DESIGN FOR MOULDABILITY (PART 3)

Coring refers to the elimination of material from a particular area of the component.  It is a means of reducing wall thickness and material content.  Imagine a ‘Frisbee’ – viewed from the top or the side and it appears a solid disc but viewed from below it is ‘dish’ shaped or ‘cored out’ in its most basic form.

Coring can also refer to the addition of holes or vent slots in a component.  Holes are associated with a number of problems as outlined elsewhere.  Irregularly shaped slots incorporating sharp internal corners should be avoided.  The weld or flow-lines created by holes may be aesthetically unacceptable.

To minimise these problems:

• Shortest distance between edges of any two holes or slots should be greater than twice nominal wall thickness.
• When positioning a hole or slot near the edge of a component the distance between edge of hole or slot and edge of component should exceed twice the nominal wall thickness.

With a normal cavity and core the cavity is stationary and the core is attached to the moving platen.  In the majority of cases the part ‘sticks’ to the core due to shrinkage of the material moving it away from the cavity and tightening it onto the core. In order to facilitate part removal from the mould it is necessary to incorporate draft angles.  0.25 degrees to 2 degrees per side for both inner and outer walls is generally sufficient.  If the surface is polished the draft angle can be at the lower end of the scale, for textured surfaces more draft is required if ‘scuffing’ or sticking of the part in the mould is to be avoided.  Cavities with texture require an additional 1° – 1 ½ ° of draft per 0.025mm of texture depth.  If there is texture on the core then even greater draft angles are required as the material tends to shrink around the core.

It should be remembered that even ribs, bosses, depressions and holes must have draft.

Recesses or depressions in the surface of the plastic do not usually present problems. However, they can impact on the flow of the plastic causing weld lines, flow marks and possibly holes in the surface due to trapped air. Recesses can be eliminated by reshaping the wall to maintain a uniform section but if this is not possible then problems can be minimised by using ample radii.