January 2012 Design Tip

Tips on Clips Part 2: Make it Snappy

Plastic’s ability to flex without permanently deforming allows moulded parts to incorporate a variety of snap fasteners other than the common hooked cantilever clip. These include:

  • annular (round) snaps
  • torsional snaps, which store return force by twisting rather than bending
  • compressive snaps, which work by compressing and then returning to hold the fastener in place.

Annular Snaps

Annular snaps are used in a variety of everyday applications, from the tiny snaps that close the wind flaps on jackets to larger ones that fasten caps to pens to the still larger lids on plastic yogurt containers. While these are all designed for ease of opening and closing, annular snaps are also used, with slight modification, in “childproof caps,” which can be easily opened in one position but are virtually impossible to open in any other. And while many applications support repeated opening and closing, annular snaps can be used in industrial applications for permanent fastening, typically ensured by the angle of the locking edges of the snap.

Whatever their application, annular snaps operate by elongation and recovery, typically of the female component. This restricts the materials that can be used to those with relatively high elastic deflection limits, the point where material fails to fully recover from deformation. Maximum permissible strain varies for different materials, from about 50% of the strain at break for most reinforced plastics to more than 70% of strain at break for more elastic polymers. Detailed information on annular snap fit design can be found here.

Torsional Snaps

Torsional snaps store closing force when opened by imparting twist to a torsion bar at the pivot point of the arm, as opposed to bending the flexing arm as in a cantilever clip. But in most other respects—hook design, etc.—the torsional snap is similar to the cantilever clip. Rocker clips are typical torsional snaps. For detailed information on the design of torsional snap fits, click here.

Compression Snaps

Compression (or interference) snap fits can take many forms. The compound dovetail snap used in the Protomold sample design cube (Figure 1) is one example. This clip is dovetailed in two directions to lock together two of the cube’s six faces. In one direction, the male component is highly tapered providing an unbreakable connection between the two components. In the other direction, the male connector is only slightly tapered providing a connection that can be easily undone to disassemble the cube. As the male connector is pushed into position, material in both male and female components is compressed and then released as the snap moves into its locked position.

Figure 1: Clip is dovetailed in two directions to lock together two sides of a box.

Figure 1: Clip is dovetailed in two directions to lock together two sides of a box.

Another example of a compression snap fit is found in the Reptangles building system. The challenge here was to design a connector that would mate when the parts came together in any direction within a 90° arc. The patented connector (Figure 2) consists of a triangular arch that fits into a corresponding slot. “Fingers” in the slot walls grasp the hollow under the arch to link the pieces firmly together while still allowing easy disassembly.

Figure 2: Arch-shaped connectors fit into a slot in an interference fit.

Figure 2: Arch-shaped connectors fit into a slot in an interference fit.

A compression snap fit can create mouldability challenges. In some cases the connector can be formed as a “bump-off,” in which the part is slightly undercut but the material flexes to allow ejection. (For more information on bump-offs go to read this past Protomold Design Tip). In the case of Reptangles, bump-offs were not required, as both arch and slot components of the connectors were formed by sliding shutoffs passing through holes in the part to form the undersides of protruding features.

The wide variety of clip and snap options allows designers a great deal of latitude in creating integrated connectors.

To read Tips on Clips Part 1, click here.