Key takeaways
- Superglue is quick and convenient but gives a brittle joint. Parts that take impact or vibration should use epoxy, or move to a mechanical joint.
- ABS and ASA are the only common materials that can be solvent-welded: acetone dissolves both faces and lets them set into one piece.
- Threads tapped straight into printed plastic survive only a few cycles. A heat-set insert turns the load on a few thread flanks into pressure spread across the whole bore wall.
- Screws driven into 3D prints often pull the layers apart. The part's print orientation decides whether the joint holds more than the type of screw does.
A lot of good parts are printed and then fail at assembly. The cause usually isn't the wrong glue but a wrong assumption: that the joint only needs to be strong enough to hold two pieces together. On a 3D print, the joint is often the weakest point of the whole assembly, so it has to be designed, not handled after printing.
Adhesives by plastic
| Material | Adhesives that work | Notes |
|---|---|---|
| PLA | Superglue, two-part epoxy | Very easy to glue. A superglue joint is more brittle than the PLA itself. |
| PETG | Superglue, epoxy | Key the bonding faces; PETG is slick and solvent-resistant. |
| ABS / ASA | Acetone, solvent cement, epoxy | Solvent gives a true weld — the strongest in the table. |
| PC | Epoxy | Avoid solvents: they cause stress cracking, sometimes only visible days later. |
| PA, PA-CF | Epoxy with surface treatment, adhesives for hard-to-bond plastics | Low surface energy. Better to switch to a mechanical joint. |
| TPU | Shoe glue, flexible PU adhesive | A rigid adhesive cracks right at the joint edge when the part flexes. |
| Cured resin | Superglue, epoxy, or the resin itself cured with UV | Gluing with liquid resin gives a uniform joint, very handy for models. |
Three rows are worth stopping on. Superglue is brittle — strong in straight tension and very weak in bending or impact, so a vibrating part comes apart at the glue line rather than breaking in the plastic. Epoxy fills gaps, which matters for FDM parts because mating faces are never perfectly flat. PA is best not glued at all: nylon is notoriously hard to bond, and a nylon joint that feels strong when you test it by hand often doesn't survive heat and humidity cycles.
Solvent-welding ABS: why it's different
With glue you add a third material between two pieces, and the joint is only as strong as that layer. With acetone on ABS there is no third material: the solvent dissolves the outer layer of both faces, the two softened layers mix, the solvent evaporates, and what's left is continuous ABS. That's why a properly made ABS joint can be nearly as strong as the part itself.
- 01
Dry-fit first
Check that both faces sit tight. Solvent fills very small gaps, not 0.5 mm ones. If there's a gap, sand the faces flat or dissolve a few ABS scraps in acetone to make a thicker slurry.
- 02
Brush a thin coat on both faces
With a small brush. Wait a few seconds for the surface to turn glossy and slightly tacky — the sign the plastic has dissolved but not yet run.
- 03
Press and hold
Press just hard enough to see a bead of softened plastic squeeze out along the joint. Clamp and leave it; don't adjust once pressed, because every shift breaks the layer you just dissolved.
- 04
Let the solvent leave before doing anything else
The joint sets in minutes but holds solvent much longer. Sanding or painting too soon leaves a sunken line along the joint as the remaining solvent leaves. For thick parts, count in days, not hours.
Three ways to make threads, and which one holds load
| Method | Handles | Use when |
|---|---|---|
| Screw driven into a printed hole | Light load, few assembly cycles | Housings, covers, trim |
| Brass heat-set insert | Medium load, many assembly cycles | The default for every functional part |
| Through-bolt and captive nut | The highest load of the three | Load-bearing structures, fixtures, frames |
The difference is where the load goes. A screw driven straight into plastic puts all the force onto a few thread flanks cut into printed plastic — and when the plastic is PLA, those few flanks strip after three or four removals. A heat-set insert is pressed in hot, the plastic flows around its knurled exterior, and the load spreads across the whole bore wall. A through-bolt is better still: the clamping force sits between the bolt head and the nut, and the plastic is only in compression — the load state 3D prints handle best.
Heat-set inserts: what makes or breaks it
- Hole diameter
- Exactly as the insert's data sheet says, not estimated
- Iron tip
- A dedicated insert tip, on a temperature-controlled iron
- Temperature
- Hot enough for the plastic to flow around the insert, not so hot it boils and wells up
- Speed
- Press slowly, square to the hole. A crooked insert never straightens
- Wall thickness around the hole
- Thicker than you think — thin walls bulge as the plastic flows
- After inserting
- Let it cool completely before driving a screw in
Orientation decides the joint, not the screw
FDM parts are weakest perpendicular to the layers. A screw tightened into a hole whose axis is parallel to the layers creates a spreading force that splits the layers apart like a stack of paper. The same screw in the same part, printed rotated 90 degrees, can hold many times more. Before going up a screw size, revisit the orientation — why layers are a mechanical property is covered in print settings for strength.
A joint in a 3D printed part is a design feature, not a post-processing step.
Splitting a large part into pieces
Joining isn't only for repairs. For parts bigger than the bed, or ABS parts wider than the chamber can handle, splitting and gluing often beats one monolithic print: each piece sits in its own best orientation, needs less support, warps less, and a failed piece means reprinting only that piece. The size at which the chamber becomes decisive is covered in enclosures and chamber heating.
- Put the split where it's both hidden and lightly loaded. A joint right where the part bends most is a crack already scheduled.
- Add locating pins. Two holes and two pins make assembly faster and straighter than any alignment by eye. Leave clearance for the pins at the right fit tolerance — the clearance section of the handbook has starting numbers.
- Leave a glue channel. A shallow groove along the joint gives the adhesive somewhere to sit instead of being squeezed out when you clamp.
- Increase the bonding area with a stepped or tongue-and-groove joint. A flat cut gives the smallest possible area; an L-shaped step can double the bonding area without changing the outside dimensions.
- Glue first, paint after. Adhesive bonds better to plastic than to paint, and a filled seam disappears under primer — the process is in how to paint 3D prints.
Frequently asked
Does superglue work on PLA?
Yes, very well in terms of adhesion. The issue isn't adhesion but the nature of the joint: cyanoacrylate cures to a brittle layer, so a part that takes impact or vibration comes apart right at the joint. For display models superglue is a sensible choice; for functional parts, use two-part epoxy or switch to bolts.
Why is there a white film around my superglue joint?
That's cyanoacrylate blooming: glue vapour meets moisture in the air and deposits as a white film around the joint — most visible on dark parts and in humid weather. Avoid it by using less glue, pressing faster and working somewhere ventilated. Once it's there, a light sanding removes it.
Can I tap M3 threads straight into a PLA part?
Yes, and it holds well the first time. But threads cut in PLA wear with every removal, and PLA also creeps under sustained clamping load — so the joint loosens over time even if you never touch it. If the part will be opened repeatedly, use heat-set inserts from the start.
How do I set inserts without a dedicated tip?
A temperature-controlled soldering iron and a conical tip can do it, but it's hard to keep the insert square and easy to push plastic up onto the surface. For a few inserts, accept that and sand off the excess. For volume, a dedicated tip costs much less than the parts you'll spoil.
What's the best way to glue PETG?
Two-part epoxy after thoroughly sanding both bonding faces. PETG resists solvents, so there's no chemical weld like ABS, and its slick surface gives glue little to grip. If the joint has to carry real load, design in locating pins or a through-bolt and treat the glue as locating and sealing only.