Advice needed regarding Bowden extruder

Hi,

I built a RepStrap that uses a Bowden system. The extruder (Compact Bowden Extruder: [www.thingiverse.com]) seems to have plenty of power, with the motor getting about 0.6A. It pushes and retracts filament fine outside the hotend, but when inside the hotend, it seems to have a severe compression problem. When the hotend is heated to 190C, the extruder pushes out about 3mm very well before it gets stuck--the drive gear still spins but it only chews up the filament and doesn't push it forward. Though if I wait a few seconds for the nozzle to ooze out the filament, I can extrude another 3 or 4mm again before it gets stuck. Retraction still works fine while the extruder is stuck.

How could I remedy this? I suspect that my Bowden tube is way too long (1 meter), but I googled and found a person working with an 80cm tube just fine. What is an ideal length for a Bowden tube?

Thanks!

Hi,
An actual picture of your setup would be very helpful to diagnose your problem. But based on what you wrote above, I can give you the following tips:

1. The extruder stepper is not getting nearly enough current, you need to increase it to 1.2A at the very least. Direct drive extruders require a lot of torque.

2. The ideal length for a Bowden tube is zero i.e. you always want to make it as short as possible. Indeed 1m is way too long. 60cm would me what I personally consider an acceptable max length. YMMV.

3. Make sure you are cooling your hotend properly, otherwise heat may creep up along the heatbreak, expanding the filament inside the hotend and causing a jam.

Finally, what nozzle diameter are you using? With a Bowden setup, 0.4mm is the minimum nozzle diameter recommended. Back pressure increases a lot when going from 0.4mm to 0.3mm nozzle.

If it's chewing through the filament at such a low current, you need to tighten it up to make the hobbed part dig into the filament more.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks very much for your responses!

Here is a picture of my setup: [www.dropbox.com]
I've shortened the Bowden tube to 50cm. I forgot to mention that I'm using PLA, and the nozzle diameter is 0.4mm. The hotend is an E3D clone, but its fan seems to be good enough; the upper fins of the heatsink only get comfortably warm to the touch.

Increasing the current to 1.2A still results in the drive gear chewing through the filament, so I think it's likely that I haven't tightened the bolt enough. How tight is "tight enough," however? I'm wary of snapping the shaft off, which I've seen happened (albeit on the Internet), and I'd hate to have to buy another motor. Does it have to get to the point of "it hurts to turn but it still won't move"?

What happens to the hot end temperature when you extrude? You may need to go through the PID tuning process to get enough power to the the hot end in order to stay stable enough. Extruding will put 'cold' filament into the nozzle cooling it down. If it cools down too much the filament turns solid and will jam until it's warmed up again.
see
http://reprap.org/wiki/PID_Tuning

Thanks for the PID advice! I had totally forgotten about it and been using the default values for Ultimaker (Marlin firmware).

I can push out about 6mm at 3mm/s now before the thing slips at 220C. I found that the higher the temperature goes, the further I can extrude in one go. Though 6mm at a time at 3mm/s seems awfully slow...

Regarding the hotend itself, manually pushing the filament through is relatively effortless and I don't need to wait out the "stuck" stages, so perhaps it's the drive gear that isn't doing its job well enough. Before I buy another MK8 gear, though, is the Compact Bowden extruder that I'm using recommended? Is there any other Bowden extruder, especially one with more grip, that I could use?

I use the same bowden system, albeit with a MK7 drive gear. I have as much tension as possible on the filament against the MK7 gear.

3mm/s is quite fast - that's 180mm/min. Try reducing your print speed.

Also as has been mentioned, your temerature is probably dropping too quick while the filament is passing through it. Check what happens to the hotend temperature while your passing filament. If it drops over 5 degrees, either the PID is wrong or the heating element isn't getting enough current to heat the block fast enough.

I found that the temperature remains a straight line on the graph in Repetier-Host, so then perhaps I'm pushing the extruder too fast.

Does your extruder gear slip when it's going too fast? Or does the motor stall instead? I have never seen mine stall so that leads me to think my MK8 is somehow worn already, as it's supposed to have quite a good grip according to its reputation.

My thoughts are it is not reading correct temp. With the knockoff clones the thermistor has a lot of variances. Are you sure you have the right one configured in marlin? Just because it reads 220 doesnt make it 220 dont suppose you have a meter with a k type temp probe?

What about the PSU, is the 12V line stable or does the extruder clog every time the heatbed-FET is ON?
A voltage drop could also be the cause for a "freezing" nozzle, as well as a bad heat transfer between heater element and hotend.
I´m just trying to think out of the box
-Olaf
I upped the 12V to 13.5V . Now I´m happy with the overall temp-stability

I suppose it was wishful thinking to tell myself that the thermistor is good enough. I use thermistor #11 in Marlin since a round of Google told me that it is the "common eBay thermistor." What would you recommend to use as a temperature probe? Any reliable brand?

Since I don't use a heated bed, the 12V line should be stable enough, but I don't think I can confirm that with only a digital multimeter. Thanks for telling me of that potential issue!

A thermocouple with the external thermocouple board is the most reliable, but a bit more expensive.

A thermistor can be very accurate but often needs to be calibrated. I used my multimeter with thermocouple to calibrate rhe thermistor for the heated bed and changed the lookup table values in tempsens.h IIRC.

I've also checked the hot end thermocouple with the multimeter and gives the same temps within 2 degrees.

For a E3D v6 hot end the thermocouple is much easier to fit into the tiny hole. Glass thermistors can easily break and short circuit when using the fastening screw. Another advantage is that a thermocouple supperts higher temps.

Edited 1 time(s). Last edit at 07/16/2015 02:25AM by imqqmi.