Technical Expertise Snap-Fit Design Manual

1 Technical ExpertiseSnap-FitDesign Manual TopicPartIntroduction.. IntroductionSnap-Fit Design Applications .. ITypes of Snap-Fits .. IISnap-Fit Beam Design Using Classical Beam Theory .. IIII mproved Cantilever Snap-Fit Design .. IV U & L Shaped Snaps .. VGeneral Design Guidelines .. VIEnglish/Metric Conversion Chart .. Inside Back CoverTable of ContentsAbout BASF Performance PolymersBASF Plastics is a fully integrated, global supplier ofengineering resins from production of feedstocks to thecompounding, manufacture and distribution of hundredsof resin is committed to continuous product development tosustain rapid growth in the nylon resin market.

2 In ourPlastics Technology Laboratory, a highly experienced staffof research and development engineers continues todevelop new resins to further extend the horizons ofproduct offers high-quality engineering resins, including: Ultramid (nylon 6 and 6/6)Nypel (a post-industrial nylon 6) Petra (post-consumer recycled PET) Ultradur PBT Thermoplastic PolymerUltraform Acetal (POM)Ultrason High Temp PolymersThese resins from BASF, coupled with the company sconcept-through-commercialization Expertise , cancombine to help make possible the most efficient, cost-effective Snap-Fit for your product. Our Technical support isready to help you with all your needs.

3 And for moreinformation, you can always visit our web site DesignThis Manual will guide you through the basics of Snap-Fit Design , including: typesof Snap-Fit designs and their applications;how to calculate the strength of the unit andamount of force needed for assembly; and thethree common causes of failure in snap-fitsand how to overcome them. IntroductionSnap-Fit Design ApplicationsWhy use snap-fits? This chapter will give you a thumbnailsketch of the benefits of snap-fits and the materials usedto make are the simplest, quickest and most cost-effective method of assembling two parts. When designedproperly, parts with snap-fits can be assembled anddisassembled numerous times without any adverse effecton the assembly.

4 Snap-fits are also the mostenvironmentally friendly form of assembly because of their ease of disassembly, making components ofdifferent materials easy to snap-fits can be designed with many materials,the ideal material is thermoplastic because of its highflexibility and its ability to be easily and inexpensivelymolded into complex geometries. Other advantagesinclude its relatively high elongation, low coefficient offriction, and sufficient strength and rigidity to meet therequirements of most applications. The designer should be aware that the assembly may havesome play due to tolerance stack-up of the two matingparts.

5 Some snap-fits can also increase the cost of an injection molding tool due to the need for slides in themold. An experienced designer can often eliminate theneed for slides by adding a slot in the wall directly belowthe undercut or by placing the snaps on the edge of thepart, so they face outward (see Figure I-1). REQUIRES SLIDE IN MOLDUNDERCUTNO SLIDE REQUIREDSLOTNO SLIDE REQUIRED,MOLD LESS COMPLEXF igure I-1I-1 Part ISNAP-FIT Design APPLICATIONSI-2 Concluding points: Snap-fits solve the problem ofcreating an inexpensive component that can be quicklyand easily joined with another piece. Thermoplastics are the ideal material for snap-fits because they have theflexibility and resilience necessary to allow for numerousassembly and disassembly handle bezelBackside of bezelDetail of backside of bezel, cantilever designII-1 Types of Snap-FitsThis chapter provides an overview of the different types ofcantilever snap-fits and gives an idea of when they are engineering material applications with snap-fits use thecantilever Design (see Figure II-1) and, thus, this Manual willfocus on that Design .

6 The cylindrical Design can beemployed when an unfilled thermoplastic material withhigher elongation will be used (a typical application is anaspirin bottle/cap assembly). YCANTILEVER U SHAPED CANTILEVER L SHAPED CANTILEVERF igure II-1 When designing a cantilever snap, it is not unusual for thedesigner to go through several iterations (changing length,thickness, deflection dimensions, etc.) to Design a snap-fitwith a lower allowable strain for a given types of snap-fits, which can be used, are the U or L shaped cantilever snaps (see Part V for more detail).These are used when the strain of the straight cantileversnap cannot be designed below the allowable strain for thegiven material.

7 Concluding points: Most applications can employ acantilever type Snap-Fit in the Design . In applications withtight packaging requirements, the U or L shaped snap maybe oil filter snapsCordless screw driver housing, cantilever snap-fitPart IIIII-1 OVERHANG DEPTHENTRANCE SIDERETRACTION SIDEA Design engineer s job is to find a balance betweenintegrity of the assembly and strength of the cantileverbeam. While a cantilever beam with a deep overhangcan make the unit secure, it also puts more strain on thebeam during assembly and disassembly. This chapterexplains how this balance is typical Snap-Fit assembly consists of a cantilever beamwith an overhang at the end of the beam (see Figure III-1).

8 The depth of the overhang defines the amount ofdeflection during assembly. Friction Coefficient = tan Mating Force= WW = P tan( + ) + tan W= P 1 tan Figure III-2 Figure III-1 The overhang typically has a gentle ramp on the entranceside and a sharper angle on the retraction side. The smallangle at the entrance side ( ) (see Figure III-2) helps toreduce the assembly effort, while the sharp angle at theretraction side ( ) makes disassembly very difficult orimpossible depending on the intended function. Both theassembly and disassembly force can be optimized bymodifying the angles mentioned main Design consideration of a Snap-Fit is integrityof the assembly and strength of the beam.

9 The integrity ofthe assembly is controlled by the stiffness (k) of the beamand the amount of deflection required for assembly ordisassembly. Rigidity can be increased either by using ahigher modulus material (E) or by increasing the crosssectional moment of inertia (I) of the beam. The product ofthese two parameters (EI) will determine the total rigidity ofa given beam length. ' RWPWPRFRICTION CONE + } MATING FORCESnap-Fit Design Using Classical Beam TheoryPart IIISNAP-FIT Design USING CLASSICAL BEAM THEORYIII-2 The integrity of the assembly can also be improved byincreasing the overhang depth. As a result, the beamhas to deflect further and, therefore, requires a greatereffort to clear the overhang from the interlocking , as the beam deflection increases, the beamstress also increases.

10 This will result in a failure if the beamstress is above the yield strength of the material. Thus, the deflection must be optimized with respect to theyield strength or strain of the material. This is achieved byoptimizing the beam section geometry to ensure that thedesired deflection can be reached without exceeding thestrength or strain limit of the assembly and disassembly force will increase withboth stiffness (k) and maximum deflection of the beam (Y).The force (P) required to deflect the beam is proportionalto the product of the two factors: P= kY The stiffness value (k) depends on beam geometry asshown in Figure or strain induced by the deflection (Y) is also shownin Figure III-3.