Problem with print speed?

[donmilne 2]

Can anyone suggest what the problem is here? Below is an OpenSCAD rendering of a part I'm trying to make. It's a sleeve designed to slip over the standard UM2 spool holder and expand the diameter to one that fits the hub of other PLA filament I have. Inner_diam=53mm, outer_diam=74mm, height=90mm. Note also the retainer "ears" designed to stop the filament reel sliding off the adapter.

 

[donmilne 2]

Now below is a picture of the result - I've actually been having this problem quite a lot recently - the printer does the job perfectly until it reaches the flat top, and then it messes up. Filament stops coming out, my UM2 is printing air.

 

[donmilne 2]

I had an earlier result that looked much worse than that. So, I increased the resolution, increased shell thickness to 1.2mm and reduced the print speed to 40mm/s. The result was that the printer printed the body perfectly - no real blemishes at all, until it reached that flat top, then it instantly messed up, as you can see. That can't be a coincidence.

Another thing I noticed was that the "air printing" at the seemed to me to be faster than I expected, but I don't see anything in the Cura settings that controls top layer speed similar to the bottom layer one.

I also wonder if those sticky up ears might be confusing Cura, making it treat the top layer as... not the top layer.

I'm thinking that it's trying to print the top shell too fast - the filament can't feed that fast, I've no doubt that when I remove the filament I'll find a gouge ground out of it.

Any suggestions as to how I might kill off this problem? It's really a pain in the ass for it to work for 18 hours only to mess up in the last 5 minutes.

 

[donmilne 2]

Oh: Cura 14.6.1, IRobertl's bugfix firmware, which I have not updated since I've not seen a changelog for the latest official firmware.

 

[donmilne 2]

One thing that occurred to me in the wee small hours last night is that setting speed limits in Cura is getting it backwards. What we should be setting is flow rate limits, i.e. if the extrusion "demand" increases per unit time then it should automatically drop the speed to avoid stressing the feeder.

 

[gr5 2,210]

Did you look at the part in cura "layer view" to see if something strange is happening there?

Anyway I'm guessing maybe there is lots more infill at the top - perhaps it is trying to fill in the entire top - not sure and that your infill speed is much higher than your shell speed - say 100mm/sec for infill and 40mm/sec for shell?

If the above two suggestions don't help, at least try sinking the part into the print bed such that cura only prints the "top" so that you don't waste so much time before getting to the problem spot.

Also make sure you don't have any plugins enabled on the plugin tab such as "tweak at z".

 

[donmilne 2]

Hi, thanks for replying. I'm not sure what you mean by "infill at the top" - it is supposed to have a solid top, with a 1.2mm thickness just like on the body. Are you saying that the top is considered infill rather than shell? You are exactly correct that the infill speed is set to 100mm/s with 40mm/s elsewhere. [Edit] No, I take that back. Infill speed is set to 0, it was travel speed which was set over 100mm/s (in fact 150mm/s). So I'm stumped: even if the top was considered infill it should have printed at the same speed.

I have not yet used the "tweak at z" feature - I've just checked and none are listed as enabled.

I don't really understand what the layers view is supposed to tell me - it has little or no mention in the Cura user manual.

Maybe I should revert to the standard firmware before I try again.

 

[gr5 2,210]

definition of "infill":

When a layer is printed the gcode usually has a shell printed for that layer which prints out one or more "islands". The shell can be one pass or many passes. It also prints the infill between the shell paths. The infill is sparse on most layers but on top/bottom layers it is filled in 100% I still call it "infill". When your print got to the top infill you greatly increased the flow rate because you are doing long straight lines so the printer can finally get up to "full" speed. Full speed in this case is only 40mm/sec - not particularly fast.

It wasn't 100% clear to me if the printer stopped printing entirely (went silent - all axes stopped) or if the X,Y,Z axes kept moving but the E axis (extruder) stopped. Or if E was still moving/spinning but the filament stopped coming out.

So I'm trying to figure out which of 3 failure modes it was. I still dont' know.

The "check cura layer view" is to check to see if the gcode even has the commands to print the top. Maybe the printer was *told* by cura to not print the top. There are many reasons this could happen but I don't want to get into them until I know which of the 3 failure modes occurred. Also this first failure mode could be caused by you pulling out the sd card before it was fully written to.

In the second failure mode - E axis not moving - this is strange but can happen if the printer thinks the print head is below minimum temp. You can check what temp it thinks it is at in the TUNE menu while the failure is occurring.

Third failure mode indicates ground filament or slipping sleeve on extruder or clogged nozzle. This is very common but I would have thought you would have noticed when you printed it again (no filament coming out at *start* of next print). Unless maybe you changed filament between prints?

If you don't remember what happened or weren't there you can reslice just the top of the part so you don't have to wait so long to see the failure and pay more attention this time.

 

[gr5 2,210]

I'm thinking that it's trying to print the top shell too fast - the filament can't feed that fast, I've no doubt that when I remove the filament I'll find a gouge ground out of it.

 

Did you find a gouge? If so decrease speed, or layer height, or increase temperature. Here is a graph that shows absolute printing speed limits based on layer height, speed, and temperature - the dark blue line - I like to stay at half this speed or slower:

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

 

[gr5 2,210]

"air printing" at the seemed to me to be faster than I expected, but I don't see anything in the Cura settings that controls top layer speed similar to the bottom layer one.

 

Sounds like failure mode 2 or 3. Probably 3. It may seem faster but it's probably just that it has more and longer straight paths so it can go faster.

Was it extruding a small amount? Maybe 1/10 of usual? Or nothing at all?

It sure sounds like a nozzle clog or grinding filament fixed by printing slower and/or hotter. I guess I'd like to know your layer height/ print speed (40mm/sec) and nozzle temp to see if you are close to the limit.

 

[donmilne 2]

The layers view showed a solid top, so nothing wrong there.

The printer didn't stop, it kept going through all the motions, printing on and with air, nothing at all coming out. I haven't yet looked for the gouge - I'm afraid I've just turned my back on it this last couple of days as these feeder failures and firmware bugs are beginning to be discouraging.

Temperature was UM2 standard for PLA - not tuned.

As to speed, I'm fairly sure it was faster that it was printing the bottom or the shell. I.e. where the base and shell was zzzzz,zzzzz,zzzzzz, the top was zop-zip-zop-zip, rather like when it was doing the honeycomb. I guess setting an explicit speed for infill is certainly something to try.

The irony is that this widget I'm making is designed to reduce drag on the filament spool, reducing the risk of underextrusion. The mark I attempt (without the ears) was in place while this was printing. It works great! - visibly smoother.... but didn't prevent the problem.

 

[donmilne 2]

Ok, I checked the filament, and sure enough it had been grinding. No nozzle blockage - when I cut off the bad part of the filament it extruded just fine.

While I was there I noticed that the UM2 now has a "Flow%" parameter stored for the material. Do you know what that's about?