Inner perimeter prints before "infill" that bridges to it above the first layer
I've been struggling to get rid of some stringing and curling printing this model: https://www.thingiverse.com/thing:3306454 which has a couple of tubes recessed off the base of the print with flat faces. I've used supports and had difficulty removing them cleanly, but the spaces are small enough bridging should work, so I've been tweaking all sorts of temperature and cooling settings and trying other filaments, but I haven't come up with a really good solution.
The problem seems to be the order in which the gcode is executed when it gets to the new layer with the overhang - it prints the perimeters for the outer part of the model as expected, but then prints the innermost perimeter around the cylinder, which has no material next to or under it at the time, then it treats the rest of the downward facing "bridge" area as infill and prints it after the now failed circle, making it impossible to support, so it takes several layers of mangled looking strands before it the consecutive layers find enough purchase to actually make the intended shape.
I've looked through the options in slic3r and will be trying the "Infill before perimeters" option, but I assume this will impact the exterior perimeter visual quality... so is there any way to tell slic3r to NEVER print something above the lowest layer that is not touching another layer? Is there a way to change the order of the print instructions for this layer to make sure it prints the "bridging" material from the outer to the inner perimeter before it prints the inner perimeter? Have I somehow goofed it up and overlooked something (I've lowered the bed temp to reduce curling in the close-to-the-bed confined area, I've lowered print temperatures, I've slowed bridges and perimeters, I've even used the cooling fan for bridges on ABS)?
Re: Inner perimeter prints before "infill" that bridges to it above the first layer
Could you post a picture of the layer in Slic3er? I believe it isn't that easy to understand what you are talking about 🙂
Re: Inner perimeter prints before "infill" that bridges to it above the first layer
Tried the infill before perimeters option and while the external quality wasn't so badly degraded, my problem wasn't solved. I'm wondering if I'm seeing the perimeter of the next layer printed and it actually is printing them in the right order..... next attempt will be even more fan speed on bridging, as ill-advised as that may be for ABS.... maybe I'll even go back to trying supports at some point. I've tried printing in a couple of other orientations (upside down from this and on the largest flat side) and while printing on the side does clean this up, it also makes layer lines run close to the axis where this part will need the most strength, so I'd prefer to avoid that.
The overhang I'm talking about is the internal bridge infill and overhang perimeter in the center of the model in the image (looking up through the bed at an angle). Those first couple of layers printed simply don't hold their shape, and then further up the cylindrical hole there's a second similar overhang with similar poor results.
Is there a possibility the Bridge Infill section is being printed with Infill speeds instead of Bridge speeds/settings (and the fan isn't spinning up at all?)
EDIT: Slicing up another time for a new print attempt and I may have found a bug. When setting the Top/Bottom Infill Pattern to Concentric, only the top layer gets the new pattern, the bottom one (the one I'm working with) remains rectillinear. Seems a couple years ago the base Slic3r added different settings for both top and bottom - perhaps the Prusa edition has a single pulldown box for the two and it's only setting the top infill pattern now?
Re: Inner perimeter prints before "infill" that bridges to it above the first layer
Well, I see the issue now. Did you, by any chance, design the model yourself? If so, you can simply add 1-2 solid layers right at the base of that hole. That would make it bridge all across the hole and you can simply cut those layers out later on.
There is a description around the 2 min mark in this video.
If you didn't design it yourself, then I guess you are somewhat out of luck as it is really not that easy to print and certainly not optimized for fdm printing. Support is probably your best shot then.