How to Replace a Bowden PTFE Tube and Upgrade to Capricorn Without Under-Extrusion

A Bowden tube swap can fix inconsistent feeding, but it can also trigger under-extrusion if the filament path isn’t sealed the way your hotend expects. The good news: most problems come from preventable setup mistakes—especially how far the tube is pushed in and how cleanly its ends are cut.

This guide walks you through removing the old PTFE tube, installing Capricorn with a proper push-fit, and then re-running extrusion calibration so you get consistent flow again.

Prep and safety checks before the tube swap

• Let the printer cool fully (you can work safely around the hotend area).
• Unload filament from the extruder so nothing drags through a partially open path.
• Note where the Bowden tube routes from extruder to hotend so you can keep the new run similarly straight and kink-free.
• If you have a removable fan duct or shroud that blocks access to the hotend inlet, plan on removing it so you can see what you’re doing.

Replace the Bowden PTFE tube step by step with Capricorn

Do the swap in a clean, repeatable order: remove the old tube, replace it with a properly trimmed piece, and seat it all the way in.

1. Unload and remove the old tube from the extruder side. Loosen the coupler at the extruder end, then release the clip/collar (if your fitting has one) and slide the PTFE tube out. Inspect the tube end—if it’s crushed, melted, or deeply scratched, it’s a strong sign feeding friction was already building.
2. Release the tube at the hotend inlet. Access the hotend side coupler and remove the tube. If it resists, don’t yank aggressively—use the correct release method for your fitting (pushing the collet release ring while pulling the tube is usually safer than brute force).
3. Remove any debris and check the inlet. Look inside the hotend inlet area for filament dust, PTFE crumbs, or softened plastic. Wipe the area carefully. If you recently had clogs, it’s worth checking that the nozzle/hotend aren’t partially blocked before continuing.
4. Cut Capricorn with a square, clean cut. Use a sharp blade. Capricorn performs best when the end is flat and not shaved at an angle. Aim for a cut that looks uniform from edge to edge.
5. Trim only what you need and clean the ends. If your tube length was the problem (too long or routed with tension), shorten it now so it sits without stress. After cutting, remove any whiskers or dust from the tube end so it can seat flush.
6. Seat the tube fully using a push-fit technique. Insert the Capricorn tube into the hotend fitting until it bottoms out at the designed stop. Then firmly tighten the coupler/retainer. The key is consistent insertion depth—under-extrusion often comes from a tiny gap that lets filament buckle or compress before it reaches the hotend.
7. Manage routing so it doesn’t kink. Ensure the tube run is straight enough to prevent tight bends. A kink or sharp bend can increase friction and mimic the symptoms of a partial clog.
8. Reinstall the extruder-side connection with the same care. Fit Capricorn into the extruder coupler until it’s seated, then secure it the same way. Keep the tube aligned with the extruder to avoid side-loading the connector.
9. Heat-soak briefly and test feed before long prints. Heat to your normal printing temperature and manually extrude a small amount. Watch for smooth flow and consistent extrusion without clicking, grinding, or stopping.

Why these steps matter: Capricorn’s tighter internal surface and smoother channel can reduce friction, but only if the filament still has a sealed, straight path. A slightly angled cut, incomplete insertion depth, or a routing kink can create resistance or a gap—both show up as under-extrusion.

Re-run extrusion calibration to confirm the filament path is right

Even though a tube swap usually doesn’t require permanent parameter changes, under-extrusion symptoms can be subtle at first. Re-running extrusion calibration helps you separate “tube seating problem” from “slicer/extruder settings drift.”

1. Do an extrusion test at the same temperature you print with. Warm the hotend, then command a known extrusion distance (commonly a short test run). If your printer supports it, keep the fan and other conditions consistent with your usual prints.
2. Measure what actually extrudes. Use calipers or a ruler to compare commanded vs actual filament movement/extrusion. If the printer extrudes less than commanded, that’s under-extrusion in your current configuration.
3. Adjust extrusion using your usual method. Depending on your firmware/workflow, this is typically handled as either:
   • Flow/extrusion multiplier (slicer-side setting), or
   • Extruder steps (E-steps) (firmware-side setting).
4. Repeat the test after changes. A second test is important because you’re verifying that the mechanical feed path (including the new tube) is behaving.
5. Check retraction settings if you notice it only during retractions. Tube material and friction can change how easily filament moves during fast retract/prime cycles. If you see consistent under-extrusion specifically around retraction events, review retraction distance and speed to match your new feeding behavior.

The goal is simple: after calibration, a basic extrusion test should produce steady filament without stuttering. If your extrusion test fails even after a correct seat and clean tube ends, focus on the mechanical causes (next section) rather than chasing slicer settings.

Common under-extrusion failure modes to troubleshoot

Under-extrusion after a tube swap is usually traceable to one of these issues. If the printer starts extruding poorly right after the change, check them in order.

• Tube not pushed to the intended stop. Even a small gap at the hotend inlet can cause the filament to compress, buckle slightly, or not feed into the melt zone.
• Uneven or angled tube cut. A slanted Capricorn end won’t seat correctly in the fitting and can leave a discontinuity that increases resistance.
• Debris in the hotend inlet or coupler. PTFE dust, filament crumbs, or melted residue can prevent a clean seal.
• Kinked or overly bent Bowden routing. Extra friction from a bend can show up as thin lines, clicking, or inconsistent extrusion—especially on longer moves.
• Loose couplers or worn clips. If the coupler can move, the insertion depth can drift under vibration, recreating the same gap problem mid-print.
• Extruder friction or partial clog from before the swap. If the original problem was caused by a partial clog, the tube upgrade won’t fix it by itself. Confirm the nozzle/hotend are clear.

A quick practical trick: if manual extrusion at temperature is smooth but print output is thin, it’s often slicer/flow or temperature-related. If manual extrusion also struggles or the extruder clicks, it’s much more likely a mechanical path issue (seat depth, cut quality, debris, or routing).

When you get the tube seated correctly and the ends trimmed clean, swapping to Capricorn is typically a low-effort reliability improvement for Bowden setups. If you want the best odds of avoiding under-extrusion, treat the push-fit insertion depth and square cutting as the “non-negotiables,” then verify everything with an extrusion test and calibration before committing to a longer print.

PriceIt3D may earn a commission when you buy through links on this page, at no extra cost to you. Affiliate notice.