Part Details Review
| Wall Thickness |
Boss sidewalls should be sufficiently thick to be able to withstand hoop stress generated by screw installation. Thread forming screws produce much higher levels of stress than thread cutting screws and are, therefore, not usually recommended for use with polycarbonate or polycarbonate blends. Molded-in metal inserts also produce hoop stress in the bosses surrounding them due to differential cooling of the metal and the plastic during injection molding.
Because of the hoop stress, the wall thickness at the top of the boss is of critical importance. In bosses with blind holes, the thickness at the base is always more than that at the top due to the effect of the draft on the core and the cavity. A boss which is too thin at the top may crack under hoop stress. A boss which is too thick at the base has the negative effect of producing an unsightly sink mark there.
Some thick sections at the base are created inadvertently by limiting the depth of the blind holes to be just enough for good thread engagement. The hole should be long enough for the screw so that it does not bottom out in it. Extra space at the bottom provides good clearance for the material deposited by the thread cutting screws. See figure 2 for this and other tips on what to avoid and what to do for a good boss design.
