Print time cooling calculation done "per part" or total?

I am curious - the Slic3r settings of "slow down" and "enable cooling" if "layer time is below X seconds" are pretty nice features. However I'm wondering how it calculates layer time if I am doing multiple parts, or if I am printing a part with both large and small features. I can see two scenarios:

A: By "layer time" it means the area currently being processed and infilled. For example, when printing a Yoda bust, the top of his head may not trigger the cooling fan, but the tips of his ears would.
B: It computes the total time required to print the layer. In the example above, while Yoda's ears would trigger cooling, the tip of Yoda's head would increase the print time of that layer to the point where cooling was never enabled.

I am hoping that the answer is A, since I would prefer to be able to just dump a whole bunch of parts into my build area and have the ones that need it receive cooling. If the answer is B, I suppose I would have to print all of my small parts separately, which would be a hassle.

I've thought of a few ways I could test out the logic, but I was hoping someone here could save me the time and would just know the answer

Thanks!

I believe it is total layer time. The main issue being addressed is that when printing a layer on top of another one that hasn't solidified yet, the previous layer gets dragged and distorted. You can see this happening very clearly if you print a tall narrow pillar.

Slic3r also has a setting for speed of printing small perimeters, which may be of some use to you.

Edited 1 time(s). Last edit at 10/14/2014 03:36AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for the reply! I did run a print with a couple of small parts and as far as I could tell you are absolutely correct. The larger of the two parts received the same amount of cooling (fan speed was set the same) as the very tiny part.

I'm very aware of that particular issue, as I've had it happen to me on a few occasions. I'm thinking that the way Slic3r currently works is sub-optimal. A good example print would be one of the cell phone cases with gears on the back. I'd love to be able to just throw all of the parts on the tray and have it invoke cooling on the small gears and pegs, and not cool the case itself, so as to prevent warping. The best way for me to run this print right now is to do all of the larger parts, then come back and do the smaller ones. The trouble with this is that running all of the small parts together will not quite work either, as together the total print time is higher than the threshold. So I just turn cooling on for the entire "small batch".

I'm not a huge fan of the slowdown feature for small perimeters as it seems to radiate more heat into the part by dwelling there longer. However if the fan could be enabled for small perimeters I think that would be much nicer! I print inside a heated enclosure so the really small pegs can stay a bit soupy for a little while, particularly if I do a very slow print.

I do see the point of having the whole layer time be part of the equation, as you may not need the fan given a long enough time between layers on a small part, but perhaps those of us with heated enclosures could use that option Unless there's a trick that I don't know about haha

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KickMe0869
I'm not a huge fan of the slowdown feature for small perimeters as it seems to radiate more heat into the part by dwelling there longer.

Quite so, I have made the same point frequently. On small columns it makes the problem worse rather than better. What would make more sense would be when the layer time is less than the set minimum, to have the nozzle move to a defined park position and wait for the remaining time. For parts that have small sections I have to create a sacrificial column and position it a distance from the part, forcing the nozzle to move to and fro between part and column. The issue there is that it also slows down any lower layers that do not require the additional time - on a pyramid for example, it is only the upper layers that are small and require the extra cooling time.

Dave

That's a good idea to use a sacrificial column! Prior to building my enclosure I had luck with making a really tall wall around the part, but sometimes it would pop loose and mess everything up. The nice part was that it was something I could do in Slic3r though. I wish I had thought of just having a basic column part like that!

The last few prints I've done have come out well. I turned off the slowdown feature, and I've got the fan coming on at just the right point (even a little bit for ABS) such that I haven't had any "mushy" failures thusfar. I've also got the printer going at 90 mm/s, which actually seems to help a lot as well.

I do wonder though, the fan turns on when a layer time is supposed to be below a certain threshold. My ETE calculation is always off by a factor of 2. Is that the same calculation used in determining layer time? Someday I want to poke through the firmware and see if I can't find a way to scale my ETE to display properly. If I did that, would I have to scale my values for "cooling"?

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KickMe0869
That's a good idea to use a sacrificial column! Prior to building my enclosure I had luck with making a really tall wall around the part, but sometimes it would pop loose and mess everything up.

The other way of course is to simply drop 2 of the item into Slic3r if they are small enough to both fit on the build plate, then you have a potentially useful spare instead of a lump of plastic fit only for the dustbin.

Dave

Nice idea! I just happened to be modifying my end g-code last night and I noticed that slic3r has a gcode for layer change. You could put a pause or an orbit in there as well, or whatever you'd like to delay the print

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KickMe0869
Nice idea! I just happened to be modifying my end g-code last night and I noticed that slic3r has a gcode for layer change. You could put a pause or an orbit in there as well, or whatever you'd like to delay the print

It should work - however it would execute for every layer change, not only the layers containing a small cross-section, thus making total print time unnecessarily long.

Dave