Best HP printer that can print 18' x 24'?

In the very pro, industrial category HP might have a solvent printer that is 5.5 meters wide.

For 18"x24" one could use the Designjet Z3200 24" wide with pigment inks that do not fade or the Designjet 130 with dye inks that last less long. Pro and sub-pro category. The first will deliver more consistent B&W prints too.

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Met vriendelijke groet, Ernst
500+ inkjet paper white spectral plots: OBA content etc.
http://www.pigment-print.com/spectralplots/spectrumviz_1.htm