What causes pressure buildup while printing?

[donmilne 2]

I've noticed that, quite often, my larger prints will start off quite well, they can get 90% through without any problem, and then it will start skipping. The skipping problem will gradually get worse and worse, it's a race against time to where the print either finishes (with defects), or I'll abort it, or the extruder will stop altogether.

I assume the motor is skipping because it doesn't have the torque to overcome the back pressure. Can anyone suggest what a gradual back pressure buildup might be caused by?

I don't believe there is any blockage: I tried the atomic cleaning method, some gunk was indeed removed, but it didn't cure the problem. At the same timing I felt little resistance for the bit of filament I used for cleaning, so I doubt teflon deformation as well... and in any case, why would that get better on the start of the next print?

I don't know why a pressure buildup wouldn't just result in a bigger volume being squeezed out?

 

[cleven 4]

As many issues it is probably the PTFE part located above the printer head. After long use it can deform slightly causing a restriction on the material :(. The second is the feeder itself. If the feeder is slipping it can chew into the material causing friction in the tubing/ptfe exacerbating the friction/problem. I noticed that this with me increased over the printing time as you have described.

1 Clean the feeder or try some of the secondary options posted on Youmagine,

2 Inspect the bowden tube for damage

3. Take apart the printer head and inspect the white PTFE part for inner roughness. You can possibly drill out the hole slightly increasing the ID to redress the tube profile. Ultimaker is now providing a glass filled PTFE which has slightly better properties compared to the original. If there is damage perhaps you can put in a ticket or just order a new one.

There are many posts about this extrusion problem, I posted some of my tests here http://umforum.ultimaker.com/index.php?/topic/7024-teflon-spacer-replacement/

Or maybe it is something totally different

Cheers,

Kris

 

[donmilne 2]

Hi, thanks for the response.

1. I've already moved to Robert's feeder a few weeks back. It seems to be working well, but doesn't prevent the skipping problem, which seems to be purely a matter of torque on the motor.

2. The bowden tube seems fine to me. It mates well at both ends, and in any case I don't see why damage to the bowden tube would get worse during a print.

3. As I mentioned in my first post, while I was cleaning the print head using the atomic method, I didn't notice that the teflon (a.k.a. PTFE) part offered any obstruction. In any case I've only had the printer a few months and it has not been heavily used. I don't expect that it's seen more than 30 hours of use.

I was thinking more along the lines of slight overextrusion causing a backlog of filament inside the head, but I couldn't explain why that wouldn't just cause the filament to exit faster.

 

[illuminarti 18]

It's not a build up pressure, I don't think. All things being equal, that would indeed just cause the filament to extrude faster.

But it might be a gradual increase in resistance due to e.g., heat spreading into the teflon part, causing either a constriction of the bore, and/or a softening of the PLA that results in greater friction.

 

[donmilne 2]

Both of those sound like brilliant ideas. Might drilling a wider bore in the teflon part reduce both issues?

 

[ian 32]

there is a new teflon hotend part made from fibre glass.... I think this comes with the new machines as standard... this does not deform easily in the hotend and reduced greatly the chance of blockages.

Also you can put a little normal machine oil in the teflon tube that transports the filament that can help things along.

Ian :-)

 

[eldrick 65]

From experience with my UM2 when new, you may have bits of the OEM black feeder assembly in the nozzle. It can build up during a longer print, and cause under-extrusion as you go along. If you are seeing tiny black specks in your prints, those are probably worn-off chunks of feeder, which can partially clog the nozzle.

The atomic (cold-pull) method is not a panacea - you will probably need to do it several times until the filament comes out clean. Also, Never let PLA sit in the nozzle without printing at temps over 165C, to avoid carbonized plastic buildup in the nozzle.

Repeat the cold pull several times until it comes out completely clean (use white filament so you can see the junk) and you will probably be good to go. I've got over 1000 hours printing on mine, most of it 24-hour or longer prints, and it's now quite reliable (with |Robert|s feeder).

 

[ian 32]

can you show us a photo of your feeder and we can see what version you have ?

there are a few different modifications that ultimaker released in different ultimaker 2s to help the feeder.. a photo could answer that straight away.

Ian :-)

 

[donmilne 2]

Thanks for a replies, however as I mentioned in my first post, I cleaned the nozzle using the cold pull method just before the print, so I'm satisfied that isn't an issue. I'm also not convinced that deformation of the Teflon part is relevant either - I felt no obstructions while cleaning. I'm using Robert's feeder - which I'd rather not be distracted into discussing as I've already discussed that issue plenty in another thread: I think I've got a good handle on how to get it working well.

I've pruned my area of investigation down to a fairly narrow question: from what source(s) does the back pressure on the filament originate? and why (if my observation is correct, which it may not be) does it seem to increase over the course of a print, so that skipping gets worse and worse? (or alternatively, is max motor torque reducing?).

Actually I do have an additional thought: if I understand correctly, the UM1 didn't skip, it would grind the filament instead, killing the print, and possibly other problems too (head overheating?). Now that I know a little more about how stepper motors work, I'm guessing that the UM2 design used a weaker (lower torque) motor in an attempt to fix this issue. In fact it just replaced the issue with a different one: heavy underextrusion producing a useless print.

I'm thinking the correct solution is a stronger motor, with multiple points of contact on the filament so that one spot won't get chewed up. I'd rather not go down the drive belt route - belt failure seems like a likely common fault. I'm thinking some kind of geared mechanism.