printing with a 0.25" nozzle  

Re: printing with a 0.25" nozzle

[...] what is the reason for such a limitation? May I print during more than 24 hours with such settings?
It just going to print very, very slowly. Think of a print as a volume of plastic. Whatever nozzle or speeds you're using, your printer has to move that same amount of plastic in order to produce your print. The smaller the nozzle, the narrower your extrusion width and lower your layer height. The amount of plastic moved per second is layer height X extrusion width X speed. At 0.25mm, your layer height and extrusion width are limited to about 40% lower than when using a 0.40mm nozzle. Let's use an example of printing at maximum layer height and extrusion width the nozzle can reliably support at 50mm/s:

Even though the 0.25mm is a bit more than half the size of the 0.40mm nozzle, it's going to move significantly less than half the volume of plastic at the same speeds. It's the square-cube law at work. The nozzle produces extrusions that are smaller in multiple dimensions. (See my signature below.) You can conceivably make up some of this by increasing speed, but quality starts to suffer and there's a maximum at which the printer itself can move.

So for practical purposes, the smaller nozzles are going to produce prints significantly slower than larger nozzles. My general rule is to use a nozzle as small as needed to show small details, but no smaller. If your smallest detail can be shown with a 0.30mm nozzle, you won't gain much using a 0.25mm nozzle.

Re: printing with a 0.25" nozzle

Thank you for these very clear clarifications.
I am not in favour of increasing the speed which reduces quality and could generate extrusion problems in case of thin nozzle.
So in the end, it's just a matter of printing time. But is it really a problem if you are looking for high quality?
I wonder if the printer can handle such a so slow printing for such a long time?

Re: printing with a 0.25" nozzle

[...] So in the end, it's just a matter of printing time. But is it really a problem if you are looking for high quality?
It's not a problem so long as you are actually getting better quality. I lurk in many of the 3D printing reddit groups, and I see a lot of posts by people excited by printing miniatures who automatically assume that if a 0.25mm produces good results, 0.15mm will be even better. These posts are often followed by "I can't get good prints with a 0.15mm nozzle" threads. The smallest details on my mini prints show well with a 0.25mm nozzle, some are lost with a 0.30mm nozzle, and using a 0.20mm nozzle doesn't add anything.

In the end, it depends on what you're printing and what your parameters for success are. I can print a 1:100 miniature that looks good using a 0.25mm nozzle with 0.10mm layer heights in 3 hours, or go with 0.05mm layer heights for 6-7 hours. In the end, they look the same at tabletop distances to me, so there's nothing gained by going "ultra-detail". 24 hours is the upper end of my patience for prints, but I've seen beautiful work done that took 40+ hours.

I suspect E3D puts that language there so they have a "told you so" out when uninformed buyers complain about print times.

Re: printing with a 0.25" nozzle

You are right. interesting lighting. I'll take that into account because it looks to make sense.
Just to set the contours of the discussion:

Here is a loco scale 1/160: 11cm/1.5cm/2cm. Small but beautifull ……….if well detailled print.
Based on Cura, the printing time is about 19h with a 0.25 nozzel, 0.08 layer and 10mm/s with HIPS filament. Just acceptable but not as expected. I am not able with my current printer (not yet a PRUSA) to go down under this nozzel size. My dream was to succeed to use their E3D experimental nozzel and greatly improve the surface quality as well as the details.
Indeed, I expect that the PRUSA printer will allow to obtain a print of the desired quality with a 0.25 nozzle. That's what you're finally talking about. Good point.
On the other hand, my stress concern is about security when the printer is working without any physical presence (of course during a so long printing process). Fire is not an accident that only happens to others.
Tks for your input
Philippe

Re: printing with a 0.25" nozzle

If you design your model well, you can assemble it from parts, so the detailed parts could be done in 0.25 and the larger slabbed parts could be printed with a 0.4 nozzle, that way you get the best of both worlds and since you have now broken the printing down to smaller parts, you would not be printing each part for extremely long times.
Just think about how they would build that train for real, the side would not be a single continuous sheet of metal but smaller panels riveted together, this could be a natural join line for parts printed and would look more authentic than a single slab.
Just an idea for thought!.