3D printer setup: Extrusion geometry and voids

Introduction

Extruding filament through a round nozzle always creates some voids. The flow expresses their amount: If the track shape creates 5% voids, you must use at most 95% flow or filament will be squeezed to undesired places.

The void shape and volume depends on the ratio of the extrusion width to nozzle diameter, the ratio of the extrusion width to the layer height, the viscosity and probably more. It is possible to extrude tracks that are smaller or larger than the nozzle diameter. The rule of thumb is a range of 60–150% of the nozzle diameter.

Extra voids offer room for uncontrollably extruded filament, which hides some problems caused by extrusion speed variance, but voids also reduce the strength of parts.

To measure the volume of voids and investigate the track geometry, I created a small test stick. I cut off thin slices with a razor blade and observe those with a transmitted light microscope using a 10x lens, which works great for PETG. Distances and areas can be measured with imagej.

Script to generate test stick

Examples

This is a sample with 100% flow and the geometry of 0.3 mm extrusion width at 0.2 mm layer height. The flow is clearly too much, leading to smearing filament all over, which ruins the surface and causes the tracks in x direction not to be straight. The red area is 0.062 mm², which about matches 100%, so the extruder works correctly. This sample has a particularly good quality:

The green area is what would be a correct track at 0.053 mm². That indicates to use 85% flow, but measuring a few more tracks, the average is more like 90%. Indeed setting the flow to 90% stops the smearing and leaves a good surface as well as straight tracks in x direction. This sample is not as thin, which shows less of its structure, but still enough:

If flow adjustment still results in too many voids, they can only be reduced by a better track geometry, e.g. increasing the width of the track. Using the nozzle diameter with a flow of 95% contains much less voids due to the more wide tracks:

Further increasing the flow to 96% shows beginning smearing at the surface and again curved tracks in x direction:

The print process reacts very sensitive to flow changes. 1% often determines between too many voids or too much filament. If in doubt, optimize for more voids, because at 1.75 mm diameter, 0.02 mm tolerance already exceeds 1%.

Using this approach, the best flow for any track geometry can be determined.

Summary

The extruded track geometry determines the amount of unavoidable voids and the flow adjustment compensates the material volume loss from voids. More voids make a part weaker and offer room to hide unwanted extrusion.

A display model builder may desire weak parts that use less material and have enough voids to hide extrusion problems, optimizing towards a profile with just enough flow that the surfaces stay closed by reducing the line width at constant layer height and decreasing the flow if possible.

Structural parts should be optimized towards less voids and a flow as high as possible, just below where it would cause smearing and squeezing, by increasing the extrusion width and possibly reducing the layer height.