How to "fight the sag": Mastering overhangs in FDM printing

Has this ever happened to you?

To understand what this is and why this happened, let's first understand overhang in FDM printing.

What is overhang?

Overhang is any area of a 3D model that extends outward horizontally, and does not have solid material (or with only limited amount) directly beneath it to serve as a foundation.

Because FDM printers build objects layer by layer from the bottom up, each new layer should be deposited on top of an existing one. Overhangs violate this principle, as the nozzle is essentially trying to print in mid-air. Therefore, overhang areas might sag before they cool and solidify due to gravity, leaving an uneven surface and a poor appearance.

How can I successfully print overhang?

Usually, support is required for the overhang area for optimized printing result. But not all overhangs require support. You'll notice that in ElegooSlicer, the threshold angle for support is 30° by default, which means support will not be automatically added to overhang areas under 30° angle. This is because the layer above has enough contact area with the layer beneath, and would not lead to printing failure.

So I'll just add support to all overhang areas and that'll fix it all?

This would be an overkill because of several disadvantages:

1. Using much more filament than actually needed

2. Increasing total printing time and after-processing (support removal) time

3. On the final print, the contact area with the support might look terrible (Optimizing support will be our next tutorial!)

In the next part, we'll show you how you can successfully print overhangs without support and with great results!

How to fine-tune overhangs

At its heart, printing an overhang is a race, a race for the melted filament to cool down and solidify faster than gravity can pull it down. We'll recommend 4 key settings for overhang optimization, and every setting is either helping with the cooling or reducing the effect of gravity.

1. Overhang speed

Printing fast over an overhang is like taking a corner in a car at full speed—you're likely to lose control. Slowing down gives the filament time to cool in place.

• Slow down for overhangs

This setting is usually enabled by default. But if you are slicing a 3mf file with a preset, it may be disabled. So don't forget to check it!

1. Line width

When printing overhanging outer walls, a portion of the wall line is printed on the previous layer. The more it is printed on the previous layer, the tighter it adheres, reducing the likelihood of sagging.

• Increase outer wall thickness

You can set the outer and inner wall line width to 0.7 mm. This will make more of the wall lie on the layer below, helping it counteract sagging caused by gravity.

1. Wall and surfaces

There are 3 orders for outer/inner wall printing order. Let's see the differences they have for printing overhangs.

a. When using the "Outer/Inner" order: The printer prints the outer wall first, then the inner wall.

In this case, the overhanging outer wall is "floating" in mid-air. The only thing it can adhere to is the line of filament directly below it. This small contact area offers limited adhesion, making it very prone to sagging.

b. When using the "Inner/Outer" order: The printer prints the inner wall first, followed by the outer wall.

Here, the freshly extruded outer wall can press tightly against the solid inner wall that's printed just before it. The inner wall acts as a kind of "built-in support," providing a sturdy anchor for the vulnerable outer wall.

c. "Inner/Outer/Inner" order: It requires a minimum of 3 walls to be effective as it prints the internal walls from the 3rd perimeter onwards first, then the external perimeter and, finally, the first internal perimeter.

In this case, the outer wall cannot press against the inner wall when it is being printed. If the wall loops are 2, it will print like the "Outer/Inner" order.

• Set wall printing order to "Inner/Outer"

You can find the walls printing order in "Quality" settings and set it to "Inner/Outer" when there are overhangs.

1. Layer height

A thinner layer height provides a smaller overhang span, thereby reducing the risk of sagging.

• Reduce layer height

You can reduce layer height in "Quality" settings to ensure better overhangs.

When you change this setting, the overall layer height will change, and the print time will increase accordingly. If you're concerned about the long print time, you can try the variable layer height feature to modify only the layer height of the overhang areas. (See the tutorial on variable layer height in our blog or YouTube video)

Action Plan for Successful Overhangs

1. Print a Test Model: Use a test model with overhangs to do a calibration test. It’s the fastest way to dial in your settings for your specific printer.

2. Try Different Combinations: Adjust one or two settings, or even all four of them if necessary. Print the test, and observe the print for the best result.

3. Material Matters: Always re-tune when switching materials. What works for PLA won't work for PETG or ABS.

Mastering overhangs is a process of understanding and tuning. Follow the guide and try adjusting the settings, and soon overhangs will be just another feature your printer handles with ease!