Coloring UV resin

In theory, we can use pigments to colour or tint UV resin, either as dry powder or as liquid dyes or dyes with a liquid carrier. In practice, there are complications:

• If a very effective dye stains the FEP film at the bottom of your VAT, you may have to replace it.
• Pigments that are very opaque may interfere with the ability of UV to penetrate the mix of resin and pigment, requiring longer exposure times and longer post-build cure times.
• Some powders, such as titanium dioxide white, are quite abrasive (I don't know whether TiOx added to resin can scour the FEP film, but haven't felt inclined to try).
• Some mineral pigments may scatter the UV light, causing a lack of precision in the resulting prints.
• Some mineral pigments may be fluorescent! In other words, they may be quite efficient at absorbing UV light and reemitting it at another colour. Not ideal.
• Some powder pigments may have trouble staying in suspension. In theory, one can mix mica pigments with resin, in practice they tend to settle out and sink to the bottom of the vat over the course of a few hours, which can be the amount of time needed to make a larger print. Mica powders also seem to seem to align their flakes horizontally, perhaps as a side-effect of the model pushing down on the FEP film. Adding gold flakes to a clear resin can give a nice gold effect on all the “final” horizontal surfaces, but leave the other five sides of a cube looking a muddy brown.
• Adding too much powder to a resin alters its mechanical properties. The final model may be softer, weaker, and more crumbly. It might not even print successfully.
• Water-soluble dyes are probably no good for non-water-soluble pigments. If you add a water soluble dye to a water-soluble pigment, bear in mind that you are adding water! The final print may later shrink and perhaps crack and crumble as the included water eventually evaporates, or may partially dissolve when you try to water-wash it.
• Alcohol-soluble dyes might work with non-water-soluble resins, but again, bear in mind that if you are adding alcohol to an uncured resin that can be washed away with alcohol, you are preloading the resin with solvent that may predispose it to being washed away during cleaning, and that the final print … incorporating trapped solvent that will eventually evaporate .. may not be mechanically stable.
• Also bear in mind that a commercial UV resin is not a single substance, but a complex mixture of chemical components (polymers, monomers, fillers, activators) that have been chosen to interact together in a specific way. This is why you’re advised to always shake a bottle of resin before using it, as these components may start to separate out over time. Even if an additive doesn’t alter the reaction chemistry, it may affect the microstructure of the final cured resin, changing its mechanical properties – perhaps for the better, but more likely for the worse.

Experimenting with dyes

If you want to experiment with dyes or pigments, it’s a good idea to use small batches of resin, and perhaps to wait until your FEP film needs replacing anyway, in case it gets stained or damaged. If using powder or mica powder, this may wedge itself into the cracks around the edge of your FEP film, and you might not be able to get it out without changing the film. This will normally be outside the print area, but may mean that the film surface is not quite as perfectly flat as it was before.

Bear in mind that your failure rate will likely be higher, that you may have to experiment more with exposure settings, and that the resulting successful prints might not be as stable, and might develop problems years later (which is more of a problem if the prints are being sold commercially).

Mixing coloured resins

More adventurous users might decide that the opaque (or the clear) resins from a single manufacturer //might// be based on the same base formulations and might only differ in the colour additives, and might then decide to take the risk of mixing these colours together (say, darkening a grey opaque resin by adding some opaque black from the same manufacturer) and crossing their fingers.

In the world of household home decorating paints, the manufacturer of a range of, say, egghell finish wall paints, may use the same water/resin suspension formulation across the whole range and only vary the added pigments. This simplifies the production process and allows customers to mix paints. But what one shouldn't do is mix paints from different ranges, or different manufacturers, in case the different formulations interact badly, producing mixtures that clump or curdle, or refuse to set. Similar rules probably apply to UV resins.

CMYK-mixable resins

It is occasionally possible to find four-packs of bottles of compatible UV resin in CMYK – Cyan (a blue), Magenta (a pinky-red), Yellow, and “Key” (black). These can be mixed together to produce pretty much any colour that you could achieve using CMYK printing. The price that you pay for this convenience is literal – these "niche" packs are extremely expensive.

Dedicated resin dyes

Monocure3D make a range of resins, and also make a range of compatible liquid pigments, available in packs of four 30ml bottles, in CMYK colours. While only certified for use with their own white resin, the adventurous may want to experiment with these colours and existing stocks of “standard” alcohol-washable white from other makers. If one doesn’t already have a large quantity of white that needs using up, and there’s not too much of a difference in price between the Monocure resin and what one normally buys, it may be worth buying the Monocure white to use as a base, if only for the knowledge that this combination has been properly tested.