Part Details Review
Providing proper radii at the corners in plastic parts is more than just specifying "Radius all corners". A blanket statement, such as this, does not do justice to the part design. You must consider several issues, such as stress concentration, molded-in stress, sink marks, internal voids etc., some of which may be caused by too little radius and some by too much. You have to make a trade-off, depending upon the part geometry and the end use requirements, to strike a balance between the "too much" and the "too little". Specifying a maximum allowable value, "radius 0.030" max.", invites trouble. Toolmakers like to leave intersecting machined surfaces as they are generated by the machine tool. Unfortunately, this practice sometimes results in an internal sharp corner in the molded part, causing stress concentration. External corners radii are easily generated by the cutter or the grinding wheel, which are both round. It is better to specify radii as a range rather than a specific maximum value. Ideally, all corners should be individually considered for radius specification. Avoid universal radius specification that calls for often unneeded radii on the outside edges (e.g. at the parting line), which increase mold cost. Outside corners need different treatment than inside corners. Radius around a bend, as in a box or channel shaped part, should be as large as possible as long as a uniform wall section is maintained. On the other hand, fillet radius at a T-junction, as in a rib or cantilever snap feature, has to be carefully considered due to opposite effects of too much or too little radius. |