Printing air and filament stopppage

I have been having an ongoing problem with my printer since I first built it. The problem is printing air. The cause is filament stoppages.

I will be printing and everything is going well, so I leave and return later only to find my extruder has worked its way up the "Z" axis by about an inch and while it plods endlessly following the path dictated by the slicer there is no filament being deposited.

I kill the print and reverse the filament feed (I use Repetier host) the filament doesn't move, so a further reversal aided by gently pulling the filament and I see that the filament has a nice curve cut into it. The filament has jammed and the hobbed drive has cut the "nice curve" without driving the filament.

The filament was feeding fine at the start, and there was plenty of heat in the print head and it was still at the required temperature while printing air".

Initially I had a Wade's extruder on my printer but was often having filament jams and breakages caused by "heat creep" up the print head and each time it was a hour or more tear-down and cleanup to get restarted.

I finally got a new extruder (MixShop G-1) which did a pretty good job and further modified it to add both a heat sink, a fan (directed just above the heater block), and insulated the heater block with Kapton tape.

My breakages dropped significantly and heat creep became a thing of the past. I do however still get stoppages of filament. I have tried all manner of pressure spring adjustments on the filament drive's hobbed drive/bearing interface with inconsistent results (I wonder if I'd be better off without the springs and apply a set number of nut turns once the bearing is in contact with the filament thus limiting how fare the hobbed drive can cut into the filament in case there is more "heat creep" than I think there is).

It seems that if you have melting occurring in the printer nozzle at a uniform temperature and speeds are slow (defaults) and pressure on the filament drive interface the flow of plastic should remain constant. Has anyone experienced these problems or have any thoughts on how to resolve my stoppages?

Bob

PS, my printer is a Prusa/Mendel, my extruder a Mixshop G-1 and it's standard print head with an added heatsink and fan. My filament is 1.75 PLA extruded at 190C. My nozzle is usually 0.35 and occasionally 0.50, both are regularly cleaned and free of blockage or debris.

I know you did a few of these, but I figure I would try and make it as thorough as I can.

A few things can cause this, the first is obviously a jammed nozzle, another is trying to print too fast. Most heads are actually only rated for around 100mm/s, if that. You would be shocked at the pressures inside a head while printing at high speed.

Too tight or too loose on the extruder pinch can also cause it. Too loose obviously because the filament just slips, but too tight can crush it, making it harder to get through a head or bowden, this extra force can cause the extruder to slip. This generally happens more on 1.75mm since most hobbs are meant for 3mm, but measuring a bit of filament as it leaves your extruder can check. Expect some deformation, just not a lot. What's a lot? If it binds in your head or bowden.

Another thing that can cause it, is too much retraction.
Too much retraction causes hot filament to be pulled repeatedly up into the hot end, heating it, and causing it to start grabbing the liner. Finding the sweet spot is harder on 1.75mm and/or a bowden. the easiest way to tell if it's a retraction issue is to look at the print. If it works fine until a section where there is a lot of retraction happening, and does it on more than one print, consistently, where it's doing heavy retraction, then it's likely a retraction problem.

I mentioned it has to be consistent, the reason is over or under heating the filament can also cause it to happen. If you over heat the filament, the head will start heating up and can cause the filament to soften too soon and start to stick, just like with retraction. This is time based, not retraction based. If you under heat it, the extruder has to work extra hard and when it or the stepper driver over heats, it will lose power. This can happen regardless of hot end heat levels.

Almost certainly nozzle jamming - either due to poor nozzle design, high speeds, low temperature.

That is not a hotend I have heard much about.. Is it all metal? All metal hotends often have issues with pla. If it has a ptfe liner, the liner may be damaged.

Try upping the temperature to 200C or even 210.

Watch the printer for an hour or so - monitor the hotend temperature it should stay within 2 degrees of the set temperature.

Try printing slowly and see if the issues go away (20mm/s)

Try turning off retraction for now.

Oh and insulate the hot end of your hotend!

Thanks guys,

I'll recheck speeds to ensure they are not too high, recheck nozzles for irregularities, and look at retraction.

I have not done anything with retraction and have only the defaults in play. I will certainly check this function out further to understand it more.

I will monitor the actual temperature fluctuation over time, I feel its OK, but will track more closely for variation. I willl add more Kapton insulation around the heat chamber (do I also understand you to suggest wrapping the nozzle as well?).

I will try higher extruding temperatures, I have been operating around 190 -195C.

The mix G-1 extruder can be seen here: [www.mixshop.com]. Mine differs in that I have added a heat sink above the heater chamber and assembly here [forum.mixshop.com].

Thanks for the advice, I'll post my results after some experimentation.

Bob