Today's KNOWLEDGE Share:Coating on moldings

Today's KNOWLEDGE Share

If you browse through the documentation of mold coating suppliers (PVD, WS2, others) they all claim better flow (i.e. lower pressure to fill) after coating the mold with just a few microns of their coating (typically a metal oxide, salt, or some other complex).

These layers do impart lubricity indeed, which is great to help ejection of the molded part.

But in Injection Molding there is no slip against the wall. Actually if/when you get some, you invariably end up with a surface defect !

So improved lubricity, as claimed by suppliers, DOES NOT explain better flow.

What is most probably happening is that the added layer, which is not "metallic" has always a lower Thermal Effusivity.

Effusivity is the material property that controls interfacial temperature.

So what we really have here is a slightly thinner frozen skin resulting from a somewhat higher interfacial temperature between plastic and coated steel. In a flat flow, effective available thickness for flow has a quadratic effect on pressure drop. For a runner/gate (cylinder) it is even a cubic dependence.

So the smallest reduction in the frozen layer, especially for thin cavities, can immediately explain the observed 5-10 % decrease in pressure drop (or, conversely, increase in flow length).

source:Vito leo