Why Do My Prints Look "Dull"? COEX 3D

Why Do My Prints Look "Dull"?

Sometimes when you're printing functional pieces, the finish of your printed part isn't very important. Other times you really want the print to look a certain way. Part of that look is how shiny or matte the part is.

We usually think of shininess or matteness being a fundamental property of the filament itself. Manufacturers kind of encourage this by the fact that specific Matte and Silk filaments exist that accentuate these features. But did you know that you can take the exact same filament and simply adjust your print settings to change how shiny your parts are? First, let's talk about what causes the finish to change in the first place.

What causes this?

FDM 3D printing is a pretty simple process, when you get down to it. A thermoplastic is fed into a hotend, it melts, gets deposited where you want it, and then solidifies again. To explain the dull vs. shiny phenomenon, we have to look closely at what is going on in the hotend itself when the filament is being melted and pushed at the same time.

When extruding any material, thermoplastics included, there will be shear forces. A shear force is one that acts parallel to the filament and causes it to slide against itself at different rates rather than evenly and smoothly. This is caused by the extruder pushing the filament through the nozzle while the nozzle itself is exerting friction on the filament. This shear force is the root cause of dullness in a finished part, as it causes slight variations in the surface of the part which scatter light and cause the part to appear dull or matte. See below for a diagram illustrating shear forces.

A diagram illustrating shear forces in a 3D printer nozzle

Maybe you've noticed this effect doesn't happen with slower printers and only with high-speed ones.This is because when printing slower and not extruding very quickly, the filament spends plenty of time in the hotend to completely melt. Where this effect comes into play is typically with much higher speed printing setups, such as on a Bambu Lab X1C or a Voron or RatRig printer that's been built for speed.

Due to the high extrusion rates when printing at speed, the filament simply doesn't get time in the hotend to melt completely, increasing the shear forces. This then causes a part to appear dull or matte.

How do we reduce shear and therefore dullness?

The basic goal when reducing shear force is getting the filament to become more evenly liquefied in the nozzle. This can be done one of two ways: Increasing print temperature or lowering print speed.

Looking at an example below, these were printed in Coex 3D Magenta Pink PLA on a Bambu Lab X1C. The part on the left was printed at 220°C and the part on the right was printed at 245°C. The change in finish is very apparent, with the part printed at a higher temperature being significantly shinier.

Two 3D printed models, the one on the left being matte in finish, while the one on the right is noticeably shinier

245°C might seem like a very hot temperature for PLA, and well above our recommended printing temperatures for this material. But when printing with high speeds and extrusion rates, the filament won't have time to fully reach this high temperature.

Another option is lowering printing speed. This might feel strange given the goal of these higher-speed printing setups is well... speed! But there's a way we can get our desired effect without sacrificing too much print time.

Specifically, you can lower printing speed on just the external perimeters, as that's what you're seeing on the outside of the print. This way you can get the aesthetic you're going for without sacrificing too much print time!

Exactly what temperature and speed to use will take experimentation. Increasing temperature in steps of 5-10°C is a good start, and decreasing speed in 5-10mm/s steps is a good way to go about it too. If you have a slower printer that already gives you the effect you want, you can always use that printer's external perimeters speed.

Why does one filament exhibit this effect more than others?

We've heard that in some cases, it's specifically Coex 3D filament that exhibits this effect. Even at high speeds, some filament will still come out with a glossy finish. This comes down to filament manufacturing.

It's important to understand that thermoplastics have "thermal memory". That is, when the material is melted and allowed to solidify again, it will melt at a lower temperature next time. Repeating the melting and solidification process continues to lower the melting point of the material.

At Coex 3D we use only virgin resin to make our materials, without any filler that has been processed before. This means that the resin we're using has much less "thermal memory" and therefore melts at a higher temperature. You may have been clued into this if you've noticed that our printing temperature recommendations (210°C - 225°C for PLA) are a bit higher than other manufacturers' (often 180°C - 210°C or thereabouts). This is because the budget brand filaments often use previously processed material, which lowers the melting point of the filament. This also has the drawback of making the filament less reliable. If you've noticed that you have to tweak print settings even though you're using the same filament from the same manufacturer, just a different batch, this is why.

So because our filament has less "thermal memory", it melts at a higher temperature and therefore is more prone to increased shear when printing. This then leads to the dullness that other filaments may not exhibit.

Conclusion

Now you know what causes a dull or matte effect in printed parts, how to solve it and why our filament does it while others might not. We hope this guide helps you get exactly the look you're going for in all your prints!

If you have any questions or want to better understand how to get your parts looking perfect, feel free to reach out to us at support@coex3d.com

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