Screw Boss Design

Screw bosses are common in assembled plastic parts designed for future disassembly. As with everything else in plastic part design, there are general guidelines for creating screw bosses.

Figure 1 shows that thick sections should be avoided (this goes back to the rule of uniform wall thickness). As shown in the top picture, thick sections lead to sinks because thicker sections cool slower and shrink more during injection molding. Use thin standoffs to avoid this problem. The thickness of the standoffs should use the same rules that apply to the thickness of a rib. Gussets can be used to increase the torsional strength of the boss (Figure 2).

Figure 3 shows the guideline dimensions for designing a boss with a blind hole. In general, for any blind hole, the length should be no longer than 2.5 times its diameter. Injection molding uses enormous pressures (up to 30,000 psi) that can bend the piece of steel that forms the hole if it is too long. The blind hole protrudes slightly past the inside edge of the wall (the 0.7w dimension) to reduce sink. Regardless, the boss will still produce a ring of sink because of its thick base. Figure 4 shows a technique to reduce this.

Figure 5 shows that if the boss's hole is a through hole, its length can be twice as long. Since the piece of steel that forms the hole is supported on both sides, it can be 5 times the diameter of the hole.

Figure 6 shows a method to reduce the length of the blind-holed boss. For easier molding, it is better to make the through-holed boss longer than the blind hole, as shown in the right picture.