Blobs and Oozing
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The failure of this print is caused by excess material oozing from the nozzle when no extrusion should occur.
Retraction is a process where the printer pulls filament back during travel moves to minimize unwanted material discharge. Higher retraction settings are generally required when the distance between the extruder and the hotend increases. For example, Bowden setups demand significantly higher retraction values compared to direct drive extruders. Additionally, even among direct drive systems, a smaller gap between the extruder and hotend—such as in the Hemera system—reduces the need for high retraction and makes it easier to prevent blobs and stringing.
Material selection also plays a critical role. While PLA can often be tuned to print cleanly with minimal blobs or strings, PETG is much more prone to stringing, even with optimized settings. When printing with PETG, some post-processing with a razor blade or heat gun may be necessary to remove residual strings.
Blobs and strings typically form when the hotend is moving without printing. Increasing travel speed reduces the duration of these non-print moves, thereby limiting the opportunity for material to ooze out. Provided the machine’s stepper motors and frame can accommodate higher speeds and accelerations, travel speeds can be significantly increased. For instance, settings of 200mm/s travel speed and 2,500mm/s² acceleration are effective on many CoreXY machines. However, if excessive vibration or skipped steps are observed, speeds should be reduced accordingly. Maximizing travel speed without compromising machine stability can greatly reduce stringing and blobs, and since no printing occurs during travel moves, overall print quality remains unaffected.
That said, for the majority of inexpensive Bowden printers, you will likely see an improvement in print quality when you utilize Coasting. This is particularly true when printing in something like PETG which is more likely to cause stringing issues than PLA.
Coasting is still in the “Experimental” section on Cura, though other slicers have made it a standard setting. I expect it to move out of that “Experimental” section in coming updates to Cura.
Change Z Seam Alignment Changing your Z Seam Alignment will help more so with blobs than stringiness. It will be essentially impossible to avoid a minor seam on your print no matter what you do, but you can mitigate its effects by changing the Z Seam Alignment.
There is a setting for Z Seam Alignment where you can choose “Random”. I do notknow of a time when you would want to use a random Z Seam Alignment, but if you do choose it, you will likely see ugly little blobs all over your print. The best option for Z Seam Alignment seems to be “Sharpest Corner”, which hides these artifacts as best as your slicer can, though it will be near impossible to remove them entirely on a print that has no corners, such as a cylinder.
Many slicers set the "Minimum Layer Time" too high for most materials. This setting causes the hotend to pause after completing a layer faster than the specified minimum time. For example, if the minimum layer time is set to 10 seconds and a layer finishes in 5 seconds, the print head will pause for an additional 5 seconds before starting the next layer. Such pauses often result in oozing, leading to blobs and strings on the print. For most materials, a minimum layer time of 3 seconds is sufficient. As long as there is no curling of layers, maintaining a 3-second minimum is recommended.
Some printers offer power loss recovery by saving progress after each layer, allowing resumption after a power outage. However, this feature can introduce brief pauses as the printer saves progress, which can cause blobs to form throughout the print. Disabling power loss recovery eliminates these pauses and prevents the formation of blobs, though this also removes the ability to resume printing after a power outage. Not all manufacturers have implemented this feature optimally, so its use may negatively impact print quality.
Blobs may also result from over-extrusion. Excess material extruded from the nozzle not only affects the appearance of the print but can also create artifacts where surplus material accumulates. Careful calibration of extrusion settings is essential to prevent this issue.
Poor-quality filament or material that has absorbed moisture can also increase the likelihood of blobs and stringing. Ensuring that filament is dry and of good quality helps minimize these print defects.
Summary of Fixes and Precautions •Increase your retraction settings. • Increase your travel speeds. • Turn on coasting for Bowden printers. • Try “Sharpest Corner” for your Z Seam Alignment to reduce the visibility of the seam. • Decrease your Minimum Layer Time so that your printer isn’t paused in place when it isn’t required. • Check to see if your printer is pausing after each layer due to its power loss recovery function. • Check to see if you are over extruding. • Dry or swap to new filament.
Coasting replaces the last part of an extrusion path with a travel path. Replacing the last section of a print with a travel path will cause any oozed material to be used to print your part to reduce stringing. As you can see in the “” article, if you have coasting on when you do not have issues with stringing, it can result in holes on the side of your print.
