Snap fit

Snap fits are widely used for both temporary and permanent assemblies, principally in injection- and blow-molded parts. Besides being simple and inexpensive, snap fits have been superior qualities (see Figs. 5-41). Snap fits can be applied to any combination of materials, such as metal and plastics, glass and plastics, and others. All types of plastics can be used. 

When used as demountable assemblies, snap fits can compete very well with screw joints. The loss of friction under vibration can loosen bolts and screws. A snap fit is vibration proof, however, because its assembled parts are in a low state of potential energy. There are also fewer parts in a snap fit, which means a savings in component and inventory costs. 

A snap can be characterized by the geometry of its spring component. The most conmion snaps are the cantilever type, the hollow-cylinder type (as in the lids of pill bottles) and the distortion type. These snaps include those in any shape that is deformed or deflected to pass over an interference. The shapes of the mating parts in a hollow-cylinder snap are the same, but the shapes of the mating parts in a distortion snap are different, by definition. This three-part classification is rather nominal, because the cantilever category is used loosely to include any leaf-spring components, and the cylinder type is used also to include noncircular section tubes. 

For high-volume production, snap-fit designs provide economic, rapid assembly. In many products, such as inexpensive housewares or hand-held appliances, they are designed for one assembly only, with no nondestructive means for disassembling them. Where servicing them is anticipated, provision is made for the release of the assembly with a tool. Other designs, such as those used in the battery compartment covers for calculators and radios, are designed for easy release and reassembly many hundreds of times. 

There is always some part of a snap fit that must flex like a spring, usually past a designed-in interference, and quickly return, or at least nearly return, to its unflexed 

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Snap-Fit and Press-FitJoints. Snap-fit joints offer the advantage that the strength of the joint does not diminish with time because of creep. Press-fit joints are simple and inexpensive, but lose hoi ding power. Creep and stress relaxation reduce the effective interference, as do temperature variations, particularly with materials with different thermal expansions. 

Assembly features - torqued eonneetions, shrink fits, snap fits, shear pins and... 

Component assembly Snap fits difficult Very good... 

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. 

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. ... 

Action of package design combining a hinge with snap fits. 

When used as demountable assemblies, snap fits can compete very well with screw joints. The loss of friction under vibration can... 

For high-volume production, snap fit designs provide economic, rapid assembly. In... 

Fig. 4-16 Basic snap fit design for a cantilever beam with a rectangular cross-section.

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... 

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

Mobay/Bayer Design Manual Snap Fit Joints in Plastics, Mobay Engineering Thermoplastics Bulletin, Bayer Chemical Corp., 1990. 

These groovs lock the two halves together after filling (SNAP-FIT principle)... 

Snap fittings and fixing parts for interior linings. .. 

The impact strength of certain plastics allows their assembly by stretching as click-and-spring work, clamping, snap-fit, etc. To make this task easier, the assembly angle must be sh low, generally lower than 30°, and the strain must be much lower than the elastic limit. 

Milacron is collaborating with Foboha on the development of a machine that simultaneously moulds two parts in two different materials and snap-fits them during injection of the next parts. A robot removes the finished assembly allowing a cycle time of the order of 6-7 seconds to be achieved. 

Implantables are less likely to use adhesives because of their longevity, performance, and toxicity requirements. With these parts, self-assembly (snap-fit or interference fit) or solvent and heat welding processes are commonly used for assembly. 

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