First Layer Height Slicer vs Actual

[Jeff Ratner]

Let's say you adjust your nozzle height with the common 0.1mm thick piece of paper shim trick.

If you set your first layer height in your slicer to 0.28mm and you have your Z offset or babysteps set to -0.04, what should the printed first layer actual thickness be if measured with a caliper or micrometer?

Not sure if this is a stupid question or a valid question since it is early Sunday morning and I haven't had any caffeine for a while.  

[Petr Ptacek]

The answer to this question is surprisingly complex, bit here are some points that might help.

Extruded material is liquid and incompressible. If your first layer is a single extrusion width, nozzle will squish it so the lack in height is compensated with larger width. If your first layer cover larger area, liquid has nowhere to go but up if you are soaking it too much so in general, height will be what you are extruding (simple conservation law), but likely with different textures making it difficult to measure thickness accurately.

So, if you are closer to the bed than you are actually attempting to print, the only variable is the pressure applied by the liquid plastic coming out of the nozzle on the bed. That is the factor that might help with how much the first layer sticks at the expense of some shape distortion described above.

If you are too far, pressure drops significantly to the point the plastic just drops on the bed (gravity) giving you air gaps between individual extrusions and that means you now have larger height of the first layer than intended.

That sleep makes it somewhat clear that finding the right height is a balancing act between too much pressure and distorted layer and not enough pressure and air gaps and adhesion issues 

So TLDR:

If your nozzle is (somewhat) closer to the bed than first layer height, then ...

Single extrusion width will (somewhat) have the height defined by the nozzle distance from the bed.

Large areas will have (somewhat) same height that defined in slicer.

If your nozzle is (somewhat) further away from bed than the first layer height, you get air gaps and the first layer height is (somewhat) same as distance off your nozzle from the bed).

All these statements are made based on assumption that your extruder will extruder what is asked for and there is no volume drop with increased pressure.

I'm also ignoring viscosity and surface tension of molten plastic but this should give you some general idea.

TLDR for TLDR above?

Not sure, too complex to say.

p.s. Just got my morning coffee, my brain is happy.

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