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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.

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It has additives to the PLA to change it's characteristics.

Typically, “silk” filaments are enhanced PLA filaments, owing their glossy result to various additives. As such, they tend to show most of the same pros and cons of PLA filament. In the case where silk filaments have a different base, these would be expected to have similar properties to the base material. For this article, we’ll focus mostly on PLA-based silk filaments. -- https://all3dp.com/2/silk-3d-printing-filament-brands-compared/

The article goes on the say that this does modify how the material prints and thus may need different parameters on your printer from normal PLA. You can get silk PLA in different brands. There doesn't seem to be a standard composition, so each brand my behave differently.

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Silk PLA is a descriptor

Silk PLA is a description of how the PLA looks. It is a separation from Glossy and Matte that is taken over from paints: Gloss, Silk, and Matte are the three general types any paint can come in, indicating different reflectiveness of high, intermediate, and low respectively.

As a result, Silk-PLA is designed in such a way to get some sheen without looking all shiny/glossy.

Most PLAs are modified with various additives anyway to achieve good printing or specific looks, silk PLAs use theirs to get the sheen often attributed to silk. As a result, you need to tune for each material, brand and color combination anyway.

Further reading

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