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Pictures please. Please send a picture of your extruder. Also maybe you could send a picture of a failed print or a video of when the print stops working and show the extruder in part of the video? I can't tell you how many times we give bad advice up until the moment we see a photograph. Maybe you should disable retraction in Cura to eliminate that as part of the problem. Then show a photo of what is going wrong.

Make sure the fan is on and make sure it is taking at least 5 seconds per layer. If you don't do both of those then you will get exactly what you see here (although I think illuminarti is right and it's printing over air). For example the "minimal layer time" should be at 7 seconds and more importantly, "minimum speed" should be 0 (under expert/cool).

Good detective work! An alternative would be to change the model maybe to have walls that are .8mm thick. It seems to me cura should be able to print .4 to .6 mm thick walls just fine with a single pass. That would be a nice feature!

As illuminarti says - it is supposed to do this. You can disable this feature in cura (disable retraction) but it's very useful for ultimakers. Please post a picture of your print that shows what is going wrong if it isn't coming out how you hoped.

On your electronics board under the UM are drivers for each of the steppers. There's a little potentiometer on each one. They can turn. See if one of them is at a different setting than the others. It may be that it got bumped or wasn't adjusted properly and is putting out more current than the other 3. More info here: http://wiki.ultimaker.com/Electronics_build_guide#Tuning_the_stepper_motor_drivers

1) Your bed levelling might actually be just fine. But you need to clean the blue tape with isopropyl alcohol to get the PLA to stick to it. Also called rubbing alcohol. It is found at any supermarket or drugstore where they sell bandages. The non-stick side of blue tape has something that allows the tape to be rolled up and not stick to itself. That must be removed. 2) Push the print head around with your hand. You should be able to do it with one finger. Check the endcaps (as SandervG said) - try loosening all of them to see if that makes a difference. Make sure the belts are directly over or under the rods. Make sure the belts aren't rubbing against the wood frame - especially near X stepper.

The idea for printing slow is that there is a delay between when you tell the feeder to start and stop and when the filament actually stops and starts. With lower speed printing, the pressures in the nozzle are lower and it responds faster when you retract. And blobs should be hopefully too small to see. I have been very happy with the default 4.5mm retraction. Although I did add this part to my feeder side of my bowden tube which may or may not help with the retraction/blob issue: http://www.thingiverse.com/thing:46157

That pyramid looks pretty damn good regarding stringing/blobbing. Hmm. Some slicing bug probably then. But to answer your question: In the "start/end gcode" section of Cura you can add a line that simply says "M302". Don't forget to take this out at some point as cura keeps using your previous settings on future prints. When you power cycle the ultimaker it should go back to normal mode. I've never tried any of this but plan to some day. It seems straightforward. But be careful not to put too much pressure. Another thought I had was maybe you had too much retraction. If you look at the bowden tube, when you retract, the filament should be no longer touch the top of the arc of the tube and should be resting on the bottom of the tube. If you retract too much you will suck air into the nozzle which will rise (bubbles rise in liquid as they are lower density), then the pla will reseal at the tip of the nozzle, the the air bubble heats up and expands, then some pla comes out (blob) and later you will get a bubble of underextrusion somewhere else.

Your blobbing is also called "stringing". For some colors/brands of PLA there's nothing you can do. For other's you can get rid of it. I found that temperature was the most important factor. Build two test towers like in the below photo and start at 200C and slowly lower the temp in 5C increments as the two towers print and keep good notes. Consider disabling the extruder-temperature-shutdown feature (M302) so you can go below 175C if needed. Definitely print slow (maybe 10 or 20mm/sec) with colder temperatures: http://umforum.ultimaker.com/index.php?/topic/1872-some-calibration-photographs/

Tereza I have a heated bed but don't use it very often anymore because of what I learned... non-heated-bed: If you clean the blue painters tape with isopropyl alcohol (aka rubbing alcohol) it cleans off the wax on the non-stick side (which keeps the tape from sticking to itself). This will make your parts stick about 5X more powerful and you will have a new problem - getting parts off the blue tape. Or if the blue tape lifts at the corners then use wider blue tape. heated-bed: It should be at 70C. I found that at 20C to 60C the heated bed does nothing for me. But at 70C something suddenly happens and things stick real well to kapton tape. Some people even turn off the heated bed after the first layer and claim it sticks even better when it cools below 60C. heated or non: 1) Brim, brim brim brim brim! Brim is great. Use it. Around 10 passes (4mm) is usually enough. 2) Rounded corners helps (but brim gets around that anyway). It's easier for the corners to lift as they have lots of force on a tiny tiny point. Rounding them helps a lot. Or use brim (did I mention brim?). 3) putting vertical holes through your part helps - from top to bottom. Also consider turning off the bottom layer if you don't need it. This shouldn't be necessary but can really help. 4) Print first layer thick, slow, and hot. .3mm, 10 to 20mm/sec and 240C. I no longer do .3mm first layer as I am better at levelling but the point is to shove that blue plastic hard onto the kapton or blue tape.

Hopefully someone else can answer this as I don't know techniques for printing directly on aluminum but... Have you considered getting larger kapton tape? Would 6 inch wide be enough for most of your prints?

You don't need flyback diodes as they are already built into the UM. The mk2a acts mostly like a resistor. Read about ohms law on wikipedia. It is the basis for everything electronic. If you don't understand that then you should read it again. The mk2a is specified to be 1 to 2 ohms but which is it? I guess it's closer to 1 ohm when cold and 2 ohms when hot? That's a huge difference. A factor of 2X. How annoying not to be more specific. I=V/R (current = voltage over resistance). So if V is 12V and R is 1 ohm then you will get 12 amps. If you double the voltage you get 24V/1ohm is 24 amps. 24 amps at 24 volts is 4X the power of 12 V at 12 Amps. 12V at 12 Amps is 144 watts.

Or in Cura in exper settings you can simply check "fix horrible" "extensive stitching". This works for your model.

Oh - and you can try "painting" the corners with a brush soaked in acetone when the first layer is going down.

1) What temperature did you try for the bed? I'm guessing not hot enough. Where did you measure the temp - and how. Also you should try turning off the heated bed once the first layer is down. This will keep edges from lifting but has side effects. 2) Can you post your STL file? Changing the model slightly can make a huge difference - post it for suggestions. Or post it somewhere else and post a link to it. 3) Did you use the brim feature? It helps. Did you turn off raft? You should.

That last picture - that's pretty bad. I can see that your belts are loose as you can see there are two lines close together followed by a gap, then two lines again. This is probably due to the short belts being loose. Here is an explanation: http://umforum.ultimaker.com/index.php?/topic/1872-some-calibration-photographs/?p=14474 However, that's not the main problem. There is not enough filament getting laid in there. Strange. Is the feeder screw tight? It should be compressed to about 11 to 11.5 mm. The feeder should be able to pull about 22 pounds force with no load (no pressure) at the print head. You might want to test this by pulling back on the filament. You could increase "flow" at the UC. Not sure this is a good solution. Could it be that you set the filament diameter to 3mm in cura yet your actual filament diameter is less - around 3.9mm? This would only reduce infill by 2.5% yet you appear to have 50% underfill. Maybe your nozzle got clogged near the end of the print? Some dust or wood chips might have gotten in there. Maybe you don't have the fan on? With the fan off, sometimes the nozzle messes up the layer below because it hasn't had time to harden. That's all I can think of. It sure looks strange. Like the feeder stopped feeding filament. Or the nozzle got jammed.

This post is confusing. Could you post a picture of the view in Cura of the two parts, label them, and explain exactly what the problem is in fewer sentences? FYI - if you set all the x,y,gantry distances in cura/preferences it handles multiple objects better.

Cura sends to Marlin line segments and print speed. It estimates the time by calculating how long it would take at that speed. Marlin then has default acceleration (and "jerk") parmeters and the actual time to do a short segment is much slower because it doesn't get up to speed. So for parts with lots of small line segments e.g. small parts, it is off by a lot. For large parts with long straight line segments it's pretty damn accurate. Cura always gives you the minimum. Noone has ever said it took 1/4 the time Cura expected. For cura to get accurate times it would need to import a dozen or so parameters from Marlin and know that you have Marlin as different firmware does different things. There are more important things for daid to do at this point.

I prefer lower temps with retraction: http://umforum.ultimaker.com/index.php?/topic/1872-some-calibration-photographs/ I haven't noticed a grinding noise but that doesn't mean I don't have it also. Note that 40mm/sec is a little faster than the extruder can reasonably spin. But there was a bug in Marlin such that it didn't matter because Marlin was moving the filament much slower. But if you have the latest Marlin and are still using 40mm/sec for extrude/retraction speeds that might be a problem. Illuminarti wrote a blog entry about this somewhere. I'm not sure what a reasonable speed is. Probably 20mm/sec.

Need more details. Are you saying no plastic is coming out of the nozzle until it starts the 3rd pass/layer? If so maybe the extruder retracted at the begining before printing and it takes a while for the plastic to catch up? Maybe something is wrong with your gcode? Please explain more what the problem is.

You should have two sets of cabling. You can try the spare set (on both ends of the cable bundle).

If your bed isn't flat you can compensate in several ways. One is to use a thicker bottom layer. This is fine for most things. Another trick is to use the "raft" setting. But this will make the bottom layer look pretty bad and is even more likely to look bad on an ipad case (but for most things it doesn't matter). You could also choose either of these settings and print the ipad case sideways. Personally for an ipad case, I think .2mm layers would look better.

The bottom is usually as smooth as the print bed or in your case as smooth as blue painter tape. The top surface doesn't have a nice flat surface that smooths it out (like a putty knife).

That's actually pretty good so if you are dissapointed, prepare to be dissapointed a lot. If you look carefully at the infill passes, you can see every other pass has a larger gap. This is classic example of "play" or "backlash". This is easily fixed by tightening belts - usually the short belts to the motors which are also the easiest to tighten. Here's an explanation of backlash which can also cause an issue of not reaching the sides: http://umforum.ultimaker.com/index.php?/topic/1872-some-calibration-photographs/?p=14474 Make sure to also look at the photo of the cubes 2 posts earlier than the above link. As far as bumpy - probably not much you can do about that other than making your parts not-flat on top maybe? For example a pyramid would look better but I assume you don't want to only print pyramids, lol. You could also look into "laybrick" filament which is bumpy everywhere so it looks the same everywhere so it looks really perfect. I've not tried it - it's still very new.

The old cura does. You can install both and switch back and forth at will. When you install a new major version (where the second number changes e.g. 13.03 to 13.04) it appears to install a separate installation so you can go back.