First layer after raft preview not matching print behaviour

[jgrouse]

Hi,

When using a raft, when I preview the first layer of the object (layer 6) and then move forward to the next layer (layer 7) it shows the base rectilinear fill at 45 and then the 7th layer is at the same z height with a follow up -45 fill with a narrower extrusion. Now this seems to make sense as it is attempting to really fill in that base layer as its underside has a larger gap making the first extrusion gaps larger. However when I then take this to my printer with a Ramps 1.4 controller running Marlin that 7th layer appears to print at the next layer thickness up (0.15mm in my case) rather than, as the preview suggests, the same height as layer 6. This is what the 6th and 7th layers look like in the preview -

The Gcode looks like this -
; layer 6, Z = 1.73204
M105
; tool H0.196 W0.360
; inner perimeter
G1 X83.919 Y109.563 F6000
G1 Z1.732 F3000
G1 E0.0000 F3000
G92 E0
G1 X61.081 Y109.563 E0.7182 F4500
.....
; layer 7, Z = 1.71204
M105
; tool H0.150 W0.360
; inner perimeter
G1 X76.994 Y83.199 F6000
G1 Z1.712 F3000
G1 E0.0000 F3000
G92 E0
G1 X76.966 Y83.115 E0.0021 F4275

Notice here that the 7th layer actually lowers down a fraction from the previous layer to really flatten out that first "fat" layer?

Unfortunately it looks like the Marlin firmware detects this as a mistake in the Gcode and disallows the merging of the layers, resulting in the 7th layer just going over the top like a regular layer at the next height up.

I'm just about to go and run this code on the printer and will take a video/photos of it doing layer 6 and then 7 to show the result.

Back in a bit...

[jgrouse]

Well it does actually look like the follow up 7th layer is in fact at much the same Z height when it prints as I could not see a change in the lead screw rotation positions between layers. So I guess what I am seeing is that the 7th layer is actually pushing the 6th layer down a bit rather than completely fusing with it giving the impression it is higher?

One thing I have found helpful is that I can force a pause after the raft is complete so I can spray a very light coat of hairspray over it to reduce the bonding of the object to the raft as I as having a hell of a time separating them. This also allows the gap to be reduced giving a flatter bottom layer on the object.

Have to be very sparing on the hairspray as too much causes practically no bonding between the object and the raft.

[dkightley]

This is exactly the same subject as this thread:

viewtopic.php?f=9&t=3246

Note it's always worth doing a quick search before starting a new thread. It'll make it easier for the Simplify3D support staff to review issues we raise if related posts are all put in the same thread.