SELF-WATERING POT
Tools and Skills: Material Analysis, CAD in Fusion 360, Static Simulation, Finite Element Analysis, Formlabs SLA 3D Printing
Project Brief
In groups (shoutout to Team SLA-y), create an assembly of 3D printed parts that leverages the unique properties of at least one rigid material and at least one softer material from two given lists. Parts can be assembled with hardware, joints, or slip/press fits but no adhesives.
Challenges
My team decided to use the SLA Formlabs resin printers since none of us had used them before. Not only was this new territory for us, but resin is also much more expensive than FFF filament and is much longer to print and finish, which meant that we had less opportunities to prototype and test.
Results
Despite this being our first rodeo with the SLA printers, our project was quite successful! I learned a lot about using the resin printers, and my team worked and communicated very well together. We were able to contribute according to our strengths and pick up slack for each other as needed.
MATERIALS RESEARCH & CONCEPT IDEATION
My group was curious about materials that are flexible and/or watertight, which led us to a self-watering planter whose parts would showcase both of these properties.
Typically, self-watering planters consist of two parts: the “inner pot”, which holds the plant, and the “outer pot” or reservoir, from which the plant uptakes water. We were then able to incorporate our desired material properties into our design choices:
The outer pot must be watertight and rigid so that it doesn’t leak and maintains stability when filled with water.
The inner pot should be watertight so that its layers stay adhered together, but it must also be flexible/squishy so that the user can squeeze the pot to easily remove the plant if desired and to mitigate suction force in case the water creates a vacuum.
We also wanted the inner pot to have tabs so that it could be easily lifted out of the outer pot.
MATERIAL ANALYSIS
In our research, we found that an SLA (Stereolithography) or resin 3D printer would produce a better result for this project than would an FFF (Fused Filament Fabrication) 3D printer. Finer layers and better adhesion between these layers allow SLA prints to have more isotropic behavior and watertightness, which was essential for both the inner and outer pots. We decided on the following materials:
Rigid 10k for the outer pot: this resin is 7x less absorptive of water than other rigid resins, and its extremely high stiffness allows for really thin walls, which meant that we could use less material and reduce costs.
Flexible 80A for the inner pot: this resin is at least 3x less absorptive of water than other flexible resins, and it prints clear, allowing the user to easily see if their plant is root-bound.
CAD FILES & ASSEMBLY
We were able to follow our initial concept sketch pretty closely for our CAD. Two aspects we were unsure about until prototyping and testing were the amount of clearance we had to leave between the inner and outer pot and the wall thicknesses — we wanted to use as little material as possible while ensuring watertightness.
FINITE ELEMENT ANALYSIS (FEA)
While the mechanical properties and robustness of Rigid 10k were straightforward, we were less certain about the durability of Flexible 80A. We used Fusion 360’s Static Simulation feature to model how the inner pot would behave under the physical conditions that would be seen in a proper and improper use case.
PROPER USE CASE: INNER POT SITTING IN OUTER POT
Constraints: fixed bottom faces of all tabs, where the inner pot would be supported/held in place by the outer pot
Loads: 2N load — based on the volume of the pot and estimated weight of the plant and soil — distributed across the inner bottom face
Takeaways: the highest stress concentrations formed a ring where the drainage holes met in near the center of the bottom face. While the estimated safety factor was already very high (19), we increased the thickness of the bottom face and moved the drainage holes out toward the edges of the pot.
IMPROPER USE CASE: LIFTING THE POT BY ONE TAB
Constraints: fixed the bottom outside face since the pot is at rest
Loads: 2N applied to the bottom face of a singular tab to simulate lifting force of a user attempting to remove the inner pot from the outer pot by only one tab
Takeaways: cantilever bending stress was highest at the attachment point between the tab and pot body. We increased the thickness of the tabs to ensure a high safety factor of 6.
Whether these FEA results would directly translate to reality was uncertain, so we aimed for high safety factors just in case.
SLICED FILES
PreForm is a slicing software used for SLA printers. Rather than extruding material like FFF printers, these printers cure layers of resin with a UV light, which means that the print bed is above the resin and the print essentially prints upside down. As a result, we had to take special considerations about orientation in order to prevent printing errors.
Thanks to the drainage holes at the bottom of the inner pot, we were able to orient this print right-side up. This would reduce the amount of supports necessary for this print and therefore the amount of material used and the cost.
Printing the outer pot right-side up would have caused a cupping error. This occurs when the formation of a cup shape causes resin to be suctioned up from the material vat, which ruins print quality due to uneven layers of resin. Unfortunately, this meant that this print would require a lot of supports.
MINI TEST PRINT
This was a prototype that was scaled down to be 25% the size of our actual design. We were testing for watertightness, functionality, and print quality.
Water flowed up through the inner pot drainage holes as intended. Also, after leaving the pot filled with water for three days, there were no leaks, and the print quality didn’t deteriorate!
It’s a little difficult to see, but there’s a small rip near the center of the bottom face. This is due to the drainage holes being too close to each other, which was an issue that we predicted in our FEA.
While the inner pot could be easily removed from the outer pot, we thought the fit could be a little tighter. We also found that the wall thicknesses of the pots were ideal and updated our full-sized design accordingly.
HURDLES & PRINT FAILURES
While we caught onto the SLA printing process quite quickly, we still ran into some obstacles. Luckily, these weren’t major set backs, and they were valuable learning experiences.
Supports that were printed inside the outer pot prevented excess resin from being washed out.
Walls detached from the bottom due to either a modeling error or the transition fillet being too thin.
This was an outer pot print failure. We’re not sure why it happened, but it could have been an issue with the resin.
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POST-PROCESSING PHOTOS
Unlike FFF prints, SLA prints must be post-processed before they are used. Uncured resin is toxic, and without additional curing, the print can warp since its structure isn’t fully solidified yet.
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The print immediately gets washed in isopropanol to remove excess resin.
Repeat with the outer pot. This took much longer due to excess resin and longer cure time.
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Post-wash! Any remaining resin is removed with additional isopropanol.
Remove the supports and assemble! We were very excited to see our complete assembly!
Print is complete! Notice upside-down orientation above the vat of resin.
The print is cured in a UV light box. The time and temperature varies depending on material.
FINAL PRODUCT
Overall, our design was successful and works as intended. If given more time, we would have liked to design the water spout differently so that it doesn’t jut into the overall pot capacity. Also, we would have liked to see if the petal tabs experience any creep deformation over time and if results of the same quality could have been produced with FFF printing, which would have been much more efficient cost- and time-wise. That being said, our planter is now home to a small succulent and lives happily with my other plants.
