Silver Heat Block

With the VDE-100 Schnekenstruder working as expected, and diamond nozzles on the way, it was only fair to make a proper heat block to mate with my tri-metal heat break.

So I ordered 3 troy ounces of pure silver casting grain, and spent some time making molds from 3D printed forms. Why silver?:

Cons:
1) It's expensive. Currently $20-$30 per troy ounce. The heater block will weigh about an ounce.
2) It's heavy. Over 4 times heavier than aluminum, and heavier than copper.
3) It's a bear to machine. It's sticky and work hardens instantly.

Pros:
1) Has the highest heat conductivity of any metal at "normal" temperatures.
2) Has a very low specific heat capacity.

This all means that I should end up with a block of silver that will respond to input from the heater cartridge roughly twice as fast, and transport the input roughly 4 times as fast, as a standard aluminum block. Will this make any difference in the final printed result? I suspect that inter-layer adhesion will be better, and that there will be fewer surface artifacts from temperature variations at the nozzle. The brass nozzle mount for the diamond is the "thermal resistor" in this enire Fourier equation.

I won't actually mate all of this together until November, when I get the Diamondback nozzle, but may post reports on the pure silver block process (casting, machining, etc.).

... my best bet would be bronce or brass

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Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

I would be interested to see what your results are as I looked in some depth at various materials and geometries for hotends and came to the conclusion that the race is a lot closer than you might think.

One starting assumption that I used is that having a high specific heat is a good thingas it acts a a heat store while the heater is trying to catch up. From this point of view, while Silver has a specific heat of 0.233 J/g.K and Aluminium has a specific heat of 0.900 J/g.K, both have the same specific heat per unit volume at 2.43J/cm³.K. Again, in terms of conductivity, the conductivity and area where the heat transfer from metal block to plastic takes place is much more important than the conductivity of the bulk of the metal: If this is through a stainless steel tube having a 1mm wall thickness and with a contact area area of 220mm² then the difference between the ability of Copper to deliver the heat is less than 5% better than Aluminium.

Having said that, I use Copper as other considerations make my hotend uncomfortably small, having a 3W wirewound resistor for a heater - no cartridge is available in the size I need.

I have to (as I very seldom do) disagree with VDX as Brass and Bronze both have fairly horrible conductivities and are not even particularly easy to machine.

Edit:
A late edit to put this into perspective.Thermal conductivities in W/m°C

Aluminium          237
Copper             386
Silver             420
Brass              115
Bronze             26
Tellurium Copper   360  - my favorite on thermal conductivity and machinability but hard to get

Mike

Edited 1 time(s). Last edit at 08/14/2021 11:13AM by leadinglights.

... hmmm ... didn't look into thermal conductivity, as the main part of the alloy is copper

My guess was more to brass/bronce than copper, as it's much better to machine with my small lathe and CNC-mill ...

---

Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

Quote
leadinglights
I would be interested to see what your results are as I looked in some depth at various materials and geometries for hotends and came to the conclusion that the race is a lot closer than you might think.

One starting assumption that I used is that having a high specific heat is a good thingas it acts a a heat store while the heater is trying to catch up. From this point of view, while Silver has a specific heat of 0.233 J/g.K and Aluminium has a specific heat of 0.900 J/g.K, both have the same specific heat per unit volume at 2.43J/cm³.K. Again, in terms of conductivity, the conductivity and area where the heat transfer from metal block to plastic takes place is much more important than the conductivity of the bulk of the metal: If this is through a stainless steel tube having a 1mm wall thickness and with a contact area area of 220mm² then the difference between the ability of Copper to deliver the heat is less than 5% better than Aluminium.

Having said that, I use Copper as other considerations make my hotend uncomfortably small, having a 3W wirewound resistor for a heater - no cartridge is available in the size I need.

I have to (as I very seldom do) disagree with VDX as Brass and Bronze both have fairly horrible conductivities and are not even particularly easy to machine.

Edit:
A late edit to put this into perspective.Thermal conductivities in W/m°C

Aluminium          237
Copper             386
Silver             420
Brass              115
Bronze             26
Tellurium Copper   360  - my favorite on thermal conductivity and machinability but hard to get

Mike

The purity of the metal makes a huge difference. As an example, for a heat break in which thermal conductivity is NOT a good thing, Grade 5 Titanium is about three times better than Grade 2 commercially pure titanium. Three times better meaning that Grade 5 has one third the thermal conductivity of Grade 2.

Sterling silver is about comparable to OFHC copper, while pure (electrolytically refined) silver is considerably better as a heat conductor.

Using values for pure metals, I get these volume heat capacities:
Aluminum: 3.92 Joules/cubic centimeter-Kelvin
Copper: 3.38
Silver: 2.51

Pure silver has volumetric heat capacity 64% that of pure aluminum. It will take 64% fewer watts to increase the temperature of the silver by one Kelvin, or given the same watts, 64% less time.
And then the silver will transport that heat input faster by 75% (240 Watt/meter-Kelvin for aluminum versus 420 W/mK for silver). These values are from [www.engineeringtoolbox.com], at 127C, and are for pure metals. Common aluminum alloys, as are normally used in this application, are considerably worse in all regards.

The common brass and steel nozzles are the sticky wicket in the entire heat path. I may have to ask Diamondback to make me a custom diamond nozzle in a pure silver mount. The entire assembly should be thermally comparable to a 60 watt E3D Volcano, but with a "normal" size and only 30 watts.

This is ultimately an exercise in curiosity, since I generally aim for precision rather than speed when printing.

Edited 1 time(s). Last edit at 08/14/2021 01:15PM by rq3.

The figures I get for volumetric heat capacity are from [www.tibtech.com] and I get

              J/g.K        g/cm3       J/cm3.K     
Aluminium     0.900         2.7        2.4300
Silver        0.233        10.5        2.4465

Copper        0.386         8.9        3.4354

The thermal conductivity of Aluminium alloys is significantly worse than pure Aluminium, but compared with Copper alloys it still remains in the usable range. For example, 6082T6 Aluminium is about 180W/m.K but 75% Cu 25% Sn Bronze is only 26W/m.K. Intrigueing bit of info is that isotopically pure C12 Diamond has a thermal conductivity up to 41,000W/m.K [en.wikipedia.org]

Quote
Linus Pauling
Satisfaction of one’s curiosity is one of the greatest sources of happiness in life.

Mike

Quote
leadinglights
The figures I get for volumetric heat capacity are from [www.tibtech.com] and I get

              J/g.K        g/cm3       J/cm3.K     
Aluminium     0.900         2.7        2.4300
Silver        0.233        10.5        2.4465

Copper        0.386         8.9        3.4354

The thermal conductivity of Aluminium alloys is significantly worse than pure Aluminium, but compared with Copper alloys it still remains in the usable range. For example, 6082T6 Aluminium is about 180W/m.K but 75% Cu 25% Sn Bronze is only 26W/m.K. Intrigueing bit of info is that isotopically pure C12 Diamond has a thermal conductivity up to 41,000W/m.K [en.wikipedia.org]

Quote
Linus Pauling
Satisfaction of one’s curiosity is one of the greatest sources of happiness in life.

Mike

Mike, many thanks for that! I re-checked my sources, and had visually grabbed AlBuMet for Aluminum. Our figures are now in close agreement, with only the conductivity of silver giving it an advantage. I knew that the volumetric heat capacity for solid elements tends toward a constant, so should have known better when I had such a discontinuity. One reason why these discussions are so valuable. Thanks again.

Just waiting for the molds to come out of the curing cycle, and I'll actually try casting a pure silver block tomorrow. I confess to NOT looking forward to the machining!

Edited 3 time(s). Last edit at 08/14/2021 05:32PM by rq3.