These models are 3D printed capacitors! What are they good for? Good question. It was just an experiment but might be an interesting STEM or introductory physics class project.
Let’s take a look the design. They are printed using two filaments and in 3 layers. The two outside layers are Protopasta Electrically Conductive Composite PLA – these are the capacitor plates. The interior layer, is basic PLA to act as the insulator between the plates.
The original design was a 30x30 mm plate area and printed with a 0.08 layer height with 100% fill. The design is first changed by selecting the Protopasta filament in the AMS. To create the insulator, I used the paint-by-height feature in Bambu studio. I first tried to paint one layer, the center layer, but this did not provide good insulation as an ohmmeter showed a few thousand ohms from plate to plate. So, I then painted two layers of 0.08mm - the middle two layers.
Using online calculators, or your physics book, you’ll see that 30x30 mm plates separated by an insulator of 0.16 mm and having a relative permittivity of 2.72 (PLA’s relative permittivity) should be around 135 pF. When measured by a LCR meter, I got 151 pF. The difference may be due to the insulating layer not being consistently 0.16 mm but having some air spacing also.
The LCR meter gives some other interesting data: The capacitor has a series resistance is 2.56 Kohms. This is high for a capacitor but is due to the conductivity of the conductive filament. It is not conductive like a wire it has significant resistance.
I then scaled up the X and Y by 300% to get a 90x90 mm capacitor (the number of layers and layer height remained the same). This resulted in a 1120 pF (1.12 nF) capacitor. This works out as it has an area 9 times the 30x30 mm cap – so it should be 151 pF times = 1215 pF).
I then built a 74HC14 (Schmitt trigger inverter) based oscillator ( https://resources.pcb.cadence.com/blog/schmitt-trigger-oscillators-a-simple-square-wave-generator ) using a 100k resistor for R and the 3D printed capacitor for C. The 30x30 capacitor gave a frequency of 35.6 kHz and the 90x90 capacitor gave a frequency of 12.4 kHz. Note that this is quite different than an online calculator would show. The main difference is the particular 74HC14 I used had a high level hysteresis threshold far from the 2/3 Vcc that is typical – it was closer to 0.5 Vcc. The other unaccounted piece is that there is a series resistance in the capacitor which is unaccounted for in the equations used (using a 100k resistor for R should be large enough to minimize the effect though).
A tip: When creating this circuit make sure to ground the input of the 5 unused inverters or you will get odd effects in the created square wave.
Note that the 90x90 had a slight issue spanning the gap between the tab and main capacitor area. It works ok and adding supports could result in conductive filament being used and shorting the capacitor.