ID my print problem?

Hi folks. Thanks in advance for trying to help!

We've had our RepRap for a couple of months now and I love it but it's driving me crazy. It seemed to start out printing fine, then keep getting worse. It's not even consistent in how it messes up, which is throwing me even farther off.
The basics: Prusa Mendel 2; 1.75 PLA; Pronterface; Slic3r; Sprinter firmware...what else should I include in that list?

I think the biggest problem is that the filament isn't extruding consistently. I've cleaned the hot end, cleaned the filament, calibrated the extruder (many many times), adjusted the tension screws (both tighter and looser), with no noticeable difference. If anything, it seems to be getting worse.

Basically, I can calibrate the extruder, then immediately start a print, but by as early as a few layers in, the filament isn't feeding in a steady stream any more. It seems to slow to drips and drabs, which means I end up with a mess of gaps and strings. But then, if I stop the print and check the extruder calibration, it's fine.

I've tried slowing it down to 30 mm/s, speeding up to 60 mm/s; turning the temp down to 160, up to 190. Sometimes it messes up on the first layer, sometimes not until an hour in. I'm going nuts! Sometimes it evens out when I help feed the filament in, but I don't know how to correct for that. It doesn't need that when I'm calibrating, so what do I have set wrong?

So, what should I try next? What other info would help you guys? Should I just take the whole thing apart and start from scratch? (Please don't let that be the answer!)

Photo 1 - seemed to be going well, but messed up an hour into the print. Oddly, seemed to recover at the end and the top looks pretty good.
Photo 2 - Long print full of strings and gaps. Extruder calibrated right before, checked right after, and was fine.
Photo 3 - proof that the darn thing sometimes prints fine.
Photo 4 - today's trial. The perimeters looker fine, but the fill was gappy and stringy. The solid section is where I was "helping" the filament by giving it a bit of feed pressure on the way in. Obviously not a sustainable fix.

You aren't stripping the filament / grinding so its skipping step ? maybe look at the current to the stepper.

I did try turning down the current; no change.

My biggest confusion is, why does it extrude fine before and after prints, but not during?

Where is the hot-end from?

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Waitaki 3D Printer

Did you measure the filament diameter to see if it varies wildly over a few feet/meters? Could the problem be that the filament gets too narrow as the diameter changes for the extruder to get a good pinch on it and starts slipping?????

It sounds like it is slipping, but intermittently.
Have you cleaned out the teeth that push your filament? Send a pic of it. The teeth may be too shallow or uneven.
Are you using a Bowden cable? This may be creating extra friction, slowing your extrusion

some hotends (esp. JHeads not from the original creator) tend to clogg when getting too hot. This is because a small part of the filament is not guided between the peek and the metal part. When the hotend is manufactured wrong, there is a possibility that the filament gets too warm in this place, so that it gets soft and cannot be pushed anymore.
This would explain that it does work when calibrating (only short period of heating) and fails on longer prints.
Try mounting a small fan on the peek-part of the hotend (or on the upper metal part for E3D), this will probably help. Or, if all else fails, get a new hotend.

If you're using a bowden extruder, friction can increase as z-height increases due to the curl of the tube, resulting in exactly what you see in the first picture. I had the same problem when I switched from a 0.5mm nozzle to 0.3mm - the increased pressure in the melt chamber was just enough to cause the extruder to skip at about 1cm high, but lower than that it was fine.

This was fixed by lowering the print speed and repositioning the extruder so there was less curling of the bowden tube as the print got higher.

tl;dr: try lowering your print speed.

Thanks for all the responses!

I do not use a bowden cable.

I'm not sure where the J head is from. I bought a build kit, and it didn't specify. If that's the problem, and the fix is to get a new J Head, who is the "original"? It sounds like that's who I should buy from, but I'm not sure how to tell.

I've also noticed the J head assembly seems loose. Not up & down, but like it can rotate. Not matter how I tighten the bolts going through the mounting plate and the extruder assembly, the hot end still has some rotational play. Normal? Problematic? Fixable?

There is a fan blowing on the hot end already.

I guess filament could be a problem. I haven't tried different reels because I only have 2 and they're from the same maker. I did measure it and it seems pretty consistent. It's not oval or dented, it's kept in a very dry environment. Is it worth trying another manufacturer? The stuff I have is from MatterHackers. Anyone have thoughts on the best PLA?

I'm going to try changing filament before disassembling down to the bolt, but I'll post pictures when I get that far.

Ok, tried switching filament and slowing things down with pretty good results.

I switched to a different reel, measured and averaged a good 2 meters of it and came up with 1.72 instead of 1.75.
In the slicer I changed that and also slowed the print speed way down.

It printed fairly well at 15 mm/s. My vertical edges seem kind of jagged, but I'll work on that next.