First layer Problem

Hello, i was wondering if somebody knows what kind of problem my Hephestos 2 has with this first layer. the nozzle prints okay until a certain point, then these little "hills" appear (those on the right side) and after that nearly no filament comes out. the rest of the lines seem okay.
the bed is levelled correctly, the nozzle is clear. I am printing PLA at 220° (but also tried down to 200°) and I tried all kinds of different retraction and speed settings. Nothing seems to work.



Edited 1 time(s). Last edit at 01/31/2016 09:42AM by BobBuilder.

Way too close. Your z gap seems like its 0.02mm, when it should be around 0.2mm. The hills are probably a result of slight bumps in the bed. If you print the object in a different location, does the ripple stay put or move with the print?

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Master Tinkerer

Agreed, Too low for the first layer.

Thanks a lot! I still haven't found the right setting, but it's a lot better now. How do you calibrate the Z-Gap properly? I first tried the "Take a sheet of paper and adjust the nozzle so that the paper gets stuck under it" method, but as you can see in the picture it did't turn out so well.

Make sure the bed and the nozzle are at print temperatures before you try to level and zero the bed.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

It's not unusual for heatbeds to be bowed up in the middle, so if you do the obvious thing and level at the four corners you can end up too close in the middle. This is the situation I'm in as I still haven't heeded DD's advice to get a decent flat bed. The solution I used for now is to level at the four corners (iteratively, small adjustments each time), and then move to the middle and make equal adjustments to each screw until the gap at the center position is ok. That leaves an unequal and overly large gap out near the edges, so if I'm printing larger objects I have to start with a thick first layer. I've been meaning to try printing on a raft to see if that would also be useful for soaking up the bow.

Try setting the z=0 at the center first, then move the extruder to one corner and adjust the level, then to the diagonally opposite corner and adjust it. This makes the bed pivot on the other two screws, hopefully, instead of flexing. Then adjust the last two screws the same way. I think that that will allow the leveling with minimal flexing of the bed and undercarriage, and the zero position should be pretty close, but you may have to go around and do the whole procedure a couple times until its right. Finally move the nozzle about 1/2 way between the center of the bed and any edge and set the zero for the Z axis.

The reason to zero away from the center assumes that the surface isn't perfectly flat (due to the 4 corner leveling scheme) so by setting the zero point off center, the first layer will work well away from the center and will be squished a little flatter/thinner at the center which is usually bowed upwards. Squished flatter/thinner is usually not as big a problem for print adhesion as having the nozzle too far from the bed. The result should be a larger area that is flat enough to print on.

I'm just guessing at this procedure. 4 corner screws is problematic for leveling...

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I found out that the problem mentioned above is not jut caused by the z-gap. I can't seem to properly level the bed. My bed has three screws (a Hephestos 2 by BQ), and when all corners are levelled, somewhere in between it isn't. this drives me mad, any tips for this?

If all three corners are levelled than the rest will be too unless you have some warping in your build plate.
Three point calibrations is far easier than 4 point so I guess your problem is with the bed itself.

Quote
BobBuilder
I found out that the problem mentioned above is not jut caused by the z-gap. I can't seem to properly level the bed. My bed has three screws (a Hephestos 2 by BQ), and when all corners are levelled, somewhere in between it isn't. this drives me mad, any tips for this?

You need to adjust the screws in the right order. Post a picture of the bed. Make sure the bed and extruder are both at print temperature before you try to level/zero the bed.

Two bed leveling screws should be in a line on one axis. You use one of those as a reference and don't touch it, and the other adjusts the pitch in that axis. Start by moving the nozzle to the first screw (the reference) and set the nozzle to pinch a piece of paper by moving the Z axis up or down, then move the nozzle to the second screw and adjust the leveling screw to pinch the paper. Now if you move the nozzle back and forth along the line between the two screws you should see that the bed is level along that line. Finally move the nozzle to the third screw and adjust it- it will not affect the previously set level because it is adjusting the roll around the previous axis. That should get the bed level. Now zero the z axis by moving the nozzle somewhere near the center of the bed and adjust the Z axis up or down to just pinch a piece of paper and set the z=0 screw.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

What surface are you printing on? You may want to try a mirror since they would probably have higher quality flatness. I use a 4.8mm aluminum plate from OnlineMetals, just bolted on above a pcb heater.

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Master Tinkerer

Putting a mirror on top of a bowed pcb heater doesn't work out that great - that's where I'm at. If you only constrain one side of the mirror it stays flat, but you get an enormous gap between the glass and the heater on the unconstrained side and a corresponding thermal gradient across the surface. It also flaps around as the print head moves over it in a way that will cause deep unhappiness to anyone with an engineering background. Clamping all sides of the mirror improves thermal contact and reduces unwanted movement, but distorts the surface. It's not as bad as the original bow in the pcb as the pcb is more flexible than the mirror, but it's noticeable none the less. The only way to do the print bed right is to have a surface that is flat to the tolerances that matter for printing - say about half the thickness of the first layer height that you want to use over the entire printable area. I don't think if actually makes much difference if you use 3 or 4 point mounting, provided a suitably rigid sub-frame, although I concede that 3 point has practical advantages when setting up.