Snap-in design

Fig. 7.5.14 Sensor with brass housing and front side O-ring snap-in design for coolant bypass for exhaust-gas catalyst retrofit control unit...

Fig. 7.5.17 Sensor with brass housing, snap-in design, snap-in locked with female connector for sensing coolant return temperature, fan-speed control...

Fig. 7.5.19 Sensor with plastic housing, NTCwith direct air contact protected by cage, snap-in design for quick assembly for sensing intake-air temperature in passenger cars...

Fig. 7.5.24 Sensor with plastic housing, snap-in design, NTC directly exposed to air, heat radiation shield, additional port for air flow for sensing passenger-compartment air temperature...

Bottle caps made from different plastics are extensively used. Some closures are of the simple cork snap type design, but most are of the screw type. Strong, accurate threads can be molded, which represent undercuts. Simple designs should be used when permitted, such as wide-pitch threads. The thread should be designed to start about 1 /32 in. (0.08 cm) from the end of the face perpendicular to the axis of the thread. It is usually practical to mold up to 32 threads per in. more than this number can give certain molders trouble. 

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

Snap-in tabs, above each end of the output meter, indicate the direction the final control element moves for a change in the output signal. Tabs normally read "open-close" for control valves and "slow-fast" for variable-speed motors, or other appropriate designations. 

Mechanical Design The vertical basket centrifuge is equipped with a cylindrical basket rotating about the vertical axis. The basket shell is normally perforated and lined with filtration media consisting of filter cloth, and backing screens to provide liquid drainage paths to the basket holes. Securing the filter cloth may be by hook and loop attachment or with snap-in retainers. 

Mechanical fasteners and the design of parts to accommodate them are covered in detail in later sections of this chapter. The following section describes the use of press-fit and snap-fit designs that are integrated into the molded part to achieve assembly. 

Shrink fit n. A method of joining circular and annular parts in which the outer member, having a slightly smaller inside diameter than the inner member s outside diameter, is heated, causing it to expand, then slipped into place over the inner member and allowed to cool. Alternatively, one can chill the inner member in liquid nitrogen, slip it into the outer member, and let is warm. Care must be taken in designing the joint to have the final stresses in both members well below yield values so as not to lose the joint to creep over time. Compare snap fit. 

In the electronics segment there are components built to tight tolerances with moulding flexibility needed for intricate snap-fit designs. This permits the mass production of highly complex finished components which have no need to be machined further. Other applications include handling equipment for semiconductor and LCD flat panel fabrication. 

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. 

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. 

In the cantilevered snap-fit design, the retaining force is essentially a function of the bending stiffness of the resin. Cantilevered lugs should be designed in a way as not to ex-... 

Snap-fit or integral attachment includes built-in design elements for attaching or joining one plastic part to another part [5]. They generally include apart... 

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. 

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. 

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