When building our first full suit of armor, a common problem arose when affixing inflexible armor pieces to a flexible body (a person). What needs to move, what doesn't, and how to make such compromises.
We found there were two areas on the Mk49 that were going to need to stay flexible: the neck, and the boots. For the neck, we originally printed it in PLA and in testing found head turning was painful and very limited. For the boots, we figured a person's foot flexes in at least two major areas when walking and the sleek design of the Rescue boot did not lend itself to being sectioned.
We chose to experiment with flexible filament. After purchasing a roll of Ninjatek's Ninjaflex thermoplastic urethane (TPU) we made a small modification to our printer to print with this material. *These pieces were printed on our Anet A8 which is a direct-drive printer, versus our CR-10 which has a Bowden-tube extruder. Although this is possible on a CR-10, generally a direct-drive printer works better with flexible filament which is why we used our Anet A8.
Due to the flexible nature of TPU filament, it can easily bind in the extruder and cause a print failure. In order to combat this, we printed a filament guide from Thingiverse: https://www.thingiverse.com/thing:2366523 Search for your model of printer and you'll probably find something similar.
For settings, we did change a few things in order to print TPU:
Increased printing temperature to 230* C
Reduced speed to 30 mm/s
.1mm layer height - you can't sand and fill TPU like you are used to with PLA, so we'd recommend using the highest available resolution you can print with.
20% infill - this is mostly personal preference but the shape of some of these parts in our case required internal structure.
We split the neck into two pieces, a front and back.
The shoe/boot piece we printed in one section, trimmed based on how much would be covered by the shin.
Due to the flexible nature of the completed parts, we could not utilize the same painting process as the rest of the suit. We have great luck in painting cosplay parts that require flexibility with Angelus leather paints. This acrylic paint stays flexible and wears much better than most acrylic paint.
We custom mixed these colors to match the rest of the armor. The result is "close enough".
Using flexible armor pieces for joints and areas that require movement made a huge difference in our build success. Not only wearability and comfort were improved, but mobility and posing became much better. Being able to turn your head, stand on tip-toes for an "in-flight" photo, etc became possible without having to resort to building something that doesn't match out of foam.
A similar effect could be achieved by printing and finishing a part, then creating a mould and casting it in urethane rubber. This is a much more costly method which we were not interested in experimenting with at this time. Moulding and casting pays dividends when making multiples of one part, but for this build every piece would be unique so there would be nothing gained by mould reuse.
All in all, we had success using this method and are planning to continue using TPU filament for more of our cosplay 3d print projects.