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Injection molding surface defects

I trouble shot splay and blistering for 12 years (until three years ago). Silver streaks were (in our blended products) always a first sign of marginal mixing, and of course contamination (this covers humidity, larger filler than specs allowed, other contaminating materials). I was the optical and electron microscopic morphologist. These silver streaks (some of the thinnest and hardest surface defects to microscopically visualize), splays (ultra-thin), then blisters (largest) all contained separations that could easily be seen from the surface with the correct microscopic conditions, and in cross section as an interface that at one end usually displayed a thin cavity usually at or very near the skin layers.

It was very difficult in the first 3-4 years to convince traditional staff at first that it was not only humidity, understandably, but repetitive micro-analysis seemed to produce very predictable results, even before surface silver, or small or this, splay could be seen on production lines. So predictability and rapid solutions became an important issue.

Obviously humidity, wet product and contamination, including low levels of incompatible rework or previously splaying material (poorly mixed) could also easily and predictably produce similar results, since they are all in a way related (out of spec. defect material sources). Reduction in the defects could usually be minimized in final product production by slowing the extrusion or molding speed, but this was not generally acceptable, since this was a profit issue. The defects were enhanced by increasing higher final product extrusion speed (injection) and pressures (shear and mixing issues), but these apparent solutions only usually temporarily masked, changed, moved, and even enhanced the defects size, or even increased the defect-depth from the surface.

These were the most interesting of structural surface defects. What was more interesting was the ability to reproduce most of these defects in the lab at scaled down processing levels and be able to manage and predict their size, shape, and location.

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