The classic Bowden tube extruder has the following components:
- The drive motor
- The fitting that you attach the tube to
- The teflon tube.
- The Bowden fitting at the print head end
- A heat sink, usually finned, and with a powered fan
- A steel or brass tube that has a teflon liner
- A hot block with heater
- The nozzle.
For reliability, the bowden fitting should be tight to the steel tube, screwed right against it. The top part of the steel tube is teflon lined, and the bottom part is solid steel. That screws into the hot block (there may be a thin part that restricts heat flow). The nozzle should be screwed right up against the tube.
In the above setup, the bowden fittings have a lip that stops the tube from being inserted past the fitting. This setup has two places where filament can hang up. One is where the second Bowden fitting is up against the lined steel tube, and the second is where the steel tube meets the nozzle, However, that area is so hot in operation that a hang will self resolve.
The newer HE3D printers use a different scheme, The Bowden fittings don't have a lip, so the teflon tube can be pushed right through the fitting. This is what you need to do. The steel tube doesn't have a teflon liner. The long tube becomes the liner. You need to push the tube well into the fitting on the extruder, probably 40mm. Push it in until it bottoms.
So what happens if you fail to push it in?
The steel tube is hot enough to melt the plastic, but under normal operation it does not get quite hot enough to become a large melting chamber. So the plastic partially melts. You might pull the filament out and see a wide spot. The printer might stop printing completely. You have a slump.
The fix is to get the teflon tube all the way into the steel tube.
If you can, you are done.
But it is also likely that when the plastic melts, some will stick to the walls of the steel tube. This will stop you from inserting the teflon tube properly and it will give the filament a place to slump into.
There are two fixes for a slump. You can remove the extruder, take it completely apart and clean it. Careful. Screws may be stripped in the process and there is a lot of the extruder that should only be taken apart while hot, For example, never try to remove the nozzle when it is not hot enough to melt the plastic that has oozed everywhere and sealed things up. Don't try to remove the steel tube from the hotend either.
The second fix is simpler. First, remove the teflon tube from the extruder. Adjust the filament so that it forms a plug - it should be flush with the end of the extruder.
Heat the extruder very hot. If you have been using PLA, get the extruder to 240C. If you have been extruding ABS get the extruder all the way to 260C. Let it stabilize at that temperature for a good 5-10 minutes, so that the heat is traveling up the steel tube. We want the plastic stuck to the wall to be molten hot.
Using your favorite pliers, grab the hotend. Point the hotend somewhere safe, like at the build surface. Do not point it at skin or anything that can be damaged. With a single firm motion, before anything has a chance to cool, push the bowden tube into the fitting, until it bottoms out in the steel tube. About 40mm should go in,
The hot plastic should be pushed out the nozzle. You might see a spray of plastic on the surface you pointed the nozzle at.
When done, your printer should print without slumping.
It is a good idea to mark your bowden tube before removing it. If it is a new bowden tube, then you might want to measure it against the side of the extruder - it should go most of the way through the heat sink.
The new design for extruders is better - many people are switching to it. But you have to watch out for this one quirk.