This aims to be a single thread to track all raft contact z distance related issues, all of which don't seem to be gaining enough attention individually, nor are the suggestions given in any way conclusive.

Also, I haven't seen some conclusive research onto the topic that provides data as to what actually currently happens under different scenarios.

Version

<= 2.2.0-alpha2, testing in this post with 2.1 stable

Operating system type + version

several if not all

3D printer brand / version + firmware version (if known)

seemingly irrelevant

Behavior

The z distance / air gap that is left between the last raft layr and the first model layer is vastly incorrect, leading the object in several cases to be printed in mid-air, far above the last layer of the raft.

Previous threads mentioning the problem

• Distance from Raft as Air Gap like in cura #2823
• Erroneous "Contact Z Distance" #2766
• Unstable first object layer with raft #118
• Make support separation distance configurable to negative numbers to keep ABS from warping #102
• first layer of support for multi filament support oddly spaced #1445 [different but related]

Project File (.3MF) where problem occurs

raft_issue.zip

Analysis

First off, there needs to be clarity and documentation about what we're talking about.

I'm using MK3S printer [standard 0.4 nozzle] with generic PLA and 0.15 quality profile. 4 layers of raft and 0.1mm contact z distance. supports are enabled but don't seem to affect this, but are helpful to understand the thickness of the first model layer by visual comparison in front view mode [shortcut: 3 key]

1. In prints with raft enabled, does the first layer height value apply to the first raft layer or to the first model layer, or both? Apparent answer: first raft layer only

2. In my tests the first model layer above the raft is printed at 0.4mm layer height [with a 0.4mm nozzle by the way!], respecting the contact z distance setting, so after the last raft layer the toolhead moves 0.5mm up and prints a 0.4mm layer 0.1mm vertically off the raft. The setting seems to be: first model layer above raft height = nozzle diameter. This doesn't work!

3. Raft layer height [except first raft layer] is not customizable by the user, in my tests it's printed at 0.3mm layer height with 0.4 nozzle and 0.45mm layer height with 0.6 nozzle. The setting seems to be: non-first raft layer height = nozzle diameter *3/4. This is reasonable.

4. Number of raft layers doesn't seem to affect the issue.

5. Supports generated over parts of the raft [such as the edges often times] if supports are also enabled other than just the raft don't seem to affect the issue, they only appear to introduce [as expected] micro-layers of supports between the last full and proper raft layer and the first model layer, without affecting their positions or heights.

6. If you set first layer height to 0.4 in the above 3mf prusaslicer file:

• First layer / first raft layer: 0.4mm
• Second layer / second raft layer: 0.3mm
• Third layer / third raft layer: 0.3mm
• Fourth layer / fourth raft layer: 0.3mm [toolhead height 1.30mm]
• Fifth layer / first model layer: 0.4mm with 0.1mm z-airgap [toolhead height 1.80mm]

If instead you set first layer height to 0.2mm in the above 3mf prusaslicer file:

• First layer / first raft layer: 0.2mm
• Second layer / second raft layer: 0.3mm
• Third layer / third raft layer: 0.3mm
• Fourth layer / fourth raft layer: 0.3mm [toolhead height 1.10mm]
• Fifth layer / first model layer: 0.4mm with 0.1mm z-airgap [toolhead height 1.60mm]

Expected behaviour:

1. The first model layer printed at 1:1 nozzle size layer height is in my opinion way too high, and produces in almost all cases a random print in the middle of the air. Its value should be customizable, if not at least set to a reasonable fraction of the nozzle diameter.
2. The first layer height setting must specify that it's being applied to first raft layer and not first model layer above the raft in prints with a raft.
3. Perhaps the user should be allowed to customize the non-first raft layer height, otherwise it should be specified what it's going to be calculated at.
4. super-mega feature request: allow the raft contact z distance to be different than the support contact z distance. This will in so many cases need to be vastly different! And the support option of the height range modifier feature was removed! I've found myself splicing together 2 gcodes several times to achieve both a functional raft with a negative contact z value [which is in and of itself something that was requested and is now available but we absolutely don't know why it's necessary and if so even more to the point, why are we printing so high up off the raft?] and supports that didn't actually physically conflict with the model but were in fact removable.

Bonus issue

support_issue.zip

What on earth is going on with support [not raft!] contact z distance? In the 3mf below the last support inside the X letter of the calibcube is printed at 1.44mm height, and the ceiling is printed at 2.00mm height. you can tell that on the sides it's printing at 0.15mm height, correctly, because otherwise it would be printing inside the gyroid infill if it were printing a thicker layer, instead above the X void you can see from the front view [shortcut: 3 key] that it's printing a thicker layer. How thick? It would need to be 0.36mm thick to "start" at 1.66mm height and therefore respect the 0.2 contact z distance. Is it? Is it not? How do we know? Why is it doing this? How does it calculate it?

Thanks