DESIGN FOR APPEARANCE

These typically occur over projections such as ribs or bosses.  They are caused by localised thickening of the sections which results in an above average shrinkage.  To minimise, attempts should be made to keep the thickness of ribs or bosses to 50% of that of the main wall.  In certain circumstances, gas assisted moulding may be used to minimise the effect such as in the case of TV surrounds.  Sink marks may also be disguised by the use of a textured surface or a styling feature e.g grooves, beads and surface texture as shown above.

These occur where two flow fronts meet. Tool design can be critical in minimising the effect of these. Problems occur when there are holes in the component as the material has to flow around these and weld on the other side. In this instance, there are not only small weld lines adjacent to the hole but there may also be flow lines where the smooth passage of the material has been interrupted. Weld lines can cause a physical weakening because the joint at the weld is usually weaker. They look like cracks on the surface of a moulded part.

There are other problems such as streaks, delamination, jetting and gate marks which are more the concern of tool designer rather than the product or component designer. Rutland Plastics can take your component design and advise on any necessary changes, tool design and manufacture is then managed in-house.