How wide is extrusion line at given layer height?

Technical question, important for design ( I understand this is somewhat variable with head temperature , speed and material ):

when you print a line, of a given layer height, how wide is it? THis is extremely important, for example, if you want a strong thin wall, you want the print lines smashed up against each other to get nice lateral fusion and set the wall thickness to an integral number of line thicknesses.

Also important, how wide does slic3r think the print lines are?

For example, how wide are the lines with PLA printing to a layer height of 0.3mm, at 210 degreees set at speed of 60?

Well you already answered to your own question if you know it is a variable. It's roughly the diameter of your hole drilled under ideal condition which is never the case. Over and under extrusion makes the value off, but even for a functional design, that should not be a major issue.

As richard said, line width is nominally the diameter of your nozzle. So a .4mm nozzle will produce a .4mm wide line. I would argue that speed and feed rates are going to effect the actual line width just as much as layer height. As material is extruded it's going to come out at the diameter of the nozzle but once it hits the platform (or the previous layer) it will flatten some causing it to get somewhat wider. This will happen regardless of your layer height. The more material is extruded per unit of length traveled by the printhead, the more it will get smashed into the platform and the wider the line will get. So fast speeds with slow feeds (underextrusion) will result in a thin line while slow speeds and fast feeds (overextrusion) will result in thicker lines. If you need actual numbers my best suggestion would be to model a part that is a line the same width as your nozzle diameter. Print that line over and over using different speeds and feeds at the same layer height to see what results you get. I suspect that there is a mathematical relationship between speed/feed/line width but I don't know that anyone has ever actually figured out what that relationship is. Also realize that if you change the layer height, the same effect applies BUT your speeds and feeds from the previous layer height will result in different line widths because you're applying the same amount of material per unit length but your vertical height has changed so width will have to compensate for that.

Actually, from what I understand (still noobish here), there is also die swell to account for. Normal filaments (ABS/PLA) exhibit a tendancy called die swell. They are under pressure behind the die and when extruded that pressure is released and the material expands slightly. I have noticed that the amount of swell is affected by backpressure on the head (thin layers typically at the beginning of the print) as well as moisture content of the filament. I noticed that I got thicker walls before I started locking up my filament in buckets with descant.

I don't think anyone will be able to tell you the thickness. There are simply too many parameters that affect it. Idler tension, moisture content, filament material, extrusion temperature, moisture content, layer height, etc.

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Originally Posted by soofle616

I suspect that there is a mathematical relationship between speed/feed/line width but I don't know that anyone has ever actually figured out what that relationship is.

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Not sure, but I think Simplify3D has figured it out. In the extruder tab on in the FFF settings panel, there is a seting for line width. The auto setting seems to change based on layer height and speed. I don't know if temperature affets it. In my normal FFF settings for ABS, the auto setting most often reads 0.42 for a 0.35 nozzle. That meas its accounting for a 0.07mm swell. Somehow. I don't know what all goes into the calculation but clearly the folks over at S3D got something figured out.

Slic3r allows you to specify extrusion width and layer height. It will then calculate the appropriate extrusion speed to achieve the desired result, although as previous posts state, environmental conditions will have an effect.