DESIGN FOR MOULDABILITY (PART 4)

BOSSES

Bosses are usually incorporated to facilitate mechanical assembly.  They can be designed to accommodate self-tapping screws, push-in or moulded-in inserts or used for ultrasonic welding.  Therefore, the boss may have to withstand a variety of forces – tension, torsion, compression, shear and flexing. 

Design suggestions:

     Wall thickness of the boss should be 50% to 70% of the nominal wall.  However, this may not be sufficient to withstand the stresses imposed by an insert but a thicker section can cause sink marks.  Frequently, a compromise is required.

     Minimum radius of 25% of the wall thickness at the base of the boss is recommended.

     Further strength may be achieved through the use of support ribs.

     Strength can also be increased by attaching the boss to a nearby wall using a rib.

All of the above Boss layouts will result in sink marks and/or voids

Typical layout for optimum strength and appearance

UNDERCUTS

Undercuts should be avoided if at all possible.  Ideally mould tools should be straight open and close.  In certain circumstances mouldings with slight undercuts can be ‘bumped’ off the core.  The same results as an undercut can be achieved using certain design techniques. For some complex components, it may still be necessary to have side movements in the mould tool but it has to be accepted that this will increase tooling costs. 

 There are various alternative solutions that  would be happy to discuss and advise on.

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