3d printed PLA is notoriously difficult to glue, but I have discovered an easy-to-handle, non-toxic glue that works really well with it: Polycaprolactone. I get mine in pellet form for about $20/lb — a search on Amazon for polycaprolactone will turn up several different brands, and you can also find it at makershed.com.
Here, you can see a 3d-printed bracket that cracked due to poor layer orientation. No big deal — I just heated up a couple of pellets of PCL in hot water, pressed them into the gap like plumber’s putty, and squeezed the parts together. 2 minute fix, vs a couple of hours to re-print…
The color even matches pretty well! I didn’t bother to clean up the squeeze-out, but that would have been easy enough to scrape off while the glue was still warm.
As another example, I used a single pellet of PCL to carefully glue each gear shaft of this elliptical gear set.
Elliptical PLA gears, with a single bead of PCL to glue the shafts together.
It wasn’t hard to keep the gears from locking up — I just spun the gears for the 30 seconds or so that it takes the glue to cool off.
[Edit: Updated to add PLA and ABS to the list of compatible materials]
Polycaprolactone or PCL is a white plastic that is very hard and slick at room temperature, but melts to a putty-like consistency when heated under very hot water. Make Magazine has an excellent article summarizing some of PCL’s many uses, and in fact that article is where I first learned of it.
Depending on your application, PCL’s tendency to stick to other plastics may or may not be desirable. I compiled this table of compatibility based on my own experiments:
Material Type
Common Uses
Result
Notes
PLA (Polylactic Acid)
3d printing
sticks very well
Be careful not to overheat and distort PLA
ABS (acrylonitrile butadiene styrene)
3d printing, Lego
sticks very well
Custom Lego!!!
PVC (Polyvinyl Chloride)
white plastic pipe
sticks very well
preheat pieces for best bond
CPVC (Cross-linked Poly Vinyl Chloride)
yellowish plastic pipe
sticks very well
preheat pieces for best bond
Polycarbonate (aka Lexan)
very strong glass substitute
sticks very well
preheat pieces for best bond
PEX
flexible water pipe
does not stick
PET / PETG
soda bottles
does not stick
Metals
copper, steel, aluminum
does not stick
Brands
Polycaprolactone is sold under a number of brand names including InstaMorph, Shapelock, Friendly Plastic, Missing Link plastic, etc.
As far as I can tell they are nearly identical products, with the exception of Missing Link — they sell a “flow formula” that is a bit less viscous when melted, which may be useful for some applications.
Two PVC pipes joined with Polycaprolactone. This joint can be undone with gentle heating.
Fans of PVC pipe know that once a PVC joint is cemented together, it is impossible to take apart without destroying the pieces. Until now. The formula is simple:
PVC + Polycaprolactone = Awesome
Poly-capro-what you say? You can read More about Polycaprolactone aka PCL here. The awesomeness of PCL is well documented by Make Magazine in their article The Many Uses of Shapelock, but I have discovered one important use that they missed!
While trying to use a bit of PVC as a mold for PCL, I was quite displeased to discover that the PCL instantly bonded to the PVC like pine sap on glass. The PCL stayed pliable, and could be scraped off with great effort, but it really, really wanted to stay put on the PVC. And once cooled and hardened, the two couldn’t be separated at all, even with pliers, screwdriver or hammer.
When PVC meets melted PCL, the result is sticky
My displeasure at the failed experiment was short lived when I realized the many implications of this happy accident, which I summarize with a simple photograph.
Two PVC pipes semi-permanently joined with a ball of low-melting point polycaprolactone plastic
As you can see, the putty like nature of the PCL combined with its adhesion to PVC allows for some very non-standard joints. I also found, as you can see below, that it works very well on traditional joints that need to be strong, yet not permanent.
Recipe for making reversible, semi-permanent PVC glue joints
1) Heat beads of PCL in very hot water until clear and putty-like, then form into an O-ring like shape
A ring of PCL ready to glue two pieces of PVC together
2) Stick the o-ring onto the male side of the joint. Pre-warming the PVC with very hot water will ensure a good bond and keep the PCL from cooling too quickly.
The PCL O-Ring has been attached to the male side of the joint. When pressed together, some will be forced into the gap, and the excess will squeeze out.
3) Press the fittings together and wait for the PCL to cool
Once cool, the result is nearly as strong as a cemented joint. For extra strength, you can CAREFULLY pour near-boiling water over the entire assembly after the joint is pressed together to ensure a good bond.
Finished joint is watertight, nearly as strong as cement, and can be undone by gently heating
If you ever need to take the joint apart, (gently!) heat the joint under hot water and pull the fitting apart.
Some thoughts on safety
Although this should go without saying, I’ll say it anyway:
– be careful around hot water
– heating plastic with a heat gun or hair dryer is a bad idea due to fire risk
– joints formed with this technique are neither suitable nor advisable for any plumbing application
– joints formed with this technique may unintentionally come apart if heated (duh)
