How do I choose the right nozzle size for large format 3D printing?

Why is choosing the right nozzle important?

Nozzle size and bead geometry have a close relation. If you have the wrong nozzle selected for your current print process, a lot of problems and defects may occur. Problems we see happening when the wring size nozzle was selected are. 

1. Corner dragging: When the corners of your print have a waving pattern. Divided over multiple layers. But this pattern is repeating consistently. Then there is a good chance that you have selected a to big of a nozzle for your current print settings. It could also be that you have chosen the wrong bead geometry, from which you can read more in this article: Bead geometry guidelines
2. Voids in your print: If you have chosen the wrong nozzle size. It could happen that this causes voids (air pockets) right in your print. This is unwanted behaviour since it will impact the structural integrity of your final part or tool. Nozzle size in comparison to your bead geometry, has an influence on the voids. There could be other reasons as well, however make sure that you have the correct nozzle selected to eliminate at least 1 part of the potential causes. 
3. Ugly side wall: When chosen the wrong size nozzle, it can cause very ugly prints with a lot of ripples in the walls. If you are looking for high quality looking large prints with smooths surfaces, then it is really important that you have chosen the right size nozzle. 

How to select the right size nozzle?

Selecting the right nozzle size depends on the bead width, bead height, and material. In practice, the most important relationship is between the cross-sectional area of the nozzle and the cross-sectional area of the printed bead.

The bead width and bead height determine the amount of material that must be deposited. By calculating the bead cross-sectional area and multiplying it by the machine feedrate in mm/min, you can estimate the required volumetric output in mm³/min.

As a rule of thumb, the ideal bead area should be around 75% to 80% of the nozzle area. This helps balance material flow and bead quality.

• If the nozzle is too large relative to the bead, the material flow speed through the nozzle can become too low compared to the laydown speed. This can lead to dragging, corner defects, and voids.
• If the nozzle is too small relative to the bead, the material may move too quickly through the nozzle, which can lead to surface defects and poor bead quality.

 Use the following formulas: 

Nozzle area = π × (nozzle radius)²
Bead area = bead width × bead height
Nozzle bead ratio = bead area / nozzle area × 100%
Aim for a nozzle bead ratio between 75% and 80%.

Calculate bead size, based on the used nozzle. 

1. Calculate the nozzle surface area
2. Take 78% of that area
4. Determine the bead width x height ratio
5. Calculate ideal bead dimensions

[Example]
Nozzle size 6mm, we want a bead ratio of 4:1 (width x height)
1. Nozzle area = π × 3² = 28.3 mm²
2. Target bead area = 0.78 × 28.3 = 22.1 mm²
3. For a 4:1 width-to-height ratio:
   - Width = 4 × √(22.1 / 4) = 9.4 mm
   - Height = √(22.1 / 4) = 2.35 mm
4. Check:
   9.4 × 2.35 = 22.1 mm²
5. A practical rounded bead size would be 10 × 2.2 mm

Nozzle size x bead dimensions cheat sheet

 The table below shows standard nozzle sizes, their nozzle cross-sectional area, and example bead dimensions based on a bead area of roughly 78% of the nozzle area.