As an alternative to hand-painting, spray painting, or transfer film, models can also be created directly using color printing on filament printers. This is possible for all model sizes down to 1:160 scale (220?), although the requirements naturally increase for smaller models, even if many parameters remain the same.
I would like to explain the process using an Erie Lackawanna "Thru Line Coach," a modernized heavyweight passenger car.
Several prerequisites must be met for success. You should be proficient in 3D design, especially in dividing a complete model into subassemblies.
The next step is to be familiar with the "slicer," which generates the gcode for the printer. A 0.25 mm nozzle is recommended for N scale. This allows the layer height to be set to 0.1 mm normally, but also down to 0.03 mm for curves.
Before attempting a color print, you should have successfully printed a monochrome model.
Now you need colored filament. It's important to ensure that not only is the color an exact match, but also that the filament isn't slightly transparent, otherwise sharp color separations won't be possible. Living in Europe, I use ColorFabb PLA, matching the Industrial RAL color chart of German origin.
Now we can really get started: Import the STL file of the car body into the slicer and set the layer height to 0.1 mm (.0039 in) which is then the minimum line thickness. Generally, lines of 0.3 mm (.011 in) gave a good result, smaller need some testing, they may simply dissappear for our eye.
The colors chosen are RAL 7042 gray, RAL 1018 zinc yellow, and RAL 3005 wine red. With very few exceptions, American railroad colors can be reproduced very accurately using RAL, as I've personally verified in several railroad museums in the USA.
Using photos and drawings, the color scheme is then transferred to the model. In this case, gray is printed up to 0.35 mm (the first layer is always 0.15 mm), then yellow up to 0.65 mm, gray again up to 5.65 mm, yellow up to 5.95 mm, burgundy up to 11.86 mm, yellow up to 12.15 mm, and the rest is gray again.
A skirt is printed around the actual part, fulfilling three functions: keeping the print at a consistent temperature, keeping the nozzle clean, and, most importantly, catching the inevitable drips when starting the print after color changes.
I do all color changes manually; this saves plastic and allows for high precision. When switching to lighter colors, enough material must be spooled through to achieve a pure color.
After printing, the car body is cleaned of any imperfections, optionally with a heat gun (rarely), a cutter, sandpaper, a wire brush, etc. PLA is very durable, and since the color is like "mass dyed" even the finest stripes are not damaged during post-processing.
Next, I printed all the remaining parts: the chassis, the bogies, the bellows, the seats, and the roof (in two parts), all in the final color. I also added window panes made of clear plastic, laser-cut.
I created the same model in two other color variations: one for the ERIE, in two shades of green with fine yellow stripes, and one for the DELAWARE & HUDSON, in gray, blue, and yellow. In these versions, the ends are a single color, so I logically separated these components and attached them to the multicolored center section. The glue joint on the two-part roof is slightly visible; I might add some filler and spray paint if it doesn't disappear with weathering.
Making one car takes about a day, with the actual color printing taking three hours. Except for the MTL wheelsets, the model is entirely scratch-built.
Software and hardware used: Cinema4D, Formware Online STL repair, Prusa Slicer 2.9, Prusa mk4 / 0.25 mm nozzle, ColorFabb RAL PLA, wire brush, cutter, CA glue, sandpaper
