PLA is a nice one, and gluing has been a topic on some of our most favorite maker's channels. For example Stefan from CNC kitchen (this video) and Joel the 3DPrinting Nerd (this video). Here some information from them together with my own experiences. Most of these glues are not exactly PLA specific by the way and work for many other materials too. Be careful with PLA containing infil though, as that can seriously alter the properties.
Step 0: Safety First!
Some of these methods are working with chemicals that can irritate the skin (resin, cyanoacrylate), have irritating fumes (acetone), or are flammable (acetone). Others (Cyanoacrylate) are not heat-stable and break down into their components under heat.
Use proper protection when working with glue! Eye protection and respiratory protection, as well as gloves, are to be used when necessary. Read the manual of the products you are working with!
For most glues, it is advisable to prepare the surface: sand it to increase the surface area, remove grease from fingerprints etc. Follow the manual!
- Cyanoacrylate - yep, the "one kind for all" is a solution for PLA too: Superglue. However, look out for what type you get! Some are clearly better than others, and using an accelerator can change the properties of the glue spot.
- Together with talcum powder, CA glue can fill gaps easily.
- CA is not stable under heating and when heated too much it breaks apart into a rather noxious fume! This can be handy to break metal-CA-metal bonds in machining of small parts but keep this in mind if you want to use inserts or plastic-soler pieces on the same areas! Don't tack with CA in those cases.
- Epoxy resin - Epoxy is a favorite for very tough gluing, a few droplets can stick a car to the roof... and it warms up in curing. If you take a slow curing resin, you can safely use it to glue PLA without the part deforming.
- Uncured Resin and their hardeners are strong skin irritans.
- Urethanes - 2 Component Urethanes offer strong but flexible bonds and work great according to Joel. Their curing process is also exothermic, so take care to not 'cook' your piece.
- 2-phase Putty - in a similar vein come 2-phase putties like Green Stuff or Miliputt, which harden after mixing. Their heat generation isn't too big and they allow to fill gaps easily. My favorite stuff though is not the expensive modeling putty but the stuff from the home depot: stuff like Pattex Repairsorry, no English site for this or UHU Repair All Powerkitt harden within an hour, are surprisingly cheap and get a smooth surface.
- Acetone - We all know that you can smooth and glue ABS with Acetone or an acetone-ABS slurry. Tom (Thomas Sanladerer) made a few experiments with it. He discovered that it works for at least some types of PLA in the following fashion: apply some acetone to a spot and press the second piece (that also was prepared this way) to it and they might melt themselves together after some time.
- Effectivity of this depends on the exact PLA you got.
- Acetone is highly flammable.
- There's a type of glues commonly called "Kraftkleber" or "Alleskleber" in Germany, for example, UHU Hart or Pattex KraftkleberGerman. While they often stick to PLA, I personally don't like their gluing power and find them often quite messy to work with.
- Wood Glue - Yes, Wood Glue. PVA Wood glue as well as its non-water-soluable cousin ("Express") have proven themselves to me as a rather nice surface coating to smooth over print-lines as well as a good solution to affix paper and wood to prints. It is less of a solution for plastic-plastic bonds but works OK-ish.
Glue? Why glue?!
What better way is there to combine parts than welding or soldering?! Often none. My personal all-time-favorite PLA glue is PLA itself, by using it as PLA solder. This method also works for most other filament types, but is not advisable for ABS and other plastics that emit fumes without wearing respiratory protection!
- Take the pieces and make sure on both sides is a cavity that can be filled.
- Take a soldering iron and set it to around 200°C.
- Take a length of PLA filament.
- Melt the filament with the soldering iron and use it as solder when combining the two pieces. Make sure that at least some filament gets into the cavities and sticks there - it can help to stick the soldering iron into the goop in there to force it to merge with the infill/walls and press together to hot PLA goop filled pieces against the iron before pulling it away, pressing the pieces together.
- As the PLA cools and hardens, the joint is usually tougher than the actual layer boundaries.
Instead of using a soldering iron, one might also use a 3D printing penOne that eats filament, not one for PCL or some gel!, but I don't like those personally.
On a different note, a soldering iron is also a very good solution to make inserts into PLA - heat up the metal insert (like a nut) and press it into an undersized hole, and it will mold the plastic around itself into a perfect fit without any glue.
An alternative to using direct heat from a soldering iron is friction welding. For this, take a rotary power tool and some filament. Insert the filament into the tool, tighten and cut so that about an inch is reaching out of the claw. Turn it on at medium speed, about 800 to 1200 RPM. Now, once you press the tip of the spinning filament against other PLA it gets hot and melts, creating a welding seam. Joel has a good explanation.