issue:
For a long time I was annoyed about the bottom surfaces of parts printed with support looking really bad (I’m mostly talking about engineering parts).
software and data
Using Orca Slicer V2.2.0
My project is available on makerworld:
problem:
bad bottom surface of the parts supported area
identification of the cause:
What causes this bad surface? It’s bridging! The slicer applies “bridging” on support structure, even when the interface layer is a completely closed surface.
But what characteristics does bridging have?
- bridging means less extrusion, therefore the extruded strands have a smaller cross-sectional area than regular strands (outer wall, inner wall, …)
- another bridging characteristic is a higher fan speed to cool down the extruded material as quickly as possible (so it doesn’t squish between the support interfaces strands - in most cases people use the slicers standard properties, therefore the interface layer has gaps)
- the slicer applies a Z-gap between the top support layer (interface) and the following part layer (Top Z distance)
solution (unsatisfying):
There is the option using a different support material. The most common example is using a PETG interface material for a PLA print.
With this option, you can set the top Z distance to zero as well as the top interface spacing. This leads to a nicely closed surface on which you could theoretically directly continue printing PLA.
Unfortunately there are some downsides:
- You can’t expand the support or the support interface beyond the outer boundries of the supported section of the part (ok, there is an option called “normal support expansion” but this never worked for me). The support or the support interface just ends on the part boundary, so those small bumps from the strands beginnigs/ends will directly transfer to the above part layer.
- since the support interface material is alien to the support material, it sometimes doesn’t stick well enough and will eventually detach from the support (which usually kills your print). The slicer doesn’t provide any interlocking mechanisms.
goal:
create a surface from an alien material which is as smooth as possible but still has a good connection to the underlying support structure. The surface must be two or three milimeters larger than the supported part, so the strand beginning and endings are not in the supported area of the part. This surface shall be seen or used as a kind of build plate.
solution: (explained relating to this example):
6.) constructing a support part (PETG red) with the following properties:
- some elevated pins to interlock with the underlying support
- supported area is larger than the supported part
7.) constructing a modifier part for the first layer on support (height: layer height, in this case 0,2 mm)
8.) slicer settings:
properties global:
support:
- manually select the supported areas of the red PETG part
- support type: normal (manual), snug
- support/raft and support/interface: PLA black
- top/bottom Z distance: 0
- top/bottom interface spacing: 0
- support/object xy distance: 0 (so those red PETG pins are really embedded in the surrounding black PLA)
properties for the red PETG support part:
- wall loops: 1
properties for the modifier part
- wall loops: 1
flush volume:
- increase the flush volume from PETG to PLA to a minimum of 600, more is better to ensure the PLA is not mixed with PETG residues (otherwise the print will break easily since the impure PLA has bad strength)
- flush volume from PLA to PETG can be smaller since it is just support
I’m quite satisfied with the result, but there still is some room for improvements.
issues/improvments:
The following issues can improve the surface quality even more, but I don’t have a solution yet:
- as you can clearly see, the top surface of the red PETG part is not continuous, every strand is interrupted twice. These interruptions have the shape of the parts outer boundary but with a 1-2 mm offset. First step in improving the surface would be to figure out how to print without interruption. Any idea?
- ideally you could apply ironing on the top surface of the red PETG part. But this doesn’t work with “all top surfaces” because the area which is supported is being spared out and I don’t want to iron every layer.
So the question is: is there a possibility to force ironing for a specific layer? - Material pairing: For a PLA print im using PETG as interface material. But the other way round hardly works, since I usually print larger engineering parts which means I heat my chamber up to 45°C for a PETG print to (successfully!) avoid warping. Unfortunately, this temperature sometimes caused clogging within the print head, since the temperature in the print head is much higher than the surrounding chamber air - so a PLA material is guaranteed to clog immediately. What alien interface material can I use for PETG (with same or higher softening temperature) , what is your experience?
- same goes for ASA, here I heat my chamber up to 65°C to (successfully!) avoid warping, but what alien interface material could I use?
- I had problems with the large amount of flush material. The PLA kind of sticked to the nozzle or just didn’t slowly move down the waste chute. How can I avoid this?
So far, my experience. Perhaps it is useful for some of you 
Best regards
Alex