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

[HendriXML]

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).

I haven't compared the PLA spec's but it may be a bad heat conductor by itself (with air pockets making it worse off cource). Because of pretty high thermal capacity it doesn't feel very warm at first like for instance PTE foam (just an example, that is  a much worse conductor). What I mean by that is that vent holes are necessary even at low powers. Until now the experiments don't show this, whether that's time related I'll try to figure out.

[HendriXML]

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

The printing went very well, this is an excellent method of creating a mesh. However the infill lines should have a bit more distance between them Now the maze is very fine, which can be obstructive. No way stuff thicker than 0.8 mm can go through. (I think even 0.5 mm)
The infill line distance (1.2 mm) used is probably from center to center.

When I look at the previous print, its clear that that solution is a bit less obstructive and almost optimal within the constraints.

[HendriXML]

Made some photo's of the air holes. The 1.2mm maze looks better in real life, how much light that shines through is very angle dependent. But the photo might show also the issues that are less obvious when a (bright) light is shining trough. (That seems to make the holes look bigger)

Because the top has some 4 mm thickness, the holes can only be penetrated by pointy objects via a certain angle. This changed my mind a bit on how small they really need to be.

A newly printed 2 mm maze gives a lot more air throughput and still delivers a lot of protection against stuff you don't want to fall inside. Maybe this top might actually perform.

All 4 will be tested and monitored using a scope. Each tests will be stopped if the monitoring shows long term stability (15 min).

[HendriXML]

First about the setup:
I made a SCPI script that controlled the
PSU: setting the voltage and on/off,
AWG: ref voltage (Temp) and trigger signal (0.2 Hz)
Scope: measuring amplified current, acting on trigger.

The amplified shunt voltage was measured using the average of 14.0kpts every 5 seconds. After each measurement the VOffset of the channel was "centred", so the readings could be done using 50 mV/div.

The current regulation had a bit of an oscillation. Also the temperature regulation was not oscillation free. This is not really an issue because both are averaged. One using the 14.0 kpts. The other using the last 100 seconds of those averaged values.

I made a modification of R11 (100K -> 33K) that made the oscillating worse at higher powers.

The measurement where done in the shade/dark, that actually makes a difference compared to normal daylight. Also I left the room, so no incidental air movements.

The best top was as expected the large mesh. The solid top and covered side holes only performs 8.7% less.

At 80 deg celcius the top became a bit softer. I wouldn't design for that temperature, 60 deg would be ok though. For my own usage I would like it to run up to 40 deg environment temperature, thus only 20 deg difference, that would mean 1.6 W would be the average power it could dissipate. With a lot of headroom, if one doesn't deform the plastic when it's hot.

The graph doesn't show that there's a lot of heat buffer in the case. I ran the tests long enough for that. The heatsink has a little bit of buffer, and also the top, but both of them don't weigh much. The rest of the box doesn't heat up much.

But now a 8.7% difference is shown between a good top and a solid one. Not much, but if there wasn't a difference I would have found it necessary to dig further into that mystery.

What did the experiment do for my intuition?

It enforced my believing that one cannot dissipate much heat in small plastic boxes. It changed my thinking that airflow can make it a lot better.

Printing a thinner top, with surface enhancing structures (inside and outside) together with a plastic that conducts heat better might give similar or even better results.

I also learned not to just trust the dissipation values in the data sheet.

[tggzzz]

It enforced my believing that one cannot dissipate much heat in small plastic boxes. It changed my thinking that airflow can make it a lot better.

It is instructive to look at existing products. Small boxes with significant heat dissipation have forced air ventilation.

[HendriXML]

I really dislike fan-noise, when choosing products that is for me a criteria that's high on the list.

Maybe that's why I like aluminum boxes, those extruded ones aren't that expensive. (But won't be very great in dissipation either)

Having noisy fans as a first option make product designers lazy. Especially in psu's, awg's and scopes.

I would gladly pay 30 euro more for near silence. That should be in most cases more than enough for better solutions.

[DaJMasta]

For dissipation in aluminum chassis, the trick is to bolt the device to the chassis (with a silpad/insulating hardware when appropriate).  Aluminum conducts heat well, so if you can transfer it to the chassis, you get much of the surface area of the box as your effective dissipation surface.  You can probably get similar or better performance with good coupling to an aluminum chassis as you could with slots in a 3d printed chassis with no forced air provided the boxes were similar size.

[HendriXML]

Sure, the slots seem not to work very well.

Aluminum will perform much better indeed, but I dare not say how much better.

Having a 70 deg alu box, is not completely user friendly (it also transfers heat fast to skin), so bolting stuff directly to it is in many instances not necessary if lets say max 50 deg case temperature is a constraint.

[Conrad Hoffman]

The heat has to dissipate somewhere. Remember that if you have a 5 watt heater inside a sealed aluminum box, the box will come to the same temperature regardless of whether the heater is screwed to the box, or floating in mid air inside. The gradient from the heater to the outside world will be wildly different, hot spots will be different and the time constant will be wildly different, but the outcome will eventually be the same. To lower the temperature of the box, you need ventilation or a heat pipe or something, to dissipate the heat to a larger sink.

[HendriXML]

That's what I meant. When mounted to the case because of the low resistance path, the temperature is technically allowed to get higher, but if it should be constained by safety it can not reach those temps in reality.

For power stuff large cases are better suited imo. If I would choose for active cooling, then just a breeze, or somewhat more powerful in extreme conditions.

The psu of Rohde and Schwarz shown here Teardowns of the Rohde & Schwarz HMP4040, HMP2020 ... looks like a great design. I wonder how silent it can function.