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

Choosing the right nozzle size, depends on the bead width, bead height and the material that you are going to print with. In this article, we will give you the tools to make sure you are selecting the right nozzle size!

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 size nozzle for your print, can have quite some different influences. From materials to bead geometry. However, the biggest influence on having the right nozzle, is the bead width and the bead height. 

The reason being, that the bead width and the bead height, will determine the volume that needs to be extruded. You can look at the cross section of the bead and measure the surface area, then multiple this area by the machine feedrate in mm/min. Then you have calculated the cubic millimeter per minute your extruder is pushing through your nozzle. 

What you want, is that the surface area of your nozzle, is not to big but also not to small in relation to the bead surface area. To big, would cause dragging of the material, because the speed at which the material is flowing through the nozzle, is slower then it is with laying down. So then your your are pulling in the material causing drag, causing issues such's as corner pulling and voids in your beads. To small of a nozzle, would cause the print quality issues, the reason is that if the nozzle is to small, the material will travel to fast through the nozzle in relation to how fast it is being placed in your 3D print bead. This causes wrinkling of the bead and leads to ugly surfaces finishes. 

So what size do I need then? The rule of thumb says that you want to have a nozzle surface area that represent between 75% to 80% of your bead area. The formula for the nozzle surface area would be:

Nozzle Area = π x Radius of Nozzle2
Bead Area = Bead Width x Bead Height

Nozzle bead ratio = Nozzle area / bead area x 100%

Aim for a nozzle bead ratio between 75% and 80% for optimal result. 

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 surface area: π x Radius of Nozzle2 = π x 32 = 28.3mm2
2. Ideal surface area is 78% = 0.78*28.3mm2 = 22mm2 
3. Calculate width for ratio 4:1 -> 4 x (22/4)^(1/2) = 9.4mm
4. Calculate height for ratio 4:1 -> 1 x (22/4)^(1/2) = 2.35mm 
5. Verify your calculation: 9.4 x 2.35 = 22.09mm2  (some rounding errors, but good)
6. Determine more practical dimensions if you want: 10 x 2.2 would work perfectly as well. 
  

Nozzle size x bead dimensions cheat sheet

In this table, we mention the standard nozzle sizes. The surface area of the nozzle and the ideal bead dimension. That has a nozzle bead ratio of roughly 78% and a width x height ratio described in the table.

Note: Custom nozzle can be manufactured on request. Please ask you support engineer for a quote on support@ceadgroup.com