While, in principle, there's no reason a good functional helmet can't be 3D-printed ... more than a few examples of step stools on 3D printing model sites. These will come with exact instructions ...

The entire helmet is 3D printed as one piece using a semi-transparent PLA filament with NeoPixel strips (144 pixel per meter) laid inside. It takes about 3 days to complete the printing job ...

At this point, everyone knows that the print quality you’ll get from even an entry level UV resin printer far exceeds what’s possible for filament-based fused deposition modeling (FDM) machines.