Almost always missing layers / underextruding

[Nicolinux 288]

Thanks for the tip. I think most tests were performed at 230° or higher, so given differences in filament, at these high temps, the effects should be negligible. That said, when I printed the extrusion test (no retraction) at 250° it failed only at 10mm^3/s. Don't know what I should make of it though. The temperature is pretty high and on the far end of what PLA can take.

 

[Nicolinux 288]

Alright guys, I took the head apart. This time right down to the bone, you know, nozzle in hand and all.

I am pretty sure the nozzle is not blocked. Everything looks smooth right down to the nozzle tip. The teflon coupler looks fine too. I can slide filament without resistance. But there is a slight lip at the end where it sits on the nozzle:

 

So if I insert filament and push it sideways a bit, when I move it through, it cants/hangs a bit on its way out. Also the area where the teflon piece and the nozzle make contact is dark. Is some sort of coating or just some burnt stuff that got in there?

Next thing I found, the temperature sensor was covered in some gunk. Again it looks like something that got in there and burnt up completely. It fell off when I took it out of the heating block, and only a bit remained on the temp sensor:

 

And speaking of the sensor. It looks like a little piece is missing. It is not the mark from the screw that holds both the heating cartridge and temp sensor in place. Just a little pice is gone (or is this intended? I have no idea how the sensor works. I hope I am playing captain obvious, pointing out something that every temp. sensor has)

I think I need to file the lip away from the exit of the teflon coupler. I am not convinced that it is the cause for my problems though, because even with no retraction, my tests fail early and the filament is not moved back where it could hang on that lip.

 

[gr5 2,177]

I did the extruder weight test!

Quick answer: 12.1 pounds or 5.5 kg.

I used a bunjee cord that stretched more and more as the filament went up into the tube. I had the other end tied to weights on a scale and watched the "weight" drop down to lower and lower values as the extruder lifted more and more weight. I was at 3.3mm/sec extruder speed.

The extruder can definitely hold more weight when stopped. I repeated the experiment a few times. Everytime it failed it spun backwards (I was staring at it and watched it zip around backwards). Another way to say it is it missed steps rather than grind the filament.

I used the same light blue PLA from UM that I think you used nicolinux. I got a variation of only 2 ounces or 1%.

So my extruder had 10% to 50% more power than yours. Not sure how relevant this is as I'm not sure how repeatable your test was and how accurate. I should have taken a picture of my setup. I'll describe it in words:

I put a spaghetti pot on a scale, connected the two handles with very short bungee. inserted a 2 foot PLA. Clamped to the bottom with vice grips with bungee also going through vice grips grip. This second bungee reached both ends down to the bungee on the spaghetti pot. The spaghetti pot had 14 pounds of weights in it. After I was done I measured everything together including the weight of the vice grips. Subtracted total weight from the weight when it failed.

 

[aaronalai 2]

I think I need to file the lip away from the exit of the teflon coupler. I am not convinced that it is the cause for my problems though, because even with no retraction, my tests fail early and the filament is not moved back where it could hang on that lip.

Do you think there is any way that this ring could be at a location where the filament is sometimes soft and sometimes harder as a function if it's location to the hot-end nozzle and fluctuating temperatures of nozzle? This may be why you were getting better prints at higher temps, this location in the hot-end may have always been soft and not prone to getting stuck in the ring. Also, perhaps at higher throughput rates the ring may impede flow; which is why there is under extrusion at higher printing speeds.

 

[gr5 2,177]

Maybe but remember that you *always* get better extrusion at higher temps. There isn't some sudden melt point where extrusion stops getting any better - viscosity keeps getting lower.

http://umforum.ultimaker.com/index.php?/topic/4127-um2-extrusion-rates-revisited/

 

[aaronalai 2]

Maybe but remember that you *always* get better extrusion at higher temps. There isn't some sudden melt point where extrusion stops getting any better - viscosity keeps getting lower.

http://umforum.ultimaker.com/index.php?/topic/4127-um2-extrusion-rates-revisited/

 

Yeah, I agree; what I was trying to get at is that there may be an eddy type phenomena forming in the teflon piece. So when he is trying to print the top of the cup at 10mm/s, instead of the liquid plastic flowing vertically through the hot-end there is a little swirling action happening at the ring interface inside the teflon piece, as the plastic decreases in viscosity it would be more susceptible to swirling inside the hot-end effectively minimizing throughput out the nozzle.

 

[chrisp 10]

I just wanted to add another finding that may or may not be an issue. Under my extruder album I have a few pics of the bearing that is mounted in the tensioner, which seems to be rubbing against the black plastic so tight that the bearing has I hard time turning. The tension screw is as loose as it goes and the tensioner with the bearing is so tight it sits. I know once the filament gets feed through the extruder, pressure is pushed against the bearing, which would cancel out this problem, but I don't know how to confirm it. Also the black plastic from the extruder case always wears down making the hot end susceptible to pieces clogging the nozzle. What type of plastic is the black case enclosing the extruder?

 

[ian 32]

this problem within the extruder housing design is being offically addressed and resolved at the moment I believe.

These problems correctly addressed should lead to far improved extrusion performance and also reduce the incidents of under extrusion that some users are experiencing.

Ian

 

[IRobertI 520]

Well, I think I've removed one variable from my system at least...

 

I'll make the files available Soon

 

[Nicolinux 288]

Well, I think I've removed one variable from my system at least...

 

I'll make the files available Soon

 

WAAAAAANT!!!

 

[nallath 1,122]

The material of the feeder is ABS.

 

[gr5 2,177]

What I think is happening is that with retraction, the sharp pieces of PLA can rub against the black ABS housing shown in Chrisp's photos above. This rubbing breaks off pieces of black ABS, some of which make their way to the nozzle and cause a clog. Having straight filament pulled from straight below the UM might keep the PLA from rubbing on the ABS. Although I'm not certain that will help.

Maybe the extruder needs some stainless steel parts or something much more scratch resistant than ABS? I don't know what the solution is.

 

[gr5 2,177]

@chrisp - I don't think the bearing ever touches the drive gear directly so I don't think anything bad happens when there is no filament in there and the drive gear is rotating.

But the drive gear chews up the PLA a little bit making for sharp areas. As that goes in and out (retraction) it could potentially grind the black PLA. Although I would expect all the grinding to happen on the drive side - not the bearing side so maybe you are right.

 

[takei-naodar 0]

Or maybe make it out of PLA so no clog chance?

 

[gr5 2,177]

there is a little swirling action happening at the ring interface inside the teflon piece

 

I can't wait until you get your own printer so you can get a better feel for these things. First of all the pressures in the nozzle are pretty high. 8 to 12 pounds force are common over surface area of 3mm meaning about 1000 psi. That's a lot of pressure!

Also if you play with PLA and heat it in boiling water and bend it around you will see that it gets very soft like gum. Glass temp is around 50 or 60C where there is a rather sudden transition. Melting temp is closer to 180C where there is another transition. Those temps (50-60C) may be occurring up in the nylon. Or maybe colder. In either case it takes a bit of force to get coolish PLA around this "lip". Maybe. Certainly not a "swirling" issue. It would be easier to swirl cold honey in a jar. PLA's viscosity is similar to honey I'm guessing around 240C. In the area of the nylon it should be much colder and more like the viscosity of modeling clay (plasticine). Around 170-180C the viscosity is closer to toothpaste.

 

[gr5 2,177]

r maybe make it out of PLA so no clog chance?

 

Brilliant! Make it out of clear or natural PLA so that it also doesn't leave a blackish spot in your print. Daid says he has the STL for a printable version of the extruder.

 

[gr5 2,177]

So Nicolinux - while you had the head apart - I hope I hope you did two things:

1) Heat the hot end in a flame to burn out any ABS in there. Or soak in acetone for an hour to dissolve any ABS.

2) Drill out the nylon piece with a 3mm drill and make sure the filament slides through there with less than a few ounces of force.

I suspect there is a little piece of ABS in your nozzle tip sticking to the wall of your nozzle but not clogging the tip directly.

 

[Nicolinux 288]

Sorry for the delay. I don't have time right now (working... this hobby of mine ), but I didn't assemble the head yet. I will also try to reproduce your extruder weight test since it doesen't compare with mine and the difference in "power" might result thereof.

 

[chrisp 10]

Gr5,

I have been feeding from just below the extruder, however it's not lined up perfectly straight. The hole in the extruder that the filament enters is slightly curved to the bottom left to accommodate the roll of filament being mounted on the back of the UM2. I have my roll on bearings, feeding at that exact angle from underneath.

Correct, the bearing never touches the gear because it is against the black ABS in the extruder and cannot go any further.

I was just thinking about designing a new extruder that is beefed up out of PLA and there happens to be one already? I love it!

*Is there a problem with quoting for anyone else? I can't seem to copy and paste anymore.*

 

[gr5 2,177]

I will also try to reproduce your extruder weight test s

 

Well then you should DEFINITELY get prontrface. It's free and very helpful for this test. You can set the extrude distance and speed. I think I used 50mm for the distance and 200mm/minute for the speed. prontrface is a gui the connects to your UM2 (or UM Original) through a USB cable. It gives you SO MUCH MORE CONTROL over your printer which is especially useful when doing tests:

printrun/pronterface download:

http://koti.kapsi.fi/~kliment/printrun/

 

 

[gr5 2,177]

I rebuilt my test jig just now so I could take a photo (I should have taken a picture originally). The bungee is critical so that you can tell the printer to extrude 50mm and it slowly reduces the weight on the scale until suddenly - ZIP - it falls back many steps on the stepper - maybe 100mm.

Let me know if the picture isn't visible and I will upload it to the forum instead.

 

[IRobertI 520]

I tried reproducing your tests a few minutes ago but it failed on account of my shitty digital scales... Since it doesn't give a continuous readout it's very difficult to get an accurate reading. But I have a few very crude numbers to share.

I use PowerBlocks which are great for working out but have a fairly coarse stepping: 2.5 - 4.5 - 7 - 9 - etc. I used a couple of good old fashioned plates of 2.5 and 1.25 to try and get a bit more granularity.

On static hold I got somewhere between 7kg-9kg. I had a 7kg weight hanging fine and then slowly added the weight of a 2.5kg weight. It failed somewhere in the middle. Science!

Using the Move material function I could move 4.5kg without issue. Slowly adding the weight of a 1.25kg plate it failed somewhere along the way... Science strikes again!

 

[gr5 2,177]

I reposted my data, Nicolinux's data, and IRobertI's data regarding pulling force of extruder here:

http://umforum.ultimaker.com/index.php?/topic/4222-pulling-force-of-um-extruder/

Please post any test results there and I will continue to refine the summary at the top of that topic (although IRobertI is a moderator and welcome to edit my post directly if he has new data).

 

[aaronalai 2]

I can't wait until you get your own printer so you can get a better feel for these things. First of all the pressures in the nozzle are pretty high. 8 to 12 pounds force are common over surface area of 3mm meaning about 1000 psi. That's a lot of pressure!

Also if you play with PLA and heat it in boiling water and bend it around you will see that it gets very soft like gum. Glass temp is around 50 or 60C where there is a rather sudden transition. Melting temp is closer to 180C where there is another transition. Those temps (50-60C) may be occurring up in the nylon. Or maybe colder. In either case it takes a bit of force to get coolish PLA around this "lip". Maybe. Certainly not a "swirling" issue. It would be easier to swirl cold honey in a jar. PLA's viscosity is similar to honey I'm guessing around 240C. In the area of the nylon it should be much colder and more like the viscosity of modeling clay (plasticine). Around 170-180C the viscosity is closer to toothpaste.

 

Trust me, I can't wait until I get my printer either!!! That aside, I understand to some extent the forces generated inside the extrusion nozzle, that being said it doesn't matter how much pressure the molten plastic is at or what it's viscosity is, it is still undergoing flow (even really slow flow still counts, honey clay whatever) which is susceptible to drag. Swirling, eddies... these are just the byproducts of irregular drag on the flowing medium (which admittedly is the word I should have been using all along) independent of the pressures the medium is under. From cutaways of the hot end I've seen, it doesn't look like the method for shoving the filament through the hot end can handle anything but perfectly smooth walls and if there is a little burr, cut, abrasion ... it will cause drag and it will be significant because of the tight tolerances inside the hot end structure. Now just how significant this is to Nicolinux's problems is debatable.

 

[Nicolinux 288]

@gr5: Did I said thank you already? So thanks for the weight test. Regarding your other comments - already using Pronterface This was my goto tool for our crappy Pursa Mendel.

Now I've got almost all ingredients here. Hypodermic needles, acurate temperature sensor, acetone (the nozzle is taking a bath in it right now). I've only missed those stupid bungee ties and here in Germany the shops are already closed. I will reproduce the weight test tomorrow.

@AaronAlai: About the lip inside the teflon coupler. I am not sure any more if it is a good idea to file it away. Because if I do, it would increase the available space right where the coupler and the nozzle meet. If retraction occurs and the plastic is already soft, then it would have more room to expand sideways and on then hang on subsequent retraction action.

@Chrisp: Quoting is alright for me (Chrome, Mac OSX). By the way, are you sure the extruder case is made of ABS? The finish and sturdyness feels more like those Delrin pieces from the UM1.

@IRobertl: Just wanted to remind you that your new filament spool holder looks nice. I whish I had one like it. But I don't. So sad...

@Ian: Where, how do you know about the extruder improvements? Is this another moderator insider thingie? Then I'd whish I'd be part of this Freemasonry underground movement