Designing of Plastic Products for Injection Moulding
Bosses are the cylindrical elements in the plastic parts which are used as a mounting point or a location point. Boss may receive a screw, insert or a shaft. Their cylindrical form is easy to machine and is the strongest shape in molded part.Bosses have to be rigid and it is achieved by supporting the boss with buttress ribs or linking it to a side walls or corners of the part. Extending the boss to meet side wall or corner will increase the diameter of boss which will give a thick section resulting in the sink marks. To avoid this boss is linked to side walls with a flat rib recessed below the boss or edge. Function of a boss is to accept screw fasteners. The screw hole of a boss is provided with a counter bore to reduce the stresses at open end and prevent its splitting. Dimension of boss and hole is decided by two factors screw thread diameter and the material of plastic used.
Design considerations
- Consider the forces acting on it during assembly and service.
- If forces are less then use buttress ribs otherwise extend the boss to walls or corner for greater forces.
- Avoid complex mold machining.
- Use counter bore to reduce stress at open ends.
