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Without a heated bed its difficult to impossible to print materials with high thermal expansion coefficients, as those tend to warp, leading to deformed prints and parts getting loose from the print bed.
Personally I stick to materials that don't need a hbp (PLA, PET(G)) and I'm happy with it.

Scaling up the i3 design to 500mm is harder than you might imagine. The stronger shafts that you'd need alone would ultimately lead to a complete redesign of the whole thing.

Regarding large heated build plates, I see some problems associated:

  • power requirement: a 500 mm square print bed would have a rather high power requirement (750 Watts or so)
  • thermal expansion makes most print bed materials bulge, leading to an unlevel printing surface. with large print beds this becomes worse, requiring quite elaborate solutions