I picked up some no-name "silk" PLA in a multi color pack as part of a project where I needed some additional distinct colors and structural properties didn't matter, but now I'm playing with the excess I'm not using for that, and its behavior is really weird. On overhangs with detail on the downward-facing surface it's like everything flowed/melted together rather than retaining the extruded shape. And on vertical surfaces where one would expect to see layer lines, the lines are present and can be felt and slightly seen, but don't seem to contibute to the sheen/reflective properties like how they would in normal PLA or other materials. Together these observations make me suspect there's an additive that melts to a state where it flows much more than the base PLA at PLA printing temperatures, coalescing into more of a uniform smooth surface.
What are the likely additives in "silk PLA" filaments, and what printing/mechanical properties should we expect from them?
Some further observations: pushing it out of the hotend by hand, it has a lot of die swell, almost up to the original 1.75 mm diameter, and if tension is released it tries to retreat back into the nozzle. This suggests to me there might be some sort of foaming type additive involved, and also explains the behavior on overhangs.