Very Poor Print Quality On Some Features

[RayLivingston 1]

I have a TEVO Tarantula, with modified extruder and hot end, which is capable of producing really excellent quality prints.  But I'm finding for certain kinds of features, the g-code being created by Cura gives really terrible results, and don't know why.

 

Here is a link to the Fusion360 model for my current part:  https://a360.co/2Ts9cBH

 

Note that the model consists of three different cross-sections:

 

Bottom "plate", 1mm thick.  This prints perfectly.

Lower profile, ~4mm thick, which consists of two concentric rings, each 0.040" wide, with a 0.040" gap between them.  This prints perfectly.

Upper profile, same as lower profile, but with several segments of the two concentric rings removed.  This prints horribly.

 

The bottom "plate" and "lower profile" both print 100% perfectly, every time.  They are very solid, excellent adhesion between layers, and dimensionally dead-nuts accurate.  The upper profile is a complete mess.  Surface finish is awful, and looks like too much plastic was extruded, at too high a temperature, with no more than ~50% fill.  This whole area is very weak, and very ugly.

 

I'm sure this is a matter of settings, but I've been unable to find any that make a substantial difference in quality.  It appears to me the problem is in the way it forms the walls.  On the lower profile, it does mostly nice, concentric inner and outer walls, then a single line right down the middle  between those two walls.  On the upper profile, it alternates between a single line down the middle, and lots of very thin, parallel diagonal lines jammed between the two walls.

 

The g-code file is attached.  The problem area starts at layer 25, on line 25519 in the g-code file.  This is being done using the "normal" profile. 

Normal.gcode

[RayLivingston 1]

Here is a photo of the part.  As you can see, the lower part printed fine, but the upper is awful.

 

Regards,

Ray L.

 

[gr5 2,172]

oh wow.  Picture helps a lot.  How big is this part in diameter?  I'm thinking it's pretty small and the problem is it doesn't have enough time to cool.  If it's smaller than 20mm in diameter then definitely try putting a rectangular "tower" on either side that is the same height or taller.  While it prints the towers the fan will blow on this and it will have time to cool a little more.

 

Putting the towers to the left and right is good as the cooling fans are on either side of the nozzle in the print head.

 

[RayLivingston 1]

That makes sense!  It is small-ish.  The OD is 1.8".  I will add some extra features to make it take longer, and let you know what happens.

 

Thanks!

 

Regards,

Ray L.

 

[RayLivingston 1]

As a quick test, I simply ran two of the parts at one time, and the result was greatly improved.  Still not perfect, but MUCH closer.

 

It's not clear to me how much cooling is "right".  Is the idea for the part to be fully cooled before the next layer is extruded, or is there a range of temperatures that is good for putting down another layer, and anything out side that range, whether above or below, will cause  problems?

 

The fan on my printer is currently on all the time, which, for most designs, works very well.  Is there a real advantage to having the fan actively controlled during the print?

 

Regards,

Ray L.

[RayLivingston 1]

OK, I've played with this some more, and things are looking better.  But....  While the appearance is greatly improved, and almost acceptable, the upper layers are still MUCH weaker than the lower ones, and it takes almost no force at all to break them off the rest of the print, while breaking the lower layers is extremely difficult.  Those areas are still (visually) much less dense than the lower layers, hence much weaker.  There is still a distinctly visible line at the separation between the two areas, and that is exactly where it breaks when stressed even the tiniest bit.

 

Regards,

Ray L.

[yellowshark 153]

What material are you using? If PLA I would suggest you set your minimum layer time to at least 10 secs. Controlling your print cooling during the print; well it is quite normal to slowly increase your cooling up to 100% over the first few layers. I am not aware of any advantage in changing the cooling once it has got to 100% - but I could be wrong :)

[RayLivingston 1]

Thanks for the info.  The printer I was given has the fans wired on all the time.  Perhaps I'll try to re-connect them for computer control, and see if that helps with bad plate adhesion.

 

I've decided to re-design the part to eliminate the thin walls that are causing the problem.  The timer per layer setting also sounds helpful.  I looked for something like that, but did not find it.  I'll have to look again.

 

Regards,

Ray L.

[Smithy 1,144]

It is called "Minumum Layer Time". 

Use the search field above the settings, then it will come up regardless of your visibility settings.

[geert_2 557]

A minimum layer time may not be enough: if the object is very small, the nozzle may keep sitting on top of the object, and radiate heat, so it can not cool down and solidify. Setting the nozzle outside of the print is also not good, because then it leaks, causing defects on the outer walls. And this interrupts the flow, causing different material temperatures and viscosities, which also shows up. The flow also has to be as constant as possible.

 

So that is why printing a dummy block next to the real model is usefull.

 

Further, in my experience it is best to keep the printing area per layer constant for small objects. Sudden changes from a large surface to a small one, also show up.

 

So, for small objects that need more cooling time, I often make the dummy block less or more into the inverse of the real model. Then the total printing time per layer is constant, and while the nozzle is printing the dummy, the real model can cool.

 

I recommend adding a flange to the base of the dummy, to make sure it sticks well and is not knocked over.

 

This first pic shows the effect of not enough cooling (dimensions mm and cm). The pictures below show dummy blocks: the theoretical concept, and a real example.

 

 

 

 

 

 

[RayLivingston 1]

VERY helpful.

 

The re-design of my part has completely resolved my problem and the new parts print perfectly.  But I'm sure I will run into this problem again, so the above information will be very helpful in resolving those future headaches!

 

Thanks!

 

Regards,

Ray L.