I recently picked up a filament splicing tool – the SUNLU FC01, designed to join leftover bits of filament to fresh spools. As someone who often ends up with 20cm or less on a roll, this seemed like the kind of tool that could actually save material and some guilt.
At first glance, the product looks pretty solid. The construction feels decent, and the concept is smart: A central fixed heating block melts the ends of two filament strands, and you manually press them together to fuse. What sets this apart from other DIY splicing setups is that it lets you choose the temperature based on the filament type, which is a nice touch.
The kit also includes a small PTFE cutting tool. You slide a short PTFE tube over the two filament ends to help hold them aligned and pressed against the heated plate during splicing. Once the joint cools and hardens, you cut off the outer PTFE sleeve with the included cutter. Simple but effective.
But as always, marketing is one thing—actual use is another. The idea is great, but the reality？Kind of frustrating.
I slid the PTFE sleeve onto the two filament ends, pressed them together on the heated metal plate, and powered the machine on. I set the temperature and waited for it to heat up. But very quickly, I ran into a problem: I had to hold both ends of the filament manually the entire time. One hand on each side—because the filament isn’t straight, it’s curved and springy. If I let go, the natural bend and tension would cause the joint to shift or pop off the platform.
When the temperature reached the target—200°C in this case—the machine beeped to signal that the filaments inside the PTFE sleeve were melting and starting to fuse. At that point, I had to act: turn off the heating and press the eject button to release the upper metal cover so I could take the filament out for cooling. But here’s the awkward part – as soon as I took one hand off to press the button, the free filament snapped back due to its elasticity,
shifting the still-soft joint and often ruining the weld. So yeah… failure rate? Pretty high.
To fix this, I decided to design a custom clamp to free up my hands—something that could hold both filament ends firmly in place during the fusion process. The goal was to securely anchor the two pieces on either side of the splicer without me needing to hold them manually.
So I ended up designing and publishing a complete splicer base mount. You can place the SUNLU FC01 directly on this printed base, which adds a clamp on each end—one for each piece of filament. After aligning the two filaments inside the PTFE sleeve, you place them into the clamps, press the metal cover down, and begin the heating process.
I iterated through a few versions before finalizing the design, and the current model has a few nice advantages:
- No supports required – The entire base prints in one go, cleanly, without the need for any support structures.
- No extra parts needed – The clamp mechanism doesn’t require any external springs or screws. I embedded a printable spring structure into the model itself, and after real-world testing, it turned out to be surprisingly strong and reliable.
- Added a rear release button – Just press to release the clamp, making it much easier to remove the filament after welding.
After putting the whole setup to real use, the results have been great – with the clamp base in place, my filament splicing success rate has jumped to 100%. The process is now smooth and repeatable, and I can finish each splice in just one or two minutes.
I went ahead and shared the design on Makerworld, and to my surprise, people actually started downloading and printing it. Apparently, I’m not the only one struggling with SUNLU’s original design. Turns out many people found it nearly impossible to use without some kind of external assist. Honestly, I’m just happy this tiny fix could help others too. There’s something satisfying about solving a small problem that a whole product team somehow didn’t think through.
And to be honest, I seriously doubt the engineers behind this product ever actually used it themselves. Because if they had, they’d know that trying to splice two springy pieces of filament—without any way to hold them in place – is borderline impossible. You can’t expect people to press, align, and hold melted plastic with both hands while also operating the machine. That’s not user experience. That’s a two-handed juggling act with hot materials.
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