support structures?

[dgsharp 0]

I have some parts that require supports, and I'm trying to find a reasonable solution. Normally it's fine to try and design such that you require no supports, use Cura's built-in supports, put in custom specific supports where necessary, or even chop the model up into pieces to be assembled later, etc. MeshMixer's supports can be extremely helpful, but don't do much on a fully overhanging surface. Ideally I would use a second extruder with a soluble support material (and hopefully we won't have to wait too much longer before this is an option! <nudge nudge>). But there's got to be something else to try. In particular I am making some small experimental propellers. They need to be as strong as possible, so gluing parts together isn't ideal. Some of them have a flat, pure-overhang face on the bottom of the hub that absolutely needs some kind of support, and the default supports leave a really nasty surface finish. Most of the blades can print reasonably well without support but a couple of areas inboard require support and come out with a really nasty surface finish when you use it.

I was getting ready to start some experiments to try and find a support method I like, and thought I would ask here to see if anyone has any good ideas. Let's say the design criteria is that it has to be able to support a full overhang (a horizontal surface with nothing beneath it), and the surface finish needs to be as good as possible after removing it.

Here are some ideas:

- A series of closely spaced pegs. In a simple case maybe just a single extruder width wide if possible, or to make it easier to both fabricate and cleanly tear off, they could be wider (like 0.8mm or 1.2mm diameter) and then taper down to a single extruder width point of contact or close. By varying the spacing I could try and find a good balance between nice flat overhanging faces and minimal pock marks where the supports were torn off.

- Very narrow (ideally single-extruder-width) zig-zagging vertical supports.

- Wonky idea inspired by some comically under-extruded parts I made when my UM2 was having trouble with very stiffly coiled PLA: print a relatively "solid" section but force the g-code to produce them severely under-extruded (presumably by writing some kind of plugin). This produces a sort of wispy lattice of diagonal blobs that you can see right through. Not sure how well this would work when mixed in with areas of proper extrusion (might just clump up in areas or something).

How can I guarantee that a small feature I create geometry for comes out in the g-code? I tried making features that were 0.4mm wide and they get ignored most of the time. I found that even 0.6mm didn't always make it through to the toolpath. Should they? How can I design support structures that are minimally small/thin, but which come through to the printed model? Is there a straightforward way?

Thanks for any thoughts you may have! Hopefully that second extruder will be an option in not too long.

Edit: Not mentioned was the option of just tweaking Cura's native support settings. Know of any particular settings to try?

 

[burki 1]

So far i have very good results using a second extruder and printing support with PVA.

Using the Cura support features is not your friend there, though: If you need surfaces that require as little finishing as possible i suggest you create a second model for the support structure and print that using the "merge objects" feature.

If you use cura's support feature, you may need a high value for the support infill. like 50% or so, since otherwise you get a rather wide grid, and depending on temperatures that will leave you with very uneven structures,

When you have two models, you get closed top surfaces of the support model.

Be advised to leave some space between the objects: if you desing with zero space you will find that the strings of plastic are fused together leaving ugly surface structures after removing the PLA.

For a lateral spacing you can easily have several mm spacing, although 1 mm is more than enough.

for the vertical spacing i don' t have that much experience yet. you might try 0 mm, or one layer height.

I'd be interested to hear how you fare!

 

[dgsharp 0]

You're making me jealous Burki! I am dying to do dual extrusion. It's ultimately the only way to really do it right I think.

I did a test today using TweakAtZ to print a solid tower, alternating the material flow percentage between 100% for 1mm, to a lower number (starting at 50% near the bottom and ending at 10% near the top), resulting in a number of 1mm-thick plates connected by fluffy, severely under-extruded support (I've been calling it "CUE support", for "Comically Under-Extruded" support).

I found that after tearing away the clumps with a pair of pliers, the underside of the 30% section (the side that was built on top of 30% fluff) looked the best of all of them. It's not smooth, but it is relatively uniform (comparable to Cura's built-in support at 25%). The top side (the side with 30% fluff sitting on top of it) cleans up quite nice, better than the corresponding side of the test print I did using Cura's built-in support set at 25% (though maybe tweaking the Z offset could help here) and not that much worse than the top side of a solid print.

I don't know that this is a viable approach on its own (is this how Netfabb does its "fluff"? I've never seen it), but it could come in handy for some specific situations where the material is easily accessible for removal with tools. It would probably work better with a direct-drive extruder as the pressure in the Bowden takes a certain amount of time to dissipate, resulting in a slow ramping up or down of material flow following a setting change.

I tried the new/old "lines" support in the Cura 14.02 RC5 today and wonder why the lines aren't connected into zigzags at the ends. That's more like what the Dimension machines did IIRC and it worked pretty well, probably easier to remove all at once. I like how easily it removes but the bottom (supported) surface left something to be desired for my purposes. I will see if I can do some more experiments, maybe playing with Z offset can help as you suggest.