Spiralize generating a z-scar?

[mechamecha 64]

I'm trying to print a cup that I designed in Blender. I'm using Cura's Spiralize feature and blue T-Glase filament. I exported the model from Blender as an STL file. I'm trying to print a .6mm shell so the cup won't be too flimsy, but it's been difficult getting a watertight surface, especially the bottom. It seems that my Ultimaker 2 under-extrudes quite a bit when attempting to lay down a shell wider than its .4mm nozzle. It's been printing a mesh with tiny holes on the bottom of the cup. I've been gradually reducing the filament diameter setting to compensate, which seems to be helping. Is there some other setting that I should be modifying?

Another thing I've discovered while viewing the g-code in Repetier-Host is that Cura's Spiralize feature is generating a z-scar down the side of the cup. It was my understanding that Spiralize was supposed to prevent that. Is there any way to get rid of the scar?

 

[illuminarti 18]

The under-extrusion may simply be volume/second related. In 'Spiralize' mode, Cura will print the bottom layers using passes of the same shell thickness as specified for the rest of the print. Depending on your print speed, and first layer height, this can be too much plastic per second.

What first layer height, speed and shell thickness do you have set?

If you are simply trying to extrude too much plastic per second, there isn't a lot you can do to compensate, you really have to slow down or make the layers thinner. However, if you simply need more plastic to be extruded, and there's still capacity to do that, then the simplest way to do that is to go into the tune menu, and increase the flow percentage above 100%. (Reducing the declared filament diameter has the same net effect, but is conceptually a slightly less direct way to think about it).

 

[illuminarti 18]

The z-scar is an artifact of how Cura generates the spiral version, I think. It's not the same, or as severe as if it was a normal layer-by-layer print, but Cura still slices the object in layers, and then adjusts them after the fact to give the gradually climbing edge. As a result, sometimes the start of one layer doesn't quite line up with the end of the previous one - in which case the head jumps back to the start point of the next layer - and leaves a mark.

 

[mechamecha 64]

Thanks, illuminati!

All of my layer heights are set to 0.3 mm, speed is 10 mm/s, and shell thickness is 0.6 mm. The bottom prints fine when I set the shell thickness to .4mm, but develops holes when I set it higher than the nozzle diameter. I want a thicker shell to print a sturdier cup.

 

[mechamecha 64]

I probably wouldn't notice the z-scar on the print so much using normal filament, but T-Glase definitely makes it more obvious!

 

[illuminarti 18]

Going thicker than the nozzle width at the same time as printing quite thick layers may be part of the issue; you're relying on the pressure between the bed and the tip of the nozzle to spread the plastic out thicker than it's being extruded. If your bed leveling is just fractionally high, that pressure won't be applied consistently. If you pick a 0.4mm width, then it's less of an issue; even if the extrusions are just laid next to one another, and not squashed out, they'll still be touching correctly.

Probably going to a lower layer height - say 0.2mm - might also work with the wider shell thickness.

Personally, I'd prefer if Cura did the base with normal nozzle-width extrusion, and only switched to wider extrusions for the sides of spiralized prints.

 

[gr5 2,265]

Probably going to a lower layer height - say 0.2mm - might also work with the wider shell thickness.

 

Yes. Or raise the temperature. higher temperatures let the PLA flow more like water instead of toothpaste. What temp are you using - 240C is usually my upper limit with PLA. Higher than that gets a little dangerous that you will bake the PLA into a carbonized clog if you don't keep things flowing.

 

[Daid 306]

The above effect is because spiralize is a hack. It generates the layers as normal and then modifies every point on the path so the Z is increased. This means that you can have an offset in X/Y on every layer as it does not use the "proper" XY position at the end of the layer.

 

[gr5 2,265]

Having said that you should get much less Z scare (or none visible) with spiralize.

Personally I don't get Z scar at all on my "show" prints as I print them slow (35mm/sec) and this seems to eliminate the Z scar for me. So I really don't care about the randomize feature.

 

[mechamecha 64]

T-Glase really shows any kind of flaws in the surface. I chose a large layer height, because I had read that larger layers make the T-Glase surface more transparent, while thinner layers add opacity. (Is a 0.4 mm layer height possible with the Ultimaker²?)

Anyway, after experimenting for a couple of days, I produced a very nice, water-tight print of my cup! I had to set my filament diameter to 2.50 mm (even thought it's actually 2.85) to make the bottom water-tight. The final settings I used are as follows:

Layer height: 0.3 mm

Shell thickness: 0.6 mm

Bottom thickness: 1.2 mm

Print speed: 10 mm/s

Hot end temp: 220

Bed temp: 70

Filament diameter: 2.50 mm

 

You can see the z-scar in the second photo, but a person picking up the cup just to have a look would really have to scrutinize it to notice it.

 

I must say, T-Glase produces a very cool looking surface! Depending on the viewing angle, it can look glassy, metallic, or plasticky.

 

[illuminarti 18]

That's a really cool looking print. There's no reason that you can't try a 0.4mm layer height, especially if the extrusion width and/or flow is sufficiently high to ensure that the layer gets squashed down onto the lower one properly.

However, you will fairly quickly run into throughput limits in that case, so will be restricted to printing very slowly.

 

[mechamecha 64]

I decided to see what difference, if any, printing with a 0.4 mm layer height would make. It made a noticeable difference in surface clarity. I would definitely recommend a 0.4 mm layer height when printing with T-Glase filament. (Because of the additional volume of filament being extruded, I had to lower my filament diameter setting yet again—to 2.30 mm—to get a watertight bottom using Spiralize.)