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On several occasions I've wanted lettering/numbering printed as part of a design, but with the ability to make it stand out more effectively in the printed object without having to do detailed manual finishing. Is there a good material which can catch in and fill sunken (depth 0.4 mm, width 0.6-1.0 mm) lines/strokes of alphanumeric characters without sticking to the surface (including fine layer ridges) of the print? My best result so far has been with crayon wax, but I wonder if there are more suitable materials. (Polymer clay, perhaps?)

Results with crayons:

3D printed D20s with numbering filled with crayon wax

Durability is nice (and essential for some applications), but for many uses I have in mind it's not such a big deal. For example another place I've wanted clear text is on test panels to check nut/bolt thread sizes, in which case the text is unlikely to receive harsh treatment but any heat-based curing processes might effect the dimensional accuracy negatively. So both durable and non-durable solutions are interesting to me.

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I think your instinct is good. The trick is finding the right modeling compound, with the right curing properties, that will stick to your polymer dice, and will be available in colors you like.

Amazon has some epoxy-based clays that appear to be either terra-cotta or black. If these colors work, this may be the right choice. Most epoxies will cling well, and it seems to have minimal shrinkage when hardening. You could probably add some pigment and alter the color, but I doubt you could get a white, yellow, or any light color.

There are some air-drying clays, but the reviews show that these have higher shrinkage.

There are some bake-to-harden clays that harden between 250 and 300 degrees Farenheit. If you printed with ABS or a high temperature plastic, it might work. You would have to try it to know. 300 F is only 149 C, so maybe. PLA wouldn't stand a chance. If you can print polycarbonate, it might work better.

This material looks interesting: https://www.amazon.com/Polymer-Modelling-Modeling-Tutorials-Accessories/dp/B0716D1VDM?ref_=fsclp_pl_dp_6 There are lots of colors, and it can be cured in a 275 Farenheit oven, with a hot-air gun, or by boiling in water.

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  • $\begingroup$ Thanks. Note that while dice were an easy go-to example here, I've also had occasion to use inset text for thread size test plates and other places where durability (curing) of the text isn't as important as avoiding processes that might affect the dimensional accuracy of the printed piece. But I think you're right that these types of "clays" are probably the right direction for cases where I do want a durable result. $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Aug 26, 2019 at 18:39
  • $\begingroup$ IIRC there are polymer clays that are supposed to be cured at only 125-150°F, which might be compatible with PLA... $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Aug 26, 2019 at 18:41
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    $\begingroup$ One works out a process and the number of places it is applicable is amazing. I have a multi-material Prusa printer, so I first went to "use multi-material", but I actually think for this purpose filling gaps with clay could be far better, and faster. $\endgroup$
    – cmm
    Aug 26, 2019 at 18:41
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    $\begingroup$ The boiling water cure is interesting since in my experience ABS stands up to boiling water. $\endgroup$
    – cmm
    Aug 26, 2019 at 18:43
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    $\begingroup$ @Davo: And it works quite well! See my answer. :-) $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Aug 27, 2019 at 3:23
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As suggested by Davo in a comment, nail polish works very well for this.

I've seem people use fingernail polish before, wiping the excess off while it's wet.

What works even better than wiping the excess off while it's wet is letting it fill up the pits and fully dry, then using acetone or other solvents to remove it from the non-sunken surface around the text. Acetone doesn't attack PLA, but I've found it does alter some PLA dyes, so alternate solvents might be preferable if that's a problem.

Here's the result of my first test:

White D20 with red nail polish filling numbers

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