Printer, Paper & Ink for 100% Poly Dark T-shirts

Well at least you can rule out standard sublimation.

You can use systems such as the OK I white etc. which lay down a white toner underbase, but is quite expensive, or you can try special papers such as Jet-Opaque, but I've never achieved good washable results with these.

For dark shirts, without laying out a fortune in equipment, I'd stick to htv or plastisol transfers.

Thank you Webtrekker

i agree with webtrekker,
if it is under 4 colors then vinyl, unless it would be too time-consuming to weed
over 4 colors then plastisol

I found this article, I think all of you guys have the true in hands...
https://www.howtostartaclothingcompany.com/screen-printing-plastisol-transfers/