What Can Cause Your 3D Prints to Have Rounded (Bulging) Corners (Blobs on Corners)?

Problems during the 3D printing process can appear in ways that you least expect, whether they cause the printing process to fail, prevent it from starting at all, or generate less noticeable but still considerable signs such as dimensional inaccuracies on the 3D printed model.

As a result, it’s a good idea to thoroughly observe both the 3D printing process and the models you have printed now and then for any minor flaws that may not be apparent at first look, as they can be the signs of something being wrong with the printing process in some way. 

Today, the topic of our article will be a widespread 3D printing issue known as rounded corners, bulging corners, or blobs on corners, in particular, where the 3D printer fails to print the sharp corners of a model and ends up printing corners that look deformed instead, leading to poor aesthetics and dimensional accuracy.

So, what can cause your 3D prints to have the issue of rounded corners, which is also known as bulging corners and blobs on corners?

The rounded corners problem (also known as bulging corners and blobs on corners) occurs due to the release of the built-up nozzle pressure (which naturally happens during the print) as the printhead slows down to turn the corner, which triggers filament to come out of the nozzle unintentionally.

In the upcoming parts of the article, we will be taking a deeper look into the potential causes behind your 3D prints having rounded and bulging corners, discuss how to fix the problem as rapidly as possible, and finally, take a look at signs that may help you identify that your 3D prints are also suffering from rounded corners.

What Can Cause Your 3D Prints to Have Rounded (Bulging) Corners (Blobs on Corners)?

The corners of your 3D prints ending up rounded and bulging instead of sharp and defined is one of the most apparent warning signs that there is something wrong going on in the 3D printing process, as a 3D printer should be able to print sharp corners when necessary.

During a standard printing process, the nozzle gradually builds up pressure due to the filament being pushed into it by the extruder gear, as the extrusion process causes the filament to become compressed more and more against the nozzle.

When the printhead eventually slows down to turn a corner, the extrusion also naturally slows down, releasing the pressure in the nozzle, which results in excess plastic coming out of the nozzle involuntarily at that point.

The excess plastic that comes out is what creates the issue of rounded, bulging, or blobbing corners, which essentially is a form, albeit very specific, of over-extrusion, with the over-extrusion only taking place on the corners due to the phenomenon we have explained above.

Since the pressure in the nozzle is directly responsible for the amount of excess plastic released, cases where the nozzle pressure builds up to levels that are way too high will create more pronounced rounded corners.

How to Fix the Issue of 3D Prints Having Rounded (Bulging) Corners (Blobs on Corners)?

Fixing the problem of your 3D prints having rounded or bulging corners will require you to go through a few distinct solution steps due to the existence of more than one culprit that can cause the issue unless you have already identified the cause in your case.

  • Activate the Linear Advance feature. Linear advance is a feature that aims to keep the nozzle pressure constant at all times and prevent the shifts in extrusion that take place due to pressurization and depressurization cycles through compensation, which will eliminate the problem of the rounded corners and many other under-extrusion or over-extrusion problems related to nozzle pressure. Please note that not all stepper drivers support the linear advance feature, and in such a case, even if you enable it from the software side, you won’t see any effects.
  • Reduce the print speed. Reducing the print speed will reduce the amount of pressure that builds in the nozzle, decreasing the possibility of the nozzle depressurizing severely and releasing a lot of excess plastic at the corners.
  • Increase the jerk value. Increasing the jerk value will prevent the printhead from decelerating way too much to turn the corners, allowing it to maintain a decent amount of speed and, as a result, cause the nozzle to release less pressure in the process. Please note that increasing this value way too much will create the problem that resides on the opposite end, known as ringing.

Between the three solutions, activating and correctly configuring Linear Advance will be the biggest difference-maker, as the feature is essentially designed to regulate the nozzle temperature to avoid cases of blobbing due to nozzle depressurization, with the issue of rounded corners being a good example.

While reducing the print temperature and extrusion multiplier can also technically help, there is no point in lowering these values if the only issue you’re facing is rounded corners with no other signs of trouble on your print, and in fact, doing so can create more problems.

How to Identify The Issue of 3D Prints Having Rounded (Bulging) Corners (Blobs on Corners)?

As with any other issue, resolving the problem of your 3D prints having rounded or bulging corners also starts with identification and ensuring that the problem you’re facing matches the corresponding solution steps you will be applying to avoid further misconfiguring your print settings.

While the issue of rounded (bulging) corners sounds very straightforward to identify, as the issue itself is described in the name, many other problems, such as an overall case of over-extrusion, can create the same signs (and some others), but require a whole different solution.

In a nutshell, if every part except the corners of your 3D printed model is perfect, with no signs of over-extrusion or any other problem that you can notice, and the only observable issue is the rounded and over-extruded corners, it is indeed the bulging corners issue we have at hand.

As this particular problem is one that you can easily confuse with others due to the overlap in symptoms, we highly recommend taking a thorough and close look at your print for all the signs that seem unusual compared to a correctly printed model before moving on to solutions.

Wrapping Up

The appearance of rounded corners on your 3D printed models where the corners should be sharp instead definitely is a sign that something has gone wrong during the printing process, as a correctly configured 3D printer should be able to print sharp corners without any problems in a standard scenario.

To quickly recap, the main reason behind the problem of rounded corners on your 3D printed model is the nozzle pressure that gradually builds up, eventually getting released as the printhead slows down to turn the corner, which results in excess filament coming out of the nozzle in the process.

As a result, solutions to the problem often involve reducing or regulating the amount of pressure that the nozzle builds during the printing process, such as lowering the print speed or activating linear advance and making the nozzle move faster after turning the corner by increasing the maximum jerk.

Happy printing!