile yield strain value

(2)

(2) _{G.G. Trantina.}_{G.G. Trantina.}_{Plastics Engineering.}_{Plastics Engineering.} August 1989.

August 1989.

(3)

(3) _V_V_{.H. T}_{.H. T}_rumb_rumb_ull_ull_._.₁₉₁₉_{84 A}₈₄_A_SME_SME_Wi_Wi_nte_{nter A}_{r A}_nnu_nnu_{al Confere}_{al Conferenc}_nce_e (4)

(4) _{DAM - “}_{DAM - “}_Dry_Dry_A_A_s_{s M}_M_olde_olde_d_d“_“_co_co_nd_ndi_i_tion_tion (5)

(5) _BA_BA_{SF test lab;}_{SF test lab;}_{Note 4% s}_{Note 4% s}_houl_houl_{d be us}_{d b}_{e us}_ed_{ed i}_i_{n M}_{n M}_ati_ati_{ng Force}_{ng Force} Formula

Formula

Tab

Table le IVIV-II-II NOTES:

NOTES:

(1)

(1) _{Material tested against itself}_{Material tested against itself} Co

Coeeffifficient ocient of Ff Friricctitionon(1)(1) A

A) THE MATHE MAXXIMUM IMUM DEFLEDEFLECCTION OF STION OF SNAPNAP B)

B) THE THE MAMATITING FORCNG FORCEE SOLUTION:

SOLUTION: A)

A) THE MAXTHE MAXIMUM IMUM ALALLOWABLE DEFLOWABLE DEFLECLECTION OF STION OF SNAPNAP

tY

tY maxmax

∈

ooLL22QQ

∈

oo= 1= 1..5 5 _L_L——22——_Q_Q——-- ⇒⇒ Y Y _max_max= = _1.5_1.5————_t_t—— ——

L

— = 5.0

— = 5.0 ⇒⇒ Q = 2.0Q = 2.077 _{(from Q Factor Graph)}_{(from Q Factor Graph)}

t

(0.0

(0.022₅₎₍₅₎₍11₅₎₅₎22_(2.0_(2.0₇₇₎₎

Y

Y _max_max==————————————————————== 2.59 mm2.59 mm

1.5

1.5(3(3 ) ) T

Therehereffore, iore, in an an an actuactual l dedessiign,gn, a sa smallmaller valer value for ue for defdefllecectitionon (Y) would be chosen for an added factor of safety.

(Y) would be chosen for an added factor of safety. B)

B) THE THE MAMATING TING FORFORCCEE

bt bt22_E_E

∈

o o P P = = ——————————— — 6L 6L 6 6( ( 33 ) )22₍₍₄₈₃₀₄₈₃₀_)(0.0_)(0.0₂₂₅₎₅₎ P P==——————————————————————== 72.45 N72.45 N 6( 6( 115)5) µ + tan a µ + tan a W W = = P P ————————————— — 1–µ tan a 1–µ tan a 0. 0.33+ tan30º+ tan30º W = W =72.4572.45_{———————— = 76.9 N}_{———————— = 76.9 N} (72.45)¹ (72.45)¹– 0.– 0.33(tan30º)(tan30º) Therefore, it will take

Therefore, it will take 76.9 N76.9 Nmating force tomating force to as

asssembemblle pe parts, if arts, if the pthe part deart deffllecteected to d to the mathe materiterialal’’ss all

allowabowablle e sstraitrain.n.

(From Q Factor Graph,

Figure IV-1) Figure IV-1) IV-5 IV-5 DETERMINE: DETERMINE: I

IS THIS TYPE OF SNAP-FIT ACCEPTABLE FOR USE INS THIS TYPE OF SNAP-FIT ACCEPTABLE FOR USE IN

ACETAL (ULTRAFORM N2320 003) ACETAL (ULTRAFORM N2320 003) SOLUTION: SOLUTION: tY tY

∈

= 1.5 ———-= 1.5 ———-L L22_Q_Q L L — —= 3.= 3.5757 ⇒⇒ Q = 2.7Q = 2.7 t t (0.063)(0.090) (0.063)(0.090)

∈

= = 11..5 5 —————————————————— = = 66..22%% (0.225) (0.225)22_(2.7)_(2.7)

Therefore, it is acceptable for unfilled

Therefore, it is acceptable for unfilled acetal (POM)acetal (POM) (See Allowable Strain Value, Table IV-1).

(See Allowable Strain Value, Table IV-1). Concluding points:

Concluding points: Unlike conventional formulas, BASFUnlike conventional formulas, BASF i

includencludes s the the dedeffllectiection mon magagniniffiicatication facon factor itor innall all cacallculaticulationsons.. T

The examples she examples show how to calhow how to calculculate theate the mamaxiximum mum sstraitrainn duri

during asng assesemblmbly and how to predy and how to prediictct the force needed for assembly. the force needed for assembly.

Cl

Closeose-up of automotiv-up of automotive wheee wheel l cover snapcover snapss

Snap

-Fi

t Des

i

gn

Ex

am

pl

e

#1

GIVEN: GIVEN: Material

Material⇒⇒Ultradur B4300 G3Ultradur B4300 G3 ( (PPBTBT)) t = t = 3 mm3 mm L = L = 15 mm15 mm b b == 6 mm6 mm E E = = 4830 4830 MPaMPa µ =

µ = 0.30.3(From Table(From Table

IV-II, Coefficient of IV-II, Coefficient of Friction)

Friction)

α

α == 3030..0°0°

∈

oo== 2.5%2.5%(From Table(From Table

IV-I, Allowable IV-I, Allowable Strain Value) Strain Value) Figure IV-4 Figure IV-4 tt Y Y b b L L α α P P W W

Snap

-Fi

t Des

i

gn

Ex

am

pl

e

#2

GIVEN: GIVEN: Material

Material⇒⇒UnfilledUnfilled

Acetal Acetal t t == 00..00663 3 iinn Y Y == 00..00990 0 iinn L L == 00..22225 5 iinn b b = = 0.242 0.242 iinn Figure IV-5 Figure IV-5 tt bb L L Y Y PP

Uniform Beam - Type 4

Uniform Beam - Type 5

(14)

V-1

The cantilever beam snap-fit design isn’t appropriate The cantilever beam snap-fit design isn’t appropriate f

for alor all l apappliplicacatitionsons. . TThihis cs chaphapter deter deffiines nes ““L“L“and and ““U“U“sshahapepedd snap

snaps as and telnd tellls when they are useds when they are used..

Occasionally, a designer will not be able to design a Occasionally, a designer will not be able to design a ca

cantintillever sever snapnap-fi-fit cot confinfiguraguratition with a son with a straitrain bn below theelow the all

allowaowable lble liimit mit of the iof the intentendnded ed mamateteririal. al. ThiThis is uss is usuaualllly y ddueue to l

to liimimited ted papackaging spckaging spacace whie which cch can resan restritrict the ct the llength ength of of the s

the snapnap. . TThihis s iis the s the iidedeal al titime to me to consconsiideder usir using either anng either an “

“ L

L““sshaphaped ed snasnap or a “p or a “UU““shashapeped snad snap.p. T

The he ““LL““sshaphaped ed ssnap nap ((ssee ee FiFigure gure VV-1) i-1) is s fformeormed bd by desy desiigninggning i

in sn sllots ots iin the n the babasse wae walll l whiwhich ch effeffecectitivelvely iy increancreasses es thethe beam length and flexibility compared to a standard beam length and flexibility compared to a standard ca

cantintillever beaever beam. m. TThihis as allllows the dows the desesiignegner to rr to rededuce theuce the s

straitrain dn duriuring ng asasssembemblly bey bellow thow the e allallowabowablle e lliimimit of thet of the se

sellecected mated materiterialal. . IIt should be notet should be noted that add that addiding a sng a sllot toot to the ba

the base se walwall l may not be may not be acceacceptable in some dptable in some desesiigns gns fforor cos

cosmetimetic c or aior air fr fllow coow concencerns.rns. T

The he ““U“U“sshahapeped sd snanap (see p (see FiFigure Vgure V-2) i-2) is s anoanother way tother way to increase the effective beam length within a limited space increase the effective beam length within a limited space envelope

envelope. . WiWith thith this ds desesiign, egn, even matven materierials wials with lth lowow all

allowaowable sble straitrain lin limimits ts ((ssuch uch as as hihighly glghly glasasss-fi-filllled ed mamateriterials)als) can be

can be desdesiigned to meegned to meet asst assemblembly y requirequirements. rements. TThehe “

“ U

U““shashapeped dd desesiign usgn usualuallly iy incorponcorporates rates the undthe undercut on ercut on thethe outer e

outer edge dge of the paof the part to elirt to elimiminate nate the nethe need fed for sor slliide de iin then the mold, unless a slot is acceptable in the wall from which the mold, unless a slot is acceptable in the wall from which the snap

snap projprojectsects..

Figure V-1 Figure V-1 Figure V-2 Figure V-2

“

U

“

& “

L

“

Sha

pe

d

Sna

ps

“L” SHAPED CANTILEVER “L” SHAPED CANTILEVER

“U” SHAPED CANTILEVER “U” SHAPED CANTILEVER

Pa

rt

V

;; ;;

(15)

“

“UU““& & ““LL“ S H A“ S H AP E D SP E D SN AN AP S P S ( C O N S T A( C O N S T AN T C R O S S N T C R O S S S E C T I O N )S E C T I O N )

L

L Shaped SShaped Snap-Fnap-Fit Examit Exampleple A

A) ) CaCallcucullatate the e the miniminimum lengmum length (th (LL22) ) of the sof the sllot (seot (seee

s

sketcketch, h, FiFigure Vgure V-3)-3)in the main wall in the main wall for Ultramid 8233 nylonfor Ultramid 8233 nylon i

in the con the confinfiguration beguration bellow. ow. TThe reqhe requiuired dred defleflecectition ion is .3s .388 inches.

inches. B)

B) CalculCalculate ate the rethe requirquired ed fforce orce ((P) P) to dto defleflecect thet the snap

snap .38 i.38 inchesnches.. GIVEN: GIVEN:

∈

82338233 = .02= .0255 t t = = ..1 1 iinn L L11= .5 in= .5 in R R = .= .12 12 inin I

I = Mo= Momenment oft of IneInertirtia (ra (rectectangangle)le)

R R 22+ 8L+ 8L11R) + 12LR) + 12L22(L(L11+ R)+ R)22 ] ] Where: Where: L

L22= Length of = Length of slot as sslot as shown ihown in sken sketchtch

∈

oo= Allowable strain of material= Allowable strain of material

Y = Maximum deflection required in direction Y = Maximum deflection required in direction

of force of force t =

t = TThihicknescknesss L

L11= = LLength as ength as shown ishown in sn sketchketch R = Radi

R = Radius aus as s shown ishown in sketchn sketch (

(at at neuneutral axitral axiss)) P =

P = FForceorce b =

b = Beam WiBeam Widthdth E =

E = FlFlexuraexural l MMododulusulus I

I = = MMomomenent ot of If Inenertirtiaa P P P P L L11 R R L L22 A A AA Section Section A-A A-A b b tt P P P P

(16)

“ “ U

U ShaShaped Sped Snapnap Example #1 Example #1

Case 1 Case 1 A

A) ) CalculCalculate ate the the amoamount ount of f dedeffllecectition aon at the t the titip op of f thethe bea

beam fm for a 1.0 or a 1.0 pound pound lloadoad GIVEN:

GIVEN: P

R R 22}+}+ 6L 6L22(3L(3L1122- 3L- 3L11LL22+L+L2222 )] )]

∈

P P P P R R L L11 L L22 P P b b Section Section A-A A-A tt A A AA

(17)

Concluding points:

Concluding points: SnaSnap-fip-fits cts can usan use eie either the “ther the “U“or “U“or “LL““ sha

shapeped ded design tsign to overo overcome scome spapace lce limiimitatitationsons. . Both tBoth the “he “LL““ and “

and “UU““sshaphaped ed ssnapnaps es effffecectitivelvely ry rededuce uce sstraitrain dun duriringng as

asssememblybly, thus , thus making imaking it it idedeal fal for maor materiterials wials with lowerth lower all

allowaowable sble strain ltrain liimits.mits.

“ “U “U “& & ““L “ SL “ SH A PH A P E D E D SSN A PN A P SS V-4 V-4 “ “ U“

U“ShaShapeped d SnaSnapp Ex

Examample #2ple #2

Case 2 Case 2 A

A) ) CalculCalculate ate the the amoamount ount of f dedeffllecectition aon at the t the titip op of f thethe bea

beam fm for a 1.0 or a 1.0 pound pound lloadoad GIVEN: GIVEN: I = 0.833 x 10 I = 0.833 x 10-4-4_in_in44 E = 534,000 psi E = 534,000 psi R = 0.15 in R = 0.15 in L L11= 0.7 in= 0.7 in L L11= L= L22 L L33= 0.273 in= 0.273 in t t = = 0.1 i0.1 inn Y Y == 6EI 6EI[4L[4L1133+ 2L+ 2L3333+ 3R {L+ 3R {L11(2(2

π

LL11+ 8R) ++ 8R) +

π

R R 22}]}] = = 6(534,000)(0.833 x 10 6(534,000)(0.833 x 10-4-4₎₎ [4(0.7)[4(0.7) 3 3_{+ 2(0.273)}_{+ 2(0.273)}33₊₊ 3(0.15){0.7(2 3(0.15){0.7(2

π

• 0.7 + 8(0.15)) +• 0.7 + 8(0.15)) +

π

(0.15)(0.15)22_}]_}] = = 0.012 0.012 inin P P 1 1 R R P P L L11 L L33 L L22

Automotive wheel cover

Cl

Closeose-up of above c-up of above cover backside featuriover backside featuring the “ng the “LL““shashapeped sd snapnap-fi-fitt

des

desiign (fgn (from a rom a top atop angle)ngle)

I

(18)

VI-1

be

between the tween the paparts, relrts, relaxatiaxation aon at the t the jjoioint cant can resn resulult it in ln lososss of seal pressure, resulting in leakage of the contained fluid. of seal pressure, resulting in leakage of the contained fluid. A

Another probnother probllem oem offten sten seeeen in is es excesxcessivsive pe pllay betweeay between then the pa

parts drts due ue to tolerance to tolerance varvariiatiationsons, s, someometitimes mes resultresultiing inng in noise a

noise and vind vibrabratition. on. SeSeveral veral ways to miways to mininimimize thesze thesee phenomena

phenomena iinclnclude: desude: desiignigning a ng a llow stress ow stress snap snap beabeam,m, de

designisigning the ng the snasnap-fip-fit to incorporatet to incorporate a 9

a 90° return angl0° return angle e so so that it rthat it relaxes elaxes iin tensn tensiion versuson versus be

bendndiing (seng (see Fie Figugure Vre VII-2-2)). . ThiThis wils will l preprevent the vent the mamatiting png partart f

from slrom sliippppiing pang passt or t or bebecocomiming lng loooosse. e. AAnothenother r way way iis tos to use

use a la large return angarge return anglle ae and incrnd increaease se the lthe land and llength in theength in the return ang

return anglle ae area (serea (see Fie Figure Vgure VII-3). -3). IIncncreasreasiing theng the

overhang depth and evaluating the worst case scenario in overhang depth and evaluating the worst case scenario in a to

a tolleranerance ce sstudy witudy willl l allallow the ow the dedessiign to gn to retain giretain given pven pululll- -off force even after relaxation occurs.

off force even after relaxation occurs.

Fi

Figugure re VVII-2-2

Fi

Figugure re VVII-3-3 Three basic issues should be reviewed before finalizing

Three basic issues should be reviewed before finalizing a sna

a snap-fip-fit dest desiign: gn: stresstress conces concentratintration, creep/on, creep/relrelaxatiaxation,on, and fati

and fatiguegue. . BelBelow are desow are descricriptiptions oons of f thesthese problemse problems and s

and suggeuggestions to prevent them. stions to prevent them. AAlll l shshoulould bed be cons

consiidereddered as as part of part of good dgood desesiign practign practice for ce for anyany thermoplasti

thermoplastic dc desesiign.gn.

The single most common cause of failure in snap-fits is The single most common cause of failure in snap-fits is stress

stress concentraticoncentration due to a on due to a sharp cosharp corner rner between thebetween the sn

snapap-f-fiit bet beam aam and the nd the walwall l to whito which it ch it iis s attacattachedhed. . SiSincence thi

this s llococation noation normallrmally coinciy coincidedes wis with thth the e popoiint ont of maxif maximummum stress

stress, a sharp corner can i, a sharp corner can increase ncrease the stress the stress beyond thebeyond the strength of the material, causing point yielding or

strength of the material, causing point yielding or bre

breakaakagege. . TThihis s iis s more cmore crirititicacal l ffor rior rigigid pd pllasastitics cs lliike glaske glasss- -reinforced nylon, which have relatively low ultimate

reinforced nylon, which have relatively low ultimate elonga

elongatition. on. MMore dore ductiluctile me mateateririals, lials, like unreke unreiinforced nforced nylnylon,on, tend to yield and deform before they break, redistributing tend to yield and deform before they break, redistributing the pe

the peak stresak stress over a broades over a broader r regiregion. on. One soOne sollutiution ion is tos to i

incorponcorporate a rate a ffiillllet radet radiius us at the at the jjuncture buncture between etween the bthe beaeamm and

and the the walwall l ((ssee ee FiFigure gure VVII-1), so -1), so thathat the t the ratiratio oo of rf radadiius us toto wall

wall thithicknecknesss (s (R/t) R/t) iis as at lt leaeasst 50%. t 50%. GoiGoing beng beyond 50yond 50%% results i

results in a man a margirginal inal increasncrease ie in sn strength and trength and may causmay causee othe

other probr probllems ems lliike ike internanternal l voivoids ds and and ssiink marksnk marks. . IIf f ssiink nk mark

marks s are an are an iissssue, a ue, a smasmallller radier radius us can bcan be use used, bed, but iut itt may i

may increasncrease the se the stress tress iin thin this as area. rea. AAnother opnother optition ion is tos to ad

add the d the radiradius us onlonly on the tensy on the tensiille se siide de of of the bthe beaeam.m.

Figure VI-1 Figure VI-1 Creep

Creep, or more a, or more accccuratelurately stress y stress relrelaxatiaxation, con, can resan resulult it inn a red

a reductiuction oon of f the hothe holldiding forng force ce bebetween the tween the twotwo comp

components coonents connecnnected by ted by the snapthe snap-f-fiit. t. StresStress rels relaxatiaxationon wi

willl l ococcucur gradur graduallally ovy over tier timeme. . IIf f thethere ire is a s a gagassket or sket or seaeall

Gene

ra

l

D

e

s

i

gn

Gui

de

l

i

ne

s

SHARP SHARP CORNER CORNER R R == .5t MINIMUM .5t MINIMUM tt P POOOOR R DDEESSIIGGNN GGOOOOD D DDEESSIIGGNN RELAXED PO RELAXED POSITISITIONON

((EXAGGEXAGGERATED)ERATED)

P = MATING PART FORCE P = MATING PART FORCE UNDEFORMED UNDEFORMED POSITION POSITION UNDEFORMED UNDEFORMED POSITION POSITION P P PP R

REELLAAXXAATTIIOON N IIN N TTEENNSSIIOONN RREELLAAXXAATTIIOON N IIN N BBEENNDDIINNGG

RETURN ANGLE RETURN ANGLE LAND LENGTH LAND LENGTH OVERHANG DEPTH OVERHANG DEPTH

P

a

rt V

I

;;;; ;; ;; ;; ;;

(19)

VI-2

G E N E R A L D E S I G N G U I D E L I N E S

Fatigue, or repetitive loading, is the third major cause Fatigue, or repetitive loading, is the third major cause of f

of failailure. ure. FatiFatigue gue coconcencerns prns pririmarilmarily appy applly iy if f hundhundreds reds oror thous

thousandands os of f cyclcycles es are antiare anticicipapated. ted. WWhihille the de the desesiigngn s

strestress s llevel mievel might bght be wee welll l wiwithithin the n the sstrength trength of theof the mate

materirialal, the rep, the repeaeated ated apppplliicacatition of thion of this s stresstress cs canan res

resulult it in fatin fatigue gue ffailailure aure at st some ome popoiint in the future.nt in the future. Some

Some polypolymers mers peperfrform betteorm better than others r than others iin thin this s regard,regard, making them ideal candidates for snap-fits or living hinges making them ideal candidates for snap-fits or living hinges tha

that must must ft fllex repex repeaeatedtedlly. y. TThe fihe first warst way to avoiy to avoid a d a ffatigueatigue f

failailure is to ure is to chchoosoose e a ma mateateririal known to al known to peperfrform weorm welll l iinn f

fatiatigue. gue. TThihis cs can be dan be done bone by y comcompapariring the song the so-ca-calllled Sed S-N-N curves of the materi

curves of the materialals, whis, which sch show the expehow the expected cted numbenumberr of cycles to failure at various stress levels and at different of cycles to failure at various stress levels and at different temp

temperatures oeratures of f exposexposure. ure. TThe she sececond way, stiond way, stilll l using theusing the S-N curves, is to choose a design stress level, at the S-N curves, is to choose a design stress level, at the correct temp

correct temperature, that reserature, that resulults its in the requirn the required ed numbenumber of r of l

loaoad apd appliplicacationtions ps pririor to for to failailureure. . ThiThis mes methothod wid willl l ususuauallllyy be

be consconservervatiative sive sincence S-N curves

S-N curves are typiare typicacalllly generatey generated ad at muct much hih highergher f

frequerequencincies es than woulthan would bd be ae antinticicipapated ted ffor repeaor repeatedted ap

applpliicacatition oon of a sf a snapnap-f-fiit ast asssembemblly.y. For hygrosc

For hygroscopopiic mc mateateririals lals liike nylke nylon, on, the the effeffecects ts of of moi

moisture osture on fn fiinal part dinal part dimensmensiions ons and and mecmechanicalhanical prope

propertirties aes allso musso must be const be consiidedered. red. For For ffurtherurther information, please consult the BASF Plastics Design information, please consult the BASF Plastics Design Solutions Guide.

Solutions Guide.

Concluding points:

Concluding points: TThere ahere are a numbre a number of wer of ways toays to over

overcome come the ithe issssues ues of of stress stress conceconcentrntratiation, stresson, stress rel

relaxation and fatiaxation and fatiguegue. . A A welwell l thougthought-oht-out deut dessiign angn andd using the right polymer for a given application will minimize using the right polymer for a given application will minimize the

thesse ie issssuesues. . TThihis as allllows thows the ape appliplicacatition to beon to benefinefit ft from allrom all the advantages of a snap-fit design.

the advantages of a snap-fit design.

Ci

Circulrcular saar saw handle inset w handle inset shot shot featurifeaturing sng snap-finap-fit ct cllosure osure and and matingmating

Cl

Closeose-up o-up of trf truck miruck mirror patch ror patch covercover

Close-up of automotive fuel rail cover, snap-fit design

Aerator

(20)

Notes

(21)

Engl

i

is

s

h/Metri

h/Metric

c

Conve

rs

i

ion Ch

on Ch

a

rt

T To o CCoonnvveerrtt ToTo MMuullttiippllyy E Enngglliissh h SSyysstteemm MMeettrriic Sc Syysstteemm EEnngglliissh h VVaalluue be byy. .. ... DISTANCE DISTANCE i inncchheess mmiilllliimmeetteerrss 2255..3388 f feeeett mmeetteerrss 00..3300447788 MASS MASS o ouunncce e ((aavvddpp)) ggrraamm 2288..33449955 p poouunndd ggrraamm 445533..55992255 p poouunndd kkiillogogrraamm 00..44553366 U U..SS. . ttoonn mmeettrriic c ttoonn 00..99007722 VOLUME VOLUME inch

inch33 _centimeter_centimeter33 _16.3871_16.3871

inch inch33 _l_l_i_i_t_te_er_r ₀_0._.0₀₁₁₆₆₃₃₈₈₇₇ f flluuiid d oouunnccee cceennttiimmeetteerr33 29.5735 29.5735 q quuaarrt t ((lliiqquuiidd)) ddeecciimemetteerr33₍₍_l_l_i_i_t_te_er_r)₎ ₀_0._.9₉₄₄₆₆₄₄ g gaalllloon n ((UU..SS..)) ddeecciimmeetteerr33₍₍_l_l_i_i_t_te_er_r)₎ ₃_3._.7₇₈₈₅₅₄₄ TEMPERATURE TEMPERATURE d deeggrreee e FF dedeggrreee e CC ((°°FF––3322) ) / / 11..8 8 = = °°CC PRESSURE PRESSURE p pssii bbaarr 00..00668899 p pssii kkPPaa 66..88994488 k kssii MMNN//mm22 _6.8948_6.8948 p pssii MMPPaa 00..0000668899 ENERGY AN

ENERGY AND PD POWEROWER in lb in lbf f JJoouulleess 00..111133 ft lb ft lbf f JJoouulleses 11..33555588 k kWW mmeettrriic c hhoorrsseeppoowweerr 11..33559966 U U..SS. . hhoorrsseeppoowweerr KKww 00..77445577 B Bttuu JJoouulleses 11005555..11 BTU “ BTU “in / in / (hr (hr ““ftft22_“_“_º_ºF_F)₎ _W_W/_/m_m_“_“_°_°K_K ₀_0._.1₁₄₄₄₄₂₂ VISCOSITY VISCOSITY p pooiissee PPaa““ss 00..11 BENDING MOMENT BENDING MOMENT OR TORQUE OR TORQUE f fttllbb NN““mm 11..335566 DENSITY DENSITY lb/in lb/in33 _g/cm_g/cm33 _27.68_27.68 lb/ft lb/ft33 _kg/m_kg/m33 _16.0185_16.0185 NOTCHED IZOD NOTCHED IZOD f fttllbb//iinn JJ//mm 5533..44

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