Whenever possible I try to avoid having to glue or screw my model parts.
But what if you have to ?
There is a lot of dedicated glues available but which is the best?
None of them and all of them - depends on what you do an how many types of plastic you deal with.
So how about our favorite ? PLA…
On really flat surfaces super glue works quite well and is on par with the hardness of the material.
But did you know that it also can creep very well?
Meaning you can apply it to the already joined part rather than trying to get things together before the stuff sets stiff.
If a joint came out too wide or you have a small surface hole a bit of superglue and BAKING POWDER makes for a great instant filler that cures rock solid.
As super glue ONLY properly cures in the absence of air the baking powder also helps to cover joints and in return the uncured glue below sets much faster.
With a slight sanding PVA wood glue also works surprisingly well on PLA and some other plastics - it is it not for joints subjected to a lot of forces.
Contact glues however can be a bit tricky, even if you sand the surfaces.
They just won’t bond properly with PLA.
The real problems though all start with the ‘better’ filaments.
Like Nylon, PETG and such.
ABS is dead simple here as all we really need a bit of acetone to chemically weld parts together or to make a slurry to fill gaps.
PETG is great when renovating bathrooms 
I printed shelf mounts directly onto tiles to prevent having holes that could cause an issue years later.
But gluing PETG is can be near impossible for a novice - or require very costly glue…
Similar story if you try it with Nylon.
Did you know you can flame treat the sanded surfaces before applying the glue ?
A hydrogen flame is used to ‘polish’ certain plastics after sanding or cutting them.
But a carbon based flame, like from a little blow torch or lighter will destroy the non-stick surface properties by adding carbon into the plastic molecules.
After this treatment most normal glues that work well on PLA will work just fine.
Give the flame treatment a try and practice run on a soda bottle, preferably before opening it as it keeps it more rigid.
Measure the diameter and and create some small grip-pins that match the bottle’s diameter.
Print a bunch in the plastic you use for printing.
Apply strips of masking tape to dissect the bottle into thirds lengthwise.
Give the bottle a quick sanding with 400 grit paper in one third around the bottle and wipe it clean with IPA or Acetone.
Remove the tape on one side to place it over the sanded area and to get a clean line of a mm or so between the areas.
Repeat the sanding and cleaning but then do the flame treatment - quick enough to change the appearance of the surface but not so slow to stain with soot or melt things…
Sand you grip-pins and have at least two or three of each filament and glue type per sector of the bottle.
Now use your favorite and preferably cheap glue and place them on the bottle.
Allow for more than enough curing time…
Try to pull or snap them off and note the results
PLA+ can be a hit and miss depending on where it came from.
Some blends glue very easy, some just won’t properly bond no matter what.
For this case and if you require a very strong and water tight bond there is a way out - the plumbing department of your hardware store.
Get some PVC primer and PVC glue, preferably the clear stuff.
If the primer affects the surface of the plastic when rubbing it over with a cloth or such (wear nitrile gloves and do this outside or in a very well ventilated area!) it means it will bond.
But even on flame treated Nylon, PET and PETG the primer will aid with the bond.
Let the primer evaporate off (at least mostly), apply a layer of glue and push the parts together, wipe off all excess glue with good pressure and a clean cloth or such.
Depending on the joint area size it can take several hour or even a full day until the glue has set properly - don’t try to apply and paint or such until the glue is fully cured…
Filament pens…
Most of the cheap China ones are just ■■■■ = for using the to glue parts together…
But there is now half decent ones that can also do our higher temp plastics and offer a much better speed control.
The only thing you need to fix to abuse them as a welding pen or a filler pen is the tip where the ■■■■ comes out.
Get a small tube of high temp silicone.
Make a suitable mold for you pen to create a insulating silicone layer around it - like the ones on your hotend but a bit thicker.
I just press the thing into soft, not liquid wax.
The wax from those cheap tea lights that tend to leave an oily feeling on your skin.
I fill this with plaster to get my positive for later checks and use.
A quick wipe with oil on the remaining wax mold lets me fill it with soft wax (after cooling the mold in the fridge.
This is the cut to the right length to cover the front of the pen and laced on some cardboard after heating the bottom to keep it firmly in place.
An added layer of 3 to 4mm of modelling clay will represent our insulating silicone sleeve.
Make sure that you have enough length to go over the front of the pen as otherwise the sleeve won’t stay in place.
You can always cut strips and neat things out once cured.
Now build up the outer wax mold and let it fully cool down.
Pull out the clay, place back in place and fill it with the silicone - allow for plenty of time to fully sure.
Once on the pen cut the silicone so about 1mm of the extrusion tip stays free and you are good to go.
With slightly higher temps, a slow extrusion speed and a calm hand you can now melt the seam while filling it up with plastic like using a welder.
