Basf snap_fits.pdf

8/18/2019 Basf snap_fits.pdf

1/24

BASF P la s tic s

Snap-Fit

De s ign Manu al


2/24


3/24

Topic Part

Intro d uc tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Intro d uc tion

S na p- Fit Des ign App lica tion s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I

Typ es of Sn ap -Fits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II

Sn ap -Fit Bea m Des ign Us ing C las sica l Bea m Theory . . . . . . . . . . . . . III

Im pro ved Ca ntileve r Sn ap -Fit Des ign . . . . . . . . . . . . . . . . . . . . . . . . . . . IV

“U“& “L“S ha pe d S na p s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

Ge ne ral Des ign Gu ide line s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI

English /Metric Co nversion Ch art . . . . . . . . . . . . . . . . . . . . . . . Ins ide Ba ck C over

Table of Conte nts


4/24

About BASF Performance Polyme rsBAS F P la s tic s is a fully inte gra ted , glob a l su pp lier of en gine ering res ins “from p rodu ction o f fee ds toc ks to theco m po und ing, m an ufac ture an d distribution of hund redsof resin grad es .

BASF is com m itted to co nt inuous produc t developm ent tsu sta in rap id g rowth in the n ylon re sin ma rket. In ou rPlas tics Techn ology Lab oratory, a highly expe rienc ed stafof resea rch a nd d evelopm ent eng inee rs c ont inues tode velop new resins to further extend the horizons of product performanc e.

BASF offers high -q ua lity en gine ering res ins , inc lud ing :

“Ultramid® / Capron® (nylon 6 a nd 6/6 )

Nypel® (a po st-ind us trial nylon 6)

“Petra“® (po st-co ns um er rec ycled P ET)

““Ultradur® “PB T Therm op las tic P olyme r

Ultraform ® “Aceta l (P OM)

Ultrason® “High Tem p P olyme rs

These resins from B ASF, co upled with the co m pa ny’sconcept-through-commercialization expertise, canco m bine to he lp m ake p os sible the m os t efficient, cos t-effec tive sn ap -fit for your prod uc t. Ou r tec hnica l su pp ort rea dy to help you with all your nee ds . And for mo reinforma tion , you ca n a lwa ys visit ou r web site a twww.plasticsportal.com.

Snap-Fit Des ign

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


5/24

Sna p-Fit Des ign Applications

Why us e s na p-fits? This cha pter will give you a thum bna ilske tch o f the b ene fits of snap -fits a nd the m aterials u se dto make them.

Sna p-fits are the s im ples t , quickes t and mo st co st-effec tive m ethod o f as se m bling two parts. Whe n des igne dprope rly, par ts with sna p-fits c an be as se mb led anddisa ss em bled num erous t im es without any adverse e ffecton the ass em bly. Sna p-fits are also the m osten vironm en tally frien dly form o f as se m bly bec au seof their eas e of disa ss em bly, m aking c om pone nts of d iffere nt m ate rials ea sy to rec ycle.

Although sna p-fits ca n be de signe d w ith ma ny ma terials,the ide al m ate rial is the rmo plastic b ec au se of its highflexibility an d its ab ility to b e e a s ily a nd ine xpe ns ivelym olde d into com plex geom etries . Other ad vantage sinc lud e its relatively high e lon ga tion , low c oe ffic ie nt o f friction , a nd su fficient s tren gth an d rigidity to m ee t therequirem ents of mos t ap plica tions .

The des igner should b e a ware that the ass em bly may haves o m e “play“du e to toleranc e s tack-up of the two m atingpa rts . So m e sna p-fits can also increas e the costof an injec tion m olding too l du e to the n ee d for slide s in the

m old. An expe rienc ed de signe r ca n often elim ina te thene ed for s lide s by a dd ing a s lot in the wa ll direc tly be lowthe unde rcut or by plac ing the s nap s o n the ed ge o f thepa rt, so they fac e outwa rd (se e Figure I-1).

REQUIRES SLIDE IN MOLD

UNDERCUT

NO SLIDE REQUIRED

SLOT

NO SLIDE REQUIRED,MOLD LESS COMPLEX

Figure I-1

I-1

Part I


6/24

S N A P - F I T D E S I G N AP P L IC AT IO N S

I-2

Concluding points: Sn ap -fits so lve the prob lem of crea ting a n ine xpen sive com po nen t that can b e q uicklyand ea sily joined with ano ther piec e. Thermo plas ticsare the ide al m aterial for sn ap -fits b ec aus e the y have the

flexibility an d res ilien ce ne ce ss ary to allow for n um ero usas se m bly and disa ss em bly operat ions.

Door handle be zel

Backside of bezel Detail of backside of bezel, cantilever design


7/24

II-1

Type s of Sna p-Fits

This c ha pte r provide s a n ove rview of the differe nt type s o f ca ntilever sna p-fits a nd gives a n ide a o f whe n they are u se d.

Mos t e nginee ring m aterial app lica tions with s nap -fits u se theca ntileve r de sign (s ee Figu re II-1) and , thus , this m an ua l willfoc us o n that de sign. The cylindrica l de sign c an b eem ployed whe n an unfilled thermo plas tic m ate rial withhigh er e lon ga tion will be us ed (a typica l ap p lica tion is anas pirin b ottle/c ap as se m bly).

Y

CANTILEVER

“U” SHAPED CANTILEVER

“L” SHAPED CANTILEVER

Figu re II-1

Whe n d es ign ing a ca ntileve r sna p, it is n ot unu su al for thde signe r to go throug h s everal iterations (ch ang ing lengthickne ss , de flec tion dim ens ions , etc .) to d es ign a sn apwith a low er a llow ab le strain for a given m ate rial.

Other types of sna p-fits which c an b e us ed are the “U“o r “L“sh ap ed ca ntileve r sna ps (se e P art V for m ore d eta il)These are us ed whe n the s train of the straight ca ntileversna p c ann ot be de signe d b elow the a llow ab le s train for given material.

Concluding points: Mos t app lica tions ca n em ploy aca ntileve r type sn ap -fit in the de sign. In a p plica tion s w ittight pa cka ging req uirem ents , the “U“or “L“shap ed snap ma ybe req uired.

Automotive oil filter snaps

Cordless screw driver housing, cantilever snap-fit

Pa rt II


8/24

III-1

;

OVERHANG DEPTH

ENTRANCE SI DE

RETRACTION SI DE

A des ign e nginee r ’s job is to find a b alanc e b etwe eninteg rity of the a ss em bly an d s treng th of the c an tileve rbe am . While a ca ntilever bea m with a de ep o verhangca n m ake the unit sec ure, it also puts m ore strain on thebe am during as se mb ly and d isa ss em bly. This cha pterexplains how this b alanc e is a chieved .

A typica l sn ap -fit as se m b ly con sis ts of a c an tileve r be amwith an overha ng at the en d o f the b ea m (se e Figure III-1).The de pth of the o verhang de fines the am ount of de flec tion during as se m bly.

Frict ion Co efficien t µ = tanβ

Mating Force = W

W = P ta n(α + β)µ + tan α W = P ———————— 1–µ tan α

Figu re III-2

Figu re III-1

The overhang typica lly has a ge ntle ram p o n the e ntranc eside a nd a s harpe r ang le on the retrac tion side . The sm allangle a t the e ntranc e s ide (α ) (see Figure III-2) helps toreduc e the as se m bly effort, while the sh arp a ngle a t theretrac tion s id e (α“) m ake s disas se m bly very difficu lt orim po ss ible d ep end ing o n the intend ed func tion. Both theas se m bly and disa ss em bly force c an b e op t im ized bym od ifying the a ngles m entione d ab ove.

The m ain de s ign co ns ide ration of a sn ap -fit is integ rityof the as se m bly and s treng th of the be am . The integrity ofthe a ss em bly is c ontrolled by the stiffne ss (k) of the be amand the am ount of deflec tion req uired for as se m bly ordisa ss em bly. Rigidity ca n be increa se d e ither by using ahigh er m od ulus m ate rial (E) or by inc rea sing the c ros sse ctiona l m om ent of inertia (I) of the be am . The p roduc t othes e two pa ram ete rs (EI) will de term ine the total rigidity oa given be am length.

;

α ' α

R

W

P

W

P

RFRICTION CONE

α

α +β

}

β

MATING FORCE

Snap-Fit Des ign Using Class ical Beam Theory

Part III


9/24

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 AL B E A M TH E O R Y

III-2

The integrity of the a ss em bly can also be im proved byincrea sing the overhang d ep th. As a resu lt, the be amhas to de flec t further an d, therefore, req uires a grea tereffort to clea r the o verhan g from the interloc king h oo k.

However, as the b ea m de flect ion increas es , the bea mstres s a lso inc rea se s. This w ill res ult in a failure if the be amstress is ab ove the yield strength o f the m aterial.

Thus, the d eflec tion m ust b e op tim ized with resp ec t to theyield streng th or s train o f the m ate rial. This is ac hieve d byop tim izing the b ea m se ction g eo m etry to ens ure that thede sired d eflec tion c an b e rea che d w ithout excee ding thestre ng th o r strain lim it of the m ate rial.

The a ss em bly and disa ss em bly force will inc reas e withbo th stiffne s s (k) an d m axim um de flec tion o f the b ea m (Y).The force (P ) required to de flec t the b ea m is prop ortiona lto the p roduc t of the two fac tors:

P= kY

The stiffnes s value (k) de pe nd s on b ea m g eo m etry ass ho wn in F igu re III-3 .

S tress or strain ind uc ed by the d eflec tion (Y) is a lso sh ow nin Figu re III-3. The c alculate d s tress or strain value sh ou ldbe les s tha n the yield stren gth o r the yield strain of thematerial in order to prevent failure.

When selecting the flexural modulus of elasticity (E)

for hygrosc op ic m aterials, i.e. , nylon, c are sho uld be taken.In the d ry as m olde d s tate (DAM), the d atas hee t value m aybe us ed to ca lcu late stiffne ss , de flec tion o r rete ntion forceof sn ap de sign. Unde r norma l 50% relative hum idityco nd itions , howe ver, the ph ysica l prop erties de crea seand , therefore, the s tiffnes s a nd retention force reduc ewhile the d eflec tion increa se s. Both sc en arios s hou ld b ec h e c k e d .

Where:E = Flexura l Mod ulusP = ForceY= Deflec tionb = Width of Bea m

Figu re III-3

b

b

tP

L

L

t

2P

t

L

t

b4

P

I Uniform Cross Section,Fixed End to Free End

Stiffness:

Strain:

II Uniform Width, Height Tapersto t/2 at Free End

Stiffness:

Strain:

III Uniform Height,Width Tapersto b/4 at Free End

Stiffness:

Strain:

k =P

Y

Eb

4

t

L= ( )

3

e = Y1.50 t

L 2 ( )

k =P

Y

Eb

6.528

t

L= ( )

3

e =

b

0.92 Yt

L 2 ( )

k =P

Y

Eb

5.136

t

L= ( ) 3

e = 1.17 Yt

L 2 ( )

)

)

)

Cantilever Beam : Deflec tion-Strain Formulas


10/24

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

III-3

Concluding points: In a typica l sn ap -fit, the s treng thof a be am is d ep end ent on its geom etry and m axim umde flec tion du ring as se m bly. The force to as se m ble anddisa ss em ble sn ap -fit ass em blies is highly de pe nde nt on

the overhang entranc e and retrac t ion angles .

Close -up of automotive fuse box snap

Close-up o f automotive fuse box, full view

Close -up of automotive fuse box, snap on s ides of box


11/24

IV-1

The c antilever be am formu las us ed in c onventiona lsn ap -fit des ign und eres tim ate the a m oun t of strain atthe be am /wa ll interfac e b ec aus e they do n ot include thed eforma tion in the wa ll itse lf. Ins tea d, the y as su m e the w allto b e co m pletely rigid with the de flec tion oc cu rring on ly inthe be am . This a ss um ption m ay be valid whe n the rat io of be am length to thicknes s is greater than ab out 10:1.How ever, to ob tain a m ore ac curate p rediction of totalallowa ble de flec tion and strain for short be am s, am ag nifica tion fac tor sh ould be ap pliedto the c on vention al form ula. This w ill en ab le g rea terflexibility in the d e s ign wh ile tak ing full a d van tag e of

the strain-ca rrying ca pa bility of the m ate rial.BASF Plas tics has de velop ed a m ethod for esti-m atingthe se de flec tion m ag nifica tion fac tors forvarious sn ap -fit be am /wa ll co nfigurations as sho wnin Figu re IV-1. The res ults o f this te ch niqu e, w hichha ve be en verified bo th by finite e lem en t ana lysis a ndac tual part tes ting1, a re s ho wn grap hica lly in Figu re IV-1.Figu re IV-2 s ho ws similar res ults for bea m s o ftapered cross s ec t ion (be am thicknes s d ec rea singb y 1/2 a t the tip).

S na p- Fit Design Exam ples 1 & 2 illus trate this p roce du refor de signing sn ap -fits, inc lud ing c alculating the m axim umstrain de velop ed during a ss em bly and pred icting the s nap -in force req uired .

1 Chul S. Lee , Alan Dub in an d Elme r D. Jo nes , “Sho rt Cantilever Be amDeflec tion Analysis App lied to Therm op las tic S na p-Fit Des ign,“19 87 S PEANTEC, he ld in Los Angeles, Ca lifornia, U.S.A.

Improved Cantilever Snap-Fit Des ign

Pa rt IV


12/24

IV-2

D E F L E C T I O N M A G N I F I C A T I O N F A C T O R

Q

ASPECT RATIO, L/t

8.0

7.0

6.0

5.0

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 8.0 9.0 10.0 11.0

ON A BLOCK

(SOLID WALL)

1

ON A PLATE(OR THIN WALL)

2 4

5

3

Uniform Beam , Q FactorFigu re IV-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


13/24

IV-3


8.0

7.0

6.0

5.0

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 8.0 9.0 10.0 11.0

2T

5T

D E F L E C T I O N M A G N I F I C A T I O N F A C T O R

Q

ASPECT RATIO, L/t

2T

5T

;

t /2

t

Tap ered Bea m, Q FactorFigu re IV-2


14/24

IV-4

MATERIAL UNFILLED 3 0% G LAS SP EI 9 .8 %(2)

P C 4 %(1) - 9 .2%(2)

Ac e ta l 1 .5 %(1)

Nylon 6(4) 8 %(5) 2.1%(1)

P BT 8 .8%(2)

P C /P ET 5 .8 %(2)

ABS 6 % - 7%(3)

P ET 1 .5%(1)

MATERIAL µ P EI 0 .2 0 - 0 .2 5P C 0 .2 5 - 0 .3 0Ac e ta l 0 .2 0 - 0 .3 5Nylo n 6 0 .1 7 - 0 .2 6P BT 0 .3 5 - 0 .4 0P C /P ET 0 .4 0 - 0 .5 0ABS 0 .5 0 - 0 .6 0P ET 0 .1 8 - 0 .2 5

Table IV-I

NOTES:(1) 70 % of tens ile yie ld s train value(2) G.G . Tran tina . Plastics Engineering.

Augus t 1989 .(3) V.H. Trum bu ll. 19 84 AS ME Winter Annu al Co nferenc e(4) DAM - “Dry As M olde d “co nd ition(5) BASF test lab ; Note 4% should b e us ed in Mating Force

Formula

Tab le IV-II

NOTES:(1) Ma terial tes ted ag ains t itse lf

Co efficient o f Fric tion (1)

Allowab le Stra in Value , e o

Figure IV-3

MAXIMUM S TRAIN (@ BASE)

tY∈= 1.5 ———-L2 Q

MATING FORC E

µ + tan α W = P ——————— 1–µ tan α

bt2 E∈P = ——————— 6L

Where:

W = Push-on ForceW’ = P ull-o ff Forc eP = Pe rpendicular Forceµ = C oe ffic ie nt o f Fric tionα = Lea d Angle

α ’ = Re turn Angleb = Beam Widtht = Bea m Thicknes s

L = Beam LengthE = Flexura l Mod ulus∈= Strain at Ba se∈o = Allow a b le M a teria l S train

Q = Deflec tion Mag nifica tion Fac tor(refer to Figu re IV-2 for p rop e rQ values )

Y = De flec tion

t

Y

b

L

α

P

W

Improved Formulas

Wheel cover with cantilever snaps



15/24


DETERMINE:

A) THE MAXIMUM DEF LEC TIO N OF S NAPB) THE MATING FORCE

SOLUTION:

A) TH E MAXIMUM ALLOWAB LE DEFLECTION OF S NAP

tY max ∈o L2 Q∈o = 1.5 ———- ⇒ Y max = ———— L2 Q 1.5 tL — = 5.0 ⇒ Q = 2.0 (from Q Factor Graph)t

(0.015)(0.5)2 (2.0) Y max = ——————————= 0.050 in(1.5)(0.1)

Therefore, in an ac tual de sign , a s m aller value for deflec tion(Y) wou ld be cho se n for an a dd ed fac tor of sa fety.

B) THE MATING FORCE

bt2 E∈oP = —————— 6L(0.25)(0.1)2 (1.3)(106 ) (0.015)P = —————————————————— = 16.2 lb

6(0.5)µ + tan a W = P ——————— 1–µ tan a

0.2 + tan30º W = 16.2 ————————— = 14.2 lb1 – 0.2 (tan30º)

There fore, it will take 14 .2 lb m ating force toas se m ble pa rts, if the p art de flec ted to the m aterial’sallow ab le strain.

(From Q Fac tor Grap h,Figure IV-1)

IV-5

DETERMINE:

IS THIS TYP E O F S NAP-FIT ACCEP TABLE FOR USE INYLON 6 (CAP RO N“82 00 NYLON)

SOLUTION:

tY∈= 1.5 ———-L2 Q

L — = 3.57 ⇒ Q = 2.7t(0.063)(0.090)

∈= 1.5 ————————— = 6.2%(0.225)2(2.7)

Therefore, it is ac ce pta ble for unfilled Nylon 6(S e e Allow a b le S train Value , Tab le IV-1 ).

Concluding points: Unlike co nven tion al form ulas , BASFinc lud es the de flec tion m ag nific ation fac tor in all ca lcu latioThe exam ples show how to calculate the m axim um stradur ing a ss em bly and ho w to p red ic tthe force ne ed ed for ass em bly.

Close -up of automotive whee l cover snap s

Snap -Fit Des ign Example #1

GIVEN:

Material ⇒ Petra 130(PET)

t = 0 .1 0 inL = 0 .5 0 inb = 0.2 5 inE = 1 .3 (1 06) ps iµ = 0 .2 (F ro m Ta b le

IV-II, Coefficient of Friction)

α = 3 0.0 °∈o= 1.5% (From Table

IV-I, Allowa b leS train Value )

Figure IV-4

t

Y

b

Lα

PW

Snap-Fit Des ign Example #2

GIVEN:

Material ⇒ UnfilledNylon 6

t = 0 .0 6 3 inY = 0.0 90 inL = 0.2 2 5 inb = 0.2 4 2 in

Figure IV-5

tb

L

Y

;

P


16/24

V-1

The c antilever b ea m sn ap -fit de sign isn ’t ap prop riatefor all ap p lica tion s. This c ha pte r de fine s “L“a nd “U“sha pe dsna ps and tells whe n they are use d.

Oc ca siona lly, a d es igne r will not b e ab le to d es ign aca ntileve r sn ap -fit co nfigu ration with a strain be low thea llow a b le lim it of the inten d e d m a teria l. This is u s ua lly d ueto lim ited pa cka ging s pa ce which ca n restrict the leng th of the s nap . This is the ide al tim e to c ons ide r using e ither an“L“sha pe d s nap or a “U“sha pe d sna p.

The “L“sha pe d sn ap (se e Figure V-1) is forme d by d es igningin s lots in the ba se wa ll wh ich effec tively inc reas es the

be am leng th and flexibility com pa red to a sta nd ardca ntilever be am . This allow s the de signe r to red uce thestrain during as se m b ly be low the a llow ab le lim it of these lec ted m aterial. It should b e no ted tha t add ing a slot tothe ba se wall m ay not be a cce ptab le in som e d es igns forco sm etic o r air flow co nc erns.

The “U“sh ap ed sn ap (se e Figure V-2) is ano ther wa y toincrea se the e ffec tive b ea m leng th within a lim ited sp ac een velop e. With this d es ign , even m ate rials w ith lowallowable strain limits (such as highly glass-filled materials)ca n be des igned to me et asse mb ly req uireme nts . The

“U“sha pe d d es ign us ually inc orporates the und ercut on theouter e dg e of the pa rt to e lim inate the n ee d for slide in them old, u nles s a slot is ac ce ptab le in the wa ll from which thesna p projec ts .

Figure V-1

Figure V-2

“U“& “L“Sha pe d Sna ps

“L” SHAPED CANTILEVER

; ;

“U” SHAPED CANTILEVER

;

Part V


17/24

“U “ & “L “ S H AP E D S N AP S ( C O N S T AN T C R O S S S E C T I O N )

L Shaped Snap-Fit Example

A) C alculate the m inim um len gth (L2) of the s lot (se eske tch , Figu re V-3) in the m ain wall for Ca pron“823 3 nylonin the co nfigu ration b elow . The req uired d eflec tion is .3inches.

B) Ca lcu late th e req uired force (P) to de flec t thesna p .38 inche s.

GIVEN:

∈8233 = .021t = .1 inL1 = .5 inR = .12 inI = Moment of Inertia (rectangle)

I = 12 = 12 = 8.333(10-5

)E = 1.31 (10 6 )b = 1.0 in

Y = .38

(6/∈ ) Yt(L1+ R) - 4L 13 - 3R(2πL12 + πR 2 + 8L1R) A) L2= —–––––—————————————————————— 12(L1 +R)2

(6/.021)(.38)(.1)(.62)- 4(.5)3 - .36[.5π+.122π +4(.12)]= ————————————————————————–– 12(.62)2

L2 = 1.187 in

B) Y= 12EI [4L13+3R(2πL12 +πR 2 + 8L1R) + 12L2(L1 + R)2 ]

.38 = (12)(1.31)(106 )(8.333)(10-5 ) [4(.5)3+(.36)[.5π+

.122π+ 8(.5).12]+ 12(1.187)(.62)2 ]

.38 = 1.31(103 ) (6.718)

P = 74.1 lb

b t3 1(.1)3

P

V-2

“L“SHAPED SNAP–FIT

Figure V-3

(6/∈o ) Yt(L1+ R) - 4L 13 - 3R(2πL12 + πR 2 + 8L1R)L2 = ————————————————----------–––——–———— 12(L1 +R)2

o r,

Y= 12EI [4L13+3R(2πL12 +πR 2 + 8L1R) + 12L2(L1 + R)2 ]

Where:L2 = Leng th of slot as sho wn in ske tch∈o = Allow ab le strain o f m ate rial

Y = Ma xim um de flec tion req uired in d irec tionof forc e

t = Thickne ssL1 = Length as show n in sketchR = Ra dius a s s hown in sketch

(at neutral axis)P = Forceb = Beam WidthE = Flexura l Mod ulusI = Mom en t of Ine rtia

P

P

L1

R

L2

A A

SectionA-A

b

t

P

P


18/24

“U Shaped SnapExample #1

Case 1

A) Ca lcu late the a m ou nt of de flec tion a t the tip o f thebe am for a 1.0 pound load

GIVEN:

P = 1 .0 lbI = 0.83 3 x 10-4 in4 = b t3 /1 2 (rec ta ng ular c ro s s s e c tionE = 5 3 4 , 0 0 0 p s iR = 0 .15 inL1= 1.4 inL2= 0.973 int = 0 .1 inb = 1.0 in

A) Y =18EI

[ 6L13 + 9R{L 1(2πL1+ 8R) + πR 2} + 6L 2(3L12- 3L1L2+ L

Y =18(534,000)(0.833 x 10-4 )

[6(1.4)3 +9(0.15){(1.4)

(2π•1.4 + 8 • 0.15) + π(0.15)2} + 6(0.973){3(1.4)2 - 3(1.4)(0.973) + (0.973)2}]

= 0.064 in

Case 2

Y = 3(L1 + R)t[4L13 + 2L 33 +3R {L 1(2πL1 + 8R) + πR 2}]

or,

Y = 6EI [4L13 + 2L 33 +3R {L 1(2πL 1 + 8R) + πR 2}]

Where:

Variab les de fine d o n p revious pa ge .

V-3

“U“& “L“S HAP E D S NAP S

1

b

SectionA-A

t

R

PL1

L3

L2

A A

R

L1L2

P

P

U Shaped Snap–Fit

Case 1

Y = 9(L1 + R)t[6L13 + 9R {L 1(2πL1 + 8R) + πR 2}+

6L2 (3L12 - 3L1L2 +L 22 )]

or,

Y = 18EI [6L13 + 9R {L 1(2πL1 + 8R) + πR 2}+

6L2 (3L12 - 3L1L2 +L 22 )]

∈

∈

P

P

R

L1L2

P

b

SectionA-A

t

A A


19/24

Concluding points: Sn ap -fits ca n use either the “U“o r “L“sha pe d d es ign to overcom e s pa ce lim itat ions. Both theL“a n d “U“sh ap ed sn ap s e ffec tively red uc e s train du ringas se m bly, thus m aking it ide al for m ate rials with low er

a llow a b le s train lim its .

“U“ & “L“S HAP ED S NAP S

V-4

“U“Shape d SnapExam ple #2

Case 2A) Ca lcu late the a m ou nt of de flec tion a t the tip o f the

be am for a 1.0 pound load

GIVEN:

I = 0.833 x 10 -4 in4E = 534,000 psiR = 0.15 inL1 = 0.7 inL1 = L 2L3 = 0.273 int = 0.1 in

Y = 6EI[4L13 + 2L33 + 3R {L 1(2πL1 + 8R) + πR 2}]

=6(534,000)(0.833 x 10

-4

)[4(0.7)3 + 2(0.273)3 +

3(0.15){0.7(2π • 0.7 + 8(0.15)) + π (0.15)2}]= 0.012 in

P

1

R

PL1

L3

L2

Automotive wheel cover

Close -up of above cover backside featuring the “ L“shaped snap -fitdes ign (from a top a ngle)

Inse t shot of a “ U“shap ed snap -fit d esign


20/24

VI-1

be twee n the pa rts, relaxation at the joint c an result in los sof se al pre ss ure, res ulting in lea kag e o f the c on taine d fluidAnothe r prob lem often se en is e xces sive play be twee n thpa rts du e to toleranc e variations , so m etim es resulting innoise and vibration. Se veral wa ys to m inim ize the sephe nom ena include : de signing a low s tres s s nap be am ,de signing the sna p-fit to inco rporatea 90 ° return an gle so that it relaxes in ten sion versusbe nd ing (s ee Figu re VI-2). This will pre vent the m ating pa rfrom s lipp ing p as t or be co m ing loo se . Another wa y is touse a large return a ngle a nd increa se the land leng th in thereturn ang le a rea (see Figu re VI-3). Inc rea sing the

overhang de pth a nd evaluat ing the worst ca se sc ena rio ina tolera nc e stud y will allow the de sign to retain g iven pu ll-off force eve n a fter relaxa tion o cc urs.

Figu re VI-2

Figu re VI-3

Three ba sic iss ue s sh ould be review ed be fore fina lizinga s nap -fit des ign: stress co nce ntration, cree p/relaxation,and fat igue . Below a re des cript ions of these problem sand sug ge st ions to prevent them . All sho uld b econ side red as p art of good de sign pract ice for anythermo plas tic de sign.

The s ingle m os t co m m on ca us e of failure in s nap -fits iss t res s c once ntrat ion due to a sha rp c orner be tween thesn ap -fit bea m and the wa ll to which it is a ttac hed . Sincethis loc ation no rma lly co inc ide s with the po int of ma xim umstres s , a sha rp corner ca n increas e the s t ress b eyond thestren gth o f the m ate rial, ca us ing po int yielding orbre aka ge . This is m ore critica l for rigid plastics like g las s -reinforce d n ylon , wh ic h ha ve relatively low ultim a teelonga tion . More du ctile m ate rials, like u nreinforced nylon ,tend to yield an d de form be fore they brea k, redistribu tingthe pe ak stress over a broad er reg ion. One so lution is toinco rporate a fillet rad ius at the junc ture be twee n the be aman d the wa ll (se e Figu re VI-1), so tha t the ra tio o f rad ius towa ll thickne ss (R/t) is a t lea st 50 % . Going b eyond 50 %results in a m arginal increa se in streng th and m ay ca useothe r prob lem s like interna l voids an d s ink m arks . If sinkm arks are an iss ue, a sm aller rad ius c an b e us ed , but it

m ay increa se the s tres s in this a rea. Anothe r option is toad d the radius o nly on the tens ile s ide of the b ea m .

Figure VI-1

Cree p, o r mo re ac curate ly stres s relaxation, c an result ina red uction o f the ho lding force be twee n the twocom pon ents co nnec ted b y the s nap -fit . Stress relaxat ionwill oc cu r grad ua lly over tim e. If the re is a ga s ket or s ea l

Gene ral Des ign Guide lines

SHARPCORNER

;;

R =.5t MINIMUM

t;

POOR DESIGN GOOD DESIGN

; ;

RELAXED POSITION(EXAGGERATED)

P = MATING PART FORCE

UNDEFORMEDPOSITION

UNDEFORMEDPOSITION

PP

RELAXATION IN TENSION RELAXATION IN BENDIN

RETURN ANGLE

LAND LENGTH

;

OVERHANG DEPTH

Part VI


21/24

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

Fatigu e, o r rep etitive loa d ing , is th e third m ajor cau seof failure. Fatigu e c on ce rns p rim arily ap ply if hun dre ds orthous and s of cycles are an ticipa ted. While the d es igns tres s leve l m igh t be we ll within the stren gth o f them aterial, the repe ated ap plica tion o f this s tres s c anres ult in fatigu e failure at so m e po int in the future.S om e p olyme rs pe rform b etter than o thers in this rega rd,m aking the m ide al ca nd id ate s for sna p- fits or living h ing estha t mu st flex rep ea ted ly. The first wa y to a void a fatigu efailure is to c ho os e a m ate rial known to p erform we ll infatigue . This ca n be do ne by com pa ring the so-c alled S -Ncurves of the m ater ials , which s how the expec ted num be rof cycles to failure a t variou s stres s levels an d at d iffere nttem pe ratures o f expo sure. The se co nd w ay, still using theS -N curves , is to ch oos e a d es ign s tres s level, at the

co rrec t tem pe rature, that res ults in the requ ired num be r of loa d a p p lic a tion s p rior to fa ilure . This m e tho d w ill us ua llybe c ons ervative sinceS -N curves a re typica lly gene rated a t muc h highe rfreque ncies than wo uld b e a nticipa ted for repe atedap plica tion of a sna p-fit as se m bly.

For hygros co pic m ate rials like n ylon , the e ffec ts o f m oisture on final pa rt dim ens ions and m ec ha nica lprop erties a lso m ust be co ns ide red. For furthe rinforma tion, plea se co nsu lt the BASF P las tics Des ignSolutions Guide.

Conc luding points: There are a num be r of ways toovercom e the iss ues of s tres s conc entrat ion, s t ressrelaxa tion a nd fatigu e. A we ll thou gh t-ou t d es ign a ndus ing the righ t po lym e r for a give n a p p lic a tion will m inim izethe se iss ue s . This a llow s the a p plica tion to b en efit from a llthe ad vantage s of a sna p-fit des ign.

Circular sa w handle inset shot featuring s nap-fit c losure and mating

Close -up o f truck mirror patch cover

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

Aerator


22/24

Notes


23/24

English/Metric Convers ion ChartTo Convert To MultiplyEnglish Sys tem Metric System English Value by. ..

DISTANCEinc hes m illim e te rs 25 .3 8fee t m e te rs 0 .304 78

MASSounc e (avd p ) gra m 28.3 495p ound gra m 453 .5925p ound kilogra m 0.45 36U.S . to n m e tric ton 0 .907 2

VOLUMEinch3 centimeter3 16.3871inch3 lite r 0 .016 387fluid ounc e c entim e te r3 29.5735q ua rt (liq uid ) d e c im e te r3 (lite r) 0 .946 4ga llon (U.S .) d e c im e te r3 (lite r) 3 .78 54

TEMPERATUREd e gre e F d e gre e C (°F –3 2) / 1 .8 = °C

PRESSUREp s i b a r 0 .06 89p s i kP a 6 .89 48ks i MN/m2 6.8948p s i MP a 0 .006 89

ENERGY AND POWERin lbf J o ule s 0 .113ft lbf J o ule s 1 .35 58kW m e tric hors e p owe r 1 .35 96U.S . hors e p o we r Kw 0.74 57Btu J o ule s 1055 .1BTU “in / (hr “ft2“ºF) W/m “°K 0.144 2

VISCOSITYp ois e P a “s 0 .1

BENDING MOMENTOR TORQUEft lb N “m 1.356

DENSITYlb/in3 g / c m3 27.68lb/ft3 k g / m3 16.0185

NOTCHED IZODft lb/in J /m 53 .4


24/24

C a p r o n® , N yp e l® , Pe t ra® , and Ult raTough® are a reg is te red t rade ma rks of BASF Corpora t ion“Cop yrigh t BASF Corpo ra t ion 2003

IMPORTANT: W H I L E T H E D E S C R I P T I OD E S I G N S , D ATA A N D I N F O R M ATCO NTAINED HEREIN ARE PRES ENTED IN FAITH AND BELIEVED TO BE ACCURATEP R O V I D E D F O R Y O U R G U I D A N C E OB E C AU S E M AN Y F AC T O R S M AY AFP R O C E S S I N G O R A P P L I C AT I O N / U S ER E C O M M EN D TH AT YO U M AK E T ES TDETERMINE THE SUITABILITY OF A PROF O R Y O U R PA RT IC U L AR P U R P O S E P R IOUSE. NO WARRANTIES OF ANY KIND, EE X P R E S S E D O R I M P L I E D , I N C L U DWA R R A N T I E S O F M E R C H A N TA B I L I T YF I T N E S S F O R A PA RT I C U L A R P U R P O S EMADE REGARDING PRODUCTS DESCRIBDESIGNS, DATA OR INFORMATION SET FOOR THAT THE PRODUCTS, DESIGNS, DATI N F O R M AT I O N M AY B E U S E D W I T HIN F R IN G IN G T H E IN T E LL E C T UA L P R O PR IG H T S O F O TH E R S . IN N O C A S E S H ALD E S C R IP T IO N S , IN F O R M AT IO N , D ATDESIGNS PROVIDED BE CONSIDERED AO F O U R T ER M S AN D C O N D IT IO N S O F FURTHER, YOU EXPRESS LY UNDERS TANDA G R E E T H AT T H E D E S C R I P T I O N S , D E SDATA, AND INFORMATION FURNISHED BYH E R E U N D E R A R E G I VE N G R AT IS A N D ASSUMES NO OB LIGATION O R LIABILITT H E D E S C R I P T I O N , D E S I G N S , D ATA INFORMATION G IVEN OR RESULTS OBTAA L L S U C H B E I N G G I V E N A N D A C C E P T EYOUR RISK.

BASF Corpora t ion30 00 Co ntine ntal Drive - NorthMount Olive , New J e r sey 078 28-12 3

w w w. b a s f . c o m / u s awww.plas t icspor ta l .com

HELPING MAKE PRODUCTS BETTER

Basf snap_fits.pdf

Documents

Transcript of Basf snap_fits.pdf