Odd first layer behaviours

I'm working on a Prusa with makerbot gen3 electronics and a mk5 DC extruder. I've gotten some skeinforge settings that seem to be fairly good, including *very* nice rafts and good quality on most of the print (albeit, with rather low feedrates and no oozebane yet). However, the first few layers after the raft are done are coming out quite poorly. It seems like the extruder is just a touch too high when it lays these down, resulting in coiling of the filament when the extruder changes directions and poor adhesion of the laid down filaments. After two or three crummy layers, the extruder seems to settle down at the right height, with the nozzle gently pressing the filament down into the print and print quality becomes pretty good.

Any thoughts as to why this might be occurring?

When that happens to me, it's usually time for clean, new Kapton. Or not enough heat in the bed. For ABS. I've not used PLA yet, so I don't know how to make that stick.

The kapton is fairly new and in good condition. Bed is unheated. I am also a little skeptical of this hypothesis, because the rafts I'm getting are really nice -- good adhesion to the bed, cleanly separating from the model. It's the base of the model itself which is off, the first two or three layers.

L

Is your w/t at least 1.5

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Sorry, first layers after the raft. I missed that. I haven't used a raft in a very long time, and even then, not much. I just used it to make up for the fact that the heated bed PCB isn't flat enough to print large parts directly on without glass on top of it.

First layer doesn't stick to the raft... I'd try extruding at higher temperature, maybe slower on the first layer would help. What's you width/thickness ratio? Anywhere near 1.5? Are we talking about ABS?

Anyone with raft experience please chime in. DC extruder and oozebane are before my time.

Edit: looks like Sublime sniped me on the w/t ratio.

Edited 1 time(s). Last edit at 12/09/2011 01:59PM by Dale Dunn.

Width over thickness is 1.6, I'm printing ABS.

Worth bumping up the width/thickness?

L

If it is only the first layers that are wrong it would seem the first layer is too high above the raft. I don't know how you change that as I don't use rafts.

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[www.hydraraptor.blogspot.com]

As I've said, it's *not* a raft problem. It's a problem with the first few layers of the object itself.

Yes the first layer of the object must be too high above the raft or the flow rate of the first layer too low. No idea what would cause it as I don't use that part of Skeinforge.

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[www.hydraraptor.blogspot.com]

Does the object you are printing have its base at z=0?

Have a look at the generated gcode and in particular the z values in it. You should see a bunch of them all the same for the raft (you don't say what your layer height is, but the raft will probably be at about that - depends on your other settings) then you should see a bunch with a slightly higher value for the first layer. The difference between the raft z and the first layer z should be approx your layer thickness (again, I think there are other settings which can modify it so it may not be exact). Then subsequent layers should each be higher than the previous by exactly your layer thickness.

The "Operating Nozzle Lift over Layer Thickness" setting under Raft>First Layer was set to 1.2, where the default is 0.5.