How to Splice and Join Filament for Uninterrupted Multi-Spool 3D Printing

Running out of filament mid-print used to mean scrapping the job, and swapping colors mid-model used to mean a manual pause-and-resume dance. Filament splicing solves both problems by physically joining two lengths of filament into one continuous strand, so a single spool doesn't have to hold enough material for a large print and you can chain multiple colors or materials without an AMS, MMU, or filament-change macro. It's not as reliable as an automated multi-material system, but it costs nothing beyond a few minutes of setup and works on any FDM printer, from a bare-bones Ender to a Voron.

This guide covers the three practical ways to join filament — hot splicing, cold mechanical splicing, and adhesive splicing — along with when each one is strong enough to survive the extruder gears and hotend without snapping or jamming.

Why Splice Instead of Just Butt-Joining

A raw butt joint between two filament ends has almost no tensile strength. It will hold together on a spool sitting still, but the moment the extruder gear bites into it and pulls it through the Bowden tube or into a direct-drive hobbed gear, the joint separates. A separated joint inside the extruder is worse than running out of filament cleanly — the broken end can jam in the PTFE tube or hotend throat, and you may not notice until several layers later. A proper splice needs to survive being pulled, bent around a spool, and pushed through a hotend at temperature, all without a bump that catches on the extruder's idler bearing.

Method 1: Hot Splicing (Welding the Ends)

Hot splicing melts the two filament ends together directly, which produces the strongest and most reliable joint when done correctly. It works best on PLA and PETG; ABS and ASA are more failure-prone because they shrink and can void internally as they cool.

1. Cut both filament ends square with flush cutters — a clean 90-degree cut matters more than people expect.
2. Heat a soldering iron, dedicated filament splicer, or even a lighter held at a safe distance to bring both ends to a soft, glossy state without dripping.
3. Press the two softened ends together firmly and hold them in axial alignment until they cool — misalignment here is the most common cause of a joint that catches on the extruder gear.
4. Once cool, trim the joint with a splicing jig or a fine file so the diameter at the weld matches the rest of the strand. Any bump over roughly 2mm larger than nominal diameter risks jamming a direct-drive gear or a tight PTFE bore.

Purpose-built filament splicers (small clamshell tools that heat and butt-weld the ends automatically) give far more consistent results than a soldering iron, and are worth owning if you splice often — running a small print farm or working through partial spools on a Voron are both good reasons to keep one on the bench.

Method 2: Cold Mechanical Splicing

Cold splices use a small connector — heat-shrink tubing packed with hot glue, a short piece of brass tubing crimped over both ends, or a 3D-printed two-part collar that clamps around the joint — to mechanically hold the ends together without melting them. This is faster and doesn't need heat, but the joint is bulkier than a good hot splice and is more likely to catch on a Bowden tube coupler or an extruder idler. Cold splicing is best reserved for non-critical sections of a print, like infill-only layers, where a slight bump won't show on a visible surface even if the extruder briefly struggles to pull it through.

Method 3: Adhesive Splicing

Cyanoacrylate (CA) glue can join filament ends quickly, but it's the weakest option of the three — CA is brittle and doesn't flex well, so a glued joint that survives being handled on the spool can still snap under the tension of being pulled into the extruder. If you use CA, sand both ends flat first, apply a thin layer (thick globs take longer to cure and create a larger bump), and let it cure fully — rushing this step is the most common reason CA splices fail. Treat adhesive splicing as a last resort for emergency prints, not a routine method.

Where to Place the Splice in the Print

Splice placement matters as much as splice quality. Time the joint to land during infill or on an internal wall rather than an external perimeter, so a slightly rough spot doesn't show on the finished surface. Most slicers (PrusaSlicer, OrcaSlicer, Cura) let you pause at a specific layer height via a manual G-code injection or the built-in color-change/pause feature — use that pause to feed the spliced section through by hand and confirm the joint clears the hotend cleanly before resuming, rather than trusting it to feed unattended.

MethodStrengthSpeedBest For Hot splice (welded)HighSlow (2-5 min)PLA, PETG; load-bearing sections, unattended prints Cold mechanical spliceMediumFast (1-2 min)Infill-only sections, quick emergency joins CA adhesive spliceLowFast, but needs cure timeOne-off prints where failure is acceptable

Splicing for Color and Material Changes

Beyond emergency joins, splicing is a legitimate (if manual) way to get multi-color or multi-material results on a single-extruder printer without a purge tower eating filament. The catch is that a splice between two different materials — say PLA to PETG — is inherently weaker than a same-material splice, because the two plastics don't bond chemically at the joint the way two ends of the same filament do. Treat cross-material splices as cosmetic only, and never rely on one at a stress-bearing point in a functional part.

Safety Notes

Hot splicing involves an open heat source close to your fingers — use a dedicated splicer or a soldering iron on a stand rather than holding a bare heat source freehand, and give ABS/ASA fumes the same ventilation you'd use for normal printing, since a heated joint releases the same VOCs as the hotend does. Let welded joints cool fully before handling; molten PETG in particular stays tacky and can stretch into thin, sharp strings if pulled apart too soon.

Filament splicing won't replace a proper multi-material system if you need clean, repeatable color changes, but for stretching a nearly-empty spool through one more print, avoiding waste on a long overnight job, or getting an occasional two-tone part without extra hardware, a good hot splice is strong enough to trust and costs you nothing but a few minutes at the bench.