layer height indication  

RE: layer height indication

Your bed isn't level, it looks like it runs in as the extruder moves toward the center of the plate, and runs out as it goes toward the edge. The changing Z height is the mesh leveling adjustment compensating for it.

If you have an OctoPrint or similar, you can run a mesh bed level and get a visualization of where your bed isn't level.

RE: layer height indication

Thank you for your quick response....how is this possible?...my tray would be deformed, or would be dented? Should I change trays?

RE: layer height indication

First step is to make sure everything is completely clean and the sheet is free of leftover deposited filament from a previous print. Even a small amount can cause an uneven level if it's between the sheet and the heat bed. Wipe down both sides of the sheet and the heat bed with isopropyl alcohol.

Prusa has this article here which gives some basic troubeshooting to square up your mini, which can make a big difference to the bed level if you have significant runout in a given direction. Smaller adjustments can be made to individual points on the bed by adding slightly more or less tension to the torx head screws that secure the heatbed. If you're really chasing a flat printbed, consider this silicone heat bed mod.

RE: layer height indication

Unlike the Mk3S, which shows nominal Z height (i.e., 0.25 for a 0.25 mm thin layer), the Mini shows absolute Z height, that is, where the nozzle actual is (relative to home) at each point in time, which is why you see it vary as it lays down the filament. But that's what mesh bed leveling is all about. It will automagically adjust the layer height to compensate for imperfections in the height of the steel sheet. Could be caused by skew in the way the printer is assembled (see support page @amperage linked to), or just imperfections in the sheet itself. 

As long as your first layer looks good, I'd consider this normal and expected behavior. Sure, you can try to chase perfection but I'm still waiting for the day I feel I need to do stuff like silicone implants on my Minis 🙂

RE: layer height indication

Thank you for your answers... I know how to adjust the first layer, but I did not understand the numbers indicated on the screen... It's done... On the other hand, I had never thought of playing on the fixing screws of the heating plate ... so thank you to both of you ....