Snap Fits and Press Fits

The " ideal " plastic assembly would have all components joined right in the molding shop. Press or snap fitted together by operators at the machine, finished products would be ready for packaging and shipment. While assembly operations in the automotive , machinery or appliance industries are usually a bit more complex, subassemblies frequently will employ snap and press fits—especially when adequate strength, stiffness and toughness are assured by the use of such engineering polymers as Rynite ® thermoplastic polyester, Delrin ® acetal, Zytel ® nylon, and Hytrel ® polyester elastomer. Indeed, incorporating a press or snap fit into a design to take advantage of the speed and economies it allows labor reduction and elimination of assembly fixtures, equipment and/or inventories of separate fasteners—may be one of the more critical decision points in a successful product. All snap fits are variations of two basic designs, cylindrical (Fig. 1a) or cantilevered lug (Fig. 1b). Both provide strong, easily assembled joints. Snap fits work by using an undercut in one piece and, on the other, a lip or hook which " snaps " into the undercut. Assembly requires temporary deformation of one or both pieces, but parts return to an unstressed state in the final assembled position, thus eliminating the problem of stress relaxation, an inherent difficulty with press fits (Fig. 2). Although assembly costs are relatively low, mold costs for snap-fit parts may be somewhat high because of complicated cross-sections. This joint generally is not used when a pressure tight or hermetic seal is required. Incorporating an o-ring or gasket between the parts at assembly can provide a degree of sealing. Cylindrical snap joints can be designed to separate with ease, with difficulty or not at all, depending on the shaft lead and hub return angle (see Fig. 1a). A shallow return angle (30°) separates easily; a 90° angle is inseparable. An angle of 45° is Identical halves of split worm gear in Delrin® snap together permanently via integral cantilevered lugs.