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Hotend theory

Posted by Hobbes20xx 
Hotend theory
January 26, 2012 11:30PM
Hey guys, so ive been trying unsuccessfully for awhile now to get my reprap printing, i DO NOT want to buy a hot end but i will if necessary. I have been looking for a decently easy-to-make hotend. As far as i can tell, you want your hot zone to be as small as possible, and it is almost necessary for some sort of heatsink to be attached. Currently, i have a .5 m hole in an acorn nut, attached to (16mm?) threaded rod with a heater block, attached to a PEEK rod (1/2inch?) that is attached to my wades, however, it seems like after a couple minutes of printing my extruder just...gives up, rather i think that the heat spread out to much up into the PEEK and made the filament squishy so it wont push through? any advice? thanks
Re: Hotend theory
January 27, 2012 12:52AM
If you are in the uk I can send you a spare hotend tip. This with an aluminum block and the peek break should work.
Re: Hotend theory
January 27, 2012 12:53AM
If you haven't gotten it working yet, I'd really recommend a pre-made hot end. Makergear has a legendary one, but Lulzbot's Budaschnozzle is gaining popularity. If you want to build your own, it will probably be easier once you have a working one of your own that you can examine to figure out why it works.

As to the size of the melt zone, you don't necessarily want it to be as small as possible. If you do that, then you'll be limited to slow printing speeds. If you want to go faster, it needs to be larger, but then you run the risk of a jam higher up in the tube if you use PLA, and with any material, you'll have more strings. It's a trade-off.

Edited 1 time(s). Last edit at 01/27/2012 12:54AM by Pointedstick.
Re: Hotend theory
January 27, 2012 01:15AM
I find the J heads to be excellent


__________________________________________________________________________
Experimenting in 3D in New Zealand
Re: Hotend theory
January 27, 2012 04:22AM
PEEK isn't slippery enough. You need a PTFE liner to make it work.

And there must be no gap between the end of the metal barrel and the hole through the insulator.

Edited 1 time(s). Last edit at 01/27/2012 04:23AM by nophead.


[www.hydraraptor.blogspot.com]
Re: Hotend theory
January 27, 2012 06:43AM
Hang in there Hobbes20xx,

Like you, I also am trying to make my own hot end vs buying one. (its a pride thing)

I think I've tried over 19 designs, They all did the same thing you describe. It heats up, stuff starts to ooze out (yay!), then when I start to push 3mm ABS through it comes out faster, and faster, then stops. (boo!)

It took a lot of searching on these forums to discover what was happening. The phrase I've seen is what nophead is describing. I have read it being called mushrooming. The hard ABS at the point where it leaves your PEEK (or PTFE) and enters the hot brass squish's out the soft ABS from the hot brass into any crack or crevice at the joint and then re hardens creating a hard stop that the ABS can not push through.

For me, the easy/quick solution was to mount a small fan at the side of my extruder, and direct the air flow at the PEEK (or PTFE) and away from the hot end tip. I used a cut up sheet of teflon baking sheet from the local Dollar store as the air flow director. Pics might have to wait till I get back home.

------edit-----

Got home, and posted these:

[www.thepyroguys.com]
[www.thepyroguys.com]

They are a bit out of focus, but I think it helps convey the idea of some sort of barrier to keep the air flow away from the hot end. This stopped my mushrooming.

----end-----


Don't give up.

Regards

Edited 1 time(s). Last edit at 01/27/2012 06:05PM by fredlatesta.
Re: Hotend theory
January 27, 2012 07:39AM
I also am keen to build my own hot-end, because my Prusa project was an end in itself, I feel real good about the fact "I built it" - anything I could assemble myself I did..
But I did get a hot-end locally (From Vik O himself) in order to get going.

The hot-end is obviously one of (if not *the* ) most subjective design parts of the whole thing.
This one is I believe one of the very original designs, simple 1-piece PTFE barrel, 40 mm brass bolt with 3.2mm hole held by hose clamp, .5 mm drilled Acorn nut, and nichrome wire heater.

I had the problem where after a few minutes it would stop extruding - and exactly as fredlatesta describes, a wee fan hung under the carriage pointing back up at the PTFE barrel fixed that. (with a hot-tip shield made from some baking tray thick aluminium foil )

My main remaining problem is that it "lags" - slow to start extrusion and slow to stop, leading to fill gaps, and stringing etc (retraction doesn't fully correct).
(I'm using PLA)


The point I'd like to question is "size of the melt zone, you don't necessarily want it to be as small as possible."

I am assuming my lag is due to the long amount of brass barrel (about 40 mm) holding too much melted PLA, so even retracting the solid pla by a few mm leaves a compressed melted section, which, while the top volume expands back up the barrel, the plastic at the tip of the barrel would still be under pressure for a short period, causing it to continue to extrude rather than come to a sharp stop.

From the depths of my almost no experience, I assumed that if the above is correct one would want as short a melt area as possible.
Maybe 10mm for a heater block, 5mm above for the connection back into the thermal barrier cylinder, (not threaded in, external mechanical support)
and maybe 3mm below the block for a shaped tip.

So less than 20mm melt area to reduce the "compression/spring-like" effect, and if the heating block has sufficient thermal capacity to keep the temperature up, and the controller has a decent PID algorithm, surely it would continue to melt the PLA at reasonable rates.. (I guess that is also a subjective point)

(I'm also keen to build one with nothing more complex than my el-cheapo drill press. No lathe-work ! Time will not be of essence)
Re: Hotend theory
January 27, 2012 09:20AM
A small melt zone is good for less ooze but limits the maximum rate one can extrude because only the outside of the filament is heated, so it takes time for the inside to melt. That time cannot be longer than the transit time of the filament through the hot zone.

Another thing that helps, I think, is a longer hole in the nozzle. It takes more force to extrude but it seems to ooze less.

With PLA the filament needs to be dry, otherwise the water in it produces steam which propels it from the extruder.


[www.hydraraptor.blogspot.com]
Re: Hotend theory
January 27, 2012 10:07AM
If you want an easy to build hot-end try this design: [www.reprap.org]
For me this design has proven reliable. I can print at pretty high speeds (never actually tried to find the limit).
Another pro is that this design is extremely compact.

Also think about the diameter of the hole through the insulator, it needs to be quite a bit larger than the filament diameter (3.5mm for 3mm filament).
Re: Hotend theory
January 27, 2012 11:01AM
what could be happening is the nut fills with feedstock, and as it is pushed out as filament the pressure causing to fill the inside of the acorn nut, causing the abs plastic to now act as an insulator, and prevents further feed stock from reaching glass transition temp or it greatly reduces flow. maybe work on ensuring the nut is completely sealed around the nozzle hole area. ptfe tape or some liquid glass. imho. ymmv
Re: Hotend theory
January 27, 2012 12:22PM
By the way, i forgot to mention im using PLA, beacuse i dont want to mess with a heated bed right now with ABS. I also dont have a lathe, but a decent drill press.. and im located in US.

Edited 1 time(s). Last edit at 01/27/2012 12:25PM by Hobbes20xx.
Re: Hotend theory
January 27, 2012 02:45PM
Please have a look at my Wildseyed Simple Hot End design, if you haven't already. Most info there is slightly out of sync with my current methods, but the cuataway drawing of the interiors is new.

There are a few things, goals in the assembly of a hot end that make for a good extrusion:

Keep the melt pot small, and keep most of your heat there.
Keep the melt pot sealed. The PTFE in my current design pinches off to ~3mm right at the top of the melt pot. Above this, the I.D. of the PTFE is kept large enough that it minimizes the contact between filament and PTFE. This keeps the filament cool as long as possible. This is more important with PLA.

Keep the stepper motor weak. If your stepper can rotate the big gear when the hot end is off, then you are running it too hard. This will do one of two things, or maybe both. First, it will cause the mushrooming of the filament (PLA) inside the thermal barrier, especially if contact with the walls is not limited. Second, it will strip the filament on the bolt hobbing, and once this happens, you lose the ability to push.

Reduce the current so that the gears click, click, click, without stripping the filament.
Re: Hotend theory
January 27, 2012 03:12PM
I use a ptfe tube down through the peek into the nozzle tip. No leaks as its one part.
Re: Hotend theory
January 27, 2012 06:10PM
Progomez,

I have toyed with the idea of putting the PTFE tube into the hot end brass. What are you using that is large enough for PTFE tube to go into the brass?

I have read, and like what both Jcabrer, and north90ty do, and if I recall, they both use a system where the PTFE tube enters the Brass tube.

For some reason I'm stubborn, and want to limit the size of my brass to 1/4-20 (or 6mm) and the only PTFE tube I have is too big to go into this smaller size. I'm always on the look out for other ideas.


Regards
Re: Hotend theory
January 27, 2012 06:57PM
I tried a 16mm PTFE insulator turned down to 10mm and tapped M10 into a brass nozzle.It didn't last long enough for the first layer.

PTFE does not work for me unless it is totally enclosed. Otherwise it bulges in the middle.

If you have the motor current less than the force needed to strip the filament you can't get the maximum throughput of the extruder. I.e. you have builds fail due to the motor skipping steps that may otherwise have succeeded.


[www.hydraraptor.blogspot.com]
Re: Hotend theory
January 27, 2012 07:32PM
My nozzle, rev 2, works by using a 4x2 ptfe tube down into the nozzle about 5mm from the tip. The nozzle and tip are one part. The nozzle inside goes from 4mm to 2mm to 0.4. I made it this way after i tried the acorn nut way and it leaked.

Heres someone with results from this method cooled down using a fan, [3d-printer-kit.com] , airtripper.com uses it without the fan and with a peek insulator.

One last point if you want to make the nozzle yourself you need a lathe.

Edited 2 time(s). Last edit at 01/27/2012 07:38PM by progomez.
Re: Hotend theory
January 28, 2012 12:51AM
nophead Wrote:
-------------------------------------------------------
> I tried a 16mm PTFE insulator turned down to 10mm
> and tapped M10 into a brass nozzle.It didn't last
> long enough for the first layer.

There must be more than one formulation of PTFE then. My PTFE rods are 13mm, turned down to 8mm in the brass. The current one has been going for about five months. Of the others I have had, only one really failed, due to the I.D. being too narrow. I ruined it trying to clear a jam.

Industrial Plastic Supply in Anaheim, California is my source.

>
> PTFE does not work for me unless it is totally
> enclosed. Otherwise it bulges in the middle.

On the Wildseyed, the PTFE has only about 10mm enclosed in the brass coupling. Of the remaining ~30mm, ~20mm are in open air. The last bit at the top is clamped to the extruder body by M4 set screws.

>
> If you have the motor current less than the force
> needed to strip the filament you can't get the
> maximum throughput of the extruder. I.e. you have
> builds fail due to the motor skipping steps that
> may otherwise have succeeded.

Keep in mind that Hobbes20xx is still new to all this, and his calibrations are unproven, since he hasn't been able to print. What I am suggesting is a starting point that reduces the number of variables, so he can reach the next level. I think we can all agree that every hot end has a speed limit. Finding that limit is just as important as anything else.
Re: Hotend theory
January 29, 2012 12:01AM
@nophead, i have been able to print about 75% of the 40mm calibration block until catostrophic failure, it prints pretty well to my surprise... what hotend would you suggest for a beginner? i can get something lathed from a person i know, i just need to know what WILL work. preferably out of some common (aluminum, brass, peek and ptfe) materials (sourcing ptfe sleeve for the inside is difficult, and takes time), with minimum complexity.

@north90ty i do like your hotend design, can i see a picture of it mounted?

@wildseye, drilling a concentric hole in anything is hard for me with only a drill press, im not sure i can manage through that ptfe

Thanks all for the replies!
Re: Hotend theory
January 29, 2012 12:10AM
additionally, if i cave and order a makergear hotend, can i replace their heatercore (NiCr) with a resistor heater?
Re: Hotend theory
January 29, 2012 09:36AM
I use a totally enclosed PTFE tube that goes from the top of the cold end to the inside of the nozzle tip. Totally enclosed meaning that it can't move up, down or out. I tried other concpets like transitioning from PTFE to brass with no luck. The material wants to flow back behind the tube through the joint.

The hot material is at a high pressure. It doen't want to go out that small hole. You have to totally contain the hot material while using the cold material as a ram to keep that pressure up. Pretty simple concept if you imagine yourself as the material.

I also tried to build my own nozzle and found that it wasn't worth the effort when you can buy one for $20. Pride aside you have to pick which battles you can win.
Re: Hotend theory
January 29, 2012 09:22PM
where can you buy one for 20?
Re: Hotend theory
January 29, 2012 11:22PM
I do agree that the PTFE needs to be totally enclosed and held securely in place. I found that the pressure of the molten plastic can easily push the PTFE out of the stainless steel tube.
Version 2 of my hot end design keeps the ptfe really well in place by crimping the stainless steel. And it is easy to build.

Here are two pictures of version 1 and 2 mounted.
http://www.flickr.com/photos/[email protected]/6787087947/
http://www.flickr.com/photos/[email protected]/6787085925/

good luck,
Frank
Re: Hotend theory
February 10, 2012 05:00AM
Starting building a North90 stainless hot-end this weekend.
Quote
north90
Also think about the diameter of the hole through the insulator, it needs to be quite a bit larger than the filament diameter (3.5mm for 3mm filament).
How does this go as far as backflow going up, cooling and grabbing - which has ended up being the nemesis of my classic ptfe/brassbolt hotend.
Re: Hotend theory
February 10, 2012 05:22AM
My view is the opposite. The insulator should be just big enough for the filament, any bigger increases the back flow and that forms a bigger plug that is harder to push.


[www.hydraraptor.blogspot.com]
Re: Hotend theory
February 10, 2012 05:46AM
Agree. Maintain the same minimal clearance between the PTFE liner and filiment from the top of the cold section to the inside of the nozzle tip. Minimal means it sides thru without drag but isn't loose. Keep in mind that under pressure and heat the actual clearance is the result of how much the tube is allowed to expand into the stainless liner. Everything should mate up with out being loose.
Re: Hotend theory
February 10, 2012 11:43AM
Hobbes20xx Wrote:

> @wildseye, drilling a concentric hole in anything
> is hard for me with only a drill press, im not
> sure i can manage through that ptfe

If you put the stock rather than the drill into the chuck, concentricity is enforced by the spinning of the stock. Mis-positioning the drill bit would affect the size and uniformity of the bored diameter as the bit flexes towards the axis of the spinning work.
Re: Hotend theory
February 10, 2012 05:44PM
Moving from theory to how to build a hot end is a bit off topic, and I'm sure the opinions vary widely. Like I said above, why bother building something that won't turn out to be as good as something you can buy? I purchased the nozzle, liner and stainless down tube from Makerbot. I fabricated the heater block, but you can buy that too. I'm sure they purchased these parts from someone who is equiped far better then us, and I have a drill press and a Lathe. Look at the parts for the MK5/MK6 extruders.

You can probably find my other posts... but I struggled trying to follow early extruder designs where cut sections out of brass and join them up to PTFE. I even tried wood. Then support the lower end with guide rods that tend to allow the hot end to pull to one side and eventually fall apart. I'm sure someone is getting this to work, for a while

All jamming, stripping, back curling, ect went away when I redesigned it with good quality components. You bring it up to temperature and material always flows smoothly

I'm only extruding PLA and planning to build a second extruder for ABS the main difference being 60 degrees hotter. So I'll see if the same theories apply.
Re: Hotend theory
February 10, 2012 09:54PM
Quote
dean448
why bother building something that won't turn out to be as good as something you can buy?
For "practicality" - agreed - except for the pioneering spirit of "I built it myself".

I'm putting down a bob both ways, with an assembled J-head on order for "Sunday best" - but I'll still have a crack at making my own (probably many - maybe never get it as good ) in my little lo-tech workshop corner of the basement.

I have a relative who bought a T.O.M - it makes *perfect* prints - they are my hold-in-hand target for my prusa - but he's not into the building side - a discussion on the Theory of Hot Ends like this which I find fascinating and spurs me on to dream (literally) and design - he's not interested. And now he's decided it's boring and going to sell his TOM after about 2 months. Very sad.

Thusly why I bother building... smiling smiley
Re: Hotend theory
February 11, 2012 12:03AM
Don't get me wrong I'm all for 'building it yourself'. My entire machine is a mendel design without one printed part, except for two gears on the extruder. But for all you folks who say its easy to build a nozzle with .3 to .4 mm hole please give it a try. Your challenge is to take the smallest pin drill avaliable, drill thru 1mm of brass (assuming you can get to that distance via the blind 3mm hole), without snapping that drill off in the brass, resulting in a scrapped part. You break the drill, you can't recover the part and time to start over. I didn't mention that if you don't get the drill centered in the first place the drill has no chance of surviving.

Costs? McMaster Carr sells #80 pin drills - about $4 each. you will need a holder for the pin drill $10. You will need some way to spin the part, probably a lathe $300?. You might also need to machine threads on the outside of the brass. You will need to buy the brass.

After all that you might have a nozzle that works, congradulations... or not.

Or you just buy a .4 mm ptfe coated and threaded nozzle for $20 that works.

I'm not saying it can't be done...If you pulled this off, and your nozzle has been working for many hours, please tell us how it was built.
Re: Hotend theory
February 11, 2012 02:23AM
Yeah - those tiny drills scare the hell out of me... I bought a pack of 0.4mm, you don't want to sneeze while they're on the bench!
There's 1 or 2 lateral thinking suggestions out there for getting small dia nozzles without drilling at the final size,
but in the end, it may be an interesting exercise in determining one's limits...
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