How to manually add a brim in CAD to improve bed adhesion for 3D printing

This is a short how-to guide for creating your own brim on a 3D model in CAD to prevent a model from coming loose from the print bed. This can come in handy in case your slicer software does not include a correct brim suitable for your part.
When 3D printing taller objects which are relatively narrow, the printed object can possibly come loose from the bed, resulting in a failed print. The higher the object becomes, the larger the risk, due to to increasing mechanical advantage for larger height to width ratios.

I lost the nozzle for my watering can, so instead of buying a new one I designed a replacement nozzle and printed it in PLA. Or at least, that was the intention.

I made a couple of iterations on this design and for 2 of them the print came loose from the bed when the nozzle was nearly completed. This nozzle is very difficult to print in the upright position because it has a small diameter base with a very small contact area since it is hollow. Also, the part is very tall compared to the width at the base.

Rather than making a redesign for the entire part to accommodate for a different print orientation, I tried to enlarge the width of the brim, but to no avail. PrusaSlicer, as far as I am aware, does not generate a brim on the inside of hollow part.
As a solution I added a brim manually in CAD. It is basically a flat section which is almost touching the pipe on the inside. It is attached to the part with several small tabs.

The material on the custom brim is 0.5mm tick and the tabs are 1 mm wide. I did not run any tests to determine the optimal tab dimensions, but these seem to work fine for this part.
I made the sketch as simple as possible to save time. The sketch constists of a circle and 2 rectangles, which are included in a circular pattern around the contour with 6 instances.

The same sketch can be used to extrude both the cavities for hiding the connection point of the tabs, as well as the tabs and brim itself. Reusing the same sketch is efficient and also makes it easier to link dimensions of different features to each other.

The resulting surface area is much larger than before and now actually remains stuck to the bed until the end of the print. Note that I used both the custom brim integrated into the CAD model, as well as the brim on the outside of the part, which is generated by PrusaSlicer.

The model can be downloaded from the link in the description below.

If you have any questions or comments let me know in the comments section. Have fun designing!


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