negative scar on layer start

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JamesK
Grand stuff Paul, but the effect Rob is seeing seems to be much greater than normal - I wonder why. Robse, what travel speed do you use? If it's unusually slow, perhaps you are losing more to ooze than normal?

I use 130mm/s travel speed

At this point I think that what fits most is what Paul says: "- no retraction - the beginning of the next segment after the move will be starved due to any ooze"

This should be fixed by raising Z on retractions ... isn't it ?

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robse

Z lift is a separate issue, aimed primarily at avoiding contact between the nozzle and any high spots on the print surface during the travel move. Factors that reduce ooze (for a given filament) are:

increasing the travel speed
reducing the temperature
increasing retraction distance
adjusting retraction speed and acceleration

The last one seems to be a bit of a black art - I've seen suggestions that higher speed & acceleration is the way to go, and also suggestions that you can over do it and less speed sometimes works better. I guess experimentation is the order of the day.

In an odd coincidence, I saw something very much like this on a print last night. Unfortunately I wasn't watching during the print, so I have no idea what was happening, but I have 1 to 2mm gaps in the extrusion at one of the seams. The difference between this print and the previous print of the same object was that I dropped the temperature by 10C in the hope of improving some towers that looked like they were suffering from being too hot (using pla, originally at 210, reduced to 200).

James,
That's a good example of what I'd call a stall.

Imagine that the filament is melting into an cone like this (my drawing, not exactly to scale):



If the hot end is cooler, or the extrusion is faster what will happen?

I believe the un-melted point will get closer and closer to the orifice until a fast extrude causes it to temporarily plug the opening.

I did quite a bit of testing for retraction, and wrote this guide in the Wiki.

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My printer: Raptosaur - Large Format Delta - [www.paulwanamaker.wordpress.com]
Can you answer questions about Calibration, Printing issues, Mechanics? Write it up and improve the Wiki!

Thanks Paul, retraction is an area where I know I have more work to do. I started using nylon fairly early on which has horrible extrusion latency, so I threw long fast retraction at it to try and tame the ooze problem, and then used those settings for everything without giving it much thought. I've just started to realize that might not be the optimal setup

I'll put working through your wiki page on the todo list.

Edit: Ah, it looks like this sentence near the beginning may explain my case:

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Back it off if you went too far. If you retract too much then the beginning of the next extrusion can start cold (it will appear starved).

I've probably been masking excessive retraction by using higher temperatures, and when I lowered the temperature my clumsy retraction settings started to show up.

Edited 1 time(s). Last edit at 02/07/2016 07:13AM by JamesK.