Bulging at start/end of outer perimeter

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Bulging at start/end of outer perimeter

Postby Snowbound » 2016-Dec-Sun-17-Dec

I have a simple metal printer and I feel like I have it fairly well tuned. After getting some great prints a couple of weekends ago, today it's started doing a bulge at the start and end of the outer perimeter run. I don't see this on the inner perimeter. This is a test print of the open end of a box lid.

printer bulge issue.jpg


It starts and ends in the valley between those 2 vertical bulges, so it's creating a bulge when it starts the perimeter and when it gets to the end. The reason you don't see the bulging in the first few layers, or the top layers is the slicer is starting the perimeter around the corner. You can't see it but the same problem exists around the corner for those layers.

My guess is it's extruding too much filament there. Last time I was looking into settings to speed up the print but ultimately decided there wasn't much I could change. I didn't think I changed any settings but it appears that I must have. Any ideas what I've done?

Thanks
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Bulging at start/end of outer perimeter

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Re: Bulging at start/end of outer perimeter

Postby teicher » 2016-Dec-Mon-21-Dec

I don't know the answer but it is interesting. If you use the preview tab in slic3r, do you see the bulges? Also, do you have the seam positioned set to aligned?
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Re: Bulging at start/end of outer perimeter

Postby Snowbound » 2016-Dec-Wed-07-Dec

The preview looks fine. I'm not familiar with the 'seam positioned' setting and I'm not in front of the printer right now, but I'll check it out.
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Re: Bulging at start/end of outer perimeter

Postby Snowbound » 2016-Dec-Tue-07-Dec

I changed the seam position to random. It's an improvement because the bumps are scattered around the print rather than aligned, but it's still an issue.

I don't understand what's causing this. I tried adjusting the retraction settings but that didn't make any difference.
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Re: Bulging at start/end of outer perimeter

Postby Mooselake » 2016-Dec-Tue-10-Dec

The hot end doesn't have a shutoff valve and will leak a bit when stopped. The seam is where it stops moving in x and y when it moves the z up to the next layer and dribbles out a tiny bit of melted filament.

Slic3r has a retract on layer change option to reduce this, don't know about Cura and others. If all else fails there's always an Xacto or utility knife; don't forget safe cutting techniques. Sandpaper's useful too. Lower print temps can help but remember you're trading layer stick for better detail definition.

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Re: Bulging at start/end of outer perimeter

Postby musk » 2016-Dec-Wed-15-Dec

Acceleration can cause issues like this on corners and anytime the nozzle makes sharp turns, in part due to pressure in the nozzle.

1. An extremely "soft" acceleration value starts to slow the nozzle down so far ahead of the curve that the built-up pressure in the nozzle has a large amount of time to release over a lengthy stripe of extrusion.
2. A "medium" acceleration value won't slow the nozzle movement until it's fairly close to the corner. This means that all the extruder pressure gets dumped in that small area just before the corner.
3. An extremely "hard" acceleration value results in pressure that doesn't change at all before the nozzle encounters the corner, creating the potential for perfectly volumed extrusion. In practice though, the violent printing experience that comes with hard accelerations will increase the prevalence of other artifacts like ringing, skipped steps, and mechanical instability of your printer from the sustained earthquake.
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Re: Bulging at start/end of outer perimeter

Postby RetireeJay » 2016-Dec-Wed-17-Dec

musk wrote:Acceleration can cause issues like this on corners and anytime the nozzle makes sharp turns, in part due to pressure in the nozzle.

1. An extremely "soft" acceleration value starts to slow the nozzle down so far ahead of the curve that the built-up pressure in the nozzle has a large amount of time to release over a lengthy stripe of extrusion.
2. A "medium" acceleration value won't slow the nozzle movement until it's fairly close to the corner. This means that all the extruder pressure gets dumped in that small area just before the corner.
3. An extremely "hard" acceleration value results in pressure that doesn't change at all before the nozzle encounters the corner, creating the potential for perfectly volumed extrusion. In practice though, the violent printing experience that comes with hard accelerations will increase the prevalence of other artifacts like ringing, skipped steps, and mechanical instability of your printer from the sustained earthquake.


I've seen this type of thing too. I suppose that the results depend in part on what kind of filament you are using.

Musk, do you have example values to propose?

I tend to think that the "extremely soft" value would be so low that your effective print speed would be very slow. I was using a value of 750, which I guess you'd call "medium" and the corners, edges where the nozzle was supposed to stop printing for a "travel move," and layer changes all got a lot cleaner when I upped the acceleration to 4000. The shaking isn't too bad except where there is small, detailed infill.
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Re: Bulging at start/end of outer perimeter

Postby musk » 2016-Dec-Wed-18-Dec

RetireeJay wrote:
I've seen this type of thing too. I suppose that the results depend in part on what kind of filament you are using.

Musk, do you have example values to propose?

I tend to think that the "extremely soft" value would be so low that your effective print speed would be very slow. I was using a value of 750, which I guess you'd call "medium" and the corners, edges where the nozzle was supposed to stop printing for a "travel move," and layer changes all got a lot cleaner when I upped the acceleration to 4000. The shaking isn't too bad except where there is small, detailed infill.


I believe Printrbot ships with a default M205 value of 3000. This is very high for people who haven't lubricated their rails or have other issues that might contribute to skipped steps. My estimates of best values for these situations are:

"soft" - 400 - This slow speed was useful specifically on a GoLarge printer which has an insanely heavy toolhead and bed.
"medium" - 800-1000 - Bleh, worst of all worlds.
"hard" - 1500 - My preferred setting for Simple Metal and Metal Plus.
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