Rules of thumb for overhang angle settings?

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RA1981
Posts: 30
Joined: Fri Feb 03, 2017 7:56 pm

Rules of thumb for overhang angle settings?

Postby RA1981 » Mon Jan 14, 2019 7:16 pm

Hi,

I'm on the way to print my first model which might need support structures. The project can be described as a hollow sphere of 25cm diameter and a wall thickness of 1cm. The model itself is split into five parts, so each part makes about 10-30% of the height.

Even when having the model split into multiple parts, a single print would take a couple of hours. On a first quick slice with default template (0.15mm) there are 32 hours assumed by IdeaMaker for the bottom part (~67mm height) - without support.

Now, my question is about support structures and their angle. Are there any rules of thumb to estimate which angle really needs support? Or is the angle dependant on the layer thickness? When thinking about that, I'd say yes (in case of a sphere): having a smaller layer height means that the outer tracks overlaps more between layers and therefore would need less or no support structures.

I don't want to loose time for support structures which aren't needed. On the other side, I don't want to loose time for having the layer height choosen to small. So, if anyone can share his experience about this, I'd be glad.
To get the print time down, I've to check if I can also increase speed settings, but this would be another task.

Regards

crimsonyoshi
Posts: 80
Joined: Mon Sep 24, 2018 6:17 pm
Location: Manitoba, Canada

Re: Rules of thumb for overhang angle settings?

Postby crimsonyoshi » Mon Jan 14, 2019 9:45 pm

The first question is what plastic are you using? PLA, ABS, PC, (many others)…?

Some are more forgiving than others. For example, PLA is definitely more forgiving than Polycarbonate. Since the plastic is extruded in a spherical shape from the nozzle (or really one super long cylindrical shape), we can look at two stacked layers as a circle on top of another circle. Next, the material is hot, so there's some shape deformity as the recently extruded layer adheres to the layer below it. So you don't get two perfect cylindrical lengths layered on top of each other, the bottom and the top contact points are melted and deformed together with a slight deformity around the near contact points while the material fully hardens (more like a pancake shape, just not as pronounced as a flat pancake).

This is where extruder temperature comes into play. The hotter the nozzle, the more time it has to melt and deform via gravity to the layer below it. Colder nozzle temperatures don't allow for much layer adhesion before solidifying, so with lower angles, you get a bit more precision out of the process.

Just on how gravity works, any angles over 45 degrees from the prior layer means you're playing with fire. From the above circle reference, if you took two circles, offset them by 45 degrees (still touching each other) and added gravity, the top circle would want to roll down the side of the bottom circle and on to the floor. Granted, there's no plastic melting with two basic circles, so it's not a perfect example, but it applies. The centre of gravity of the higher circle is not over any part of the lower circle at 45 degrees or higher. Anything under 45 degrees offset from the prior layer is usually safe.

To counter this, PLA is an interesting plastic. I've been able to achieve short 75 degree overhang angles mostly okay (but they're definitely not what I'd consider to be amazing). Polycarbonate, I like to be safe and stick to 30 degrees or lower.

There's one last thing at play here. Depending on your model and plastic type, support isn't just for holding the next layer from falling down, support also can be used to prevent the model from warping upwards to the nozzles during the print. In the case of long thin flat surfaces of polycarbonate, that's going to warp up hard towards the nozzles at the edges of the polycarbonate pieces. I've had models where I put support down to 5 degrees just to handle some tight angles on a polycarbonate model. Without that, my model would warp upwards.

So in the end here, I don't have a great answer for your question because there's a lot of factors at play depending on what you're doing (including vertical and horizontal offset values). At it's base, 45 degrees or lower is a safe bet for the more common plastics (PLA for example). But you really need to trial and error for each plastic type you use, and complexity of the model.

RA1981
Posts: 30
Joined: Fri Feb 03, 2017 7:56 pm

Re: Rules of thumb for overhang angle settings?

Postby RA1981 » Mon Jan 14, 2019 10:06 pm

Hello crimsonyoshi,

thank you for your detailed explanation. I'm using PLA. So, I think the best way for me would be to prepare some much smaller models and make a few test prints. If I understood correctly, I've to ensure a proper temperature for the task (not too hot/cold). Can you explain the short 75 degree overhangs? Those were made without support? 75 degree is quite a lot of overhang. I'm asking because I wonder if my "theory" of thinner layers would help, meaning that it's ensured that there wouldn't be an overhang bigger than 45 degree.

Regards

crimsonyoshi
Posts: 80
Joined: Mon Sep 24, 2018 6:17 pm
Location: Manitoba, Canada

Re: Rules of thumb for overhang angle settings?

Postby crimsonyoshi » Tue Jan 15, 2019 3:34 am

I can do you one better and provide a picture or three!

I made a ring channel a few weeks ago just to test something similar to this out. Here’s the top side up.

04B666DE-2876-401B-9E4D-F7A790C0841E.jpeg


This is flipped 180 degrees.

1EA91F8C-17B4-42EA-9F75-869060CCFFC2.jpeg


And here’s a closer look from the top looking into the channel.

97D186BC-983F-4D9D-A780-479141E9035D.jpeg


You can see where I put the arrows, there’s something not quite right. The way I initially printed this was open side facing down, with support up to the outside open edge, and no support inside the channel (as it’s impossible to remove). This was a very hard print as there’s 90 degree overhang angles in here with no support. What made this work mostly okay was knowing the first solid layer on the bottom (arrowed picture) solid face was 1cm in width all around, but you can see the pattern the printer chose to print (N-S orientation). So as the layers started building on top of the L shape with a “flat roof” top, the shorter distances adhered to the edges okay and gave a nice layer. The longer distances (marked with arrows) are between 3-8cm long, and you can see they did adhere but not too well. So if you’re not going for precision, this works out okay. If you were trying to make a floating line over what you see here, good luck with that! Same goes for if that line doesn’t have two points it attaches too, one at the start and one at the end of the print line.

The way around this was instead of printing hard 90 degrees with no support, I can still print 90 degrees going bottom up, but the higher layers need to be a lower angle. I can’t show pictures of that part yet (that project is ongoing and I’m unable to post any images of it as of yet, but I do have the pieces made).

I modified the models (three similar prints to the pictures above each with one angle value) to incorporate a 75, 60 and 45 degree natural angle. So instead of (ignore the dots, needed those for proper formatting)
_____
|.........|
|_ ... _|

I did (ignore the dots, I needed to throw those in to format it somewhat properly - also note I flipped the model 180 degrees here):
__ ... _
| /......\ |
|_____|

Now, at 75 and 60, I told the printer to print the outside shell first (since that has vertical adhesion to the previous layer. Then because the next layer print was close enough, it was able to somewhat melt the recently laid shell layer to the outside of the current pass. Outward angles work better vs inward as if you imagine winding a string around a spool, it’s easier to “wind” the next layer of plastic with the print head around the previous layers (so in that case print the inside shell first).

45 degrees didn’t need anything special and it printed perfectly. 60, was maybe 95% there and 75 was closer to maybe 60% okay overall. Because I had a horizontal print line firmly adhered to the previous layer’s plastic, there was some adhesion to it. But again, over 45% you play a bit with fire here!

In regards to thinner layers, a finer print will always produce more resolution, but you still need to have something meaty to adhere too. Thinner layers means less extruded material per pass which means more precision needed.

One of my next prints is going to experiment with variable infill density per layer. We’ll see how that goes!

RA1981
Posts: 30
Joined: Fri Feb 03, 2017 7:56 pm

Re: Rules of thumb for overhang angle settings?

Postby RA1981 » Sat Jan 19, 2019 11:46 am

Hello crimsonyoshi,

sorry for the late reply. Thank you for that detailed description, very informative. I will check my model and modify if needed.

Regards


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