Design of Snap Fits

Example 2.10 The polypropylene snap fit shown in Fig. 2.22 is to have a length of 10-30 mm. If the insertion force is not to exceed 4 N and the yield stress of the plastic is 30 MN/m, calculate suitable cross-sectional dimensions for the snap fit. The short-term modulus of the polypropylene is 900 MN/m and the coefficient of friction is 0.3. The safety factor on stress is to be 2. 

The snap fit may be regarded as a cantilever and for this situation, the vertical deflection, S, is given by 

It is also necessary to check that the stress in the snap fit does not exceed the yield strength for the material (30 MN/m ). Allowing for a safety factor of 2, the maximum permissible stress will be 15 MN/m.  

The stress in a cantilever beam of this type will be maximum at point A (Fig. 2.22) and is given by 

For the conditions given, = 13.5 MN/m which is less than the permitted maximum of 15 MN/m and so the chosen dimensions are acceptable. Note that the stress is independent of the width, b. Fig. 2.24 gives a set of design curves for the situation considered so that it may be seen that other combinations of L, d and b could be chosen to meet the design specification. For example, L — 22 mm, d = 2.5 mm and b — 2 mm gives an insertion force of 4 N and a maximum stress of 14 MN/m which is again acceptable. 

Resolving vertical forces gives V = Ncosa — fiJVsina = A/(coscc — fi sin a) Resolving horizontal forces gives F = N sino -I- fjiNcosa = N sina + ficosa) 

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Another popular approach to evaluate the design of snap fits is to calculate their strain rather tlian their stress. Then compare this value with the allowable dynamic strain limits for the particular plastics. In designing the beams it is important to avoid having sharp comers or structural discontinuities that can cause stress risers. Tapered finger provides a more uniform stress distribution, which makes it advisable to use where possible. 

Table 4-3 Examples of different cross section types of snap fit designs...

Visit plastics.bayer.com/plastics/emea/en/femsnap/index.jsp and examine the example snap-fit joint, noting the shape and stress distribution at maximum deflection. Find some examples of snap fit joints on consumer products and compare their design with that on the Bayer site. 

The two most common types of snap-fits are those with flexible cantilevered lugs (Fig. 9.6) and those with a full cylindrical vmdercut and mating Up (Fig. 9.7). Cylindrical snap-fits are generally stronger but require deformation for removal from the mold. Materials with good recovery characteristics are required. In order to obtain satisfactory results, tbe undercut design must fulfill certain requirements ... 

ABS parts can be designed for snap-fit assembly using a general guideline of 5% allowable strain during the interference phase of the assembly. Thread-cutting screws are frequently recommended for nonfoeuned... 

Figure 5-47. The equations for designing geometrically complex cross-sections of snap fits.

Use of snap fits provides an economical approach where structural and nonstructural members can be molded simultaneously with the finished product and provide rapid assembly when compared with such other joining processes as screws. As in other product design approaches (nothing is perfect), snap fits have limitations such as those described in Table 4.10. 

Stress Relaxation. Another important consequence of the viscoelastic nature of plastics is that if they are subjected to a particular strain and this strain is held constant it is found that as time progresses, the stress necessary to maintain this strain decreases. This is termed stress relaxation and is of vital importance in the design of gaskets, seals, springs and snap-fit assemblies. This subject will also be considered in greater detail in the next chapter. 

Action of package design combining a hinge with snap fits. 

Snap fits are widely used for both temporary and permanent assemblies, principally in injection and blow molded products. Besides being simple and inexpensive, snap fits have superior qualities. Snap fits can be applied to any combination of materials, such as plastic and plastic, metal and plastics, glass and plastics, and others. All types of plastics can be used (Chapter 3, DESIGN CONCEPT, Snap Joint). 

A snap fit can be rectangular or of a geometrically more complex cross-section. The design approach for the finger is that either its thickness or width tapers from the root to the... 

One of the growing areas for PAB blend applications under the hood is in electrical and electronic connectors and systems for power distribution and control. Such applications often require low melt viscosity at injection molding temperatures to fill the intricate tooling that may be designed for up to hundreds of individual electrical connections. Practical toughness is required for snap-fit of the connections. Furthermore, glass-reinforcement of these PAB s may be required to improve modulus and dimensional stability. Typical blends suitable for these applications include PBT/PC, PA/PPE and PPS/PEI. 

A snap-fit, or snap-on, closure is made of a resilient material and is designed to deform as it passes over a protruding feature on the container (Fig. 11.7). Removal of the closure requires deformation again to snap the closure back over the protruding feature, which is typically a retaining ring. When the closure is in place, some resilient part of the closure system that is in contact with the container remains deformed, and provides a seal as it attempts to return to its original dimen-... 

Figure 4.8 shows a part manufactured from PVC to protect the lens assembly of a projection TV. There are many elements in the design, including a snap-fit connector (32), standoff (30), protrusions (24) and (26), used for connection (32), spatial orientation (30), and angular orientation (24), (26). 

It must be kept in mind that design for manufacture and assembly and design for disassembly may sometimes be in conflict with each other. For example, snap-fits may accelerate the assembly process but impede rapid disassembly if they are not designed properly. Furthermore, safety rules and regulations, such as for consumer electronics and household appliances, require the protection of electrical circuits to avoid accidents. In other products, easy access may also encourage the theft of valuable components [32]. 

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