Speed limit

Today I ran out of filament during a long print and so changed to a new reel on the fly. The print had been going quite nicely with speeds set to 100mm/s. When the new filament hit the hotend (i.e. several minutes after changing), the extruder suddenly started slipping on large infill areas. I had been printing at 240 degrees (ABS), and increasing the temperature to 250 degrees stopped the slipping. So the new reel probably has a slightly different plastic (though it was the same type from the same supplier). So I think the speed limit for the Ormerod is likely to be set by the maximum rate that the plastic can be heated and pushed through the nozzle rather than the X and Y movements.

Dave
(#106)

I agree with you...

I tried a really cheap Chinese product...
result
Back from the glass to the tape..
After 1 kilo the extruder sawn..
Low speed or many problems..
Bad smell...

will be 'secure print this stuff?

Be careful with ABS!
I had several friends intoxicated by the fumes.

Dario

Hi Dave, I noticed similar things with PLA and ABS when I was trying out various speeds (particularly with PLA, since I had the nozzle temp low so it wouldn't remelt the laid-down print while adding another layer -PLA is much more prone to this I find). Particularly with a blue batch of ABS - I had to strip down the extruder because the teeth clogged using that, whereas I printed hours of black bought from the same supplier without issue (the extruder didn't skip at all, it just silently ground off enough ABS to lubricate itself so it could spin freely, but no extrusion was occurring - luckily you'd mentioned the same a couple of days before that so I didn't panic too much ). I'm not sure if the difference between the two is just one of softness, or heat capacity, or "melting point".

Possibly an "all metal" hotend might be an improvement (or at least a nozzle that isn't lined throughout with an insulator) - I think it's a case of heat transfer through the PTFE and the filament itself being the limiting step, I made a clone nozzle a while ago, I'll make one that has a 2mm bore to the tip for the distance it's passing though the heat block and see if that improves. Raising the block temperature pushes more heat across the PTFE barrier in less time, but removing the PTFE from the melt zone should mean it runs at the same temperature since there's no resistor there (the system being kind of analagous to the voltage drop across a resistor, the voltage being the temperature and the heat flow from block to filament being the current, and the PTFE being a resistor in series limiting the current and increasing the voltage drop) . I don't see any problem with the nozzle temperature itself - it stays pretty constant no matter what feed rate I use, so there's more than enough power available, it's just a case of it getting to the filament in time hopefully

I might also try a more grippy and less scratchy drive nut like this [www.tridimake.com],

CHeers

Ray

I've settled on 70mm/s to be the best compromise for printing moves. Non-printing moves can be a lot faster. Any faster and the extruder lag makes the corners blobby - which ruined a print that had a large hole in a vertical wall. As the edges of the hole closed together the blobs built up on the corners of the thin closing edges and eventually one caught the extruder and made the Y motor miss a step. I did print it again at the same speed, but sat watching with a needle file in hand, and every time the blobs built up I quickly filed them off in the intervals when the head was out of the way. Which worked, but not really a permanent solution. At 70mm/s the extruder is not lagging so much and there is far less corner blobbing. I think that even if a better hobbed part fed the plastic fast enough, at faster speeds the extruder lag will be a huge issue. As I said, I feel sure that that could be compensated for in firmware. I'm not sure whether the lag is always a constant time. My feeling is that it could well be - the propagation through the extruder behaving similarly to most types of wave action, and having a fixed speed of propagation regardless of amplitude or frequency. In which case, as said previously, all that's needed in firmware is to insert a fixed time delay in all X and Y movements.

Dave
(#106)

Hi Dave, have you increased your extruder speed limit and acceleration too (going too high with acceleration may increase the chances of filament grinding, and/or loss of torque). My naive model of the lag is that the tube contributes some slack, or a distance that has to be travelled by the filament before the pressure at the nozzle is affected on a retraction or return from retraction, and accelerating the filament will shorten the time needed to take up this slack - I'm using the following:

M203 X6000 Y6000 Z600 E6000 ;set feedrates - divide by 60 for mm/s
M201 X2000 Y2000 Z50 E500;set accels
M906 X1000 Y1000 Z1000 E1000 ;set currents

and I have retraction of 6.0mm at 40mm/s in slic3r

The default speeds and accelerations are:
#define MAX_FEEDRATES {50.0, 50.0, 3.0, 16.0} // mm/sec
#define ACCELERATIONS {800.0, 800.0, 10.0, 250.0} // mm/sec^2

so I'm retracting at 2.5 times the default top speed, and with the higher acceleration, the initial part retraction is happening much quicker than using defaults and it does seem to reduce blobbing (I should cut down the retraction length maybe so that it takes less time still and the head doesn't dwell so long, which also seems to contribute to blobbing).

It's still going to take some finite time for retraction to occur (whether it's a constant time or constant slack distance with variable time), and that's why the "wipe on retract" option is so attractive, I stopped using this when people reported hangs - maybe time to see if slic3r 1.0rc3 does it differently and if it's compatible with the firmware now...

The retraction lag is different from the heat lag that you started the thread with of course, if my new tip (with only the upper part ptfe lined and the business end having a 2mm metal bore) improves that I'll feedback (also if it doesn't, if I ever get it made )

Cheers

Ray

Edited 1 time(s). Last edit at 02/24/2014 04:36AM by rayhicks.

Scrub that most paragraphs of that last post - I did some comparison slicing using cura and slic3r (with and without wipe-in-retract) when a new (mainly cylindrical) model turned out warty - I just couldn't prevent slic3r from leaving warts at high speed. Cura however gave much smoother surfaces (even with wipe-on-retract, slic3r dwelt too long, no matter how much extruder acceleration or speed or distance I tried - admittedly only 9 or so runs, but...). All prints used the same speed and accelerations, and finished in around the same time (50 minutes +/- 2)

Ray

Edited 1 time(s). Last edit at 02/25/2014 05:03PM by rayhicks.

Yes, I tried Cura and like it better than Slic3r in most ways - except for the strange effect of printing well over half the layers at very slow speed, which is a show-stopper. I cannot see what I might have set incorrectly - I tried changing every single speed setting I could find to 100mm/s, yet still many of the layers were printed very slowly - often right after printing a pretty much identical layer at 100mm/s. I have noticed that its non-printing moves are far more optimal, reducing print time considerably (or it would if I could get it printing at the right speed all the time!).

I'll probably try a different version of Cura sometime and set it up from scratch.

Dave
(#106)