What were the pixel dimensions of the image that you took to the printer service? The 300 PPI setting really has no bearing. Iregardless of what that setting was, the image still only has the same number of pixels.
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The printing services are professional and are saying that something is wrong on my side...are they right?
No they are wrong for sure. The jpeg you have above has nothing like that in it, so the lines are caused by their process. I opened your jpeg in photoshop and played with curves and I can't get any lines like that to show up. They're just not there in your file. Also since this looks like it is perfectly along the printing direction, it is highly likely that they have a problem with their ink head. That's not uncommon especially printing black and white if the printer they are using uses only one or two black/grey inks.
P.S. looking at the image, it looks like the preset you used does not use the full range of blacks available. Basically it causes a very washed-out look because of that. Was that your intention?
Thank you for the detailed reply.
My intention was to simulate a slightly vintage look, with blacks being a little less gray.
Could you recommend what I should say to the printing services so they do it correctly? Maybe they should print it in colour mode rather than black and white? Ive simultaneously printed a number of colour prints and they came out ok, however were much smaller in dimensions.
You can try that indeed. If this printer does not have a set of dedicated photo grays and only uses a single black ink, printing in black and white mode often gives these bands on the prints. Sometimes cleaning the head helps in that case, but sometimes it can be a limitation of their printer and cleaning won't help. It can also be a symptom of their print head being slightly out of alignment which is fixable. Epson has a utility you run and other printer makers have something similar. Your idea of printing in color mode will make it use its full ink set and likely will make these bands less visible. You do often end up with a slight color cast if you do this though. How visible that is depends on how good their color profiles for their printer and papers are.
P.S. this is a well known problem and they really should know about this. See here for an example: Fixing Banding in Fine Art Black and White Inkjet Prints on the Epson Stylus Pro 3880 & SureColor P800 – Field and Studi… The photographer there gives some tips that might help.
Also, if this is an Epson printer they should follow Epson's instructions here:
Hope this helps.
Will read into this!