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Determining heat dissipation of 3D printed box - final results!

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tggzzz:

--- Quote from: HendriXML on September 21, 2019, 08:13:20 pm ---If I look at the specifics of PLA
https://www.makeitfrom.com/material-properties/Polylactic-Acid-PLA-Polylactide
It show that it has a high Specific Heat Capacity (1800 J/kg-K) and a low Thermal Conductivity (0.13 W/m-K). So heating a 100 gr of PLA 62.2K would take 11 kJ.
That is 37 minutes of 5 watt all of the power was only used for heating up, no losses.

What if the most prominent cooling effect of the box came from heat capacity. That would explain the small difference between uncovered vs covered.
So to do this experiment well, I should leave it warming up for at least an hour. Maybe more.

My experiments had some duration, but not in that magnitude.

So I will redo the experiment of the uncovered/uncovered test cases at 80 deg. 90 deg is a bit too hot.

Also covered air-slots might be better than no air-slots, so there usefulness can only be proven not disproven.

--- End quote ---

The thermal capacity is only of interest when considering thermal transients. Naturally you should measure temperature-vs-time to ensure that the transients have "finished" and you are looking at the steady-state power and temperature.

Have you done the basic calculation to estimate the internal temperature rise of a box without vents? All the information you need is the power, the surface area, the thickness, and the thermal conductivity.

Have you compared that theoretical value with the measured value?

Then add in the effects of the vents. (Hint: forced air cooling is used for a reason, not for fun!)

DaJMasta:
Also remember the way your slicer actually realizes the model.  If the boxes walls are thin enough, there should be just solid perimeters for walls and the thermal conductivity will be about what it specifies, but if they're thick enough for infill, they will insulate much better because of the built in air pockets.

If you take the same box and slice it for a solid wall and a second box with walls with a little infill (like 1mm of thickness), especially if your infill percentage is low, the box with the hollow wall will hold its heat better (what we don't want, in this case).

HendriXML:
My thoughts when "designing" the box where that the chimney effect would do more than 80% of the cooling. So that a closed box would perform very badly.

The experiments until now seem to show something different.

They show a reduced cooling capacity of only +/- 50% without airflow at all, that's better than I would expect by intuition. Calculating the expected performance would be tricky. The box is partly solid, partly with low infill. The bottom is obstructed with a PCB. Calculating this kind of stuff needs a very good model.

Maybe "they" also calculated the thermal resistance of the heatsink. But in my instance it doesn't hold up. (Why?). That alone would mess up calculations.

With this experiment real values can be determined.

Looking at the thermal capacity however it seems to me it was still in thermal transient, that can take a long time at those low powers. (Something to remember!)
When things heat up from inwards to outwards more and more the effects of isolation will show.

During the first experiments I think the plastic that later become isolating where still heating up.

I'm thinking about monitoring the power with a DSO, not the best way, but the only one at hand. That way a graph can be plotted which shows the cooling capability over time. And will show stability when things are heated up to their end values.

I guess the cooling capability will drop for both uncovered and covered, but that the values will be further apart. I hope the uncovered will not be much worse. I think without active cooling the results then might be considered ok, if you take into account that just having side walls has already a dramatic effect.

HendriXML:
For further testing I created a new model, with specific slicing properties.

The ventilation part doesn't have a bottom or top layer and has 1.2 mm infill line distance, thus creating a mesh.

Using the slicer, and not the 3D modelling tool for this gives better control over those detailed structures. I hope it prints fine.

I wouldn't like a more coarse mesh, so this is in a way an optimized (top) airflow.

At layer height 13, the solids are shown where a threaded insert can be placed. I've done that in the box that I actually use.

HendriXML:
The mesh had some inner wall lines that could be eliminated, resulting in a bit more mesh...

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