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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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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. A common additive that gets a nice sheen in paints is Mica, but most PLAs are made silky-shiny by adding a slight bit of various TPE - Elastomers. That is not one but a whole group of materials that is added!

As a result of being a polymer mixed with additives, you need to tune for each material, brand and color combination anyway.

Further reading

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  • $\begingroup$ In terms of PLA 3D printing filament, the term "Silk" is NOT simply a description of how it looks. It refers to a specific set of additives that have been mixed in with the polymer (usually some form of lipid) to give it a shinier surface texture and a smoother feel. $\endgroup$
    – Mad Scribbler
    Commented May 4, 2022 at 20:31
  • $\begingroup$ TPE is not one but a whole family of thermoplastic rubbers, and you could create a silky, but very abrasive filament by adding Mica from the paint industry - which exists in natural and synthetic ways. What blend of TPE is added by which company or if they use a special pigment (also an option) is company secret. $\endgroup$
    – Trish
    Commented May 5, 2022 at 5:36
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It has additives to the PLA to change its 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.

-- Silk PLA Filament: Best Brands

The article goes on to 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 may behave differently.

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