Sunlu Filament Connector Fix: Weld Spools Without Bent Joints

The Sunlu filament connector welds two pieces of filament together, and it works perfectly on loose pieces. But join two actual spools and the tension bends the joint and ruins it. Here's the cheap fix that made mine work every time.

The quick version

• The problem: spool tension bends the soft PTFE tube, and the plastic is still molten when you open the lid, so joints come out bent or lumpy.
• You need: the Sunlu filament connector and some silicone tubing.
• Step 1: print a support holder that clamps the filament either side so it can't move.
• Step 2 (the real fix): swap the PTFE tube for silicone tubing (1.6 mm ID, 3.2 mm OD). It absorbs the shock when you open the lid.
• Take the filament out after the third beep for the best results.
• The joins came out smooth and stronger than the filament itself.

Why weld filament at all?

An AMS can switch to the next spool automatically, but the swap can happen on an outside wall and leave a mark. For decorative prints where the finish matters, welding spools together is the only way to guarantee a perfect surface.

Where the Sunlu connector falls down

• It was clearly tested on two loose pieces, not two spools. With spools, the tension is too strong for the PTFE tube, so the whole thing bends in the middle.
• The grip pads don't really grip anything.
• There's no cooling, so the joint is still soft when you open the heated cover and deforms at the slightest touch.

The printable holders I tried

I tested several community designs that were easy to print and needed no extra parts. A manual clamp couldn't hold the ends steady, one simple design had no way to get the filament out afterwards, and a "robot arm" one was too loose. The winner was a hinged support holder: the connector sits in its grooves and the filament stays perfectly still when you open the lid. That fixed the bending, but the joint was still too hot and picked up bumps.

A small PC fan blowing on it as I opened the lid gave great results, but it was fiddly to set up every time.

The fix: silicone tubing

Then I remembered the silicone tubing I used to use when welding filament with a soldering iron. I cut a piece to replace the PTFE tube. It's a little thicker, so the lid needs a firm push, but:

• It absorbs the shock when you open the heated cover, so the joint doesn't bend.
• The seam came out smooth, with nothing to sand. I did it three times with the same result.
• The joint was so strong I couldn't pull it apart.
• It still feeds through the extruder fine, and the tubing is reusable.

What I'd like in a Sunlu V2

Silicone tubes instead of PTFE, a built-in cooling fan, a way to turn off the beeping, and grip pads that actually grip.

What I use

Questions? Ask in the video comments, or join the community.

More guides

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Common questions

Why does the Sunlu filament connector bend the filament?

Spool tension bends the soft PTFE tube, and there's no cooling, so the joint is still soft when you open the lid. It works well on loose pieces but not on two full spools.

How do I fix the Sunlu filament connector?

Print a support holder that clamps the filament either side, and replace the PTFE tube with silicone tubing (1.6 mm ID, 3.2 mm OD), which absorbs the shock when you open the lid.

Is a welded filament joint strong?

With the silicone tubing fix, my joints were smooth and stronger than the filament itself.

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