Design snap fit

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). 

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. 

SBC can also be molded into overcaps. These are injection molded clear caps that are often used on bottles for food and cosmetic packaging. Overcaps are typically made by injection molding in multicavity molds and designed for a snap fit. They must have high gloss and clarity and can often have a transparent color. 

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-... 

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. 

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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