Some layers printing bad in the middle of the part

[emilio2 2]

Hi.

I have tried Cura to print this part on my UM2, but because it printed pretty badly, I used Simplify3D instead. Although quality is better than using Cura, I get a weird layer dent on the middle of the part (it is not a crack), as you may see on the following picture. I have repeated the print two times with identical outcome. Other ugly effect you can see is that the first layers are printed inwards(perhaps too hot bed?). Material is PLA at 0.2mm, using default Simplify3D parameters for UM2.

 

[gr5 2,210]

I see 3 issues. Let me address the ones you didn't notice so much first.

You have a much bigger dent near the bottom of the part. This is usually caused by the heated bed being too hot for PLA. I recommend 50C. 70C is definitely too hot.

Also I could be wrong about this but your bottom layer looks a bit rounded - not as squished flat is desireable (I means it's quite close - maybe its fine but I think it's high). Because of this parts might pop off the bed during a print.

Okay... now to your question. Those last 2 things have nothing to do with it.

You are over extruding on 2 layers I think and then normally (or under) extruding at the "dent". This can be caused by quite a few things:

1) Most likely (60%?) it's your Z axis not moving the same amount each time. I see you mention .2mm so I assume this is your layer height so I'm thinking the bed didn't move the proper .2mm (maybe moved 1.5mm) and so you got some over extrusion and it squished out a bit, then on the next few layers it moved more (say 2.2mm) and so you got normal or under extrusion (just slight) and then it repeated again but then you got consistent movement after than.

Z axis issues could be caused by many things - turn off power and push it up and down and see if it's rubbing anything (like spool holder!). Consider adding some grease (the little green tube if you got one with your printer). You may have a bad Z screw or bad vertical bearings or bad Z nut or bad vertical rods.

2) Also quite possible (30%?) is temperature issues. The fan may be coming on for the first time at the dent and you might be getting underextrusion due to cold nozzle. Also newer versions of Marlin seem to have issues related to some changes in how the bed temperature and nozzle temperature is controlled. You can verify this by watching the nozzle temp while it is printing the layers in question.

Both of these issues can happen at the same height always for different parts (if #2 is triggered by fan) but #1 tends to be more consistent. Both of these issues can be inconsistent where they happen on different layers. Other than fan turn on location #2 tends to oscillate forever or just for a few minutes after fan turns on. Whereas #1 is often at the exact same sticky spot on the Z axis.

 

[emilio2 2]

Thanks for your detailed answer!

I have performed some tests by moving up and down the bed by hand. I can hear a "sandy" noise as if the z screw had some sort of sand or dust. I cannot tell exactly where the noise come from. The z screw is recently greased with the product that come with the printer. I believe it has a lot of grease. Perhaps too much?. Moving the bed up requires quite an effort. Moving it down is easy.

The printer was bought by the original owner in March-2014. Since then the printer has been parked on a table without cover and without printing. So it had lots of dust when i bought it.

[gr5 2,210]

I would take the bed apart. There is a simple Z nut - small. Held in by I think 4 screws. I would first take off the cover (or at least slide it up and examine the Z screw and then clean some more and then maybe take that part off and play with it.

You might want to clean all the grease off and then grease it again. It's okay to have lots of extra grease but a single pea size drop should be enough.

WD40 is pretty good at cleaning things like this if there is dust and dirt.

It could easily *not* be the Z screw and might be those 2 vertical bearings. Those are more likely to make a "sand" noise. There are small ball bearings in the linear bearings. Usually the noise sounds bad but is simply the bearings falling in a normal way. You could loosen those also (remove the screws) and slide them up and down to check for resistance. Those two bars should be lubricated with a light oil (not wd40, not grease).

Any light mineral oil will work. The fewer additives the better. Some examples: baby oil, sewing machine oil, mutli-use oil, 3-in-one oil.

 

[gr5 2,210]

If there is dust in the two linear bearings that's bad. The 2 bars are held in by simple caps underneath. Remove the long bars, remove the bearings, clean well with wd40 and oil with light machine oil. Test on the rods - make sure they slide smoothly.

 

[yellowshark 153]

Hi George, I am certainly not an expert on the subject but I do wonder if WD40 is a good idea for cleaning, Sure if you want to try and release a stubborn nut it can be really helpful but as I understand it WD40 leaves a deposit. I have a paper on the subject buried somewhere which explains the chemical process. We want the rods and the z-screw to be as frictionless as possible so when I came to clean mine for the first time WD40 immediately came t mind but then I started thinking about it - and rejected it.

I could easily be wrong/paranoiac but I thought I would mention it just in case,

 

[emilio2 2]

I have recalibrated the bed, lowered the bed temp to 60C and the speed to 30mm/s. Now it prints fine.