Fixing Irregular Top & Bottom Surface Patterns in Extrusion 3D Printing


Irregular Surface Finish in 3D Printing

Have you ever seen a random line crossing the beautiful pattern on the top and bottom surfaces of your 3D print? It is ugly! Sometimes this can be caused by the nozzle making contact with printed surfaces, but there is another common cause that can be overlooked. It is related to your combing retraction settings!


In this article we will discuss one Cura setting that can cause your top and bottom surface patterns to be irregular.


Combing Mode


When you enable Combing Mode in Cura, it attempts to keep the nozzle within already printed areas and disables retraction during those travel movements. This can be a helpful setting. It speeds up your print by eliminating the time required to retract the filament and perform z-hop movements (if you have them enabled). However, the drawback is that oozing can occur in unwanted areas when traveling across the print.


Max Comb Distance With No Retract


There is a travel setting that can fix this oozing problem. It is called "Max Comb Distance With No Retract." You may need to unhide this setting in order to see it in your menu. The default value is 0mm, which means that no retractions will occur during all combing movements. If you set this to a bigger number, let's say 5mm, Cura will perform a retraction when combing movements are longer than 5mm.


Ultimaker Cura Combing Settings for 3D Printing

The Cura screenshot above shows the 9th layer of a part with a total of 10 layers. It is the layer just before the last layer printed. The Max Comb Distance With No Retract setting is set at 0mm. Dark blue lines represent travel movements with no retraction. You can see that there is a travel movement with no retraction directly across your nice surface pattern! If oozing occurs during this movement, it will be seen on you top surface! The picture below shows the final printed part using these settings. The defect runs across the part exactly where the combing travel movement occurred!


Irregular Surface Finish in 3D Printing


The Solution


To fix this problem, adjust the Max Comb Distance With No Retract to the largest value possible that turns this combing movement into a travel with retraction. You want the dark blue travel line to turn into a light blue travel line. It may take a few slicing iterations to figure out what the max value is, however, be cautious. Don't assign a random small number like 0.1mm. If you do that, it may cause excessive retractions that slow down your print or grind up filament.


Ultimaker Cura Combing Settings for 3D Printing

After a few iterations in Cura, we found that the setting for this specific print is 7mm. The travel line changes to a light blue color and eliminates the surface defect. It also eliminates the potential oozing on the second travel line shown on layer 9 (the second arrow in the photo). The final results are shown below.


Fixed Surface Finish Pattern in 3D Printing

As a final note, you can use this strategy for any other surfaces on your print! Just do a quick preview each layer that you are concerned with. Adjust the Max Comb Distance With No Retract until the dark blue travel lines turn to light blue!


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