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extrusion amount depends on temperature and layer thickness

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In some print I noticed that if I increase the speed, wall thickness seems to go down.

That can have more than one cause. So I set out and do some tests.

I print a simple hollow cylinder which is single walled. I set wall thickness to 0.5mm.

I vary speed (mm/s) and layer height (mm), and measure wall thickness with a digital caliper.

To have a smooth wall, I turn on the "Joris" option, so that the z increases steadily during the print.


 speed, layer->wall thicknessAt  205 degrees          215 degrees    50, 0.1 -> 0.48     50, 0.1 -> 0.53         50, 0.2 -> 0.44     50, 0.2 -> 0.53         100, 0.2 -> 0.28    100, 0.2 -> 0.35     


So it is clear that the more the extruder has to spit out (mm3/sec), the thinner the wall becomes

If I increase the temperature, this effect becomes less.

I my view, temperature should not influence the amount of plastic that is extruded.

So what is wrong?

My current hypothesis is that my filament drive is slipping, due to the force needed to press the filament into the extruder head.

My UM is from september 2012, and came (I think) with the latest knurled bolt.

I have a self built "Bertho" drive. Since I have no real info on needed force, I just used a spring I could find at work.

The slipping is subtle, there is a clear trend in the figures.

Before I start a big investigation in this, a few questions:

* With what force should the filament be pressed into the knurled bolt?

* Is a perfect press force a gurantee for no slipping?

* Or is there always a relation between drive force and drive length? I think for perfect volumetric extrusion, there should be no such relation.

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I my view, temperature should not influence the amount of plastic that is extruded.

Unfortunately, it depends a great deal on the temperature, or more precise, at the heating capacity of the hot end. the lower the temp the higher the viscosity, which is counter-balanced by low printing speed.

if you print faster (more mm^3/sec), you need a higher temp (as your data confirmed).

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