RepRapPro Huxley - Extruder drive slipping / too weak when reaching the hot end (idler tensioning problem)

Hello, I am having issues with my extruder drive on my RepRapPro Huxley not working properly; it is either too weak when tensioning the idler or slipping when applying too little tensioning (which is as expected). I am using a Melzi 2.0 Ebay Hybrid, which means that it uses a shunt resistor of 0.05 ohms. I have set all the other steppers for each axis to the proper current setting (ca 0.3-0.4V over pot, ~750mA), while I've gone with 0.4-0.5V for the extruder drive, ~1A. Now, when using this particular current setting for the extruder drive and tensioning the idler as described in the RepRapPro Huxley's Wiki (screw the tensioning screws so that the gap ends is just touching), the extruder drive can't handle the force when the filament is exiting the hot end (see video 1), when I increase the current ever so slightly (while extruding) it works just a little bit better, but the extruder drive is still too weak, and of course, the stepper driver IC gets hotter, so this is not a valid solution. When I instead loosen the tensioning, a little at a time; it still cannot handle the force, until I've loosen the idler to the point that the extruder drive slips instead of driving through filament (see video 2).

Just want to note that the hot end is not clogged, the filament is flowing through very nicely (except for when the extruder drive gets hick-ups).

Have anyone been experiencing the same issue and hopefully, come with a solution or maybe someone has a straight-up answer to my problem?

Regards, Henrik. Sorry for my camera focus, it got dirty from work so it woudn't focus properly...

Video 1: [www.dropbox.com]
Video 2: [www.dropbox.com]

Edit: Forgot to mention that I am using PLA (190-230 deg C) and am heating the hot end to approx 190 deg C. I will try to tweak the heat a little today and tell you if it worked.

Edited 2 time(s). Last edit at 06/16/2016 10:54PM by henriksod.

Hi Henrik, welcome to the forum.

Are you using 1.75 or 3mm filament? If 1.75, is the hotend 1.75mm specific? It's very difficult to extrude 1.75mm filament through a hotend designed for 3mm. PLA requires quite a lot of idler tension to avoid slipping, and the extruder needs to be capable of coping with that. I'm afraid I'm not familiar with the extruder design you are using, so I can't offer any specific advice - hopefully someone else will chime in.

When you say that "the stepper driver IC gets hotter", can you measure how hot it gets? It's expected that the chips will run hot, as long as they don't reach thermal shutdown that's not necessarily a bad thing. You can add a fan to help maintain a reasonable chip temperature (70C should be fine).

I am using 1.75mm filament and the hot end is indeed for 1.75mm. I will try using this extruder design and come back with the results: [www.thingiverse.com]

.5 is stock. Info on steppers says .7 is good. Mine is .68 and works fine. I just asked for help with filament tension because stepper shaft was visibly flexing under pressure. Mine only a problem under High Retract prints.

Edited 1 time(s). Last edit at 06/18/2016 04:30PM by elwood127.

I noticed when measuring the shunt resistance that my board is using 0.1 ohms, not 0.05, which means that I've delivered lower current to all steppers than recommended. I've increased the E reference to approx 0.75V (max 0.8 according to the melzi wiki) and it is working a little bit better, but not perfectly. I have now tested the extruder drive mentioned in the last post and it is working better than the geared one that is intended for use on the RepRapPro Huxley (see video).

This was the result when printing (a cube with honeycomb infill):


Video: [www.dropbox.com]

Getting closer. The motor is still stalling, so we either need more torque or less resistance. Is the motor running hot? For a non-geared extruder you're probably going to have to run it quite near it's upper limit, so it will likely end up running quite hot - say 50C or so depending on ambient. If it's not hot you can probably afford to increase the Vref still further.

When you remove the filament after the jam, what does it look like? Is it swollen and for how far? The shape often will give you a clue as to what's happening in the hotend. From your photo it looks like the first layer or two went down well, and the problems started later. With pla this often happens when heat travels up the hotend and causes the filament to soften inside the 'cold' part of the throat. Soft pla has high friction, so if you get 10 to 20mm of soft expanded filament in the throat it can make extrusion very difficult. In other cases the soft filament expands into a gap where there's a join in the feed path, say between the end of the bowden tube and the bore of the throat. When you take the filament out you will usually see that as a ring of expanded filament. Closing the gaps, ensuring really good cooling of the cold side of the heat break (cold end fan needs to be fully on all the time you're printing) and if necessary polishing and lubricating the inside of the throat can all help reduce the force needed to extrude the filament.

Going back to the extruder, the Greg's/Wade style extruders with roughly 5:1 gearing seem to be very reliable, you might want to try that style if you can (do you have another printer to use?)

This is how the filament looks like after removing it from the printer:


I do actually have a Greg/Wade extruder, I will give it a try again. Maybe it works better after increasing the motor currents.

So, the Greg's extruder works flawlessly, I think, but I still have the problem as shown in the image from one of my last posts where I showed you the result of a bad print. The first layers are printed nicely, but then it starts to create birds nests and the filament is just tangling areound the hot end nozzle... What causes this, how do I solve it?

Hmm, that's disappointing. What happens at the extruder when the print is going wrong? Does it look like filament extrudes normally at the hotend all the time, or does the flow reduce before the print fails?

If the extruder is slipping on the filament, the thing that comes to mind from your photo is that the tension springs look very thin compared to the ones I use. I'm not sure those springs will be able to generate enough force on the idler to give reliable grip. The fact that the print starts well and then fails still suggests that part of the problem is heat moving back up the hotend, but your filament photo doesn't look particularly bad, and it looks like you have a decent fan setup on your heatbreak, so I'm guessing the problem is more about getting sufficient force at the extruder than reduce the friction at the hotend (but both will help!).

The extruder seem to work normally when this happens, maybe it is slipping still. I will get my hands on stronger springs and try them out.

On another note, my X-axis has started to behave strange; when homing it travels just a bit and then stops, what could be the issue behind this behaviour?

Everything works now, the main problem was that my hot-end was too cold (185C), so I when I increased it, the prints got better until I reached 195C, where they were perfect. I also switched out the springs for stronger ones and the extruder doesn't slip anymore, thank you James.
I managed to solve my most recent problem aswell...it's a bad connection on the +12V power connector, maybe I will switch it out in the future, but as long as I keep my hands away from the wire, it won't cause any problems. I also added a filter circuit to both the X and the Y endtops to decrease inductive interference from the steppers, which may be overkill, since there are filters on the Melzi board, but I'll leave them on for now.

Thank you for helping me; if someone has any other questions regarding my issue, feel free to ask them and I will try to answer ASAIC.

Edited 2 time(s). Last edit at 07/02/2016 09:12AM by henriksod.

Great news, thanks for letting us know. Sorry I didn't reply to your previous update - I had no ideas when I first read it and forgot to follow up.

If there is any concern about the 12V connectors it's worth replacing them. If a high current connection becomes intermittent it can generate a lot of heat. Certainly enough to damage anything touching it, and potentially enough to start a fire. I was using a cheap version of the 4 pin molex hard drive power connector to power the 5V load on my machine. It went bad, melted the connector and made a very unpleasant smell. Fortunately no flames!