The spring changer is used with a fountain pen. By removing the syringe needle, use a 3.4mm converter, such as the Hongdian fountain pen.perfect for filling or rinsing pen cartridges, converters, and inks.It is easy to fill. Just immerse the pen tip into the bottled ink, press down the spring rod, and release the spring rod to suck the ink into the converter.Easy to : The stuffer has 3 components (converter, white connector and pin), just connect the converter and pin with the connector at the end. If you wrap it with a cloth and add some grease to the connector, you can do this. The first assembly may require a little forceNon sterile, for industrial use only color: redmaterial: plasticPackage Contents:5 x Ink Syringe FillerOnly the above package content, other products are not included.Note: Light reflection and different displays may cause the color of the item in the a little different from the real thing. The measurement allowed error is +/- 1-3cm.