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Can I mold plastic parts with ZERO sink-mark?

With regards to ZERO sink-marks, however, this is a Holy Grail that is practically impossible to achieve with basic injection molding, as soon as you locally thicken the general wall section by adding ribs or any other protrusions on the opposite surface. However, following the well-proven guidelines on ratio of rib-to-base wall thickness will always help minimize the physical dimensions of sink-marks. Conversely, the higher the gloss level, the more you will see sink-marks, for any given depth. On a high gloss surface, even a dip of a few micro-meters will be seen under certain light conditions. The nearer to feed points, the more you will be able to make the packing pressure do its job of "inflating" the slightly thicker section as volumetric shrinkage pulls it inwards. You can see often this effect very clearly along the length of a rib.

Another means, other than packing pressure, of resisting the pull caused by volumetric shrinkage is to use fluid-assisted molding (gas or water). Great results can be achieved compared to conventional injection molding. Also, plastic parts produced using Mucell technology benefit from microcellular pockets of inert gas to maintain the internal pressure needed to resist the localized sinking. Use of the Rapid Temperature Cycling technology can even "iron out" the otherwise "grainy" surface caused by bubble collapse, resulting in superb gloss (if the material is intrinsically glossy, that is).

Finally, material choice is crucially influential, not just with regard to intrinsic gloss levels, but also extent of volumetric shrinkage that gives rise to sink-marks (and vacuum voids in thick sections). Amorphous materials, such as PC, ABS and PVC change in volume much less than semi-crystalline ones (PP, PE, PA, POM, etc.) when they change from molten to solid. The former group of materials exhibit volumetric (Not mold shrinkage, though this is related) typically less than 10%, whereas semi-crystalline materials can shrink in volume by more than 25% as they solidify. If anyone wondered why packing pressure is needed after initial fill, look no further!

There is no shortcut to anything approaching ZERO sink-marks, as it involves an interconnected combination of plastic part design, mold design, tool finish, temperature control (melt and mold), process selection (conventional, fluid-assist or microcellular), process control and choice of material.

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