BAS BA S F Pl Pla s ti ticc s Snap-Fit De s ign Manu al
Ta bl blee of Conte Conte nts
Topic
Part
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduc Introducti tion on Snap-Fi Sna p-Fitt Des Desiign App Appllica cati tions ons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I
Types of Snap-Fits Snap-Fits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II II Snap-Fi Sna p-Fitt Beam Des esiign Using Using Class ica call Beam Theo heory ry . . . . . . . . . . . . . II III
Improved Cantilever Snap-Fit Design. . . . . . . . . . . . . . . . . . . . . . . . . . . IV
U U“ “ “&L ““Sha Shape ped d Snap S napss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V General Des Des ign Guide Guidellines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI VI
Engliis h/M Engl h/Metri etricc Conversi Co nversion on Chart Cha rt . . . . . . . . . . . . . . . . . . . . . . . Ins Insiide Back Cover
Introduction Sna p-Fi p-Fitt De s ign
This manual will guide you through the
Abo ut BASF BASF Performa nc e P oly olyme me rs
basics of snap-fit design, including: types
BASF Plastics is a fully integrated, global supplier of engiinee ri eng ring ng res ins “from production prod uction of fee eeds ds toc tocks ks to the th e comp ound oundiing, ma nuf nufac acture ture and di distri stributi bution on of hundreds hundred s of resin grad grades es .
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.
BASF is BASF is comm commiitted to conti c ontinuous nuous produc t devel de velopme opme nt to sustain rapid growth in the nylon resin market. In our Plastics Technology Laboratory, a highly experienced staff of research and development engineers continues to dev de vel elop op new res ins to further further extend the hori ho rizons zons of product perf performance. ormance. BASF offers high-quality engineering resins, including: U Ultramid “ ltramid (nylon 6 and 6/6) ®
Nypel (a po poss t-i t-ind ndus ustri trial al nylon on 6) ®
Petra P “ etra “ (pos t-cons umer recycled recycled PET PET)) ®
U Ultradur “ ltradur “PBT Thermo Thermoplas plasti ticc Po Pollymer ®
Ultraform “Acetal (POM) ®
Ultrason “ High Temp Polymers ®
Thes e res ins from BASF, BASF, coup led wi with th the c omp ompany’ any’ss concep conc ept-through-comm t-through-commerci ercial aliizati zation on e xperti xpertise se , ca n comb ine to he lp ma ke poss po ss ibl blee the mos mostt effi effici cient, ent, co steffec eff ecti tive ve sna pp-ffit for for your your prod uct. Our techni techn ica call s upp ort is is read y to hel he lp you with with all your need s . And for more informati nformation, on, you c an alway alwayss vi viss it our web we b s ite a t www.plasticsportal.com.
Pa rt I Sna p-Fi p-Fitt Des ign App lic a ti tion onss Why us usee sna s nap-fi p-fits ts?? Thi hiss c hap ter wil will gi give ve you you a thumb th umbnail nail s ketch of the bene fits of snap -f -fiits and a nd the materi m aterial alss us ed to make them. Snap-fits are the simplest, quickest and most costeffec eff ecti tiv ve method metho d of ass emb emblling two parts. When de signed properly, parts with snap-fits can be assembled and disassembled numerous times without any adverse effect on the as se sembl mbly y. Snap-fi Snap-fits ts are also also the mo st envi en vironme ronmenta ntallly fri friend end ly form form of o f ass em embly bly be beca caus usee of their ease of disassembly, making components of different materials easy to recycle. Although sna p-fi p-fits ts can be de designed signed wi with th many ma ny material materials, s, the ideal material is thermoplastic because of its high flexibility and its ability to be easily and inexpensively molde mol ded d into into complex geo geometri metries es . Other ad advantages vantages include its relatively high elongation, low coefficient of fri riction, ction, and a nd s uff uffiicient s trength and ri rigidi gidity ty to meet me et the requirements requi rements of most a pp ppllica cati tions ons . The designer should be aware that the assembly may have some play“ p “ lay“due to tolerance sta stack-up ck-up of the two mati mating ng parts. Some s nap-f nap-fiits can also also increase increase the co st of an injection molding tool due to the need for slides in the mold. An experienc experienc ed des d es igne gnerr ca can n often often elimi elimina nate te the need ne ed for sl s lide dess by ad ding a s lot in the wal wa ll dir direc ectl tly y below the undercut or by placi placing ng the s naps on the e dge of the the part, so they face outward (see Figure I-1).
UNDERCUT
REQUIRES SLIDE IN MOLD
SLOT
NO SLIDE REQUIRED
NO SLIDE REQUIRED, MOLD LESS COMPLEX
Figu Fi gu re I-1
I-1
S N A P - F I T D E S I G N A P P L IC I C A T IO IO N S
Concluding points: Sna Snap-fi p-fits ts so sollve the probl p roblem em of o f creating an inexpe inexpensiv nsivee comp c omponent onent tha t can be qui quickl ckly y and e as ily joi oined ned with an anothe othe r piece piece.. Thermo hermoplas plasti tics cs are the ide deal al material material for for snap sn ap-f -fiits b eca us usee they the y have the flexi exibil biliity and res resiilien ence ce nec es s ary to all allow ow for numerou num erouss asse as sembl mbly y and di disa sass ss embl embly y operations. operations.
Door handle handle be zel
Ba c ks id e o f b e ze l
I-2
De ta il o f b a c ks id e o f b e ze l, c a ntile ve r d e s ig n
Pa rt II II Type s of Sna pp-Fi Fits ts Thi hiss cha chapte pterr provide providess an overv o verviiew of the di d ifferen erentt types of canti ca ntillever snap-fi sna p-fits ts and gi gives ves an a n ide ideaa of o f when when they the y are use d. Mos ostt engi e nginee neeri ring ng mate materi rial al app lica cati tions ons wi with th s nap nap-f -fiits us usee the canti ca ntillever des d es ign (see (se e Figure Figure II-1) and, and , thus , this manua ma nuall wi willl focus on that d es esiign. The cyl c ylindri ndrica call de dess ign can c an be be employed em ployed whe n an a n unfil unfilled thermo thermoplas plasti ticc mate m ateri rial al with with highe hi gherr elongati elonga tion on will will be us used ed (a typical appli ap plica cati tion on is an an as pi piri rin n bottl bo ttle/c e/cap ap as asse se mbly mbly)).
When designing a cantilever snap, it is not unusual for the designer de signer to g o through s everal iterations terations (cha changi nging ng length, thicknes thi cknes s, de defflec ecti tion on d imens ions ons,, etc .) to de sign a s nap nap-f -fiit with a lower allowable strain for a given material. Other types of snap-fits which can be used are the “U“ or L ““s ha hape ped d cant c antiilever s nap s (s ee Pa Part rt Vfor more mo re deta d etaiil). Thes e are a re used us ed when the s trai train n of the the s trai traight ght ca nti ntillever sna p cannot c annot b e des d es igned be bellow the al a llowab owablle s trai train n for for the given material. Concluding points: Mos ostt ap pl pliica cati tions ons ca can n empl e mploy oy a canti ca ntillever type s nap -fi -fitt in in the d es ign. In appli ap plica cati tions ons wi with th tight ti ght pa ckaging requirements requirements , the “U“or U“or “L“s hap haped ed s nap ma y be required.
Y
CANTILEVER
“U” SHAPED CANTILEVER
Automotive oil filter snaps
“L” SHAPED CANTILEVER
Figu Fi gu re II-1
Cordless screw driver housing, cantilever snap-fit
II-1
P a rt II III Sna p-Fi p-Fitt De De s ign Usi Us ing Cla Cla s s ic a l Be a m The ory A de design sign engineer’s engineer’s job is to find find a ba ballanc ancee b etween integrity of the assembly and strength of the cantilever beam. be am. Whi hille a c anti antillever beam bea m with with a de ep overhang can ca n make the uni u nitt secure, se cure, it it also also puts more s trai train n on the beam be am during during ass emb emblly and dis dis as se mbl mbly. y. Thi hiss chap ter explai expl ains ns how this this ba ballanc ancee is a chi chieved. eved.
MATI MA TING NG FORCE FOR CE
P R
α'
W
α
A typical snap-fit assembly consists of a cantilever beam with wi th an a n overhang at the end of the bea b eam m (see Figure gure III-1). The dep d epth th of the overhang de fines the amount am ount of deflection during assembly. R
FRICTION CONE
}
ENTRANCE ENTRANCE SIDE SI DE
P
α+β W
β
α
RETRACTION SIDE
Frict Fri ct ion Co effi efficien cien t µ = tan β Mating Mati ng Force OVERHANG DEPTH
Figu Fi gu re III-1
=W
W = P ta n(α + β)
α µ— + tan —— ———— W = P — 1– µ tan tan α Figu Fi gu re III II-2 -2
The overhang o verhang typica typicallly has a gentl g entlee ramp on the entrance side and a s harpe harperr angle on the retraction retraction side. The smal s malll anglee at angl a t the entrance s ide (α) (s (s ee Fi Figu gure re III-2 -2)) helps to reducee the as se mbly eff reduc effort, ort, whil while the s harp a ngl nglee at a t the retraction retracti on s ide (α)“ makes disa disass s emb ly very very diff difficult or impos sibl siblee d ep epend endiing on the intend intended ed functi unction. on. Both the assembly and disassembly force can be optimized by modiifying the angles menti mod mentioned oned ab above. ove.
III-1
The main design consideration of a snap-fit is integrity of the as s emb ly and stre strength ngth of the be am. The integ integri rity ty of the as a s s emb emblly is co control ntrollled by the s ti tifffnes s (k) of the beam be am and the amount a mount of o f de defflec ecti tion on requi req uired red for ass emb emblly or disa di sa ss emb emblly. Ri Rigi gidi dity ty can be increas ed ei either ther by using using a highe hi gherr modul mod ulus us ma materi terial al (E) (E) or by b y incre increas as ing the th e cros c rosss s ec ecti tiona onall mom moment ent of inertia inertia (I (I) of the bea b eam. m. The produ p roduct ct of thes e two p aram aramete eters rs (EI EI)) wi will determi dete rmine ne the tota totall ri rigidi gidity ty of a gi g iven b ea eam m length. length.
S N A P - F I T D E S I G N U S I N G C L A S S I C A L B E A M TH TH E O R Y
The integrity integrity of the a ss emb emblly can al also so be improved b y increas ing the overhang dep th. As a res ul ult, t, the beam has to de flec ectt further further and, therefore, requires requires a grea ter effort eff ort to c lea earr the overhang o verhang from the interl nterloc ocki king ng hoo h ook. k. However, as the beam deflection increases, the beam s tres tresss a ls o incre increas as es . Thi hiss will will res resul ultt in in a fail failure if if the b ea eam m stres s is ab above ove the yiel yield d s trength of the materi mate rial al..
Can ti tillever Be am : Defl Deflec ec ti tion-S on-S train Formulas P
t
L b
Thus, the deflection must be optimized with respect to the yield eld strength s trength or o r strain of the the mate materi rial. al. Thi hiss is ac achi hieved eved by optimi opti mizi zing ng the bea beam m secti s ection on ge ometry to to ensure e nsure that tha t the des ired defl d eflection ection can c an b e reached rea ched wi without thout excee di ding ng the strength or strain limit of the material.
I ) Uniform Uniform Cross Sect ion, Fixed End to Free End P
Eb
3
Y 4 ( L ) t e = 1.50 ( )Y L
Stiffness:
k =
Strain:
=
t
2
The assembly and disassembly force will increase with both bo th s ti tifffnes s (k) and ma maxi ximum mum de defflec ecti tion on o f the bea b eam m (Y). The force (P) required to deflect the beam is proportional to the prod uct of the the two fac factors: tors:
P
t
2
P= kY
t
The s ti tifffnes s val value ue (k) (k) de depe pends nds on bea b eam m geometry ge ometry as s ho hown wn in Fi Figu gure re III-3 -3.. Stress or strain induced by the deflection (Y) is also shown in Figure Figure III-3. The c alcul alculate ated d s tres tresss or strai s train n value value s hould be less than the yield strength or the yield strain of the material in order to prevent failure. When selecting the flexural modulus of elasticity (E) for hygros hygroscop cop ic materi m aterial als, s, i.e., nylon, nylon, c are s houl hould d be taken. In the dry as mol molde ded d sta state te (DA (DAM), the da datas tashee heett value value may be us used ed to c al alcul culate ate stif stifffnes s, de defflec ecti tion on or retention retention force of s nap de dess ign. Under normal norma l 50% relative relative humidity humidity cond iti tions ons , however, the the physi p hysica call prope rti rties es de decreas creas e and,, therefore, and therefore, the stif stifffnes s and retenti retention on force force reduc reducee whille the d efl whi eflec ecti tion on increas increas es . Both scena sc enari rios os sh shoul ould d be be checked.
L b
II ) Uniform Uniform Width, Width, Height Height Tapers to t/ 2 at Free End End Stiffness: Strain:
t Y 6.528 ( L ) t e =0 .9 2 ( )Y L k =
P
=
Eb
3
2
b
P t
L
b 4
III) Uniform Height,Width Height,Width Taper s to b/ 4 at Free End End Stiffness: Strain:
P
Eb
t
3
( L ) t e =1 .1 7 ( )Y L k =
Y
=
5.136 2
Where: E = Fl Flexura exura l Mod odulus ulus P = Force Y= Deflection b = Width of Beam Figu Fi gu re III II-3 -3 III-2
S N A P - F I T D E S I G N U S I N G C L A S S I C A L B E A M TH TH E O R Y
Concluding points: In a typical snap-fit, the strength of a beam is dependent on its geometry and maximum defflec de ecti tion on during during as se mbl mbly. y. The force force to as se mble and disassemble snap-fit assemblies is highly dependent on the overhang entrance and retraction angles.
Close Cl ose -up of automotive automotive fuse b ox, snap on s ides of box
Close- up o f automo tiv tivee fuse bo x, full full vi view ew
III-3
Close Cl ose -up of automo ti tive ve fuse fuse box snap
Pa rt IV IV Im pro ve d Ca nti ntille ve r Sna pp-Fi Fitt De De s ign The ca canti ntillever beam be am formul ormulas as us used ed in c onventi onventional onal s nap nap-f -fiit des ign underes und eresti timate mate the amou amount nt of strain at the bea b eam/wal m/walll interf nterfac acee bec b ecaus aus e they the y do not incl include ude the defforma de ormati tion on in in the wall its tself elf.. Ins nstea tead, d, the y as s ume the th e wall to be co comp mpllete etelly rigid rigid with with the de defflec ecti tion on oc occurri curring ng only in the be am. Thi hiss ass a ss umpti umption on may ma y be val aliid when the ratio ratio of beam length to thickness is greater than about 10:1. However, Howev er, to ob tai tain n a more acc a ccurate urate p redi redicti ction on of total allowable deflection and strain for short beams, a magnification factor should be applied to the co conventi nventiona onall formula. Thi hiss will will ena ble grea ter flexibility in the design while taking full advantage of the s trai train-c n-carry arryiing ca capa pabil biliity of the mate m ateri rial. al. BASF Plas BASF Plas ti tics cs has de dev vel elope ope d a method for es ti timati mating ng these deflection magnification factors for various ous sn snap ap-f -fiit bea b eam/wal m/walll confi configurati gurations ons as sho shown wn in Figure Figure IV-1. The res resul ults ts of this this tec techni hniqu que, e, which have been verified both by finite element analysis and actual ac tual part testing 1, are shown graphically in Figure IV-1. Figure Fi gure IV-2 s hows s imi millar res ul ults ts for be am amss of tapered cross se secti ction on (bea (beam m thickness thickness dec decreasi reasing ng by 1/2 at the tip). Snap-Fit Design Examples 1 &2 illustrate this procedure for de s igning snap s nap -fi -fits ts,, inc incllud udiing c alcul alculating ating the ma maxi ximum mum strain developed during assembly and predicting the snapin force requir re quired ed..
1
Chul S. Lee, Alan Dubin and Elme Elmerr D. D. Jones Jo nes , Short S “ hort Cantilever Beam Deflec ec ti tion on Analysis Analysis Applied to Thermop las ti ticc S nap -Fi -Fitt Desi Des ign,“ gn,“198 1987 7 SPE S PE ANTEC, held in Los Angeles, California, U.S.A.
IV-1
IMPROVED CANTILEVER SNAP-FIT DESIGN 8. 0
1 ON A BLOCK (SOLID WALL) 7. 0
6. 0
4
2 ON A PLATE (OR THIN WALL)
Q R O T C A F N O I T A C I F I N G A M N O I T C E L F E D
3 5. 0
5
4. 0
3. 0
2. 0
1. 0
0. 0 0 .0
1 .0
2 .0
3.0
4 .0
5.0
6.0
7 .0
ASPECT RATIO, L/t
Uniform Uni form Beam , Q Fact or Figu Fi gu re IV-1 IV-2
8 .0
9 .0
1 0 .0
1 1.0
IMPROVED CANTILEVER SNAP-FIT DESIGN 8 .0
t/2
7 .0
t
6 .0
Q R O T C A F N O I T A C I F I N G A M N O I T C E L F E D
2T
5T 5 .0
4 .0
5T
3 .0
2 .0
2T
1 .0
0 .0 0 .0
1.0
2 .0
3 .0
4 .0
5 .0
6 .0
7.0
8.0
9 .0
1 0 .0
1 1 .0
ASPECT RATIO, L/t
Tap ered Bea m, Q Factor Figu Fi gu re IV-2 IV-3
IMPROVED CANTILEVER SNAP-FIT DESIGN
Allow ab le Stra in Value Value ,
Improved Formulas MATERIAL P EI PC Acetal Nylon Nyl on 6 (4) PBT P C/P ET ABS P ET
b t
P W
Y
α L
UNFILLED 9.8%(2) 4%(1) - 9.2%(2) 7%(1) 8% (5) 8.8%(2) 5.8%(2) 6% - 7%(3)
eo 30% GLAS S
2.0% 2.1%(1) 2.0%
1.5%(1) Table IV-I
Figure IV-3
NOTES: (1) (2)
MAX MA XIM IMUM UM S TRA TRAIIN (@BAS (@BASE) E)
tY ∈ = 1.5 ———2 L Q
(3) (4) (5)
MATING FORCE
α + tan W = Pµ— —— ———— 1– µ tan tan α 2 ∈ E— P = bt — — — — — — 6L
Where: W = Pus h-on Force Force W’ = Pull-off Force P = Perpe ndi ndicul cular ar Force Force µ = Co Coeff effiicient of Frict Frictiion α = Lea ead d Angle α’ = Return Angle b = Beam Width Width t = Bea m Thi Thicknes cknes s L = Beam Length E = Fl Flexura exura l Mod odulus ulus ∈ = Strain at Base ∈o = Allowa owable ble Mat aterial erial Strai Stra in Q = Deflection Magnification Factor (refer to Figure IV-2 for proper Q values) Y = Deflection
70% of tensile yield strain value G.G. Trantina. Plastics Engineering. August 1989. V.H. Trumb Trumbull ull.. 19 1984 84 ASME Wi Winte nterr Annu Annual al Conferenc Confere ncee DAM - “Dry Dry As Molde d “co cond ndiition BASF BA SF test lab; Note 4% s houl hould d be b e us ed in Mati Mating ng Force Formula Co e ffi fficient cient o f Fri Fricc ti tion on (1) MATERIAL
µ 0.20 - 0.25 0.25 - 0.40 0.20 - 0.35 0.17 0.1 7 - 0. 40 0.35 - 0.40 0.40 - 0.50 0.50 - 0.60 0.18 - 0.25
P EI PC Ac etal Nyllon 6 Ny P BT P C/PET ABS P ET
Tab le IV-II NOTES: (1) Material tested against itself
Wheel cover with cantilever snaps
IV-4
IMPROVED CANTILEVER SNAP-FIT DESIGN
Snap -Fi -Fitt Des ign Ex Exam am pl plee #1 Uniform Beam - Type 4
Snap -Fi -Fitt Des ign Ex Exam am pl plee #2 Uniform Beam - Type 5
GIVEN: b t
GIVEN:
W
t = 3 mm L = 15 mm b = 6 mm
Y
L
α
E = 4830 MPa
µ = 0.3 (From Table IV-II, Coefficient of Friction) α = 30 30..0° ∈o = 2.5% (From Table IV-I, Allowable Strain Value)
P
Y
Material ⇒ Ultradur B4300 G3 (PBT)
P
Material ⇒ Unfilled
Acetal
L t
b
t Y L b
= = = =
0.063 in 0.090 in 0.225 in 0.242 in
Figure IV-5 DETERMINE: IS THIS TYPE OF SNAP-FIT ACCEPTABLE FOR USE IN
ACETAL (ULTRAFORM N2320 003)
Figure IV-4 DETERMINE:
SOLUTION:
A) THE MAX MAXIMUM DEFLE DEFLEC CTION OF SNAP SNAP B) THE MATI MATING NG FORCE FORC E
tY ∈ = 1.5 ———2 L Q
(From Q Factor Graph, Figure IV-1)
L = 3.57 — 3.57 ⇒ Q = 2.7 t
SOLUTION: A) THE MAXIMUM AL ALLOWABLE LOWABLE DEFLEC DEFLECTION TION OF SNAP S NAP
∈o L2 Q max max ∈o = 1. 1.5 tY — — —- ⇒ Y max = —— —— 2 L Q
1.5 t
L = 5.0 ⇒ Q = 2.07 (from Q Factor Graph) — t (0.025)( 15)2 (2.07 ) Y max = —————————— ——= = 2.59 mm 1.5(3 ) There hereffore, in in an actua a ctua l de dess ign, a small s maller er value value for de defflec ecti tion on (Y) would be chosen for an added factor of safety.
(0.063)(0.090) (0.225) (2.7)
∈ = 1.5 ————— 2 ———— = 6.2% Therefore, it is acceptable for unfilled acetal (POM) (See Allowable Strain Value, Table IV-1). Concluding points: Unlike conventional formulas, BASF include ncludess the de defflec ecti tion on mag m agni niffica cati tion on factor fac tor in in all ca callculati culations ons . The examples s how how to calcul calculate ate the ma maxi ximum mum s trai train n during duri ng as se mbl mbly y and how to pred ict the force needed for assembly.
B) THE MA MATING TING FOR FORC CE 2 ∈o bt— E— P=— — — — 6L 6( 3 )2 ( 4830 4830 )(0.025) P = ——————————— = 72.45 N 6( 15)
+ tan a—— W = Pµ— —— —— 1–µ tan a 0.3 + tan30º W = 72.45 ———————— = 76.9 N (72.45)¹ – 0.3 (tan30º) Therefore, it will take 76.9 N mating force to as s emb emblle p arts, if the p art de flecte ected d to the ma teri terial al’’s allowab all owab le s trai train. n.
Close Cl ose -up of automotive automotive whee l cover snap s
IV-5
Pa rt V U “ “& L “ “Sha pe d Sna ps The cantilever beam snap-fit design isn’t appropriate for all all ap appli plica cati tions ons . Thi hiss chap c hap ter de fines “L“ L“a nd U“ U “ “s ha hape pe d snap s a nd tell tells when they are used . Occasionally, a designer will not be able to design a canti ca ntillever s nap -fi -fitt co nfigurati gura tion on with a s trai train n below b elow the allowa all owable ble lilimit of the inte inte nd nded ed ma mate teri rial. al. Thi Thiss is us ua uallly d ue to li limi mited ted pa packaging ckaging sp ac acee which which c an res tri trict ct the length of the s nap nap.. Thi hiss is the ide deal al ti time me to cons ide derr using using either an L “ “s hap haped ed sna p or a “U“sha pe ped d sna p. The L “ “s hap ed s nap (s ee Fi Figure gure V-1) is is forme ormed d b y des igning in s lots in the ba bass e wa ll whi which ch eff effec ecti tivel vely y increa increa s es the beam length and flexibility compared to a standard canti ca ntillever bea m. Thi hiss al a llows the d es igne gnerr to red reduce uce the s trai train n duri d uring ng as s emb ly bel be low the th e all allowab owab le limi mitt of the se lec ected ted ma teri terial al.. It should be note d that ad di ding ng a s lot to the ba se wal walll may not be acce ptabl ptablee in some d es esiigns for cos meti meticc or air air fl flow conce co ncerns. rns. The U“ U “ “s ha hape ped d s na nap p (see Fi Figure gure V-2) V-2) is is ano another ther way to increase the effective beam length within a limited space envelope.. With envelope th this this des d es ign, even e ven materi mat erials als with with low low allowa all owable ble s trai train n limi limits ts (s uch as hi highly ghly glas glas s -filled ma materi terials) als) can be des igned to mee t ass embl embly y requi requirements. rements. The U “ “sha pe ped d d es ign us ual uallly incorpo incorpo rates the und ercut on the outer edge e dge of the pa rt to elimi eliminate nate the ne ed for sl s lide in the mold, unless a slot is acceptable in the wall from which the snap proj projects ects..
V-1
“L” SHAPED CANTILEVER
Figure V-1
“U” SHAPED CANTILEVER
Figure V-2
“U “ & “L “ S H AP A P E D S N A P S ( C O N S T AN AN T C R O S S S E C T I O N ) L L“ “ “S HA HAP P ED S NAP–FIT
L Shaped Snap-F Snap-Fit it Example Example
A) Ca Callcu cullat atee the mini minimum mum length leng th (L (L2) of the s lot (see (se e s ketc ketch, h, Fi Figure gure V-3) in the main wall for Ultramid 8233 nylon in the confi co nfiguration guration bel be low. The req ui uired red defl d eflec ecti tion on is is .38 .3 8 inches.
P
L1 t
A
A b
R
Section A-A L2
B) Calcul Calculate ate the requir re quired ed force (P) to defl d eflec ectt the snap .38 inches inches . GIVEN:
∈8233 = .025 t = .1 in L1 = .5 in R = .12 .12 in I = Momen Momentt of Ine Inerti rtiaa (rect (rectang angle) le) 1(.1)3 bt 3 I= = = 8.333(10-5 ) 12 12 6 E = 1.31 (10 ) b = 1.0 in Y = .38
Figure V-3
(6/∈o ) Yt(L Yt(L1+ R) - 4L13 - 3R(2πL12 + πR 2 + 8L1R) L2 = ——— —————————— ————----------–––——–———— 12(L1 +R)2 or,
Y =
P 2 [4L13+3R(2πL12 +πR 2 + 8L1R) + 12L2(L1 + R) ] 12EI
Where: L2 = Length of slot as s hown in in sketch ske tch ∈o = Allowable strain of material Y = Maximum deflection required in direction of force t = Thi hicknes cknes s L1 = Length as shown in in sketch s ketch R = Radius Radius a s shown in in sketch (at neu neutral tral axis axis ) P = Force b = Beam Width Width E = Fl Flexura exura l Mod odulus ulus I = Mom omen entt of o f Ine nerti rtiaa
(6/∈ ) Yt Yt(L (L + R) - 4L13 - 3R(2πL12 + πR 2 + 8L1R) A) L2 = —–––––———1————— —————————————— 12(L1 +R)2
(6/.025 )(. )(.38)( 38)(..1)(.62) - 4(.5)3 - .36[.5π +.122π + 4(.12)] 12)] = —————————————2———————————–– 12(.62) L2 = 0.954 in
2 B) Y= P [4L13+3R(2πL12 +πR 2 + 8L1R) + 12L2(L1 + R) ] 12EI
.38 =
P (12)(1.31)(10 )(8.333)(10-5 ) 6
[4(.5)3+(.36)[.5π+
2 0.954 )(.62) ] .122π+ 8(.5).12] 8(.5).12] + 12( 0.954
.38 =
P=
P 5.655 5.655 3 ( 1.31(10 ) 88 lb
V-2
“U “ & “L “ S H A P E D S N A P S
U Sha Shaped ped Snap–Fit Snap–Fit
U Shaped Snap Snap “ Shaped Example #1 P P
t
L1 L1
b
L2
L2 R
A
R
A
Section A-A
Case 1 Case 1
Y =
∈
9(L1 + R)t
A) Calcul Calculate ate the amo amount unt of o f de defflec ecti tion on a t the ti tip p of o f the beam bea m for for a 1.0 pound load
[6L + 9R {L1(2πL1 + 8R) + πR }+ 3 1
2
6L2 (3L12 - 3L1L2 +L22 )]
GIVEN: P = 1.0 lb lb I = 0.8 33 x 10-4 in4 = bt 3 /12 (rectang ul ular ar cros crosss sec ti tion) on) E = 534,000 psi R = 0.15 in in L1 = 1.4 1 .4 in in L2 = 0.973 0 .973 in t = 0.1 in in b = 1.0 in in
or,
Y =
P [6L13 + 9R {L1(2πL1 + 8R) + πR 2}+ 18EI 6L2 (3L12 - 3L1L2 +L22 )]
L3
2 A) Y = P [ 6L 6L13 + 9R{L1(2πL1 + 8R) + πR 2} + 6L2(3 (3L L12 - 3L 3L1L2 + L2 )] 18EI 1 Y = [6(1.4)3 +9(0.15){(1.4) -4 18(534,000)(0.833 x 10 )
t
P L2
b
L1 A
R
A
Section A-A
Case 2
Y =
∈ 3(L1 + R)t
[4L13 + 2L33 +3R {L1(2πL 1 + 8R) + πR 2}] or,
Y = P [4L13 + 2L33 +3R {L1(2πL 1 + 8R) + πR 2}] 6EI Where: Vari ariab ablles de deffined on previ previous ous pa page. ge.
V-3
(2π•1.4 + 8 • 0.15) + π(0.15)2} + 6(0.973 6 (0.973)) 2 {3(1.4) - 3(1.4)(0.973) + (0.973)2}] = 0.064 in
“U “ & “L “ S H A P E D S N A P S
U“ U “ “Sha pe d Sna p Exam Ex am ple #2
Concluding points: Sna Snap-fi p-fits ts can c an us e ei e ither the “U“or U“or “L“ sha pe ped d design de sign to to overcome overcome s pa pace ce li limi mitati tations ons.. Both the the “L“ and U “ “s hap ed s nap s e ffec ecti tivel vely y red reduce uce s trai train n duri du ring ng as s em embly bly,, thus making it it ide ideal al for for ma teri terials als with with lower allowa all owable ble s train li limits.
L3
L2
L1
P
R
Case 2 A) Calcul Calculate ate the amo amount unt of o f de defflec ecti tion on a t the ti tip p of o f the beam bea m for for a 1.0 pound load GIVEN:
I = 0.833 x 10-4 in4 E = 534,000 psi R = 0.15 in L1 = 0.7 in L1 = L2 L3 = 0.273 in t = 0.1 in in Y = =
Automotive wheel cover
P [4L 3 + 2L33 + 3R {L1(2πL1 + 8R) + πR 2}] 6EI 1 1 [4(0.7)3 + 2(0.273)3 + 6(534,000)(0.833 x 10-4 ) 3(0.15){0.7(2π • 0.7 + 8(0.15)) + π (0.15)2}]
= 0.012 in
Close -up of above c over backside featuring Close featuring the “L“sha pe d s nap -fi -fitt des ign (from (from a top a ngle)
Inse t shot of a “U“ U“shap ed snap -fi -fitt d esign
V-4
P a rt VI VI Gene ra rall De s ign Gui Guide de line s Three basic issues should be reviewed before finalizing a sna p-fi p-fitt desi des ign: stres s conce co ncentrati ntration, on, creep/ rel relaxati axation, on, and fatigue fatigue.. Bel Below ow are des cri cripti ptions ons o f thes e problems and s ugge stions to prevent them. All sh shoul ould d be cons idered as part of good d es esiign practice practice for any thermoplasticc des thermoplasti d es ign. The single most common cause of failure in snap-fits is stress concentrati concentration on due to a sharp co rner between the snap sn ap-f -fiit beam be am a nd the wal walll to which which it is attac attached hed.. Si Since nce thiss loc thi ocation ation normall no rmally coincide coincidess wi with th the th e po poiint of o f maximum maximum stress , a sharp corner can increase increase the stress beyond the strength of the material, causing point yielding or breaka bre akage ge.. Thi hiss is more c ri riti tica call for rigi rigid d pl p las ti tics cs like glass glas s reinforced nylon, which have relatively low ultimate elongati elonga tion. on. More d uctil uctilee mate m ateri rials, als, like like unrei unre inforced nyl nylon, on, tend to yield and deform before they break, redistributing the peak pe ak stres s over a broade r regi region. on. One sol so luti ution on is is to incorpo ncorporate rate a fillet rad ius at the juncture b etween the b ea eam m and the wal walll (s ee Fi Figure gure VI-1), so tha thatt the rati ratio o of o f rad radiius to wall thi thickne ckne s s (R/t) (R/t) is at a t lea leass t 50%. Goi Going ng be yond 50% 50 % results in in a ma rgi rginal nal increas increas e in in strength s trength and may caus e othe r probl prob lems like interna interna l voi voids ds and s ink marks . If s ink markss are an iss ue, a sma ller radius mark radius can b e us ed, b ut itit may increas increas e the s tress in this this a rea. Another opti op tion on is is to add ad d the radi radius us onl only y on the tens ile s ide of the bea b eam. m.
be tween the pa between parts, rts, relaxati relaxation on a t the joi oint nt ca n res ul ultt in in los losss of seal pressure, resulting in leakage of the contained fluid. Another probl prob lem of o ften s ee een n is is e xces xcessiv sivee pl p lay betwee n the parts pa rts due d ue to tolerance var variiati ations ons,, some s ometi times mes result resultiing in noise and a nd vibra vibrati tion. on. Se Several veral ways to mini minimi mize ze thes e phenomena incl nclude: ude: des igni gning ng a low stress snap bea beam, m, designi de signing ng the sna p-fi p-fitt to incorporate a 90° 9 0° return angle angle so that it rel relaxes axes in tens ion versus be nd ndiing (see (se e Figu Figure re VI VI-2 -2)). Thi Thiss will will pre prevent vent the ma mati ting ng part p art from sli slipp ppiing pas pa s t or be beco comi ming ng loo looss e. Anothe r way is to use a large large return angl ang le and a nd increa increase se the land land length in the return ang le area a rea (see (se e Figure Figure VI VI-3). Inc ncreas reas ing the overhang depth and evaluating the worst case scenario in a to leran erance ce s tudy wil will all allow ow the de dess ign to retain given given pul p ullloff force even after relaxation occurs. RELAXED POSITION SITI ON (EXAGGERATED) UNDEFORMED POSITION
UNDEFORMED POSITION
P
P
RELAXATI ON I N TENSI ON
RELAXATI ON I N BENDI NG
P = MATING PART FORCE
Figu Fi gu re VI-2
R= .5t MINIMUM
SHARP CORNER
LAND LENGTH
t POOR DESI GN
GOOD DESI GN
Figure VI-1 Creep , or more a cc Creep, ccuratel urately y stress rel relaxati axation, on, c an res ul ultt in in a red ucti uction on of o f the hol ho ldi ding ng force force be between tween the two comp onents co nnec nnected ted by the snap -f -fiit. Stres Stresss relaxati relaxation on willl oc wi occu curr graduall gradu ally y over over time time.. If the there re is is a ga gass ket or s ea eall
RETURN ANGLE
OVERHANG DEPTH
Figu Fi gu re VI-3
VI-1
GENERAL DESIGN GUIDELINES
Fatigue, or repetitive loading, is the third major cause of fail failure. ure. Fati Fatigue gue co conce nce rns p ri rimaril marily app ly ifif hund reds or thousand thous andss of o f cycl cycles es are antici anticipa pated. ted. Whi hille the des d es ign s tres tresss level might might b e wel we ll wi withi thin n the s trength of the materi mate rial al,, the repea rep eated ted a pp ppllica cati tion on of this this stres s c an resul res ultt in in fatigue fatigue fail ailure ure a t some s ome po poiint in the future. Some poly polymers mers pe perf rform orm bette r than others in this this regard, making them ideal candidates for snap-fits or living hinges thatt mus t fl tha flex repea rep eated tedlly. The first first way wa y to avoid avoid a fatigue fail ailure ure is to ch choos oos e a mate m ateri rial al known to pe perf rform orm wel we ll in fati atigue. gue. Thi hiss can c an be d one by b y com compa pari ring ng the so-ca so -callled S-N S -N curves of the material materials, s, which which s how the expected expe cted numbe r of cycles to failure at various stress levels and at different temperatures temp eratures o f expos ure. The sec s econd ond way, stil still using the S-N curves, is to choose a design stress level, at the correct temp erature, that resul res ults ts in in the required required numbe r of loa oad d ap pli plica ca ti tion onss pri p rior or to fail failure ure.. Thi Thiss me tho thod d wil will us ua uallly be cons erv ervati ative ve since since S-N curves are typica typicallly generate d a t much muc h higher higher freque requenci ncies es than would would be b e anti a ntici cipa pated ted for repea ted appl ap pliica cati tion on o f a s nap nap-f -fiit as s emb emblly.
Close-up of automotive fuel rail cover, snap-fit design
For hygrosc op opiic mate m ateri rials als li like nylon, nylon, the eff effec ects ts of moisture moi sture o n fi final part dimens dimens ions and mec mechanical hanical prope rti rties es a lso mus t be cons ide dered. red. For further information, please consult the BASF Plastics Design Solutions Guide.
Concluding points: There are a re a numb er of ways ways to overcome over come the iss iss ues of stress conce ntr ntrati ation, on, stress relaxation rel axation and fatigue fatigue.. A wel welll thoug ht-o ht-out ut des de s ign and an d using the right polymer for a given application will minimize thess e is the is s ues . Thi hiss al a llows the th e ap pli plica cati tion on to be nefi nefitt from from all the advantages of a snap-fit design.
Aerator
Close Cl ose -up o f truck truck mirror mirror patch cover
Circul Ci rcular ar sa w handle inset shot featuri featuring ng s nap-fi nap-fitt c losure and mating
VI-2
Notes
Engliis h/Metri Engl h/Metricc Conve Convers rs ion Cha Ch a rt To Co n ve rt En g lis h S ys t e m
To Me t r ic Sy Sys t e m
Mu lt ip ly En g lis h Va lu e by by. .. ..
DISTANCE inc hes fee t
millime te rs me te rs
25.38 0.30478
MASS ounc e (avdp) pound pound U.S . ton
gra m gra m kilogra m me tric ton
28.3495 453.5925 0.4536 0.9072
VOLUME inch3 inch3 fluid ounc e qua rt (liq uid) ga llon (U.S .)
centimeter 3 liter c entime te r3 de c ime te r 3 (lite r) de c ime te r 3 (lite r)
16.3871 0.016387 29.5735 0.9464 3.7854
TEMPERATURE de gre e F
de gre e C
(°F –32) / 1.8 = °C
PRESSURE ps i ps i ks i ps i
ba r kP a MN/m2 MP a
0.0689 6.8948 6.8948 0.00689
ENERGY AND AND POWER P OWER in lb f ft lbf kW U.S . hors e powe r Btu BTU “in / (hr “ft2º“F)
J oule s J oule s me tric hors e p owe r Kw J oule s W/m “°K
0.113 1.3558 1.3596 0.7457 1055.1 0.1442
VISCOSITY pois e
P a “s
0.1
BENDING MOMENT OR TORQUE ft lb
N “m
1.356
DENSITY lb/in3 lb/ft3
g/cm3 kg/m3
27.68 16.0185
NOTCHED IZOD ft lb/in
J/m
53.4
IMPORTANT: WHILE THE DESCRIPTIONS, DESIGNS, DATA AND INFORMATION CONTAINED HEREIN ARE PRESENTED IN GOOD FAITH AND AND BELIEV BELIEVED ED TO BE ACCURAT ACCURATE, E, IT IS IS PROVIDED FOR YOUR GUIDANCE ONLY. BECAUSE BECA USE MA MANY FA FACTORS CTORS MA MAY Y AFFECT PROCESSING OR APPLICATION/USE, WE RECOMMEND RECOMM END THA THAT T YOU YOU MA MAKE TESTS TO DETERMINE THE SUITABILITY OF A PRODUCT FOR YOUR PARTICULAR PURPOSE PRIOR TO USE. NO WARRA WARRANTI NTIES ES OF ANY KIND, EITHER EITHER EXPRESSED OR IMPLIED, INCLUDING WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, ARE MADE REGARDING PRODUCTS DESCRIBED OR DESIGNS, DATA OR INFORMATION SET FORTH, OR THAT THE PRODUCTS, DESIGNS, DATA OR INFORMATION MAY BE USED WITHOUT INFRI NFRINGI NGING NG THE INTEL INTELLECT LECTUA UAL L PROP ERTY RIGHT RI GHTS S OF OTHERS. OTHERS. IN NO CASE CASE SHALL SHALL THE THE DESCRIPTI DESCRI PTIONS, ONS, INFORM NFORMA ATI TION, ON, DATA DATA OR DESIGNS PROVIDED BE CONSIDERED A PART OF OUR TER TERMS MS AND AND CONDI CONDITI TIONS ONS OF SALE. SALE. FURTHER, YOU EXPRESSLY UNDERSTAND AND AGREE THAT THE DESCRIPTIONS, DESIGNS, DATA DA TA,, AND INFORMA NFORMATI TION ON FURNISHED BY BASF BASF HEREUNDER ARE GIVEN GRATIS AND BASF ASSUMES NO O BLI BLIGAT GATIION OR LI LIA ABI BILI LITY TY FOR THE DESCRIPTION, DESIGNS, DATA AND INFORM NFORMAT ATIION GI G IVEN OR RESULT RESULTS S OBTA OBTAIINED, ALL SUCH BEING GIVEN AND ACCEPTED AT YOUR RISK.
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