First off, this was an experiment that ended with effective but unattractive results. I continue to try to experiment and improve the results. Also, please share your results if you try this.
Why: You can glue ABS with ABS/acetone glue , but what do you do for PLA? I had an idea to use a soldering as a welder while looking at a multi part mask print on Thingiverse. Ultimately, a strong weld could be much stronger than the rest of the print due to the larger area of solid plastic. I feel that this will work better than using other gluing methods depending on the requirements of the project.
I printed a case for my Smoothieboard last night that had warped a bit and figured this would be a great opportunity to test this process.
Tools: Soldering iron with adjustable temperature.
Temperature: I started at 210 because that is what I usually use to extrude PLA. If you go to high, you risk burning the plastic. I ended up going up to 230 to get more heat into the plastic and did not burn it. I haven't tested the upper limit.
Work area: I used my helping hand to hold the project in place but other things could work.
The goal of this process is to melt both pieces to be joined and the extra filament all at the same time to fill the gap and create a single piece of plastic.
The initial heat of 210 was not effective. I upped the temperature to 230 with slightly better results. I may test again with a higher temperature. Possibly using a temperature that can burn the plastic but moving fast enough to avoid the burning would work best.
Results: The part is plenty strong. The welds are a bit ugly. I couldn't get the plastic to flow like with a traditional welder so the joints are a bit ragged.
I have a few ideas for improvements.
Heated work area: Working with a heat gun or something to apply heat to the weld area would keep the weld from cooling to fast and may allow the plastic to flow for longer.
Preheat: Same Idea as above but preheating the joint with a heat gun and than keeping it warm.
Hotter Iron: I may try a higher temperature that could burn the plastic but keep it moving along quickly to get a better bead.
Larger Iron or different shape head: I have a cheap soldering iron/wood burning kit with several different shapes of heads. One with a flat triangular or teardrop head might work better to apply heat to a wider area. Unfortunately, I don't have a temperature control for that one but I'll see what I can figure out.
If I eventually find a good solution tho this I will make a better Instructible with more definitive guidance. for now this is still experimental.