When designing containers or hollow structures for 3D printing, it’s tempting to use the shell command in CAD software to quickly hollow out a solid body. At first glance, this seems like an efficient way to create lightweight or material-saving prints. However, this approach often leads to unexpected issues that can compromise print quality or structural integrity. In this post, I’ll explain why relying on the shell command isn’t in most cases the best choice and what alternative methods can produce better results.
A Straightforward Example
Let’s start with a simple example: a vase with a beautifully curved wall. The profile for this vase looks like this:
When extruded into a solid body, the vase appears as follows:
This solid model could be printed efficiently using the “vase mode” (also known as spiralized contour printing) in slicing software. This method produces a seamless, continuous extrusion, making it ideal for decorative vases. However, if you need a sturdier object—perhaps a functional container or a more robust vase—the walls must be thickened. Here’s where the shell command might seem useful, but as we’ll see, it comes with some hidden pitfalls.
Using the Shell Command to Hollow Out the Vase
In PrusaSlicer, achieving the best horizontal wall thickness for thin structures is determined in the “Vertical Shells” section. For example, when printing this vase with a 0.4 mm nozzle and a 0.2 mm layer height, an optimal wall thickness would be 1.68 mm—allowing for clean, consistent extrusion with four perimeter lines.
At first glance, applying the shell command in CAD software to create a 1.68 mm thick wall seems like a logical approach. Here’s how the vase looks with the shell command applied, including a section cut for better visualization:
However, once you export this model and inspect it in the slicer, an issue quickly becomes apparent. The wall thickness is inconsistent throughout the height of the vase. While the top section allows for an even four-line extrusion, the middle section suddenly requires five lines.
This inconsistency leads to two major problems. First, the wall thickness no longer matches the recommended 1.68 mm, meaning the print won’t be optimized for extrusion settings. Second, the abrupt transition between four and five perimeter lines results in visible artifacts on the vase’s surface, reducing its aesthetic and structural quality. These artifacts can manifest as uneven extrusion, small gaps, or ridges—issues that are difficult to smooth out post-printing.
My Recommended Approach
The best way to achieve a consistent wall thickness requires a more hands-on approach in CAD. Instead of relying on the shell command, I recommend manually designing the correct extrusion to maintain an even wall thickness in the X/Y plane. This involves creating an exact copy of the vase’s profile and shifting it by 1.68 mm along the X-axis.
One important detail to note is that the actual wall thickness will vary depending on the angle of the curve. The steeper the angle, the thinner the wall may appear in a vertical cross-section. However, in the X/Y plane—the plane that matters most for slicing—the wall maintains a consistent, precise thickness.
Once extruded, the modified vase may look identical to the original at first glance:
However, when you examine the sliced result, the difference becomes clear. The vase is now printed with exactly four uniform perimeter lines across its entire height. This ensures an even extrusion, eliminates sudden thickness variations, and minimizes visible printing artifacts.
By following this approach, you achieve a cleaner, more consistent print with improved structural integrity and aesthetics. While it takes a bit more effort in the design phase, the results speak for themselves.
Conclusion
While the shell command in CAD software may seem like a convenient way to hollow out a model, it is not well-suited for 3D printing. The issue lies in its inability to maintain a consistent wall thickness in the X/Y plane, which is crucial for achieving clean, artifact-free prints. Ideally, a shell tool would allow for fixed-width extrusions along the X/Y plane, but as far as I know, no CAD software currently implements this feature in a way that works seamlessly for 3D printing.
For the best results, a manual design approach is necessary. By carefully controlling wall thickness through precise modeling, you ensure a structurally sound and visually appealing print. While this requires extra effort, the improvements in print quality make it well worth the time.