BAS BA S F Pl Pla s ti ticc s Snap-Fit De s ign Manu al
Table of Conte nts
Topic
Part
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction Snap-Fit Design Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I
Types of Snap-Fits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II Snap-Fit Beam Design Using Class ical Beam Theory . . . . . . . . . . . . . III
Improved Cantilever Snap-Fit Design. . . . . . . . . . . . . . . . . . . . . . . . . . . IV
U “ “&“ L“Shaped Snaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V General Des ign Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI
English/Metric Conversion Chart . . . . . . . . . . . . . . . . . . . . . . . Inside Back Cover
Introduction Sna p-Fit Des ign
This manual will guide you through the
Abo ut BASF Performa nc e P olyme rs
basics of snap-fit design, including: types
BASF Plastics is a fully integrated, global supplier of enginee ring res ins “from production of feeds tocks to the comp ounding, ma nufacture and distribution of hundreds of resin grades .
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 committed to continuous produc t developme 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 develop new res ins to further extend the horizons of product performance. BASF offers high-quality engineering resins, including: U “ ltramid / Capron (nylon 6 and 6/6) ®
®
Nypel (a post-industrial nylon 6) ®
P “ etra “ (pos t-cons umer recycled PET) ®
U “ ltradur “PBT Thermoplastic Polymer ®
Ultraform “Acetal (POM) ®
Ultrason “ High Temp Polymers ®
Thes e res ins from BASF, coup led with the c ompany’s concept-through-commercialization e xpertise , ca n comb ine to he lp ma ke poss ible the most efficient, co steffective sna p-fit for your prod uct. Our technical supp ort is read y to help you with all your need s. And for more information, you c an always visit our web s ite a t www.plasticsportal.com.
Pa rt I Sna p-Fit Des ign App lications Why use snap-fits? This c hap ter will give you a thumbnail sketch of the bene fits of snap -fits and the materials us ed to make them. Snap-fits are the simplest, quickest and most costeffective method of ass embling two parts. When de signed properly, parts with snap-fits can be assembled and disassembled numerous times without any adverse effect on the as sembly. Snap-fits are also the mo st environmentally friend ly form of ass embly because of their ease of disassembly, making components of different materials easy to recycle. Although sna p-fits can be designed with many materials, the ideal material is thermoplastic because of its high flexibility and its ability to be easily and inexpensively molded into complex geometries . Other advantages include its relatively high elongation, low coefficient of friction, and sufficient s trength and rigidity to meet the requirements of most a pplications . The designer should be aware that the assembly may have some p “ lay“due to tolerance stack-up of the two mating parts. Some s nap-fits can also increase the co st of an injection molding tool due to the need for slides in the mold. An experienc ed des igner can often eliminate the need for slides by ad ding a s lot in the wall directly below the undercut or by placing the s naps on the e dge of 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 re I-1
I-1
S N A P - F I T D E S I G N A P P L IC A T IO N S
Concluding points: Snap-fits solve the problem of creating an inexpensive component tha t can be quickly and e as ily joined with anothe r piece. Thermoplastics are the ideal material for snap-fits b eca use they have the flexibility and resilience nec es sary to allow for numerous assembly and disass embly operations.
Door handle be zel
Backside of bezel
I-2
Detail of backside of bezel, cantilever design
Pa rt II Type s of Sna p-Fits This chapter provides an overview of the different types of cantilever snap-fits and gives an idea of when they are use d. Most engineering material app lications with s nap-fits use the cantilever des ign (see Figure II-1) and, thus , this manual will focus on that d esign. The cylindrical design can be employed whe n an unfilled thermoplastic material with higher elongation will be used (a typical application is an as pirin bottle/cap asse mbly).
When designing a cantilever snap, it is not unusual for the designer to g o through s everal iterations (changing length, thicknes s, deflection d imens ions, etc .) to de sign a s nap-fit with a lower allowable strain for a given material. Other types of snap-fits which can be used are the “U“ or L ““shaped cantilever s nap s (see Part Vfor more detail). Thes e are used when the s train of the s traight ca ntilever sna p cannot b e des igned below the allowable s train for the given material. Concluding points: Most ap plications can employ a cantilever type s nap -fit in the d es ign. In applications with tight pa ckaging requirements , the “U“or “L“shaped s nap ma y be required.
Y
CANTILEVER
“U” SHAPED CANTILEVER
Automotive oil filter snaps
“L” SHAPED CANTILEVER
Figu re II-1
Cordless screw driver housing, cantilever snap-fit
II-1
Part III Sna p-Fit Des ign Using Clas s ical Beam The ory A design engineer’s job is to find a balance b etween integrity of the assembly and strength of the cantilever beam. While a c antilever beam with a de ep overhang can make the unit secure, it also puts more s train on the beam during ass embly and disas se mbly. This chap ter explains how this balance is a chieved.
MATING FORCE
P R
α
'
W
α
A typical snap-fit assembly consists of a cantilever beam with an overhang at the end of the beam (see Figure III-1). The depth of the overhang de fines the amount of deflection during assembly. R
FRICTION CONE
}
ENTRANCE SIDE
P
α+β W
β
α
RETRACTION SIDE
Frict ion Co efficien t µ = tan β Mating Force OVERHANG DEPTH
Figu re III-1
=W
W = P tan(α + β)
α µ— + tan —— ———— W = P — 1– µ tan α Figu re III-2
The overhang typically has a gentle ramp on the entrance side and a s harper angle on the retraction side. The small angle at the entrance s ide (α) (see Figure III-2) helps to reduce the as se mbly effort, while the sharp a ngle at the retraction s ide (α)“ makes disassemb ly very difficult or impos sible d epending on the intended function. Both the assembly and disassembly force can be optimized by modifying the angles mentioned above.
III-1
The main design consideration of a snap-fit is integrity of the as semb ly and strength of the be am. The integrity of the assembly is controlled by the s tiffnes s (k) of the beam and the amount of deflection required for ass embly or disa ss embly. Rigidity can be increas ed either by using a higher modulus material (E) or by increas ing the cross sectional moment of inertia (I) of the beam. The product of thes e two p arameters (EI) will determine the total rigidity of a given b eam 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 E O R Y
The integrity of the a ss embly can also be improved b y increas ing the overhang dep th. As a res ult, the beam has to de flect further and, therefore, requires a grea ter effort to c lear the overhang from the interlocking hook. However, as the beam deflection increases, the beam stress a lso increas es . This will result in a failure if the b eam stres s is above the yield s trength of the material.
Can tilever Be am : Deflec tion-S train Formulas P
t
L b
Thus, the deflection must be optimized with respect to the yield strength or strain of the material. This is achieved by optimizing the beam section ge ometry to ensure that the des ired deflection can b e reached without excee ding the strength or strain limit of the material.
I ) 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 s tiffnes s (k) and maximum deflection o f the beam (Y). The force (P) required to deflect the beam is proportional to the prod uct of the two factors:
P
t
2
P= kY
t
The s tiffnes s value (k) depends on beam geometry as shown in Figure III-3. Stress or strain induced by the deflection (Y) is also shown in Figure III-3. The c alculated s tress or strain 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 hygroscop ic materials, i.e., nylon, c are s hould be taken. In the dry as molded state (DAM), the datasheet value may be used to c alculate stiffnes s, deflection or retention force of snap design. Under normal 50% relative humidity cond itions , however, the physical prope rties decreas e and, therefore, the stiffnes s and retention force reduce while the d eflection increas es . Both scenarios should be checked.
L b
II ) Uniform Width, Height Tapers to t/ 2 at Free End Stiffness: Strain:
t Y 6.528 ( L ) t e =0.92 ( )Y L k =
P
=
Eb
3
2
b
P t
L
b 4
III) Uniform Height,Width Taper s to b/ 4 at Free End Stiffness: Strain:
P
Eb
t
3
( L ) t e =1.17 ( )Y L k =
Y
=
5.136 2
Where: E = Flexura l Modulus P = Force Y= Deflection b = Width of Beam Figu re III-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 E O R Y
Concluding points: In a typical snap-fit, the strength of a beam is dependent on its geometry and maximum deflection during as se mbly. The force to as se mble and disassemble snap-fit assemblies is highly dependent on the overhang entrance and retraction angles.
Close -up of automotive fuse b ox, snap on s ides of box
Close- up o f automo tive fuse bo x, full view
III-3
Close -up of automo tive fuse box snap
Pa rt IV Impro ved Ca ntilever Sna p-Fit Des ign The cantilever beam formulas used in c onventional snap-fit des ign underestimate the amount of strain at the beam/wall interface becaus e they do not include the deformation in the wall itself. Instead, the y as sume the wall to be completely rigid with the deflection occurring only in the be am. This ass umption may be valid when the ratio of beam length to thickness is greater than about 10:1. However, to ob tain a more accurate p rediction of total allowable deflection and strain for short beams, a magnification factor should be applied to the conventional formula. This will ena ble grea ter flexibility in the design while taking full advantage of the strain-carrying capability of the material. BASF Plas tics has develope d a method for es ti-mating these deflection magnification factors for various snap-fit beam/wall configurations as shown in Figure IV-1. The results of this technique, which have been verified both by finite element analysis and actual part testing 1, are shown graphically in Figure IV-1. Figure IV-2 s hows similar res ults for be ams of tapered cross section (beam thickness decreasing by 1/2 at the tip). Snap-Fit Design Examples 1 &2 illustrate this procedure for de signing snap -fits, including c alculating the maximum strain developed during assembly and predicting the snapin force required.
1
Chul S. Lee, Alan Dubin and Elmer D. Jones , S “ hort Cantilever Beam Deflec tion Analysis Applied to Thermop las tic S nap -Fit Design,“1987 SPE 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 Beam , Q Fact or Figu 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
2T
5T 5 .0
4 .0
N G A M N O I T C E L F E D
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
11 .0
ASPECT RATIO, L/t
Tap ered Bea m, Q Factor Figu re IV-2 IV-3
IMPROVED CANTILEVER SNAP-FIT DESIGN
Allow ab le Stra in Value ,
Improved Formulas b t
P W
Y
α L
MATERIAL PEI PC Acetal Nylon 6 (4) PBT PC/PET ABS PET
UNFILLED 9.8%(2) 4%(1) - 9.2%(2) 1.5% (1) 8% (5) 8.8%(2) 5.8%(2) 6% - 7%(3)
eo 30% GLASS
2.1%(1)
1.5%(1) Table IV-I
Figure IV-3
MAXIMUM STRAIN (@BASE)
tY ∈ = 1.5 ———2 L Q
MATING FORCE
α + tan W = Pµ— —— ———— 1– µ tan α 2 ∈ E— P = bt — — — — — — 6L
Where: W = Pus h-on Force W’ = Pull-off Force P = Perpe ndicular Force µ = Coefficient of Friction α = Lead Angle α’ = Return Angle b = Beam Width t = Bea m Thicknes s L = Beam Length E = Flexura l Modulus ∈ = Strain at Base ∈o = Allowable Material Strain Q = Deflection Magnification Factor (refer to Figure IV-2 for proper Q values) Y = Deflection
NOTES: (1) 70% of tensile yield strain value (2) G.G. Trantina. Plastics Engineering. August 1989. (3) V.H. Trumbull. 1984 ASME Winter Annual Conference (4) DAM - “Dry As Molde d“condition (5) BASF test lab; Note 4% s hould be us ed in Mating Force Formula Co e fficient o f Fric tion (1) MATERIAL
µ 0.20 - 0.25 0.25 - 0.30 0.20 - 0.35 0.17 - 0.26 0.35 - 0.40 0.40 - 0.50 0.50 - 0.60 0.18 - 0.25
PEI PC Acetal Nylon 6 PBT PC/PET ABS PET
Tab le IV-II NOTES: (1) Material tested against itself
Wheel cover with cantilever snaps
IV-4
IMPROVED CANTILEVER SNAP-FIT DESIGN
Snap -Fit Des ign Exam ple #1
Snap -Fit Des ign Exam ple #2
GIVEN: b t
GIVEN:
W
Y
L
α
t= L= b= E= µ=
0.10 in 0.50 in 0.25 in 1.3 (106) ps i 0.2 (From Table IV-II, Coefficient of Friction) α = 30.0° ∈o = 1.5% (From Table IV-I, Allowable Strain Value)
P
Y
Material ⇒ Petra 130 (PET)
P
L t
Material ⇒ Unfilled Nylon 6
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 NYLON 6 (CAPRON“8200 NYLON)
Figure IV-4 DETERMINE:
SOLUTION:
A) THE MAXIMUM DEFLECTION OF SNAP B) THE MATING FORCE
tY ∈ = 1.5 ———2 L Q
(From Q Factor Graph, Figure IV-1)
L = 3.57 ⇒ Q = 2.7 — t
SOLUTION: A) THE MAXIMUM ALLOWABLE DEFLECTION OF SNAP
∈o L2 Q max ∈o = 1.5 tY — — —- ⇒ Y max = —— —— 2 L Q
1.5 t
L = 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 actua l design, a smaller value for deflection (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 Nylon 6 (See Allowable Strain Value, Table IV-1). Concluding points: Unlike conventional formulas, BASF includes the deflection magnification factor in all calculations . The examples s how how to calculate the maximum strain during as se mbly and how to pred ict the force needed for assembly.
B) THE MATING FORCE 2 ∈o bt— E— P=— — — — 6L 6 (0.25)(0.1)2 (1.3)(10 ) (0.015) P = —————————————————— = 16.2 lb 6(0.5)
+ tan a—— W = Pµ— —— —— 1–µ tan a 0.2 + tan30º W = 16.2 ————————— = 14.2 lb 1 – 0.2 (tan30º) Therefore, it will take 14.2 lb mating force to as semble p arts, if the p art de flected to the ma terial’s allowab le strain.
Close -up of 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 applications . This chap ter de fines “L“and U “ “shape d snap s a nd tells when they are used . Occasionally, a designer will not be able to design a cantilever s nap -fit co nfiguration with a s train below the allowable limit of the inte nded material. This is us ually due to limited packaging sp ace which c an res trict the length of the s nap. This is the ideal time to cons ider using either an L “ “shaped sna p or a “U“sha ped sna p. The L “ “shap ed snap (see Figure V-1) is formed b y des igning in s lots in the base wa ll which effectively increa ses the beam length and flexibility compared to a standard cantilever bea m. This allows the d es igner to reduce the strain during as semb ly below the allowab le limit of the se lected ma terial. It should be note d that ad ding a s lot to the ba se wall may not be acce ptable in some d esigns for cos metic or air flow concerns. The U “ “shaped s nap (see Figure V-2) is another way to increase the effective beam length within a limited space envelope. With this des ign, even materials with low allowable s train limits (such as highly glas s-filled materials) can be des igned to mee t ass embly requirements. The U “ “sha ped d es ign us ually incorpo rates the und ercut on the outer edge of the pa rt to eliminate the ne ed for slide in the mold, unless a slot is acceptable in the wall from which the snap projects.
V-1
“L” SHAPED CANTILEVER
Figure V-1
“U” SHAPED CANTILEVER
Figure V-2
“U “ & “L “ S H AP E D S N A P S ( C O N S T AN T C R O S S S E C T I O N ) L “ “SHAPED S NAP–FIT
L Shaped Snap-Fit Example
A) Calculate the minimum length (L2) of the s lot (see sketch, Figure V-3) in the main wall for Capron “8233 nylon in the configuration below. The req uired deflection is .38 inches.
P
L1 t
A
A b
R
Section A-A L2
B) Calculate the required force (P) to deflect the snap .38 inches . GIVEN:
∈8233 = .021 t = .1 in L1 = .5 in R = .12 in I = Moment of Inertia (rectangle) 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(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 sketch ∈o = Allowable strain of material Y = Maximum deflection required in direction of force t = Thicknes s L1 = Length as shown in sketch R = Radius a s shown in sketch (at neutral axis) P = Force b = Beam Width E = Flexura l Modulus I = Moment of Inertia
(6/∈ ) Yt(L + R) - 4L13 - 3R(2πL12 + πR 2 + 8L1R) A) L2 = —–––––———1————— —————————————— 12(L1 +R)2
(6/.021)(.38)(.1)(.62)- 4(.5)3 - .36[.5π +.122π + 4(.12)] = —————————————2———————————–– 12(.62) L2 = 1.187 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 .122π+ 8(.5).12]+ 12(1.187)(.62) ]
.38 =
P 3 (6.718) 1.31(10 )
P = 74.1 lb
V-2
“U “ & “L “ S H A P E D S N A P S
U Shaped Snap–Fit
U “ Shaped Snap 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) Calculate the amount of deflection a t the tip of the beam for a 1.0 pound load
[6L + 9R {L1(2πL1 + 8R) + πR }+ 3 1
2
6L2 (3L12 - 3L1L2 +L22 )]
GIVEN: P = 1.0 lb I = 0.8 33 x 10-4 in4 = bt 3 /12 (rectang ular cross sec tion) E = 534,000 psi R= 0.15 in L1 = 1.4 in L2 = 0.973 in t = 0.1 in b = 1.0 in
or,
Y =
P [6L13 + 9R {L1(2πL1 + 8R) + πR 2}+ 18EI 6L2 (3L12 - 3L1L2 +L22 )]
L3
2 A) Y = P [ 6L13 + 9R{L1(2πL1 + 8R) + πR 2} + 6L2(3L12 - 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: Variables defined on previous page.
V-3
(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
“U “ & “L “ S H A P E D S N A P S
U “ “Sha pe d Sna p Exam ple #2
Concluding points: Snap-fits can us e either the “U“or “L“ sha ped design to overcome s pace limitations. Both the “L“ and U “ “shap ed snap s e ffectively reduce strain during as sembly, thus making it ideal for ma terials with lower allowable s train limits.
L3
L2
L1
P
R
Case 2 A) Calculate the amount of deflection a t the tip of the beam 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 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 the “L“sha pe d s nap -fit des ign (from a top a ngle)
Inse t shot of a “U“shap ed snap -fit d esign
V-4
Part VI Gene ral Des ign Guide line s Three basic issues should be reviewed before finalizing a sna p-fit design: stres s concentration, creep/ relaxation, and fatigue. Below are des criptions o f thes e problems and s ugge stions to prevent them. All should be cons idered as part of good d esign practice for any thermoplastic des ign. The single most common cause of failure in snap-fits is stress concentration due to a sharp co rner between the snap-fit beam a nd the wall to which it is attached. Since this location normally coincides with the point of maximum stress , a sharp corner can increase the stress beyond the strength of the material, causing point yielding or breakage. This is more c ritical for rigid plas tics like glassreinforced nylon, which have relatively low ultimate elongation. More d uctile materials, like unreinforced nylon, tend to yield and deform before they break, redistributing the peak stres s over a broade r region. One solution is to incorporate a fillet rad ius at the juncture b etween the b eam and the wall (see Figure VI-1), so that the ratio of radius to wall thickne ss (R/t) is at least 50%. Going be yond 50% results in a ma rginal increas e in strength and may caus e othe r problems like interna l voids and sink marks . If sink marks are an iss ue, a sma ller radius can b e us ed, b ut it may increas e the s tress in this a rea. Another option is to add the radius only on the tens ile s ide of the beam.
between the parts, relaxation a t the joint ca n res ult in loss of seal pressure, resulting in leakage of the contained fluid. Another problem often s een is e xcessive play betwee n the parts due to tolerance variations, sometimes resulting in noise and vibration. Several ways to minimize thes e phenomena include: des igning a low stress snap beam, designing the sna p-fit to incorporate a 90° return angle so that it relaxes in tens ion versus be nding (see Figure VI-2). This will prevent the mating part from slipping past or becoming loose. Anothe r way is to use a large return angle and increase the land length in the return ang le area (see Figure VI-3). Increas ing the overhang depth and evaluating the worst case scenario in a to lerance study will allow the design to retain given pulloff force even after relaxation occurs. RELAXED POSITION (EXAGGERATED) UNDEFORMED POSITION
UNDEFORMED POSITION
P
P
RELAXATION IN TENSION
RELAXATION IN BENDING
P = MATING PART FORCE
Figu re VI-2
R= .5t MINIMUM
SHARP CORNER
LAND LENGTH
t POOR DESIGN
GOOD DESIGN
Figure VI-1 Creep, or more a ccurately stress relaxation, c an res ult in a red uction of the holding force between the two comp onents co nnected by the snap -fit. Stress relaxation will occur gradually over time. If there is a gasket or s eal
RETURN ANGLE
OVERHANG DEPTH
Figu re VI-3
VI-1
GENERAL DESIGN GUIDELINES
Fatigue, or repetitive loading, is the third major cause of failure. Fatigue conce rns p rimarily app ly if hund reds or thousands of cycles are anticipated. While the des ign stress level might b e well within the strength of the material, the repeated a pplication of this stres s c an result in fatigue failure a t some point in the future. Some polymers perform bette r than others in this regard, making them ideal candidates for snap-fits or living hinges that mus t flex repeatedly. The first way to avoid a fatigue failure is to choos e a material known to perform well in fatigue. This can be d one by comparing the so-called S-N curves of the materials, which s how the expected numbe r of cycles to failure at various stress levels and at different temperatures o f expos ure. The second way, still using the S-N curves, is to choose a design stress level, at the correct temp erature, that results in the required numbe r of load ap plica tions prior to failure. This me thod will us ually be cons ervative since S-N curves are typically generate d a t much higher frequencies than would be anticipated for repea ted application o f a s nap-fit as sembly.
Close-up of automotive fuel rail cover, snap-fit design
For hygrosc opic materials like nylon, the effects of moisture o n final part dimens ions and mechanical prope rties a lso mus t be cons idered. For further information, please consult the BASF Plastics Design Solutions Guide.
Concluding points: There are a numb er of ways to overcome the iss ues of stress conce ntration, stress relaxation and fatigue. A well thoug ht-out design and using the right polymer for a given application will minimize these issues . This allows the ap plication to be nefit from all the advantages of a snap-fit design.
Aerator
Close -up o f truck mirror patch cover
Circular sa w handle inset shot featuring s nap-fit c losure and mating
VI-2
Notes
Englis h/Metric Convers ion Cha rt To Convert Englis h Sys tem
To Metric System
Multiply Englis h Value by. ..
DISTANCE inches feet
millimeters meters
25.38 0.30478
MASS ounce (avdp) pound pound U.S. ton
gram gram kilogram metric ton
28.3495 453.5925 0.4536 0.9072
VOLUME inch3 inch3 fluid ounce quart (liquid) gallon (U.S.)
centimeter 3 liter centimeter3 decimeter 3 (liter) decimeter 3 (liter)
16.3871 0.016387 29.5735 0.9464 3.7854
TEMPERATURE degree F
degree C
(°F–32) / 1.8 = °C
PRESSURE psi psi ksi psi
bar kPa MN/m2 MPa
0.0689 6.8948 6.8948 0.00689
ENERGY AND POWER in lb f ft lbf kW U.S. horsepower Btu BTU “in / (hr “ft2º“F)
Joules Joules metric horsepower Kw Joules W/m “°K
0.113 1.3558 1.3596 0.7457 1055.1 0.1442
VISCOSITY poise
Pa “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
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