Gluing and Joining Parts

Glue, weld or fasten

Formlabs divides joining into two approaches. Chemical fastening (adhesives) suits art, scale models and complex shapes that will not take impact. Mechanical fastening (screw threads, pockets for hardware) suits functional parts that need a robust connection or repeated assembly and disassembly. Pick the approach before you glue anything, because a glued joint is usually permanent.

Method	Good for	Limits (as the makers state them)
Cyanoacrylate (super glue), gel or liquid	Small and medium rigid parts, quick repairs, parts you cannot clamp	Formlabs' text calls its impact strength moderate (its comparison table says low), says it is not recommended for high-impact use, and says it does not bond dirty surfaces well; Polymaker says super glue gels are unsuitable for nylon and may fail under prolonged stress
Two-part epoxy	Large areas, strongest adhesion, high-stress joints	Longer work time; Formlabs says most five-minute epoxies stop shifting after about five minutes and reach roughly 75 percent of cure strength in under an hour Rule of thumb (read your product's own times)
Steel-filled epoxy	Plastic-to-metal joints	Polymaker describes it as near-permanent; separation may need destructive methods
Solvent welding	Materials the solvent dissolves, such as ABS and ASA with acetone	Prusa says ASA can be glued or welded with acetone; the solvent has its own fire and health hazards (see below)
Liquid resin plus UV pen	Small joints on resin prints	Formlabs says the low-power pen cannot penetrate deeply, so it suits only small surfaces
Screws, clips, inserts	Functional parts, nylon, anything you will take apart	Polymaker recommends mechanical fasteners for nylon, which bonds poorly

Mechanical options are covered in Heat-Set Inserts and Threads and Snap Fits. Polymaker also mentions specialty adhesives sold for specific filaments, which work by partly dissolving the plastic surface; follow that product's own instructions and SDS.

Adhesive safety: read the SDS

Every adhesive has an SDS, and the hazards differ by chemistry. Two examples from one maker's sheets:

• Cyanoacrylate gel. Gorilla's Super Glue Gel SDS warns that it bonds skin and eyes in seconds, causes skin and serious eye irritation, may cause respiratory irritation and is a combustible liquid. It also says it polymerizes rapidly on contact with water or alkaline substances, generating heat that can inflame or burn skin. It calls for use outdoors or in a well-ventilated area, chemically resistant gloves, and chemical goggles or safety glasses. Its first aid for eye contact is to rinse with water for at least 15 minutes and get medical attention.
• Epoxy hardener. Gorilla's Epoxy Hardener SDS lists it as harmful if swallowed or in contact with skin, causing serious eye damage and skin irritation, possibly an allergic skin reaction, and suspected of damaging fertility or the unborn child. It calls for gloves, protective clothing and eye protection, ventilation to keep vapors below exposure limits, and respiratory protection if ventilation is insufficient.
• Liquid resin used as glue carries the resin's own SDS warnings. See Resin PPE.
• Acetone is a highly flammable solvent. Its hazards are covered in Smoothing Methods and What They Change and ABS and ASA.

Because the epoxy SDS lists an allergic skin reaction, wear gloves every time you mix it, not only the first time.

Prepare the joint

1. Test the fit dry before any adhesive goes on.
2. Sand and clean both faces. Polymaker calls surface preparation (sanding, cleaning) critical for all adhesives, and Formlabs says to clean surfaces thoroughly before cyanoacrylate.
3. Remove the sanding dust and let the parts dry fully before applying adhesive.
4. Test on scrap. Try the adhesive on a scrap print before the real part.

Design joints that align themselves

Formlabs recommends aligners such as slots, pins, grooves, recesses, lips and dovetails, so parts find their own position. It cautions that they only work if your printer makes dimensionally accurate parts, and that straight cuts with no aligners must be held in place until the adhesive bonds. Label parts with letters or numbers when there are many of them. How to cut a model into parts is in Splitting Models; check your printer's accuracy with Dimensional Accuracy.

How much clearance to design into a joint is covered in Fits and Tolerances.

Finishing the seam

Formlabs fills seams on large assembled resin prints with thin layers of automotive putty, or with resin cured by a UV pen, and sands lightly after each layer dries. The full filler workflow and its SDS warnings are in Filling and Priming.

When glue fails

• Joint pops apart cleanly: the faces were too smooth or dirty. Sand and clean, then reglue.
• Joint cracks under bending: cyanoacrylate is rigid. Switch to epoxy or a mechanical joint.
• Nothing sticks to nylon: as Polymaker says, use fasteners instead.
• The part breaks beside the joint: the glue held and the print's layers did not. Polymaker notes that one two-part epoxy it describes bonds stronger than the layer adhesion of many printed materials; reorient or thicken the part instead of changing glue.