@Andrew_Hodel that’s not necessarily true. The PTFE tube in free air has a particular surface area and heat transfer coefficient. The metal thermal barrier has a particular surface area and heat transfer coefficient at the filament/metal interface, then another surface area and heat transfer coefficient at the heatsink with forced convection. If the metal conduction+convection path has a lower total resistance to heat flux than the PTFE in free air, it can actually keep the filament cooler despite additional hear conducted up the heat break.
@Ryan_Carlyle quote “If the metal conduction+convection path has a lower total resistance to heat flux than the PTFE in free air”
It does not.
Especially when the existing extruder designs have a minimum diameter of 7-10mm + the fins of pure metal.
Stainless has a 16*4 times higher thermal conductivity at 25C.
@Andrew_Hodel show me the math or thermal FEA to prove that. Air is an excellent insulator. Free-air convection cooling of a small object can be as low as <1 W/m-K. Including the PTFE, at 0.25 W/m-K, the series conductivity will be quite low. Forced airflow over a heatsink can be >1000 W/m-K. And every functional all-metal design has much more cross-section piping heat away from the filament than it has conducting heat up from the hot block. A well-designed all-metal system can sink heat away from the filament much, much faster than free air+PTFE.
Measure the temperature of bare PTFE on a block at a distance, then measure the temperature of metal at a distance.
There’s no need to know the formula when you can directly read the measurements.
I know at 1cm from the heating block a PTFE tube is cooler than a stainless tube with fins even when the stainless tube has a fan on it. That difference just increases as you measure farther up the tube.
I get the idea of needing a formula before you invest millions into casting some huge reactor, but face it we are talking about <$10 in materials here to build, test and take direct measurements.
Yes, metal can pull heat away from the filament faster than air, BECAUSE IT IS MORE CONDUCTIVE.
The trouble with the typical extruder is that there is also a thermally connected heating element on a PID loop which is adding heat, not removing it.
@Andrew_Hodel we don’t care what the temp of the outside of the tube is, we care what temp the filament inside the tube is.
@Ryan_Carlyle Who is we, and the temp on the filament inside the tube is directly related to the temperature outside the tube.
It seems that you have given up sense.
@Andrew_Hodel let’s assume some realistic numbers so this makes more sense. The filament could be 45C, a safe temp below the glass point of PLA. The outside of a PTFE tube may be 30C in 27C air. The all-metal system above the heat break may be 35C in 27C air. This means we have a 15C temp difference across the PTFE tube with a thermal conductivity of .25, versus a temp difference of 10C across the metal tube with a conductivity of 15-20 depending on the steel grade. The conductivity of metal is so much higher that it can sink more heat from the filament EVEN THOUGH it is hotter.
@Ryan_Carlyle Then why is the actual measured temperature of the PTFE tube in free air lower than that of the metal tube WITH A FAN?
@Andrew_Hodel because you’re measuring the outside of an insulator. If you don’t get why that’s not representative of the temperature of the filament inside it, then this conversation isn’t going to go anywhere.
@Andrew_Hodel In addition to what @Ryan_Carlyle said, and I’m sorry if I missed it, but how are you measuring the temperature of the outside of the tube? Unless you’ve somehow molded a thermocouple into the fluoropolymer, any easy way I can think of would be measuring the temperature of the air around the tube more than that of the tube itself.
how many of times did you change filaments with the PTFE tube still in shape? PTFE insets insets wear out over here and need regular replacement every few months.
perhaps there’s something i’m missing here, if we leave out the argument over which design deals better with the heat transfer and management (as a software engineer, whose done a lot of cnc, machining, building, construction and electronics over the years @Ryan_Carlyle 's argument sounds both more scientific and makes more sense, but i know enough to know that i just don’t know enough…), can @Andrew_Hodel 's hotend (not extruder) be used to print exotics like carbon and nylon, i know that for some brands that i use i have to use my e3d v6 clone (cost ~11$ off aliexpress and works just as well as my friend’s original one) at 260-280C, that just does not seem possible with the PTFE going in directly into the heat block.
for me that basically settles it, because “better” is a subjective term (different people want different things), but can print and cannot print for me is a 0 & 1 absolute… since most of what i print is PETG, CF and Nylon, for me going above 250C is a must (which PTFE will not handle for a relavant length of time).
or is there something i’m missing here with @Andrew_Hodel 's design?
Ptfe is an excellent thermal insulator, as you state, so much so that when you measure the temperature outside the tube, it is meaningless and not indicative of the temperature of the filament. On the other side, because the aluminum heatsink is a great thermal conductor, measuring the temperature of the heatsink will be a much closer indicator of the filament temperature. As such, the 8C differential between ambient is more relevant to the efficiency of the cooling.
And as your hotend may work, and may even work quite well, it’s design adds considerable weight and room for failure. The transition point between the cold and hot side is the most common point of failure, typically from a bad seal or ptfe that softened and failed.
@Whosa_whatsis my finger can judge pain quite well, it also is capable of molding to the shape.
@Tom_Keidar then replace the PTFE going into the block with a turned tube of graphite and the aluminum will melt first. Why is this all so complicated to you all?
With that setup you could start printing solder, that’s all it is. Get the forge.
By the way, a diesel glow plug will reach 1200C in 10 seconds at less than 200 watts and they are 12v.
@Stephanie_A same response, if you want hotter than PTFE use graphite or ceramic in place, cost is less than $10.
Regarding the filament temperature inside the tube, you can see through it because it is clear. At most it is melted only 2mm above where it enters the hot end. Find the melting point of the material being extruded and you have an extremely scientific gradient chart, get a microscope. @Ryan_Carlyle see the inner measurement?
Also the aluminum weighs less than your typical stainless hotend with fins and a fan, it weighs a lot less.
Looks interesting, I would like to try it. I’ve been through many hot ends, starting to think they are a consumable item. Some I never got to work at all (sorry Josef :))
I don’t have a drill press, so I would have to buy one, at least a kit. PTFE does degrade over time in a hotend, but if it was an easily replaceable item that would be OK for me.
It’s only wider testing and time that determines whether a design “works” or not.
@bob_cousins I’ve tried selling things before, people just throw their problems on you. To top it off, when I originally made this I built a gcode file for an entire 4x8 sheet of them with the intention of taking them to a shop with a waterjet and having them cut.
The shop refused to do it without reason even though I was willing to pay whatever price they would ask. I don’t really know what was wrong with them but they didn’t think right in terms of buy/sell/improve/profit.
Basically at this point in the US it seems that anyone capable of local manufacturing wants to kill it and just handle army or oil field contracts. Everything else comes from China or India and I’m just not interested in shipping parts around the world when a machine is doing all the work anyway.