I'm sure the model(s) are watertight, because I checked them many times and they were used for other purposes, so I can't understand were the problem is.
I checked the xray view : it's all ok. But when I look at "layers view" some layers (1 or 2) are missing and there is correspondance with the layers missing in the print.
I will do some photos and post them. In the meanwhile, if anyone experienced the same problem let me know. I'm looking for a solution.
Thanks
Play with the "fix horrible" settings. Uncheck all of them if your model is perfect. These are meant to fix "bad" models so if your model isn't bad then you don't want them.
You didn't answer the question about if a wall was very thin (under 1mm).
Another possibility is *too many* short line segments. The way cura works is it intersects a plane with all the millions of triangles in the STL. The triangles are in no particular order so the lines are also in no order. So then Cura goes about hooking up those lines into closed loops. There's a little bit of rounding error in the points so Cura allows a little leeway or slop when deciding if two points are the same. So sometimes it gets confused if there are many tiny polygons smaller than .1mm across and the surface is "noisy". This can be sometimes fixed by reducing the qty of polygons - there's no reason to have triangles smaller than .1mm across.
If this is the problem checking "fix horrible A" might fix it.
Or you can reduce polygons this method. Really if you had posted a picture it would have helped me not think of problems that might have nothing to do with yours:
http://www.shapeways.com/blog/archives/226-polygon-count-reduction-with-meshlab.html
@gr5 : the wall is 0.8 but I have the infill, I tried also "solid" with 100% infill : same result.
I'll try with "fix horrible" tip..I'll be back with news.
Thank you, for the moment.
No, not the "shell" setting in Cura. The actual part you are printing. Cura can not print walls thinner than the nozzle and sometimes maybe not even walls under 1mm thick. It *REALLY REALLY* would be helpful for you to post a photo of the part in normal view and layer view.
Here is how to post pictures:
http://umforum.ultimaker.com/index.php?/topic/4525-how-to-upload-an-image-to-the-forum/
I've come across this at least twice this week. I missed it the first time and printed the goofed up gcode. The second time I caught it before printing.
Basically the model slices OK, but there will be a gap of a couple missing layers at a given height. There's nothing different about that part of the model (not the start or end of a feature, thin walls, etc.) So far it's happened about 2/3 up and it has been in the form of a thick gap (let's say 1mm) and then a smaller gap slightly above that (say 0.2mm)
Moving or scaling the model, changing the slice thickness, wall thickness, or fix horrible options don't change the gap. There's nothing that shows up in Xray view and the model is manifold and doesn't have thin walls or anything. The gap doesn't get support added, and they don't get top/bottom surfaces added (it exposes the infill), so Cura doesn't "see" it as a gap in the model.
You know when you rapidly scroll through the layers, Cura will draw layers with gaps in between, and then go back in a fill them in when you stop moving your mouse and it finishing loading the toolpaths? It's like that, except they don't ever get drawn in and appear as slice gaps in the output.
The only way I found to fix it was to go back to my modeling app and make a small tweak to the mesh, like moving a couple points around and then exporting again. (you could try Meshmixer or Netfabb as well.)
(using Cura 14.03 on win7 64bit with .stl files)
I tryed changing the scale of the object in Cura : the missing layers "move" on the printed object, but no way to make them disappear.
One more try I did is to rotate the object 90 degrees : in this case the layers are apparently not missing in the print although they're missing in Cura's layers view.
That sounds strange : it's not a problem with the model to print, maybe something else? It's quite strange that this problem regards different objects and not only one.
@gr5: the shell thickness parameter is set at 0.8mm that is twice the size of the nozzle diameter. Increasing this parameter do not solve the problem!
@selmo : we have the same problem as I can read in your post. If you find a solution, let me know, I'll do the same.
@ultismoother - you frustrate me. POST PICTURES PLEASE! I asked you several times. Also you don't understand what I am saying about wall thickness. I have seen the problems you talk about. They can be from several known causes. I refuse to say anymore until you post pictures. Even better send me the STL.
@selmo there are many causes for missing layers. If the model is manifold (that's a HUGE IF as it is difficult to prove) and if there are no walls that are inside the STL closer together than .8mm then there are still several other things that can cause this. It's hard to explain so I'd rather you sent me an STL with issues. For models that are organic - like sculpture the problem tends to be (somewhat) related to your triangles being too small. For mechanical or architectural models it tends to be that walls or corners are too close to other surfaces or corners.
Part of what cura does is it intersects a plane with the STL and gets a bunch of line segments in no particular order. This is important: NO ORDER. STL doesn't specify which triangles are touching which. So then Cura takes this random ordered set and tries to link all the lines up. Sometimes it links them up wrongly because they are very close together but not quite and Cura decides "close enough". This causes problems as Cura gets chains of lines that refuse to form loops and so it just discards those and prints nothing.
@ultismoother - you frustrate me. POST PICTURES PLEASE! I asked you several times. Also you don't understand what I am saying about wall thickness. I have seen the problems you talk about. They can be from several known causes. I refuse to say anymore until you post pictures. Even better send me the STL.
Thank you for your help, but don't be frustrated, gr5! 
I'll post pictures sooner or later. To do so I have to print an object with missing layers, and I have no time to print something unuseful at this moment.
The wall thickness on the models with missing layers is 1.6 millimeters or more. I'm a professional industrial designer, so my STLs (copyrighted) are really ok, they print fine using another printer and another software. They also work fine on a 5 axis CNC robot.
I found really interesting your answer to selmo : maybe you have found the cause of my problem, too. I'll try a different STL face resolution and see what happens. 
To do so I have to print an object with missing layer
No. You don't. I want to see your part in layer view and in normal view in Cura only. I do not want to see what it looks like printed. You already told us the layers are missing in cura slice view - so why would you then print it and be surprised.
Thanks. I'll try to catch it next time it happens and forward the STL. As it is, I have not seen it since my post.
"gets chains of lines that refuse to form loops and so it just discards those and prints nothing" <-- sounds accurate.
From what I remember about the issue, the areas that were affected were not thin walled but fat slices (in fact, cross sections with many perimeters and with infill). It wasn't just a hole in a wall of the model, but a whole block of multiple layers missing across the whole cross section. If there were multiple closed loops in a slice, not all slices were missing from the layer. I'd get some loops and not other(s). Which could line up with a poly that Cura is having trouble with or something like that. It also makes sense that my workaround / fix of going in and tweaking the mesh slightly fixes it. It's sorta like if a polygon or point has an exact certain coordinate or alignment it gets dropped and causes the loop to fail to close (or something like that)
For what it's worth, I usually try to reduce my polygon count (and check for degenerate polys / non-manifold edges, etc) before exporting to STL because too many polys just slow down slicing without improving quality. Having been using KISSlicer for over a year, which is less forgiving about model quality than Cura is, I've gotten some experience in avoiding the usual gotchas.
Hi, i just encountered this problem today. I printed a bad one as i didn't know this could happen. I'm using the "lulzbot version" of cura.
Both models were designed in Solidworks and are Assemblies of several separate parts. So i'm assuming something weird is going on when i'm assembling them together. Perhaps some sort of weird "zero thickness geometry"? Using the x-ray view as someone suggested seems to show a problem with the mated faces. I'm wondering if i somehow forgot to mate those faces together and that is what caused my problems. I can use the cura expert settings to fix the bad model, but i'd like to figure out how to prevent creating a bad model to begin with. Any ideas?
The second model i have since fixed, but i have no idea how. I was changing it's dimensions and now when i look at the new STL file it looks fine in cura.
Always look at layer view before you print. Although I guess you know that now. It will show you problems in 10 seconds instead of finding out 10 hours into a print.
The 3rd photo with the red and blue - that's xray view - that tells it all. The red areas are problem areas. The color is determined by passing a line from the eye through the part. If it hits an even number of polygons it is blue or white or some shade in there. If it hits an odd number of sides it is a red shade. It means there is an internal wall.
For example if you took two identical cubes and pushed them together but once together you removed one of the walls that are touching but not the other wall. That is what is happening.
You can also get red if there is a hole in the object but I don't think solidworks can do that.
Yeah, I know that now haha. Thankfully i only printed the smaller model and not the larger one. That gave me the opportunity to fix my large model before printing it.
I still don't know exactly what went wrong (although the models do have internal walls/faces since they are assemblies of smaller parts), but i figured how to fix it. I've been using 3dContentCentral to convert my Solidworks models into STL files. In my version of Solidworks when i export an assembly to STL it saves each individual peice as an STL instead of the whole assembly. Hence my using 3dContentCentral's nifty conversion tool to get around this. Apparently the problem occurs when i save the assembly as a Solidworks Part and then convert that, but i have discovered if i upload the complete assembly (and dependencies) and convert that to STL there are no problems. Weird, but at least i found a solution.
I really liked this page describing "bad edge" geometry. http://www.meccanismocomplesso.org/en/check-stl-files-3d-prining/
good link. I saved that for future when people have this same issue.
I'm going to point IRobertI to this thread and see if he has input on solidWorks as that is what he uses i think.
I got poked by gr5 to take a look. All I can say is that I never export assemblies as a single "chunk" for printing. My assemblies always consists of the individual parts that I need to print so I just save those out as individual parts.
But I'm guessing what you mentioned is exactly what's happening. You have internal walls between the different objects which is confusing the STL exporter or something along those lines.
Hi, i am seeing this problem on a print in cora, for some reason its missing layers, did anyone find the problem, im not an expert and didnt create the STL myself, just preparing it for print and its missing slices of the print
@buckyuk - this topic is years old. You should start a new one. But if you didn't create the STL yourself then try the netfabb free repair service:
https://service.netfabb.com/login.php
It's free but you have to create an account. Once you create the account the repair service is fast and incredibly simple.
It's an old post, but I faced a similar problem and found a solution. When a wall thickness is a multiple of the nozzle size (e.g., 0.8 mm for a 0.4 nozzle), Cura seems to have problems deciding whether to print the wall in it's actual thickness or thinner, in which case some layers are missing. Adding a small amount, e.g., 0.01 mm, to the wall thickness, gets rid of the problem and won't affect the actual wall thickness.
//EDIT: My solution from above is tedious and only seemed to work for certain parts (in my case, parts that did not overlap OR were not merged). Changing the "Horizontal Expansion" setting by a tiny amount seems to provide a better fix (+/- 0.001 did the job for me).
//EDIT2: The link posted above (https://service.netfabb.com/login.php) also works. I didn't expect that my CAD software messes up creating an STL from such a simple shape.
Edited by itsmiHello guys, since yesterday I've read through all your discussions. I have a problem with printing the STL file of my design on CURA. The design has circles with extruded cuts in it. On Solid view the structure looks perfect, but once on Layer view the extruded cuts disappear. The circle shows till the point of the extruded cut, then it stops. I've done everything possible to resolve this, all to no avail. I even uploaded the STL to Netfabb Online, the resultant component still had same issue. Can anyone help please?
I attached screenshots of the stuff I want to print. If you want to take a look at the full design, I can also send it to you mail. I'm stuck and in need of help. Thanks guys.
It looks like that circular part has thin walls. You could try enabling the print thin walls option but that doesn't always do a good job. Alternatively, reduce the wall line width enough so that the walls on the circular part are created, enable the overlap compensation and set the minimum wall flow to something like 50. Hope this helps.
1
@burtoogle: Thanks a lot! It worked!
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gr5 2,192
First of all never print anything without doing layer view first as you now know it saves you a wasted print.
I know you think it is "perfect/watertight" but this is certainly the most common issue (internal walls or holes). You can test it in Cura easily with XRAY view. If you see any red then there is a problem. Rotate all around the problem area.
A second reason for "not printing" is if walls are too thin. It would help if you posted a photo of the part in normal view and also layer view showing a problem layer.
To post pictures:
http://umforum.ultimaker.com/index.php?/topic/4525-how-to-upload-an-image-to-the-forum/
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