Slic3r vs. Sequential 3D Printing

The tiny posts on the fencing helmet ear grommet produced a remarkable amount of PETG hair, because the nozzle had to skip between four separate pieces on the platform at each layer:

So I told Slic3r to build each part separately:

Fencing helmet grommet - separate builds - first attempt

Fencing helmet grommet – separate builds – first attempt

Due to absolutely no forethought or planning on my part, that actually worked. Slic3r defines a cylindrical keep-out zone around the nozzle that I set to 15 mm radius and 25 mm height, but those numbers are completely wrong for the M2, particularly with a V4 hot end.

To the rear, the nuts & bolts along the bottom of the X gantry sit 5 mm above the V4 nozzle, with the relaxed actuator on my re-relocated Z-axis home switch at Z=+1 mm:

V4 PETG - extruder priming

V4 PETG – extruder priming

To the front, the bed fan doesn’t sit much higher:

M2 V4 Extruder - 24 V fans

M2 V4 Extruder – 24 V fans

As it turned out, the front washers built first, sitting there in front of the gantry and behind the fan, the rear washers appeared last, and Everything Just Worked.

However, even though the M2’s layout won’t allow for automated layout, I figured I could do it manually by building the parts from front to rear:

Fencing Helmet Ear Grommet - Slic3r layout

Fencing Helmet Ear Grommet – Slic3r layout

That way, the already-built parts never pass under the gantry / switch. For particularly tall parts, I could remove / relocate the bed fan to clear the already-built parts as they appear.

Come to find out that Slic3r, for whatever reason, doesn’t build the parts in the order you’d expect from the nice list on the far right side of the screen:

Sequential Build Order - Slic3r vs Pronterface

Sequential Build Order – Slic3r vs Pronterface

Worse, the Slic3r 3D preview shows the threads by layer (which is what you’d expect), rather than by object for sequential builds:

Slic3r - sequential preview vs build order

Slic3r – sequential preview vs build order

I don’t know how you’d force-fit a four-dimensional preview into the UI, so I won’t complain at all.

There’s no way to tell which part will build first; selecting the part will highlight its entry in the list (and vice versa), but the order of appearance in that list doesn’t tell you where the G-Code will appear in the output file. That’s not a problem for extruders with a keep-out volume that looks like a cylinder, so there’s no reason for Slic3r to do it any differently: it will manage the extruder position to clear all the objects in any order.

The Pronterface preview creates the objects by reading the G-Code file and displaying the threads in order, so, if you’re quick and it’s slow, you can watch the parts appear in their to-be-built order. The detailed preview (in the small window on the right in the screenshot) does show the parts in the order they will be built as you scroll upward through the “layers”, which is the only way you can tell what will happen.

So doing sequential builds requires iterating through these steps until the right answer appears:

  •  Add all objects separately to get each one as a separate line in the list to the right
    • Using the More option to duplicate objects produces multiple objects per line = Bad Idea
  • Arrange objects in a line from front to back
  • Export G-Code file
  • Load G-Code file into Pronterface
  • Pop up the Pronterface layer preview, scroll upward to show build order, note carefully
  • Rearrange parts in Slic3r accordingly

That’s do-able (note the different order from the Slic3r preview):

Fencing helmet grommet - manual sequential build

Fencing helmet grommet – manual sequential build

But it’s tedious and enough of a pain that it probably makes no sense for anything other than parts that you absolutely can’t build any other way.

In this case, completing each of the bottom washers separately eliminated all of the PETG hair between the small pegs. The upper washers still had some hair inside the inner cylinder, but not much. If you were fussy, you could suppress that by selecting “Avoid crossing perimeters”, at the cost of more flailing around in the XY plane.

All those spare grommets will make a good show-n-tell exhibit…

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