Hotend theory

I used to buy the bits for $2 at the local hobby shop until he raised the price to $3. I then went looking and found these..
[www.harborfreight.com]

This is the mini-chuck I have..
[www.amazon.com]

So do you use a lathe, or other setup, for drilling .5mm ?

My hot end is 100% reliable finally. It uses a 1/4" OD 1/8" ID PTFE rigid tube inside of a 16mm or 5/8" PEEK rod. The PEEK insulator is 70mm long and has a 50mm brass tube with an ID of 3.5mm threaded 15mm into it. The PTFE tube is pressed in after the brass tube and trimmed flush to the top of the PEEK. the heater block and acorn nut are then threaded onto the other end.

It never jams, it doesn't use a fan, it doesn't leak, it works with PLA.

I've been using it for 3 weeks straight and it shows no sign of fatigue or stress.

The only downside is you need a lathe to make it.



Edited 1 time(s). Last edit at 02/11/2012 06:04PM by aplavins.

nb99 Wrote:
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> So do you use a lathe, or other setup, for
> drilling .5mm ?

I use a lathe now that I have one, but I before then I used my mill as a glorified drill press. On a drill press I'd vice the mini-chuck with a mini-bit and put the work in the spindle.

Yes it is a lot of work to make your own hotend, but I view it as a nice project to increase my fabrication capabilities. Definitely one of those things that will lose the "if you value your time" arguments, but I often enjoy not valuing my time.

aplavins: nice and simple - presumably lathe required for the PEEK forming, which I imagine could be achieved with some careful drill press work..

I assume the length of the assembly mitigates the need for fan cooling
70mm of peek + 50mm of brass + acorn, more than 12 cm of hot-end?
(did you mean "brass tube ... threaded 1.5mm into it." or 15mm )

billyzelsnack:
I note you said "I used to buy the bits for....." so I'm guessing even with practice you still work through bits on a regular basis?

Do you generally need the highest revs available to drill .5mm through brass,
and I guess you feed *real* slow..
lubrication?

I wondered about drilling the last 1mm of the nozzle at a bigger size - maybe 1mm, filling with high-temp solder, shaping that, then drilling the 0.5mm ?
Would the softer material reduce the drilling problems?

@nb99 That's correct, 15mm.

I think with careful setup, like you said, you could build this hotend with a drill press only. The brass rod would be the most difficult. An M6 threaded rod has a solid diameter of only ~5mm and with a 3.5mm hole in it only leaves 0.75mm wall thickness.

Edited 1 time(s). Last edit at 02/11/2012 06:12PM by aplavins.

Here's a tip for drilling small holes in extruder nozzles:

instead of using a pin vice/chuck or mini chuck, hold the drill bit in a pair of pliers with your hands and rest your arms on something solid. I've had more success with this and fewer broken drill bits.

Here's a video link

in the world of 0.5 / 0.3mm drill bits - 0.75mm has become somewhat less daunting

I've just done the Welding tip "machining" for the north90 SS H-E on my drill press, which involves reducing part of the 6mm WTip to 5mm with a file, and a 3.5mm bore. It's all pretty approximate, but the .75mm walls don't appear to be a problem..

I appreciate that over the longer length of a bolt there's a greater chance of wandering off course.

What sort of print height can you get with the long tip... ( not that I've come across anything particularly tall to print as yet)

@nb99: I built my machine about 30% larger so I end up getting over 200mm (I haven't measured exactly) of Z travel. The vertices on my machine are 45cm apart.

> I note you said "I used to buy the bits
> for....." so I'm guessing even with practice
> you still work through bits on a regular basis?

Yep, but less than I used to.

> Do you generally need the highest revs available
> to drill .5mm through brass,
> and I guess you feed *real* slow..
> lubrication?

I think my RPM is around 500. It definitely seems pretty fast with such a tiny bit. Those HF bits are pretty flexible so it does not need to be perfectly aligned, but the closer you can get the better especially if you're going to add a nipple. As for feed, that's a by feel sort of thing.
Here's one I made the other day..
[plus.google.com]
My design is a bit different that other peoples. The nozzle is just press fit into the heater block with a vice.

> I wondered about drilling the last 1mm of the
> nozzle at a bigger size - maybe 1mm, filling with
> high-temp solder, shaping that, then drilling the
> 0.5mm ?
> Would the softer material reduce the drilling
> problems?

What's the highest temp solder you can get? I thought it melted at around the ABS temps.

The tiny drills made me very unhappy. i got 1 nozzle out of 5 drills .
I truly think that the welding nozzle (0.7mm id) with the injection needle insert (0.4mm id) works great and it is truly easy to make. You might need to order a few sizes of needles to find the ones that work the best but it IS much easier than trying to drill a hole.
See: [www.reprap.org]

I have made about 10 hot-ends this way and had no failures or leaking around the needle insert.

Normal solder has an MP of about 220 deg C - which is around or lower than ABS,
but RS sell a 300 deg C high temp solder.

> I have made about 10 hot-ends this way and had no
> failures or leaking around the needle insert.

Does the filament ever tend to curl up as it exits or is that issue completely eliminated due to the nicer interior surface of the needle? That feature alone would tempt me to give it a try.

billyzelsnack Wrote:
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> Does the filament ever tend to curl up as it exits
> or is that issue completely eliminated due to the
> nicer interior surface of the needle? That feature
> alone would tempt me to give it a try.

I have had no problems with extruded filament curling. With my makergear GrooveMount nozzles it sometimes curles so badly that it sticks right back to the nozzle. So the needle insert seems a lot better.

Buy some smaller needles (0.4mm od) as well, they are great for cleaning out the actual nozzle. I use 25GA for this.

Hi!
I am new (noob) to this 3D printing business and I discovered it by chance 2month before Christmas....and I am hooked! Wow, I should say, I absolutely love it.
Thank you to all of you who make up this forum, what a fantastic resource you have created. And I used all the information I could possibly find here to build my first printer
I have tried to put together a blog about what I did during the last couple of weeks (time is hard to come by at the moment). I decided to build a printer that I could make from home and not having to purchase any parts. This way I had to learn. And the hot end is included. I have made several hot ends all with 0.35mm hole and I do not have a lathe. I am not trying to brag here, but if you take care and don't rush, you can do it. Making a working hot end that is.
I was not going to publish my blog yet, but I can't see to find enough time to fill it in with photos and correcting my writing, so I decided to do it now (so you will most likely find errors). Maybe some one can benefit from my hot end design, if not at least it is something to read about.
If you look on my blog, you will see that I made a version of the Wolfstrap. I use rods and not belts, so it is a very slow printer. But it works, and it prints 10-15hrs straight without any problems. My hot end works very well on this printer.
Here is my blog address:

[3dprinting4u.wordpress.com]

As I said before, it is not complete but have a look if you wish. I will try to polish it as I get time, and update with new things as well.

Thanks again for a fantastic forum.

Regards
Jan
(IceMan)

Hi Jan,

That's a very worthy goal! I'm curious to hear how you made the 0.35 mm nozzle. But your blog seems to be marked private and only invited readers can read it at the moment.

Jacob

Hi,
Sorry, I opened up all the pages yesterday except for the main page (noob mistake). Please try now.
My apologies.
Regards
Jan

> @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

In order to drill concentric holes using a drill press, you need to follow these steps...

1. Using the drill bit you intend for the hole, drill into a block of aluminum, and drill a hole for a set screw from the side.
2. Chuck your PTFE rod into the drill press.
3. Set your drill bit in the block, and lock it in with the set screw.
4. Turn on the drill, and bring it down slowly to the bit. Allow the bit to self center.
5. Drill out your rod in increments, and do not let it overheat, as the PTFE will expand a lot.

So having just started reprapping I have already destroyed my first hot end and need to fix it or make a new one or some combination of the two and I would like to talk about the theory some more.

Having had a good long think, my take is possibly slightly controversial (or just wrong) but goes like this.


1. The feedstock HAS to mushroom at some point.

I am assuming that it takes quite a high pressure to feed even quite low viscosity (hot) material through a 0.5 or less hole that is 1.2mm or so long. This pressure HAS to be confined in some way, since we are talking about a 2.8-3mm piece of stock in a 3.2-3.4mm hole (my own current hot end is 2.8mm material in a 3.4mm bore) then it is obviously a lot easier for material to go UP this (in my case) approx 3mm^2 gap than down through (in my case with 0.5 tip) the 0.2mm^2 tip.

So we must have a pool of very hot low viscosity material ready to extrude, being pushed by material that decreases in temperature and increases in viscosity as we move up the barrel. And this material must be acting as a piston to confine the pressure.

So the material is going to mushroom once it reaches a suitable temperature (which I would GUESS is 100-130C) and so create this piston.

The question then becomes: should this crucial piston section be inside a high conductivity (metal) section of the hot end or a low conductivity PTFE section?

My feeling is that it should be in a low conductivity (low friction) section so that the "mushroom piston" is relatively short so having less drag on the walls.

IF (and I realise that its a fairly big if) this sounds sensible, and if we can make a sensible guess at the temperature involved then we should be able to design a hot end with a length such that the temperature drops away in a sensible way to create these conditions as we move up the barrel.


2. Why are we looking at cooling the upper part of the barrel?

We know this works well, and I would suggest that this is because it creates the conditions described above where the "piston" is relatively short, however what we want is to cool the incoming feedstock and prevent heat building up INSIDE the barrel. With (in my case) 6mm of low conductivity PTFE between the feedstock and the cooling this is going to be pretty poor. Obviously thinning the PTFE and putting something more conductive around the outside as many designs do is one solution, but my suggestion is why not just perforate the upper barrel to let air get in (and out) directly to the feedstock itself. As long as this perforation is a sensible distance above the piston then the feedstock should be fine to run "unsupported" for 3 or 4mm (probably more) at a time and heat should have every opportunity to escape.




Anyway, this is my thinking, please point out errors and/or give me guidance on how long these sections might need to be before I go ahead and cannibalise my salvageable hot end and waste my time and effort...

Thanks for reading all this rambling waffle, all input very very welcome.

George

How did you destroy your hot end?

How tight is your temperature transition zone? Some thin the metal so that they can control the location of the cool end of the melt zone.

How does your hotend deal with the issues discussed in [hydraraptor.blogspot.com] ?

Destroyed my hot end due to a design flaw. It was the one supplied with the longboat prusa from the reprapkitstore and unfortunately the parts have been re-jigged and the order changed slightly which puts a huge stress on the M8 thread on the end of the PTFE barrel (see here:- [forums.reprap.org] last few posts cover it). I can bodge it back together close to how it should be but want to make it better too.


I haven't really designed mine yet I wanted to check that I wasn't missing something in my analysis first.
Brass obviously is less than half as thermally conductive as aluminium but then PTFE is virtually zero. So an aluminium heater block and tip with a brass connector and then PTFE barrel might be the ideal, but for now I think I will just move the heater block down to as near the tip as I can and then put holes in the top part of my PTFE barrel to ventilate the inside a little more.

In terms of the design above I dont see stainless steel as much of an insulator, IF my theory about a short mushroom piston of feedstock being ideal is correct, then any metal is likely to be too conductive to get this, so aside from the damage he mentions that may be an issue.

George

@gsport: Your glass transition "mushroom" theory is correct IMO. Most designs use the brass screwed into PTFE design, The problem is that the glass transition zone happens at the junction of the brass and PTFE (because the PTFE is such a great insulator). To further the problem, this junction area is most often the only place where the walls of the bore are less than perfect. If the glass transition were to happen inside the brass would be ideal, if it were to happen in the PTFE you would have to seal the threads very well so that molten plastic wouldn't ooze out but it should still operate.

In my hotend, the glass transition still happens at the brass-PTFE junction but the plug (or piston) is not given the chance to expand into any voids.

Thanks aplavins, that seems logical.

So maybe what we need is a short connecting section between the high conductivity melting section and the low conductivity PTFE section which has a moderate level of conductivity (maybe 20-50 W/m.K) where we can be fairly sure the glass transition is occurring? In which case steel or stainless looks pretty good...

And this section obviously needs to be a very good fit to the melting zone...

George

Edited 1 time(s). Last edit at 03/06/2012 04:11PM by gsport.

The glass transition zone starts well above the junction of the PTFE and the brass. Brass is a much better conductor than PTFE. If you imagine the bottom of the brass is at 240C then the top is probably still over 200C. ABS melts at 105C and goes soft about 90C. The point that is at 90C will be about halfway between the top of the brass and the top of the PTFE.

If you make holes in the PTFE you have to be sure they are well above this point or it will jam solid. Also adding holes will reduce the thermal conductivity of the PTFE, so the area below them may actually get hotter, moving the transition region higher.

When the filament is moving faster the poor conductivity of the PTFE means it doesn't heat as much and the makes the transition move lower, shortening the piston making the force go down. So as speed increases the force reduces to start with and then increases again when it doesn't have time in the hot section to melt fully.

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[www.hydraraptor.blogspot.com]

If you want a short transition zone you need a low thermal conductivity between two high conductivity materials. E.g. stainless steel between aluminium. SS is a reasonable insulator because you can make it very thin compared to say PTFE. Even though it is about 100 times more conductive per unit area you can have a wall thickness of 0.2mm making the area much less than say 6mm, which would be about as thin as you could make PTFE.

Edited 1 time(s). Last edit at 03/07/2012 10:46AM by nophead.

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[www.hydraraptor.blogspot.com]

Thanks nophead, not wishing to be rude, but how do you know where the glass transition zone falls? It would obviously be very useful to be able to see inside or have some other way of measuring it. If I can take this as a solid number then I will probably use it when deciding the position of ventilation holes to the ID.

It seems to me that the PTFE has a thermal conductivity so low that you can almost ignore it. So even if the material inside is at 200C and outside air is at 20C then over the PTFE barrel that I have here (about 20mm of length at 16mm OD exposed to the air) you would only see 4watts of cooling through the PTFE even if the entire thing was full of 200C material from top to bottom. Similarly the heat that could come up through the PTFE from the hot end will be next to zero... so this heat can only really be coming up through the feedstock itself? Perhaps the moisture in the feedstock being converted to steam is also a significant driver of heat transfer upwards?

If the above is reasonable, then it also seems reasonable that any ventilation to the feedstock inside will far outweigh the tiny loss in conductivity (what little it has) of the PTFE. But as you point out, it is obviously vital that the holes are safely above the mushroom of feedstock.

I am not sure I follow your comparison of stainless and PTFE. My original PTFE section had a thickness of 6.5mm but loaded in compression by assembly, which doesn't make a lot of sense design wise, if it were literally just a guide rather than structural it could easily be 3mm or less, a 0.2mm stainless wall is pretty flimsy too and I doubt it would take the loads the PTFE is under if used in the same way.

Another question has to be what is the likely coefficient of friction between the feedstock and the various materials at the range of temperatures involved...


George

gsport Wrote:
-------------------------------------------------------
> Thanks nophead, not wishing to be rude, but how do
> you know where the glass transition zone falls? It
> would obviously be very useful to be able to see
> inside or have some other way of measuring it. If
> I can take this as a solid number then I will
> probably use it when deciding the position of
> ventilation holes to the ID.

If you assume the end of the brass is 200C I don't think you will be far wrong, or you could measure it with a thermocouple inside. You would also need to measure the top of the insulator. That is in the 50 to 60C range in my experience. Then assume the temperature in between is a linear gradient and work out where it will be 80C for ABS and 50C for PLA. It is an approximation, but again I think it won't be far wrong. You could slide a thermocouple through like I did here: [hydraraptor.blogspot.com] although the coupling from PTFE to the thermocouple might be too low.

>
> It seems to me that the PTFE has a therma
> conductivity so low that you can almost ignore it.

No try holding the top in your fingers with the heater on and you won't be able to ignore it for too long (without the extruder body to act as a heatsink)!

The amount of heat that flows is temperature difference times the conductivity multiplied by the area over the length.

> So even if the material inside is at 200C and
> outside air is at 20C then over the PTFE barrel
> that I have here (about 20mm of length at 16mm OD
> exposed to the air) you would only see 4watts of
> cooling through the PTFE even if the entire thing
> was full of 200C material from top to bottom.

4 Watts is quite a lot if you consider how big a heat sink you would need to keep the cold end cool. And regardless of the amount of heat flowing, the temperature gradient will be roughly linear along the length on the inside.

> Similarly the heat that could come up through the
> PTFE from the hot end will be next to zero... so
> this heat can only really be coming up through the
> feedstock itself? Perhaps the moisture in the
> feedstock being converted to steam is also a
> significant driver of heat transfer upwards?

The thermal conductivity of ABS is about the same as PTFE.

>
> If the above is reasonable, then it also seems
> reasonable that any ventilation to the feedstock
> inside will far outweigh the tiny loss in
> conductivity (what little it has) of the PTFE. But
> as you point out, it is obviously vital that the
> holes are safely above the mushroom of feedstock.

If it is forced with a fan then yes, but otherwise I suspect not.

>
> I am not sure I follow your comparison of
> stainless and PTFE. My original PTFE section had a
> thickness of 6.5mm but loaded in compression by
> assembly, which doesn't make a lot of sense design
> wise, if it were literally just a guide rather
> than structural it could easily be 3mm or less, a
> 0.2mm stainless wall is pretty flimsy too and I
> doubt it would take the loads the PTFE is under if
> used in the same way.

A 16mm diameter PTFE tube has been known to swell under the pressure of hot plastic because PTFE creeps and needs support, but a stainless steel tube is rigid even with very thin walls. It takes the extrusion force much better than PTFE does. I have used it very successfully and so now do UP, Makerbot, Reprappro and Stratasys I believe. They all use heatsinks and fans though, which is the downside.



>
> Another question has to be what is the likely
> coefficient of friction between the feedstock and
> the various materials at the range of temperatures
> involved...

I don't know the numbers but PTFE is still very slippery at the these temperatures. With PTFE you can get away with a long transition zone but with PEEK and stainless steel it has to be very short and preferably tapered. Particulary with PLA as that is like glue when it is molten and rubber when past Tg.

>
>
> George

Edited 1 time(s). Last edit at 03/07/2012 01:12PM by nophead.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
-------------------------------------------------------
> A 16mm diameter PTFE tube has been known to swell
> under the pressure of hot plastic because PTFE
> creeps and needs support, but a stainless steel
> tube is rigid even with very thin walls. It takes
> the extrusion force much better than PTFE does. I
> have used it very successfully and so now do UP,
> Makerbot, Reprappro and Stratasys I believe. They
> all use heatsinks and fans though, which is the
> downside.

I have been using the mendel-parts.com V9 extruder since last summer and it has been quite fast and reliable. It doesn't have any PTFE parts either. It has simply a cold end from brass and a hot end from brass and both of these screw into threads in a PEEK block in the middle. I wonder where the belief that you need to have PTFE in the extruder comes from. The plug shouldn't even be forming in the area where this thermal break is (right?). It's just unmelted filament that's passing through there.

There will be a plug formed in the PEEK section as halfway though it it will be at about half the extrusion temperature, which is above the melt point. To get away with just PEEK is has to be a short section (and so requires a heat sink) and preferable tapered. See this design of mine: [hydraraptor.blogspot.com]

I can't recall a hot end without a heatsink that doesn't use PTFE. You don't get a short transition zone without a heatsink, so it needs to be slippery.

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[www.hydraraptor.blogspot.com]