3
$\begingroup$

The Engineering Toolbox presents a chart of "common" materials and their thermal conductivity coefficients. I'd read or learned some time ago that diamond was superior in this respect. The chart shows diamond at 1000 W/m K compared to the next closest, silver at 429 W/m K.

My objective is to reduce the grip of a thermal pad between a heated bed and a glass surface. The heat transfer from the bed to the glass was improved by the addition of this thermal transfer pad, but the pad is very sticky, one millimeter thick and relatively fragile. I've only now discovered that this product is discontinued!

Pursuant to this objective is the ability to lift the glass, with the thermal pad (dusted?) remaining on the heater, without the pad adhering to the glass. It's not adhesive, but it sticks with the power of gekko.

I'm considering to powder one surface of the pad, but do not wish to reduce the thermal conductivity. I'm aware that diamond powder exists for lapping/grinding purposes, but have also found that most of the product listings provide the expression "synthetic diamond" in the description.

Even though SE discourages more than one question per post, these are all related to my objective.

Is there a better choice than diamond powder to provide a thermally conductive release mechanism between the thermal gap filler pad and the glass?

Related: Will synthetic diamond powder display the same thermal transfer characteristics?

Edit Added from comments: The bed is a flat electrical heater (3D printer) with a range from ambient to about 100°C. There is normally a 1mm air gap to the glass bed. As such, the air gap has to be heated before the glass reaches the desired temperature. Having installed the 1mm transfer pad reduced the elapsed time involved.

$\endgroup$
8
  • 1
    $\begingroup$ I would be wary about something like diamond dust even. That's not the same as crystalline diamond that you would be measuring the conductivity of. physics.stackexchange.com/questions/92433/… $\endgroup$
    – JMac
    Jan 3 '19 at 20:10
  • $\begingroup$ @JMac, That was a good question and answer pair and certainly twists my head in a different direction. $\endgroup$
    – fred_dot_u
    Jan 4 '19 at 0:22
  • $\begingroup$ Does the pad conform to the glass shape or is it flat? What is the bed (hot coal, a gas burner)? What is the temperature of the bed? Is the goal to heat the glass at the highest rate possible from the bed or to have the lowest temperature difference between the bed and the glass (they are to some extent diametrically opposite goals)? Addressing these questions will help understand why the pad gives improved heat transfer and what could be used instead. $\endgroup$ Jan 4 '19 at 16:29
  • $\begingroup$ answers in edited section $\endgroup$
    – fred_dot_u
    Jan 4 '19 at 21:48
  • $\begingroup$ @JMac If you care to place your comment as an answer, I'll accept it. I believe as you've suggested that there is no practical answer to this quest. $\endgroup$
    – fred_dot_u
    Jan 4 '19 at 21:49
1
$\begingroup$

Here's the answer to one of your questions: Synthetic diamond is identical to naturally- occurring diamond. It's made in a laboratory instead of deep underground and has the same properties as the "underground" variety.

$\endgroup$
0
$\begingroup$

Based on the question and comments, it seems you want something akin to this.

powder tray

This is a tray that holds a conductive powder. The powder is used to allow the glass bed to "self-level" independently of the hot plate.

As for the conductive powder, any metal powder would do. The thermal conductivity increase by using a powder in the gap versus having an air gap is significant enough, let alone that you want to use the most conductive material possible.

This would have two further issues. First, the powder should not stick to the glass bed. The choice may be have to be made through experiments. Second, the gap of 1 mm is small, and this may make such a tray design nearly impossible from a mechanical perspective. Indeed, even when you can create a 1 mm tray, the depth of the powder may be too thin to allow the self-leveling of the glass independent of the hot plate. In other words, when the powder is too thin, any thermal movement of the hot plate will be transmitted to the glass because the powder is essentially compacted.

One method may avoid the second problem. Increase the gap between the hot plate and the glass. This will allow the (thicker) powder to act both as a thermal conductor as well as an expansion/compression "blanket" between the hot plate and the glass. The downside is, as the gap increases, the temperature difference between the hot plate and the glass will increase proportionally. The end point will be that what you gain in mechanical stability by increasing the gap will eventually put you back to the same situation as having a 1 mm air gap. This will happen when $(k/d) \approx h$ ($k$ is conductivity, $d$ is gap width, and $h$ is the convection coefficient of air).

A second approach to this problem is to replace the air gap with a liquid gap. Flow water or oil between the hot plate and the glass. This will increase the convection coefficient in the gap by a factor of 10 - 100.

$\endgroup$
1
  • $\begingroup$ The glass plate has a trio of magnetic balls which engage slots in the metal under the heat bed. This maintains the 1 mm gap and creates a consistent level surface. The leveling screws adjust this metal plate and thereby the glass bed relative to the printer nozzle. I believe my thermal pad prevented a proper settling of the bearings and have abandoned the thermal transfer problem. On the other hand, I like your idea of a liquid transfer medium. It would have to encompass the bearings and slots, but the slots would allow the liquid to drip out. $\endgroup$
    – fred_dot_u
    Jan 6 '19 at 1:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.