I'm new to 3D printing and I was wondering about the risks of leaving my printer to print overnight? I'm aware that if something goes wrong I'll wake up to spaghetti for breakfast but what are the other things like having the nozzle and bed heated that long? And, when it's done it just sits there on so what could that do to the screen? What if the printer runs into some physical problem and damages itself and/or the print? I'm not looking for answers to these questions specifically just feedback on what I should do when printing overnight.
Presuming that you're talking about an 8 hour period, your printer should be designed to run for 8 hours continuous anyway, so nothing will happen regarding the bed or screen that wouldn't happen with a normal print.
If the first few layers stick to the bed, it's likely that you're print will at least be partially completed. So even over night it won't be a full 8 hours of printing while failed. Maybe half that period.
If the problem is bed adhesion, or anything that doesn't effect the filament being supplied to the nozzle, then your only problem will be wasted filament and disposing of the spaghetti. No harm will come to your printer.
If the problem is a break in the filament or filament runout, or a blocked nozzle then you could have damage to the head of the nozzle from printing dry. If you're printing with PLA this isn't really something that you need to worry about too much as you can run a printer dry for several hours without any problems.
If you're printing with ABS or something that needs a hotter head then you could cause damage to it if it's allowed to run dry for an entire night. But again this isn't really something that you need to worry about unless it's running dry for 4 or 6 hours.
Simply checking in on it once in the night should be enough to prevent any problems.
I've done 4 day prints with no issues.
I just have my printer in a place where there is nothing that can catch fire in the unlikely event that it decides to start burning.
For the rest it's just like any other electrical appliance. Common sense like not having it where things can obstruct moving parts etc,.
Assuming the printer is operating correctly, there are no lifetime/wear concerns from running it that long. Many-hours and even -days prints are normal usage, and print-farms run as businesses run these things basically 24/7 for months on end, modulo routine maintenance. Having the heaters on for 8 hours is really no big deal.
Of course, that's assuming the printer is operating correctly. If not, there are various things that can go wrong.
The worst possible is thermal runaway: the printer losing track of the bed or hotend temperature and running the heating element always-on until it melts the metal. This is a severe fire hazard. Printers with properly built firmware have thermal runaway protection to perform an emergency-stop and turn off heaters if they can't detect a reasonable temperature measurement response to operating the heater, but some popular machines have this feature intentionally turned off by the manufacturer. If you will be operating your printer unattended, absolutely make sure you test that thermal runaway protection is present and working (there should be good ways to simulate it and see that the safety is triggered).
Even with thermal runaway protection working, there is a very slight chance the power mosfet controlling a heater fails in a way that leaves it always-on, which the firmware cannot protect you against. For this reason you also want to have the printer away from flammable materials and may even want an active fire suppression system (there are "balls" you can hang above your printer that activate and release fire suppressant if it's on fire). This is a very low probability event - I've never heard of it happening - but I'm including it for completeness.
Now, the less severe stuff. As long as the first few layers go down well, it's unlikely your print will detach from the bed mid-print, but it can end up having (usually small) parts that warp upward and collide with the nozzle. This can cause lots of different types of problems:
Dislodging the print from the bed
Producing skipped stepper motor steps, which later can cause the print hed to collide against either end of its motion range due its logical position mismatching its new physical position.
Breaking parts of the toolhead, like fan ducts, fans, bed probes, silicone socks, etc.
Causing extruded material to be forced upward into the area around the hotend (e.g. between the sock and the heater block), producing a giant blob of plastic intertwined with toolhead parts and difficult or impossible to remove without damaging stuff.
Most of these things do not impose any major safety risk like fire, but they can do varying degrees of damage to the printer, requiring repair or at least maintenance. For example endstop collisions themselves usually don't damage anything but they might knock your frame out of square/alignment, requiring tinkering before the next print.
There are software solutions like "Spaghetti Detective" (recently renamed to "Obico") which can watch your print via a camera, and potentially stop the job if it looks bad.
Most of the time my print failures come early, in the form of poor bed adhesion - watch the job start for a while before leaving it.
I can also remote-check my cameras and stop the job if required, but that requires me to look.
The second most common failure is lifting from the base at any time in the print, so you have to keep an eye on it.
Filament feed issues are probably third on the common list of causes for problems, including running out.
I also have occasional issues where my Pi running Octoprint just looses connection to the printer, and it freezes in place with the bed/nozzle heaters on. This is just a waste of power, so a future plan is to wire the printer through a relay allowing me to power it on/off from octoprint's web page.
My longest print was ~30 hours - you have to learn to not touch it unless there's a good reason.
Finally, try and maintain the environmental conditions to be fairly constant. I have an enclosure even when printing plain PLA, becuase my printer's location is in a garage, and opening the main door allows the air temp to drop quickly. This would upset a running print until I surrounded it.
You can also make sure there's a smoke or heat detector in the room where the printer is, for added peace of mind.