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dojan

My take on a Hotend

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So, I've been thinking about making a new hotend for a while, and the recent discussions on the matter inspired me and made me actually come around to it. I haven't actually made the part yet, but I've put a lot of thought into the design. This is where I'm at for the moment. Any and all feedback appreciated!

Dojans Hotend - Blueprint - Photo 1

So the sketch is pretty, um, sketchy. Its designed to be made on a lathe, and is here seen as a cross-section through the middle.

The general direction of my thoughts here are that you want the hot part to be as short as possible, and as close (in the thermal sense) to the actual nozzle as possible.

The first thing I wanted to do towards that end was to not have a separate nozzle and heater block, since that creates both a thermal block at the interface between the to, and a much longer distance for the heat to travel to keep the very tip hot. This part should ideally be made from a material that has a very high thermal conductivity, but also a high (volumetric) specific heat capacity, which will help its temperature to remain stable even if extrusion is not. So I looked up Specific Heat Capacity on Wikipedia. Intriguing read. It turns out that the material that best fits this description is probably diamond. Other than that though, copper really stands out with around 380 W/(mK) (Watts per meter kelvin difference, unit of thermal conductivity) and 3.45 J/(cm³·K) (Joules per kelvin difference per cubic centimeter, unit of volumetric specific heat capacity). If I can't get hold of any copper that fits my lathe though, I'll try some brass.

The heaterblock/nozzle connects to a M6 threaded rod of Stainless Steel (one of the least thermally conductive metals, at around 18 W/(mK)), drilled out with a 4 mm drill to become a threaded tube. This part will act as a backbone and connect the other parts together, and also act as a thermal bottleneck, hopefully not allowing to much heat to creep upwards and create plugs or melt the PTFE parts. The threaded pipe screws about 5 mm into the copper nozzle, where it should be possible to make a molten-plastic-proof connection, given that copper is sometimes used as a gasket material. On the other end it screws first into a aluminium heatsink, and then into the PEEK replacement, separated by a few layers of fiberglass insulation. The bowden tube screws into the PEEK replacement from the other direction, contacting the threaded tube hopefully well above the point where the plastic being printed starts to melt. The exact parameters here will have to be experimented with.

The above mentioned heatsink is shaped like a couple of wide washers stacked on top of each other but with a distance between, much like the heatsinks on the all-metal hotends, but made of aluminium or copper for improved heat conductivity. I'm leaning toward aluminium since it is lighter, and I already have the material I need. Exactly how large the heatsink will have to be is also something I will have to experiment with.

The top part, which connects to the Ultimaker, and replaces the PEEK will be made from PTFE (Teflon, same as the bowden). It is largely the same as my current bowden, but has threads all the way through, and screws onto the bowden. The choice between PEEK and PTFE is pretty arbitrary, but I think I have some PTFE laying about.

Between the PTFE part and the heatsink I plan to put some fiberglass exhaust wrap as insulation. I also plan to wrap the heaterblock/nozzle in fiberglass wrapping (This stuff is fantastic, I have some on my current, ordinary hotend to test, and with only one layer of 1mm I can hold my finger right to the hotend while it's printing at 220 degrees C. It is rated for 800 C, with peaks up to 1'300 C :)

I have gone through a lot of design changes before getting here, this picture shows the rough stages of evolution:

Dojans Hotend - Evolution And messy desk

 

So what do you guys think? Am I reinventing the wheel? Am I onto something?

For more detailed photos of the sketch, see the gallery under my profile. Also, beware the nerdy mess on my desk.

 

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FWIW, I made a V2 brass tube replacement out of SS and used it with a V2 brass nozzle (the V2 nozzle threads into the aluminum heater block and meets the SS tube inside the aluminum block) and it didn't really work at all with ABS (never tried it with PLA). I fear you will not have a long enough SS section. The all SS hot end designs that I've seen are pretty long (Prusa).

Also copper is difficult to machine whereas brass machines easily. I would think brass is good enough!

Joergen and I both print a lot of ABS and we are using longer 36 mm MakerGear "barrels" (brass tubes) and BigHead nozzles. We both use an aluminum heatsink under the PEEK (though of different design). We have the full MG set of nozzles but I think we both like the 0.50 for general use.

I have a long SS tube design in mind that will run the enitre lenght of the wooden print head box (yes, it's tough to fit it in) but my current setup works so well that I'm not motivated to try anything else, ATM. I commonly print Ultimachine black at 275 C and I have no discoloration of my PEEK whatsoever, as I did before the change.

I can't really tell you what works best for PLA. I always thought the stock UM setup worked quite well for PLA, but admit that I have limited experience with PLA.

-Cal

 

The first thing I wanted to do towards that end was to not have a separate nozzle and heater block, since that creates both a thermal block at the interface between the to, and a much longer distance for the heat to travel to keep the very tip hot.

If I can't get hold of any copper that fits my lathe though, I'll try some brass.

The heaterblock/nozzle connects to a M6 threaded rod of Stainless Steel (one of the least thermally conductive metals, at around 18 W/(mK)), drilled out with a 4 mm drill to become a threaded tube.

 

So what do you guys think? Am I reinventing the wheel? Am I onto something?

 

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FWIW, I made a V2 brass tube replacement out of SS and used it with a V2 brass nozzle (the V2 nozzle threads into the aluminum heater block and meets the SS tube inside the aluminum block) and it didn't really work at all with ABS (never tried it with PLA). I fear you will not have a long enough SS section. The all SS hot end designs that I've seen are pretty long (Prusa).

 

 

Interesting. Why didn't it work? What happened? Did the molten ABS stick to the SS somehow? Please tell me more!

 

I have seen that the all metal hot ends usually are very long, and they of course have to be to dissipate all the heat, but since the original UM hot end kinda works with just the insulating plastic and no heat sink, my idea with this design is that by adding a small heat sink, but keeping the plastic insulation it should make a radically sharper thermal gradient, and be kinder to the plastic and wooden parts at the same time. This is a good observation though!

 

 

Also copper is difficult to machine whereas brass machines easily. I would think brass is good enough!

 

 

Ahh, this may well be true! I have never tried lathing copper, and may well fail :)

Brass obviously works, but I do think that copper would heat up and cool down more uniformly, and thus make the electronics respond quicker. You might well be right though, the difference may well be very slight :)

 

 

Joergen and I both print a lot of ABS and we are using longer 36 mm MakerGear "barrels" (brass tubes) and BigHead nozzles. We both use an aluminum heatsink under the PEEK (though of different design). We have the full MG set of nozzles but I think we both like the 0.50 for general use.

 

 

I would love to see pictures on how that fits together!

I have so far only used the original 0.4 mm Ultimaker nozzle, but trying other diameters is something I have been longing to do. Does changing the nozzle size change what layer height you can use?

 

 

I have a long SS tube design in mind that will run the enitre lenght of the wooden print head box (yes, it's tough to fit it in) but my current setup works so well that I'm not motivated to try anything else, ATM. I commonly print Ultimachine black at 275 C and I have no discoloration of my PEEK whatsoever, as I did before the change.

 

 

That's a brilliant idea! Though I think you would still want a heat sink at the bottom to create a sharp temperature gradient for the filament... That configuration would save some bowden as well :)

 

 

I can't really tell you what works best for PLA. I always thought the stock UM setup worked quite well for PLA, but admit that I have limited experience with PLA.

-Cal

 

Well your input is most welcome in any event :) I'm afraid that the stock UM setup haven't worked to well for me, though I haven't tried the V2.

 

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>Interesting. Why didn't it work? What happened? Did the molten ABS stick to the SS somehow? Please tell me more!

 

The problem was extrusion resistance was very high and it partially jammed at times. It wasn't usable.

>I would love to see pictures on how that fits together!

It's a simple setup. Joergen and I are both running MakerGear 36 mm "barrels" (brass tubes) and Big Head nozzles.

Here is my thread:

http://umforum.ultimaker.com/index.php?/topic/1798-got-abs-you-need-a-makergear-50-cal-nozzle-and-longer-barrel/

Joergen's hot end consumes a touch more Z than mine but it features more heatsink area than my hotend:

http://umforum.ultimaker.com/index.php?/topic/1925-modification-to-the-hot-end-for-abs-printing/&do=findComment&comment=13401

>I have so far only used the original 0.4 mm Ultimaker nozzle, but trying other diameters is something I have been longing to do. Does >changing the nozzle size change what layer height you can use?

 

The rule of thumb that I've seen is maximum layer height is 80% of nozzle diameter. The lower limit is typically a function of how accurately you can feed plastic (though not terribly useful, 10 and 20 microns probably require 1.7 mm filament feeders ;)) and how level you can get your bed. I usually print 80 to 150 micron layers (with perimeters as low as 40 microns using netfabb "half-layers"). At don't think I've even come close to max. layer height for my .40 and .50 nozzles.

>That's a brilliant idea! Though I think you would still want a heat sink at the bottom to create a sharp temperature gradient for the filament... >That configuration would save some bowden as well

Yes--I'll use a heatsink and probably design a fan duct to cool it, using the existing fan, which I've replaced with a stronger 160 mA unit. I'll leverage the aluminum plate as a heat sink and might even mill a replacement for it that features fins.

I don't have any drawings but here are the quick design elements. Leverage anything you like as you please, because I won't have time to build it for a while:

1. 303 SS tube (relatively easy to machine).

2. 303 SS tube can be as large as 5/16" diameter (I think) inside the wooden box (Bowden is 1/4").

3. End of Bowden will be threaded.

4. Top of 303 SS tube will be threaded into a nut coupling, used to connect to threaded Bowden tube. One set of threads will be left-hand (probably on the Bowden tube side) to aid assembly (just turn the coupling nut to connect or disconnect the Bowden tube to the 303 SS tube).

5. Bottom of 303 SS tube will thread into an aluminum heatsink to replace the PEEK and interface with the aluminum plate, providing both upward retention force and heat conduction to the aluminum plate/additional heat sink.

6. Plywood is fairly high temperature tolerant (probably won't ignite until at least 300 C ;)) but insulation between the SS tube and the wood might be prudent--especially if there is contact at the bottom of the box. The top of the box would be the preferred location for retaining the 303 SS tube on the other end of the "sandwich" (due to lower temperature of the 303 SS tube there).

7. 303 SS tube could thread into existing aluminum heater (only the nozzle would be brass). A hybrid using a brass tube in the heater block could also be investigated.

8. I have no interest in adding another fan. I will leverage my single fan, ducting and heatsinks, Perhaps a new duct with have a mechanical "ABS gate" to route most of the airflow to the aluminum heatsink when printing ABS, which requires little, if any fan cooling.

 

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Looks good Dojan. you might check out my development work in the Google group:

https://groups.google.com/forum/#!topic/ultimaker/JJuCfGdg6M0[251-275-false]

https://groups.google.com/forum/#!topic/ultimaker/zdPsd-tfB54

At the moment, I am running a V2 hotend, with a stainless steel uptube (identical to stock brass). I welded up the hole in the aluminum plate and drilled/tapped for M6. The stainless tube is threaded directly into the plate with thermal grease, providing a more rigid mount, and turning the plate into a heat spreader. I've reversed the fan flow and ducted it to blow air across the plate and tube. I have the whole heater block/nozzle insulated with a few layers of alumina tape and capped with aluminum foil. It works great!

The next step is to replace the whole print head with a printed part I designed, and attach a proper heatsink to my stainless tube.

My suggestions:

  • [*]Find a way to ditch the plastic parts all together. If you properly cool the stainless tube, you will be better off leaving some of the tube (near the top) exposed to further cool. I simply tapered the stainless tube, and countersunk the inside of the bowden tube to join them. Once that junction is well below the glass transition temp, it is not prone to plugging issues. I've run mine with the bowden tube separated from the stainless by 0.5", with the filament open air in between.

  • [*]Bring the inside of the stainless tube to a nice polished surface

  • [*]Go with aluminum for the heater/nozzle combo. I've crunched the numbers and see no reason at all to use brass. With that assembly well insulated, the heat loss into filament is something like 5W. The heat capacity you gain with brass is useless in my eyes, and you sacrifice thermal conductivity which IS usefull to us here.

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