Snap-fit assembly

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. 

Snap-fit assemblies are usually employed to attach lids or covers that are meant to be disassembled or that wiU be lightly loaded. The design should be such that, after the assembly, the joint will return to a stress-free condition. Materials with good recovery characteristics are required. 

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... 

The workhorse for connector moulding is unfilled nylon 66, because of its fitness for the underbonnet environment, and its useful combination of toughness and creep resistance, making it ideal for snap-fit assemblies which maintain positive contact. The formulators have had to respond to tightening specifications, particularly in respect of flame retardancy as a result there are grades available now which achieve the UL V-0 rating without losing the essential snap-fit-ability . 

External and internal undercuts for snap-fit assembly parts should be also taken into consideration in design. External undercuts are demoulded using retracting segments of the mould while for internal undercuts the wall of the mould should be free to flex outwards during injection. 

For snap-fit assemblies it is also important not to make the construction too stiff, because then the assembly force will be very sensitive to tolerances. This was the case in the snap-fit construction in the floating device depicted in Figure 4 and in more detail in Figure 11. 

In Figure 12 the reaction force in a snap fit assembly is schematically depicted as a function of the displacement. When a constant reaction force has to be exerted this can be achieved by a design with a high stiffness or a low stiffness. 

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

Component assembly Snap fits difficult Very good... 

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). 

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... 

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. 

Rigid sub-assembly components, e.g. filter canister bodies, and mountings for valves, eyepieces and speech modules, were originally made of metal but tend more now to be made of polymers. In the SI0, polyacetal is used as it is not only chemically hard but also has mechanical properties that allow snap-fitting, which reduces production costs and allows easy replacement of parts. 

Using familiar products, the aim is to note component shapes, to see how they are assembled, and measure the variation in thickness. Recycling can also be considered the ease of dismantling depends on whether the product was intended to be repaired, or to be scrapped if faulty. Screws may be hidden under adhesive labels, and the location of snap-fit parts may be difficult to find. 

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