PART I - IMPROVING SNAP-FIT DESIGN: GENERAL APPLICATION AND TYPES Introduction Snap-fit designs are thesimplest, quickest and most cost-effective method of assembling two parts. No screws. No rivets. No welds. And, after assembly, faster disassembly and servicing of components. Thermoplastic materials possess many characteristics and features that are ideal for snap-fit designs: • High flexibility • Integrative designs enabling the molding of complex geometries • Relatively high elongation • Low coefficient of friction • Strength and rigidity sufficient for most applications. Properly designed, thermoplastic snap-fits can be assembled, disassembled and reassembled several times – without adverse effect on the functional integrity of the snap-fit. If materials are prone to relax over time, snap-fits can be designed to self adjust and maintain sealing capability. From the broader perspective of sustainability, snap-fit’s ease of disassembly simplifies the recycling of components made from different materials, thereby encouraging more environmentally friendly behavior throughout the lifecycle of components and end products. Still, various factors keep some snap-fit designs from delivering the full benefits of their potential. BASF hopes to help overcome such obstacles through this series of articles that will provide designers a basic overview of the types of snap-fit designs, applications and principles, improved methods, and guidelines for avoiding common pitfalls in snap-fit design, processing and materials selection. Most Common Types Most snap-fit applications use one of three cantilever designs: • • • Straight beam cantilever U-shaped cantilever L-shaped cantilever. The straight beam cantilever design is broadly applicable. U- and L-shaped cantilevers, on the other hand, are specifically used in applications where space restrictions constrain beam geometry. Cantilever structures have been around for a long time. Design engineers know them from their common use in bridges and such architectural structures as overhangs and balconies, not to mention their popularity in single-wing aircraft design. Applications for snap-fit cantilever designs now range anywhere from mundane tamper-proof aspirin bottle-and-cap assemblies to highly engineered, rugged parts such as power tool housings, automotive wheel covers, air cleaner housing and door handle assemblies to name just a few. Traditional materials for cantilever structures – such as stone and metals – are strong and rigid. Evaluation based solely on those two properties, however, overlooks important elements of cantilever design. Use of thermoplastics introduces additional concepts to the mix of criteria. To take advantage of the true strengths of thermoplastics for snap-fits, designers must factor in new considerations that are the subjects of the second and third articles of this series: “Principles of Classical Beam Theory and Design” and “Improved Cantilever Design” Part II will explore the traditional formulas for calculating cantilever stresses and strain. Part III will present improved formulas that provide precise calculations applicable in the use of thermoplastics. Additional information is available through the following links: BASF Plastics Snap-Fit Design Manual BASF Plastics Snap-Fit Design Calculator BASF Plastics Seminar: Part Design - Assembly of Components (recorded course) This seminar is for engineers and designers involved in the design of injection molded components. The course provides an overview of various assembly methods of plastic-to-plastic components and plastic-to-metal components. Techniques covered are snap-fits, self-threading fasteners, welding, and press-fitting. Advantages and limitations of each will be discussed in detail, as well as several examples with actual design calculations. This seminar will help educate the engineer/designer about which assembly method to use for their particular application, allowing them to design a cost-effective and efficient joint the first time. For additional information or to ask questions about Snapfit Design, contact: • • Chul Lee:
[email protected] Emile Homsie:
[email protected] September 19, 2007 PART II - IMPROVING SNAP-FIT DESIGN: CLASSICAL BEAM THEORY AND SNAP-FIT DESIGN In every snap-fit application, the design engineer’s main challenge is to find the optimal balance between integrity of the assembly and strength of the cantilever beam, i.e., how it fits and stays together, and how it supports a structure joined to a wall. Before arriving at the desired balance of snap-fit properties, it is not unusual for the designer to go through several iterations – changing length, thickness, deflection dimensions and other factors. The typical snap-fit assembly consists of a cantilever beam with an overhang at its end (Figure 1). The depth of overhang defines the amount of deflection during assembly. Modifying the angles on the entrance and retraction sides of the overhang can optimize, respectively, assembly and disassembly forces (Figure 2 Mating Force). Assembly Integrity and Beam Strength The stiffness of the beam and the amount of deflection required for assembly or disassembly determine the integrity of the structure. A designer can increase the beam’s rigidity by using either a higher modulus material or a thicker cross section. The product of these two parameters determines total rigidity of a given beam length. Increasing the overhang depth can also improve the integrity of assembly. However, as beam deflection increases in reaction to greater overhang depth, beam stress also rises. Failure results if the working stress exceeds the strength limit of the beam material. Optimizing beam section geometry is one way to ensure that the deflection required for assembly integrity can be reached without exceeding the strength or strain limit of the beam material. Cantilever Beam: Deflection-Strain Formulas Assembly and disassembly forces increase with both stiffness (k) and maximum deflection of the beam (Y). The force (P) required to deflect the beam is proportional to the product of the two factors: P = kY The Deflection-Strain Formulas in Figure 3 illustrate the effect of different beam section geometries on stiffness, as well as the effect of deflection on beam stress or strain. Designers should be careful when selecting the flexural modulus of elasticity (E) for hygroscopic (moisture absorbing) materials, e.g., polyamide. In the dry-as-molded (DAM) state, datasheet values are valid to calculate stiffness, deflection or retention force of a snap-fit design. However, physical properties decrease under normal 50% relative humidity conditions. Therefore, stiffness and retention force can decline while deflection increases. Both scenarios require verification and the lower values should be used in evaluating the structural performance of the assembly. This will ensure a built-in safety factor. Improved Formulas in Part III Classical cantilever beam formulas work well with the most rigid materials, such as stone and metal. When applied to thermoplastic snap-fit designs, however, the same formulas overestimate the amount of strain at the beam/wall interface because they do not consider the deformation in the wall itself. The next article in this series Part III “Improved Cantilever Design” will present new formulas that incorporate the effects of wall deformation. The new formulas provide engineers the information they need to arrive at the optimal balance of snap-fit design variables, which produces the most successful snap-fit assemblies. Part III of the five-part series “Improving Snap-Fit Design” will combine the subject “Improved Cantilever Design” with “Guidelines to Avoid Common Difficulties.” The complete series of articles includes: Part I Part II Part III Part IV Part V Introduction and Overview of General Applications and Types Principles of Classical Beam Theory and Design Improved Cantilever Design and Guidelines to Avoid Common Difficulties Materials Selection Coming soon! Processing Considerations Coming soon! Additional information is available through the following links: BASF Plastics Snap-Fit Design Manual BASF Plastics Snap-Fit Design Calculator BASF Plastics Seminar: Part Design - Assembly of Components (recorded course) This seminar is for engineers and designers involved in the design of injection molded components. The course provides an overview of various assembly methods of plastic-to-plastic components and plastic-to-metal components. Techniques covered are snap-fits, self-threading fasteners, welding, and press-fitting. Advantages and limitations of each will be discussed in detail, as well as several examples with actual design calculations. This seminar will help educate the engineer/designer about which assembly method to use for their particular application, allowing them to design a cost-effective and efficient joint the first time. For additional information or to ask questions about Snapfit Design, contact: • • Chul Lee:
[email protected] Emile Homsie:
[email protected] PART III. IMPROVING SNAP-FIT DESIGN: IMPROVED CANTILEVER DESIGN and GUIDELINES TO AVOID COMMON DIFFICULTIES IMPROVED CANTILEVER DESIGN The classical cantilever beam strain formulas discussed in Part II of “Improving Snap-Fit Design” work well when a flexible cantilever beam is anchored to a rigid wall, such as wood to stone. In such cases, deformation of the cantilever under a given load is the primary cause of movement at the tip of the cantilever. However, the typical plastic snap-fit design involves a snap-fit finger attached to a flexible wall – usually a plastics plate with thickness in the 3 mm range. When applied to these conventional plastic snap-fit designs, however, the classical formulas fail to account for the amount of deflection from the beam/wall interface. They simply neglect the effect of deformation in the wall itself – a factor that becomes more significant with somewhat more flexible wall materials, such as wood, composites and thermoplastics. The classical formula does predict deflection fairly well when a beam length-tothickness ratio is greater than 10:1, but the calculated deflection deviates further from actual values as the ratio gets smaller or when the beam, in other words, gets “stubbier.” To obtain a more accurate prediction of the total allowable deflection for short beams in snap-fits, the design engineer must apply a magnification factor to compensate for the classical formulas’ shortcomings. Doing so allows the design engineer to take full advantage of a material’s strain-carrying capability and, therefore, to enjoy greater design flexibility.Magnification Factors BASF Plastics has developed a method for determining these magnification factors for a variety of snap-fit beam/wall configurations, including beams with either uniform or tapered cross sections (graphically depicted in Figures IV-1 and IV-2). BASF has verified the results of this method both by finite element analysis and actual part testing. Improved Formulas To determine maximum strain at a given snap-fit’s base, design engineers can now use the improved formula which incorporates a corrective magnification factor (See Figure IV-3). BASF also provides formulas for determining the Push-on and Perpendicular Mating Forces (Figure IV-3). To review examples that employ these formulas to determine accurate solutions for two different snap-fit designs, see Figures IV-4 and IV-5. The first example incorporates allowable strain and coefficient of friction values for a specific grade of BASF Ultradur® PBT; the second example, for a grade of BASF Ultraform® POM. Refer to BASF’s Snap-Fit Design Manual Page IV-4 for tables providing allowable strain and coefficient of friction values for eight different materials appropriate for snap-fit designs. U-shaped and L-shaped Designs Design engineers interested in the application of these formulas to U-shaped and L-shaped snap-fit designs – and a more detailed discussion – can refer to BASF’s Snap-Fit Design Manual, Pages V-1, -2, -3 and -4. Conclusion The formulas developed by BASF represent a significant improvement over the classical cantilever beam deflection and strain formulas. Working with BASF’s improved formulas, design engineers can now more accurately calculate and predict the forces encountered – and allowable deflection limits – for different configurations of cantilever-type snap-fit assemblies. With the greater degree of certainty these formulas provide, design engineers can get the most of their materials and their designs. GUIDELINES TO AVOID COMMON DIFFICULTIES Before finalizing any snap-fit design, the design engineer should review three basic considerations: • Stress Concentration • Creep/Relaxation • Fatigue. The single most common cause of failure in snap-fits is the concentration of stress caused by a sharp corner between the snap-fit beam and the wall to which it is attached – the normal point of maximum stress. One solution is to incorporate a generous fillet radius at the juncture between the beam and the wall, either on both sides of the beam or particularly on the beam’s tensile stress side (See Figure VI-1). Another cause of failure is that, over time, creep and stress relaxation can gradually reduce the retention force between the two components connected by a snap-fit. In some cases, lower retention force can lead to leakage from a formerly tight seal; in others, excessive play that creates noise and vibration (or “BSR” - Buzz, Squeak and Rumble). Ways to minimize the effects of creep and stress relaxation include designing a low-stress snap beam, incorporating a 90° return angle to avoid bending, or increasing the land length in the area of a large return angle (See Figures VI-2 and VI-3). The third major cause of snap-fit failure is fatigue, or repetitive loading, primarily in applications involving hundreds or thousands of cycles. The first, most obvious, way to avoid fatigue failure is to choose a material known to perform well in fatigue situations. Compare different materials’ S-N curves, which show expected number of cycles to failure at various stress levels and at different temperatures. A second way based on S-N curves is to select a sufficient design stress level at the application’s correct exposure temperature. In the real application, the frequency of cycles is usually a lower number than in testing, thereby providing a margin of safety for the design. Conclusion There are a number of ways to overcome stress concentration, stress relaxation or creep, and fatigue. A well thought-out design, plus the right choice of material, will allow your application to benefit from all the advantages of snap-fit design. “Materials Selection” Part IV, “Materials Selection”, of the five-part series “Improving Snap-Fit Design” will provide an overview of materials appropriate for snap-fit design and guidelines for selecting the material best suited to your applications. The complete series of articles includes Part I. Introduction and Overview of General Applications and Types Part II. Principles of Classical Beam Theory and Design Part III. Improved Cantilever Design and Guidelines to Avoid Common Difficulties Part IV. Materials Selection Coming soon! Part V. Processing Considerations Coming soon! Additional information is available through the following links: BASF Plastics Snap-Fit Design Manual BASF Plastics Snap-Fit Design Calculator BASF Plastics Seminar: Part Design - Assembly of Components (recorded course) This seminar is for engineers and designers involved in the design of injection molded components. The course provides an overview of various assembly methods of plastic-to-plastic components and plastic-to-metal components. Techniques covered are snap-fits, self-threading fasteners, welding, and press-fitting. Advantages and limitations of each will be discussed in detail, as well as several examples with actual design calculations. This seminar will help educate the engineer/designer about which assembly method to use for their particular application, allowing them to design a cost-effective and efficient joint the first time. For additional information or to ask questions about Snapfit Design, contact: • • Chul Lee:
[email protected] Emile Homsie:
[email protected]