Wipe tower: What is the purpose of the gap in the wipe tower?

[IgorTag]

I noticed a gap in the wipe tower.

What is its intended purpose?

This gap is causing problems when printing with some high-strength/rigid materials

see code:

PrusaSlicer/src/libslic3r/src/libslic3r/GCode/WipeTower.cpp

Lines 1339 to 1340 in 6f51012

	float left = fill_box.lu.x() + 2*m_perimeter_width;
	float right = fill_box.ru.x() - 2 * m_perimeter_width;

[lukasmatena]

I am not sure what I am looking at. Can you please enclose a 3MF so we can reproduce it exactly? What PrusaSlicer version are you running?

[IgorTag]

Hi Lukáš,
thanks for the quick response!

I'm running PrusaSlicer version 2.9.6
I've attached the 3MF file for you to reproduce it exactly, along with an additional picture that shows the gap more clearly.

TEST_WipeTower_forGitHubIssue.zip

I'm not sure if this is a bug or an intended feature, but hopefully the attachments help clarify.
Let me know if you need anything else.

Thanks again!
Igor

[lukasmatena]

Thanks, I understand it better now. The gap has no purpose, to be honest. When the wipe tower was first created, the structure of the empty layer was hardcoded and it looked like this:

When we made it more parametric later, we introduced the option wipe_tower_bridging, which can be used to push the internal lines closer together. It just did not occur to me at the time that someone might need to push it this close together (which is why I did not recognize it at all before), so I left the gap at the left and right edge hardcoded as it was, thinking that it is small enough for all use cases. This assumption held for 8 years, you are the first person I know about who asked about this.

What problem is there with the print? How do you handle bridging on the print itself, if bridging 1.5*perimeter width on the wipe tower is a problem?

[lukasmatena]

To be clear, I am not saying that we cannot or will not change it. I am just curious about your use case.

[IgorTag]

You're right that in most cases this probably causes no problem at all, but we have a bit of a non-standard use case: we're using a 0.25mm nozzle with a high-performance filament. Because of that, the "standalone" walls sometimes break away as it grows taller.

The idea is to lay the filament lines nearly against each other, so they hold onto one another and stay stable. It works well, except between the perimeter and infill of the tower on the left and right side.

[IgorTag]

So the bridging itself is not the problem — it's the thin wall above it.

edit: of course, I mean the thin wall below it. :-D