Hot end crashes into second layer

I've run into a strange problem while trying to calibrate my Kossel XL + DuetWifi + IR Probe delta. At some point while printing the second layer, the hot end invariably crashes into another part of the second layer.

I've got Simplify3D set to 50% first layer height. The first layer goes down fine; there's no visible curling and adhesion is good. The second layer starts printing fine as well, but as soon as the effector needs to travel over an already printed section, the hot end crashes into the existing extrusion so hard that you can hear a loud bump. It's almost like it is stepping lower on the Z axis for some reason.

The towers are seemingly square and the diagonal rods are all the same length. I've done G32 several times and it settles around 0.06-0.08. I'm not sure where to look next. Can anyone suggest a possible cause for this behavior?

You can use "z-lift" on travel moves to avoid this. Not sure, where you specify it in S3D, but it is related to retraction settings in general.

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o_lampe
You can use "z-lift" on travel moves to avoid this.

Makes sense. I'll give this a try this evening.

I wonder though if this is evidence of a mechanical problem somewhere. It seems like, assuming the mechanics are perfect, the only way this problem should come up is if the layer is warping or peeling. I'm using a 0.8mm Volcano nozzle, so maybe this is laying down so much plastic that it is slightly raising up along the side of the nozzle.

Are you getting over-extrusion on the first layer? Try printing just the first layer, then look at how smooth the top surface is.

Why are you printing the first layer at 50% of normal height? The usual reason for using a different first layer height is to use a greater height (e.g. 200%) in order to make variations in nozzle height over the bed less critical. But I've never used a Volcano.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Setting Z lift to 1mm helped some, but it would still occasionally bump into a lower layer. It turns out the problem was indeed over extrusion. When I looked at it closely, the first layer was quite rough with lots of peaks. It had a texture almost like a rasp.

Before I started using BuildTak, I had some problems with first layer adhesion. I ended up following this advice from S3D:

"if you entered 75%, your first layer height is reduced while your extrusion remains at 100%. ... This reduction in height generates extra pressure and more surface area for that layer, which will help the first layer adhere to the print bed." [1]

I went back to basics and spent some time getting the G31 calibration dead on [2]. Now I've got first layer height set to 80%, and the problem has gone away. I guess there is still something wrong in my setup though, as I find that it's still necessary to have height < 100% for reliable adhesion.

[1] [www.simplify3d.com]
[2] [duet3d.com]

Did you measure the thickness of the first layer with a caliper? You will probably find that it is a bit above the pre-set layer height. That would mean you have to adapt the probeheight (Z parameter in your G31 command in the config.g).
On a sidenote: the Z-lift that can be set in your slicer, it is not uncommon to lift 3 or 4mm, with a high speed, to help prevent stringing or webbing in your prints.Th idea is that by a high lift the string breaks before the travel move commences.