Raft separation distance and next layer height problem

[jomireyn3]

What's your layer height? I find that a separation distance set to 1.5 to 2 times the layer height seems optimal for my machine. I'm guessing it may also have something to do with the extrusion temp... an extrusion temp which is too high may not set on top of the raft, and bond too well. I normally use a rectilinear external infill, this bonds the edges of the model to the raft... because more filament is deposited when the infill overlaps with the perimeter. A concentric external infill could help resolve this...

Regarding your original question, its probably best to print a test cube and measure the Z height for accuracy... it shouldn't printing the layers lower or shorter than set.

I'm usually using a .2 layer height, and I've gone as high as 2.5 times that for the separation and I really don't see any difference no matter what the separation setting is.

Thanks for the suggestions on the temperature. I'll try adjusting that a little, although with the MakerBot "Smart Extruders" I've read that lower temps can contribute to clogging. We're getting a LulzBot soon, so maybe we can just stick with the Makerbot software with the Makerbot printer. It makes rafts that separate perfectly, but doesn't print as accurately for parts that require good precision & accuracy. I'm printing for some medical and health care applications, so that can be important sometimes.

You may be right about manually adjusting the Z height. I use the automatic leveling software that comes with the Makerbot and I do have trouble printing a good first layer without a raft. If that's off, it wouldn't matter what settings I put in the the raft, would it?

[jomireyn3]

Yes, that 8th layer should be printed lower than the 7th because the 7th layer doesn't stay at the exact height where it was printed. It falls down to the surface of the raft due to gravity. So the layers are being printed at the correct height.

That does seem to be the consensus, but if that's the case, I don't see the point in having any adjustment for the separation distance, if it's just going to drop the first layer by gravity on the top of the raft and then push it down further to with the nozzle with the subsequent layers.

Of course I'm getting the same (impossible to remove) results at just about every separation setting, so I guess that what's happening.

Would printing the raft as one process and the object itself as another help? Can you print just a raft as a process?

[KC_703]

I'm usually using a .2 layer height, and I've gone as high as 2.5 times that for the separation and I really don't see any difference no matter what the separation setting is.

Thanks for the suggestions on the temperature. I'll try adjusting that a little, although with the MakerBot "Smart Extruders" I've read that lower temps can contribute to clogging. We're getting a LulzBot soon, so maybe we can just stick with the Makerbot software with the Makerbot printer. It makes rafts that separate perfectly, but doesn't print as accurately for parts that require good precision & accuracy. I'm printing for some medical and health care applications, so that can be important sometimes.

You may be right about manually adjusting the Z height. I use the automatic leveling software that comes with the Makerbot and I do have trouble printing a good first layer without a raft. If that's off, it wouldn't matter what settings I put in the the raft, would it?

If going to .5 for a separation distance doesn't work, then my bet would be on the temp of the extrusion. If you've specified a 3 layer raft, I think you would decrease the temps by 1-2C at layer 3, and return to the original temps at 5. Thinking out loud - this would make the top layer of the raft and first layer of the model extrude cooler... hopefully reducing adhesion. Then after the first layer of the actual model, the extrusion temps would return to normal. This logically would seem to work, as long as the model has a relatively "big" surface area.

Yep. I printing on a raft, then the first layer wouldn't be as important. The challenge of printing on a raft may be warping since more air is getting beneath the model. Often times, I'll add a solid raft to hard to print models... like models with a thin base. I do this through SketchUp, but it could easily be done by stacking multiple STLs in S3D.

The Lulzbot has challenges too... but my TAZ 5 prints really well. I haven't built an enclosure, but I would highly recommend one... reduces challenges of the moving bed.

[dunginhawk]

Im having the same issue as the poster here.
I am testing this latest cube print at .35mm layer separation and its just as bonded as the last.
Its abs though and only running 225 degrees. no WAY it should be doing this.
And watching it lay down the 2nd layer of the print (after the raft and base layer) it does almost look like its smooshing down in to the first layer, which could of course be causing this binding.

3 raft layers.
.75% infill
.35mm separation.
ive tried .15, .2 and .35 all have the SAME bonding. this is NOT happening on other slicers. its absolutely an s3d issue. maybe not a problem, but there is something going on

[dunginhawk]

[krouviere]

"Yes, that 8th layer should be printed lower than the 7th because the 7th layer doesn't stay at the exact height where it was printed. It falls down to the surface of the raft due to gravity. So the layers are being printed at the correct height."

I strongly disagree. That is not control of the process. That's a bug.

[krouviere]

For what it's worth, I have sent a message to support and I hope it gets fixed in a future release. I really like Simplify3D for being similar to CNC CAM software with regard to control of the process. If it generates toolpaths below the plane that separates the raft from the model, it's not behaving well.

[dkightley]

"Yes, that 8th layer should be printed lower than the 7th because the 7th layer doesn't stay at the exact height where it was printed. It falls down to the surface of the raft due to gravity. So the layers are being printed at the correct height."

I strongly disagree. That is not control of the process. That's a bug.

I'm with JoeJ here.....purely on the basis of common sense.

Ignoring the preview....which shows what has been programmed and not real life!! What is the difference between no gap and a gap??

With no gap, the nozzle is one layer thickness above the previous layer and the molten plastic is extruded so it is amalgamated with it. With a gap, the nozzle is one layer thickness PLUS the size of the gap above the previous layer and extruded molten plastic is laid on top of the layer below it by extruding it into fresh air. The extruded plastic will, depending on the size of the specified gap, either lay on or drop down onto the layer below it....sticking where it touches (we do actually need it to stick!..but only where it touches).

Now lets look at the next layer... Where there's no gap, it's exactly the same as described above. Where a gap was programmed, the nozzle needs to be the appropriate distance above the previous layer so it can be amalgamated into it.....and the software has to determine what the exact position should be by making an allowance for the thickness of the air-extruded layer...which may have dropped down as described above. Hence, this layer MAY be printed lower than the previous layer.

Going back to the preview screen...how can it cater for the above two scenarios? Where a first layer gap has been printed, it cannot simulate any possible dropping of the layer...and it is simply shown as being printed in fresh air. The layer after is then shown printed at the calculated height...which as previous posters have illustrated using an exaggerated gap....looks wrong, but isn't.

[dkightley]

On the subject of the first layer "gap" not seeming to yield an easy break-away despite the gap being large, I wonder if the issue is not the gap, but the state of the top layer of the raft at the point when the first layer is "laid" onto it.

The theory behind having a gap is for the first layer to be laid on top of the (now solidified) top layer of the raft. This goes out the window if this layer has not yet solidified sufficiently ....because it has not been allowed to cool enough...by say the bed temperature being high, there being no cooling fan running, the printer being enclosed and heated, etc.

I print in PLA and have had very few problems with separating rafts from prints...and I probably use rafts far too often! I have no experience with ABS or other materials that need a heated bed.....so I can't provide any comments based on experience.

Just a thought that nobody else had had...... EDIT after having looked through the thread in detail, others have suggested this - but not explained why.

[dunginhawk]

im going to run a test right now, turning on layer fan at layer 3 (of the raft) and turning it off at layer 7, which is the start of the print.
Hopefully s3d realizes the raft is not the part and will cool it like it should, not assume the part doesnt start until 7