Summary

A jig for welding 1.75mm PLA filament.
Welded filament slides smoothly through 2mm inner diameter tubing without sanding.

A PTFE tube and coil spring are required separately.
-PTFE tube: Length: 40mm, ID x OD: 1.8mmx2.6mm, h ttps://amzn.to/3AWOsoN
-Coil spring: Wire Diameter: 0.3mm; Outer Diameter: 3.4mm, h ttps://amzn.to/3KVjfqB

When attaching the tube, bend and insert it from the inside of the donut.

WHY DO YOU NEED A SPRING?
When you heat the PTFE tube, it softens. Two filaments are pushed in from both sides in that state, and the tube expands at the butted part. A spring is installed to prevent this expansion. Therefore, when you choose a spring, its inner diameter should be approximately the same as the outer diameter of the tube.

How to use
https://youtu.be/2PpO4AbOWgg

This welder is designed to join the last filament of a roll to a new roll.
When using this welder, remove the filament from the printer, connect it to a new roll, wind it up, and re-insert it into the printer.

I made a jig for winding filament with an electric drill.
https://www.thingiverse.com/thing:5557597

WELDING TIPS
(1) Insert two filaments into a doughnut-shaped jig and butt them in the center.
(2) Heat each side for about 1 second.
(3) Lightly push in the two filaments.
(4) After waiting for about 5 seconds, sandwich the center of the donut with two coin-shaped jigs.
(5) Gently push out the filament until the weld comes out.
(6) Done.

I searched for springs on US Amazon.
I think this probably works.

Coil Spring:
https://www.amazon.com/Xianglaa-springs-Diameter-Stainless-Compression-Compressed/dp/B09DKZTT8R/ref=sr_1_51?crid=1WSUUVU7N5HY7&keywords=coil%2Bspring%2B3.2mm%2Bod&qid=1663861249&sprefix=coil%2Bspring%2B3.4mm%2Bod%2Caps%2C218&sr=8-51&th=1

PTFE Teflon Tubing:
https://www.amazon.com/Aicosineg-Teflon-16-4ft-1-8mm-Connector-Printer-Milky/dp/B092QR92XZ/ref=sr_1_5?crid=1LH9VR8P4MSNP&keywords=ptfe+tube+1.8mm+id&qid=1663859311&sprefix=ptfe+tube+1.8mm+%2Caps%2C204&sr=8-5

Sorry if it doesn't work.

1.75mmのフィラメントを溶接するための治具です。
溶接部のヤスリ掛けが不要で、内径2mmのチューブを滑らかに通ります。

別途PTFEチューブとコイルバネが必要です。
-PTFE tube: h ttps://amzn.to/3AWOsoN
-Coil spring: h ttps://amzn.to/3KVjfqB
（PTFEチューブは40mmにカットしてください。）

使い方
https://youtu.be/2PpO4AbOWgg

電動ドリルでフィラメントを巻き上げるための治具を製作しました。
https://www.thingiverse.com/thing:5557597