If you really want to tempt your fate, just start an overnight print. I mean, what could possibly go wrong? Or maybe your print is taking longer than expected and you simply want to get some sleep, so you decide to stop all the noise. Well, we all know what will happen just that one time...
Whatever the reason, you're now left with half finished prints that you would normally throw away. You'll be happy to know that you can save most of them. Using a soldering iron, you can actually salvage just about any failed print. Let's see how.
Grab the unfinished object and measure its height as precisely as you can. The top of the object should be flat and clean, so cut away any debris if you have to.
Load the model in your slicer and then move it in the Z axis. Enter the height that you've measured, but use a negative number. This will effectively cut away the bottom part of the object that's already been printed, so that you will only be left with the missing upper part. Use the same settings as before and let it print.
Check if the parts fit together and do a bit of sanding if you have to.
If you have a larger surface, you can simply glue the parts together. This might leave a visible gap, but you can fix it later with a soldering iron if it really bothers you too much.
If you do decide on using glue, then use the accelerator as well. You just spray it on and the glue will cure in seconds. Once you try it, you'll never go back to using super glue without it.
Right, let's see how you can use soldering iron to weld the pieces together. You don't need anything special, the cheapest soldering iron will do just fine. I'd even recommend buying one of those simple irons that don't have any controls at all and just plug in into the outlet. Save your good iron for actual soldering. I do recommend getting a flat tip though, it really makes the process faster.
Heating it to around 200'C is a good starting point. Too high and the plastic will burn and setting it too low will barely melt the surface.
Then simply work your way around the object and slowly weld the two pieces together.
How strong are welded parts? Very strong. You can use a lot of force and it still won't break. Banging it against the table doesn't even make a dent.
Here are a few examples that I did in the past. You might notice the weld up-close, but it's barely visible and you can always lightly sand it to hide the joint.
If you have a larger gap, then you can actually use a piece of filament as fake solder. This also works great for repairing extensive damage.
Here's the amount of waste that I've collected in only half a year of printing and I'm sure I'm not alone.
Salvaging as many prints as you can will really save you a lot of plastic and it reduces the pollution as well.
I hope that was helpful and don't hesitate to get in touch with any questions you might have.