Snap Fit

The words snap fit indicate exactly what is going on with this type of assembly. Some detail of one part is deflected from its rest position by some detail of the other part and it snaps back to rest position to lock the assembly together. A post with a hook detail going into a slot is the most easily imagined form. There are in reality a vast array of snap fit possibilities. It is impossible to relate more than general information in this format, but the imagination of designers has not yet been exhausted when it comes to the endless variety of configurations. What remains constant, however, is the need to deflect the material enough to get sufficient engagement to do the job without stressing the material to the point of weakening it. This is why it is generally advantageous to have many smaller details rather than one larger one. Another key consideration with snap fits is the need to incorporate the necessary undercuts without creating unnecessarily complex and expensive mold features. Often, snap fits can be designed so the part will snap out of the mold without requiring side action, other times, the part wall can be relieved so the undercut can be formed with a straight-draw detail. Snap fits can be designed so the part can be disassembled, but quite often a snap fit is a one-way street. Given the creep issues with plastic parts as they age, careful thought and attention needs to be paid to the dimensions of the details so that assemblies do not loosen up over time. Also, if the assembly requires a seal, the seal will have to be provided for mechanically and separately from the snap detail.