Joining 3D prints: CA glue, epoxy, solvent welding, pins and dovetails

When a part is bigger than the bed, or its shape cannot print in one go, you split it and join it back. There are three main options: CA (super glue), two-part epoxy, and solvents that melt the surface so the plastic welds to itself. Each suits different plastics, and no glue beats a joint that was designed well in the first place.

This lesson is post-processing. For smoother surfaces from the slicer side, see Smooth surfaces on 3D prints.

Which glue for which plastic

Glue	PLA	PETG	ABS / ASA	How fast	Strength
CA (super glue)	yes	yes	yes	seconds; full hardness 8–24 h	easiest; bonds well to slightly porous surfaces
Two-part epoxy	yes	yes	yes	under 90 s to over 12 h, by formula	fills gaps, doubles as putty
Acetone (solvent)	not recommended	no	yes	seconds to minutes; big parts take days to fully cure	true weld, cannot be undone

The table follows Prusa’s gluing guide, which says CA is “almost equally suitable” for common printed materials, while PETG is “inert to all chemicals normally available to a hobbyist”, so it cannot be solvent-welded. The solvents that genuinely dissolve PLA are far more hazardous than acetone. Don’t go looking for them.

TPU takes CA only if the mating surfaces are at least somewhat stiff. Prusa’s guide gives no advice for nylon and notes some plastic glues don’t work on PP (polypropylene). Test those on scrap first, or design in pins or screws instead.

Real numbers: which glue is stronger

Prusa ran an informal pull test on glued samples (few samples, uncontrolled conditions). The numbers are still useful for comparison.

Glue	PLA (MPa)	PETG (MPa)	ASA (MPa)
Regular CA	9.8	13.1	9.4
5-minute epoxy	6.3	5.1	2.0
Solvent-type plastic cement	0.9	2.5	2.5

Worked example: a shelf split in the middle, cross-section 40 × 20 mm.

1. Contact area = 40 × 20 = 800 mm²
2. CA on PLA: 800 × 9.8 = about 7,800 N in theory
3. Solvent cement on PLA: 800 × 0.9 = about 720 N, nearly 11 times less

Do not design a load-bearing part from these figures. A straight pull in a test rig is very different from the prying and peeling forces a real joint sees. Use them as a ratio when picking a glue, and notice that contact area is the multiplier. Split where the cross-section is wide and the joint gets stronger.

Heat matters too. Regular CA keeps roughly 50% of its strength at 80 °C and about 20% at 100 °C. Heat-resistant CA keeps about 80% at 120 °C.

Prepare the surfaces

1. Mating faces should be flat and clean. Sand them until they sit flush, then remove the dust.
2. Get rid of grease, fingerprints, paint and old glue.
3. Dry-fit first. If it rocks or shows a gap, fix the part before any glue goes on.
4. CA likes a slightly porous surface. Wicking glues (extra-thin CA or solvent) need dead-flat faces, or the glue runs along the layer lines and ends up somewhere else.

A tip from Prusa’s guide: tack the parts in place with small drops of CA, then fill the rest of the joint with epoxy. You get speed and strength. Mix epoxy exactly to the ratio on the label. Extra hardener does not make it set faster; it makes it hot, bubbly and weaker.

Design the joint: pins and dovetails

Let the glue keep things together. Let the geometry handle alignment and shear.

• Pins (dowel / plug). A round pin in a hole aligns the two halves and resists sliding. PrusaSlicer’s Cut tool can generate a plug (pin attached to one half), a dowel (separate printed pin) or a snap connector.
• Dovetail. A trapezoid key that locks against pulling apart in one direction. PrusaSlicer’s Cut tool has a Dovetail mode. Good for long flat panels joined edge to edge.
• Screws or magnets. When the part must come apart, embed nuts or magnets mid-print with a pause at the right layer.

Pin clearance follows the same rules as any fit: about 0.1–0.2 mm per side for a hand-push fit with no wobble, a little looser if glue has to live in the gap. Details are in Dimensional accuracy and fits. For choosing the cut plane and printing pins lying down so they are strong, see Part orientation.

Split where the load is low and make the cut face as wide as you can. The best joint is one where the glue barely has to work.

Welding ABS/ASA with acetone

Acetone softens the surface of ABS and ASA so the two halves fuse into one piece, permanently. Small parts grab in seconds to minutes, but large ones can take days to cure completely, and the joint shrinks slightly as it dries. Holding the parts together longer gives a better bond, though it still won’t match the strength of the surrounding material.

Solvent cannot fill gaps the way CA or epoxy can. Push in scraps of the same plastic or bits of filament, or dissolve ABS scraps in acetone to make a thicker glue.

Safety

• Ventilation matters even more than when printing. CA fumes irritate the eyes and airways. Solvents smell strong and are flammable.
• Acetone is highly flammable. No flames, heaters or sparks in the room. Cap the bottle every time.
• CA bonds skin instantly. Wear glasses. CA on cotton or wool can heat up enough to burn the fabric, so don’t wipe it with a cotton rag.
• Uncured epoxy is a skin sensitiser. Germany’s occupational safety institute (DGUV IFA) lists epoxy resins among the most frequent causes of work-related allergic skin disease; once sensitised, a person reacts on every contact. Wear nitrile gloves and keep it off your skin. Cured epoxy no longer sensitises. If a rash appears after contact, see a doctor.

Read the label and safety data sheet (SDS) for the exact product. Formulas differ, and so do the precautions.

Common mistakes

• Gluing over sanding dust. The glue sticks to the dust, the dust doesn’t stick to the part, and the joint peels off.
• Using acetone on PETG or PLA. Nothing happens except wasted time and a fire risk.
• Too much CA. It squeezes out onto the surface and leaves a white bloom (frosting). Apply thin and press firmly.
• Plain butt joints. No pins, so the parts slide while the glue sets, and the joint takes little shear.
• Old glue. Opened CA lasts months, not years. Old epoxy can look fine and never cure.
• Moving the part too soon. CA grabs in seconds but needs 8–24 hours to reach full hardness.

Next: Sanding, priming and painting prints

Sources

• The ultimate guide to gluing and joining 3D prints — Prusa blog
• Cut tool — Prusa Knowledge Base
• ASA — Prusa Knowledge Base
• Epoxy resins — DGUV Institute for Occupational Safety and Health (IFA)