It is always a disappointment when a finished 3D print comes off the build platform with defects and imperfections. Whether it’s a 3D print that has become unstuck from the print bed and suffered from warping, or a print with stringing or other surface irregularities: a ruined print amounts to wasted time and material costs. We’ve already addressed how to fix problems like poor first layer bed adhesion—which can lead to 3D print warping—but in this article we’ll be focusing on another type of defect: 3D print blobs & zits. Keep reading to understand what causes blobs and zits and how to prevent them from happening in the future.
In 3D printing, blobs (also known as zits) are extrusion irregularities that appear on the surface of a 3D printed part. Blobs and zits are distinguished from other surface defects, like Z-banding, by their inconsistency. Whereas Z-banding produces a ribbed pattern on the 3D print, blobs are quite literally bits of oozed filament that stick out from the print’s surface.
3D print blobs & zits are primarily caused by over-extrusion and the motion of the filament extruder. If, for example, you are printing a part with a circular perimeter, blobs can occur when the print head returns to its starting position. The material builds up at this point, which can result in the formation of a filament blob. Similarly, if the 3D printer extruder starts and stops several times over the course of a 3D print job, blobs and zits can also occur.
Blobs and zits can also be caused by 3D printer nozzle retraction. In the FDM 3D printing process, retraction is a technique in which the hot end briefly pulls filament back instead of extruding it out. This method is used to reduce unwanted material extrusion, like stringing, as the print head moves to a new print location. However, if the retraction settings on your 3D printer are not optimized, the process may actually have a negative effect, causing blobs or zits.
Other 3D printer settings, such as nozzle temperature, can also increase the risk of blobs. For example, if the nozzle temperature is too high, filament can come oozing out too quickly and create defects on the print surface. Printing speeds can also be a factor if they are too fast and don’t allow for sufficient cooling time between layers. In other words, if new layers are deposited before the underlying layer of filament has had time to adequately solidify, it can be squished and deformed by the weight of the layers above it.
Recommended reading: What Causes 3D Print Warping and How to Prevent It
Fortunately, there are a number of ways to minimize the risk of blobs and zits in FDM 3D printing. Below are some of the key strategies you can try to prevent the 3D printing defects from occurring.
In slicer software, you can adjust retraction settings to control how much filament is pulled back into the nozzle and at what point this happens. Finding the right retraction settings can minimize material buildup either at the beginning of the print head’s path along the part perimeter or as it completes a layer and rejoins its starting position (for this, see coasting settings below).
In general, it is advisable to use a smaller retraction distance (this is the amount of filament pulled back into the nozzle). This is because if too much filament is retracted, air can also be pulled into the nozzle. When the printhead then goes to extrude filament, the air is heated along with the filament, which can cause extrusion inconsistencies, like blobs and bubbles on the print surface. Too little retraction, however, can lead to over-extrusion and defects like stringing and blobs and zits. It’s all about finding the sweet spot, so you might need to try out a few different retraction distance settings. The amount of retraction will also depend on the type of FDM 3D printer: systems with direct drive extruders only require a retraction distance between 0.5 mm to 2 mm. Bowden extruder systems, on the other hand, can require higher retraction values, as much as 15 mm.
Retraction speed is also worth mentioning. This setting controls how quickly the filament is pulled back into the nozzle. If the retraction speed is too slow, your print may suffer from oozing filament. A standard range for retraction speed is between 20 mm/second and 100 mm/second.
Coasting is a setting built in to most 3D printing slicer programs, including Simplify3D and Cura. Coasting is often found under retraction settings and controls if the print head stops extruding filament at the end of each layer perimeter. Coasting is therefore helpful in reducing blobs or zits that are caused by filament buildup when the extruder comes back to its starting point.
When you activate coasting, the slicing program will essentially tell the 3D printer nozzle when to stop extruding filament—usually this will be triggered a fraction of a millimeter before the printhead completes the part’s perimeter. Coasting therefore anticipates that there will be over-extrusion when the printhead stops extruding and stops the buildup of material at these starting/ending locations. Coasting also helps to relieve pressure inside the nozzle that causes over-extrusion. It is advisable to begin with a smaller coasting distance, such as 0.2 mm, and increase the value incrementally until you no longer see blobs appearing on the part’s surface.
Another group of slicer settings that can help minimize blobs and zits are the wipe parameters. With wiping activated, the 3D printer will drag the nozzle at the end of every printed layer along the outer wall of the printed part to remove any material buildup. With this setting, it is common for retraction to happen simultaneously, which can help prevent blobs and zits. A typical wipe distance—the length that the nozzle will wipe along the outer wall—is 0.4 or 0.5 mm. Wiping can also help to reduce the appearance of the z-seam.
Because 3D print blobs and zits are primarily an aesthetic issue on the outer shell of a part, tuning your 3D printer’s shell parameters can improve the overall print quality by preventing or covering up blobs and zits. For instance, simply activating the “outer before inner wall” setting inf your slicer can improve the quality of the part’s outer shell. This setting tells the 3D printer to extrude the outer layers of a part before printing the inner structure and infill. Not only can this reduce the risk of blobs and blemishes, it can also improve the dimensional accuracy of your print.
As we saw, a too-hot print head can lead to over-extrusion, and rapid printing speeds can lead to defects like blobs and zits due to lack of cooling. Therefore, lowering the print temperature and print speed can help to reduce the risk of blobs and zits. The temperature of your nozzle will depend on the type of filament you are 3D printing. For example, PLA filament typically requires a nozzle temperature of between 200-210 °C. If blobs are still occurring in this range, try lowering the nozzle temperature in 5°C increments.
Reducing the print speed can also reduce the occurrence of blobs and zits. In general, a lower printing speed can improve print quality because the nozzle has more time to deposit the filament and the extruded layer has more time to cool before the next layer is applied. Try reducing your print speed by 5 to 10 mm/s to see if that helps minimize the appearance of blobs.
Recommended reading: ABS print temperature considerations: Nozzle, bed, enclosure
In addition to finding the right print head temperature, you can also reduce the risk of 3D print blobs and zits by increasing your 3D printer’s fan speeds. (Or, if your 3D printer doesn’t have a built-in fan, it can help to integrate one.) A fan helps to regulate the temperature of the 3D printing environment and can help prevent overheating.
This, in turn, ensures that the layers of melted thermoplastic that are deposited onto the print bed are cooled and solidified at a controlled rate. Because over-extrusion and squished print layers are two potential causes of blobs and zits, increased cooling in the print chamber using fans can promote better quality prints. Of course, over-cooling your print environment can also impact layer adhesion—it’s all about finding a balance.
In the end, the first step to preventing 3D printing defects like blobs and zits is to understand where they come from. Factors like temperature and slicer settings can be the cause and solution to blobs and zits. So if you’re experiencing imperfections in your 3D print’s outer shell, be sure to play with your retraction, coasting, and wipe settings, as well as adjust your nozzle print temperature. It may take some time to find the right combination of settings for a given material, so start out with a small test print. The results will be worth it!
 Simplify3D, 2022. “Stringing or Oozing.” [Internet] https://www.simplify3d.com/support/print-quality-troubleshooting/stringing-or-oozing/ [Accessed April 20, 2022].
 Ultimaker, May 28, 2021. “How to Print with Ultimaker PLA.” [Internet] https://support.ultimaker.com/hc/en-us/articles/360011952740-How-to-print-with-Ultimaker-PLA [Accessed April 21, 2022]