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:
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:
- Part 1: Introduction and Overview of General Applications and Types
- Part 2: Principles of Classical Beam Theory and Design
- Part 3: Improved Cantilever Design and Guidelines to Avoid Common Difficulties
- Part 4: Materials Selection
- Part 5: Processing Considerations (coming soon!)
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