Today's KNOWLEDGE Share
Have you ever observed failure due to delamination in injection molded parts ?
The very high shear at the boundary of the frozen skin can indeed trigger some localized weakness. In HDPE for instance, extended chain crystallization ("shish-kebab") due to flow induced crystallization in the frozen skin can compromise the degree of entanglement between the frozen skin and the core section. As a result, it is very easy to peel-off the (much stiffer) skin, something that shows up dramatically in higher Mw grades, when parts fail in impact. It is also very common to observe any low molecular tail or actual added lubricants migrate out inside of the skin due to hydrodynamic forces (see image, problem happens on both sides of course).
Such an accumulation of "plasticizer" or lubricant expelled by the high shear stress can also trigger delamination failure. Higher mold temperature and more gentle filling conditions can often improve the situation.
Source:Vito Leo
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#plastics #injectionmolding #mold #delamination #hdpe
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