Hotend theory 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 https://reprap.org/forum/read.php?1,115865,115865#msg-115865 Sat, 18 Jul 2026 08:31:41 -0400 Phorum 5.2.23 https://reprap.org/forum/read.php?1,115865,194115#msg-194115 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,194115#msg-194115
ASTM D21398 Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer

[www.ineos.com]

And this [plastics.ides.com]

I also found this plain english explanation of "Melt Flow Index" from here...

Cabot Melt Flow Index

One of the properties most often quoted for both natural polymers and masterbatches is Melt Flow Index (MFI). What is MFI and why is it significant ? Melt Flow Index is the output rate (flow) in grammes that occurs in 10 minutes through a standard die of 2.0955 ± 0.0051 mm diameter
and 8.000 ± 0.025mm in length when a fixed pressure is applied to the melt via a piston and a load of total mass of 2.16 kg at a temperature of 190°C (some polymers are measured at a higher temperature, some use different weights and some even different orifice sizes).

Melt Flow Index is an assessment of average molecular mass and is an inverse measure of the melt viscosity; in other words, the higher a MFI, the more polymer flows under test conditions. Knowing the MFI of a polymer is vital to anticipating and controlling its processing. Generally, higher MFI polymers are used in injection moulding, and lower MFI polymers are used with blow moulding or extrusion processes.

And then I found Wikipedia's Melt Flow Index

I've seen numerous posts in the RepRap forums lamenting the absence of data on ABS material properites so I thought I should post these tidbits.I didn't know where to post this info so I dropped it here. This info might already exist elsewhere in the forums but I didn't encounter it.

TC]]> TC General Fri, 22 Mar 2013 21:07:49 -0400 https://reprap.org/forum/read.php?1,115865,150493#msg-150493 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,150493#msg-150493 Andrew Smith General Sun, 26 Aug 2012 06:02:07 -0400 https://reprap.org/forum/read.php?1,115865,150459#msg-150459 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,150459#msg-150459
Motivation
++++++++
I’ve been wondering about the possibility of using infrared semiconductor lasers (or LEDs) as the heat source in the hot end of an extruder. The speed at which the heat can be applied and removed seemed to be appealing particularly with respect to nozzle oozing.

I searched the reprap forums and elsewhere to see if someone had tried to use lasers for ABS (or PLA) filament melting but wasn’t able to find anything definitive, or informative. Hopefully, this post will provide some meaningful data for those interested in the possibility of using lasers in the hot-end of an extruder.

Assumptions
++++++++++
I make the assumption that the filament is contained in a glass tube and the IR laser must heat the ABS filament through the glass.

I’m NOT assuming that using IR lasers as the heat source in an extruder is practical in any way. This is really just a theoretical and experimental exercise to satisfy my curiosity. I’m specifically ignoring application issues such as user safety, cost, construction, component sourcing, weight, etc.

Please feel free to correct any errors I have made.

Energy Formula
++++++++++++
Energy Required = DeltaT * Mass * Specific Heat + Mass * Latent Heat

ABS material properties
++++++++++++++++++
I am deliberately trying to use conservative values so that the calculated energy required to melt ABS will be worst case.

ABS is Acrylonitrile Butadiene Styrene

Specific Heat = 2.1 KJ/Kg-DegreesK = 2.1 J/g-DegreesK
Latent Heat = 0 (ABS is an amorphous material)
Density = 1.25 g/cm3

ABS melt temperature = 105 DegreesC
ABS becomes soft at about 90 DegreesC

Other Parameters
++++++++++++++
Maximum feedrate = 90 mm/s (high)
Extruder diameter = 0.5 mm

NOTE: at a given feed-rate (i.e. 90 mm/s) a larger diameter nozzle extrudes more mass than smaller diameter nozzle in a given time (i.e. 1 second), and therefore requires more energy.

Calculations
++++++++++
Area of Extrusion = PI * D^2 / 4
Area of Extrusion = approximately 0.2 mm^2

Volume of Extrusion / second = Feedrate * Area of Extrusion
Volume of Extrusion / second = 90 mm/s * 0.2 mm^2 * ( 1 cm / 10 mm)^3 = 0.018 cm^3 / s

Mass of Extrusion / second = Volume of Extrusion / second * Density
Mass of Extrusion / second = 0.018 cm^3/s * 1.25 g/cm^3 = 0.0225 g/s

Specific heat * Mass of Extrusion / second = 2.1 J/g-DegreesK * 0.0225 g/s
Specific heat * Mass of Extrusion / second = approximately 0.05 J/s/DegreesK

1 J/s = 1 Watt
A temperature Delta of 1 DegressK = Delta of 1 DegressC

0.05 W/DegreesK… or … 20 DegreesK/W
0.05 W/DegreesC… or … 20 DegreesC/W

90% IR absorption efficiency by ABS
90% IR glass transmission efficiency

90% * 90% = approximately 80% of IR optical power from laser, through glass, into ABS filament

20 DegreesC/W * 80% = 16 DegreesC/W (when filament is behind glass)

@ 90 mm/s feed-rate
++++++++++++++++
If filament is pre-heated before laser heating to melt filament

89 DegreesC pre-heat + 1 W laser @ 16 DegreesC / Watt = 105 DegreesC melt temperature
73 DegreesC pre-heat + 2 W laser @ 16 DegreesC / Watt = 105 DegreesC melt temperature
57 DegreesC pre-heat + 3 W laser @ 16 DegreesC / Watt = 105 DegreesC melt temperature
41 DegreesC pre-heat + 4 W laser @ 16 DegreesC / Watt = 105 DegreesC melt temperature

If filament is at Room Temp (no pre-heating)
25 DegreesC + 5 W laser @ 16 DegreesC / Watt = 105 DegreesC melt temperature

NOTE: Less optical power is needed as slower feed-rates.

Conclusions
++++++++++
The calculated laser power required is dangerous particularly given that an IR laser is invisible.

However, the calculated laser power isn’t so high as to rule out the possibility of using IR lasers to melt ABS filament altogether. It should be practical to do some bench experiments in the lab with the right precautions and safety equipment.

Pre-heating the filament will reduce the laser power required (obvious, but worth noting).

Power decreases with reduced feedrate.

Power decreases with reduced extruder diameter.]]> TC General Sat, 25 Aug 2012 19:25:11 -0400 https://reprap.org/forum/read.php?1,115865,149309#msg-149309 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,149309#msg-149309 peddiparth General Mon, 20 Aug 2012 01:27:22 -0400 https://reprap.org/forum/read.php?1,115865,148889#msg-148889 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,148889#msg-148889 -------------------------------------------------------
> Cameron, please share your PID settings of
> Sprinter so that I can fix my settings.


The PID settings depend heavily on the hotend. I use the PID autotune functionality of Marlin (M303 SXXX). You could even upload Marlin, do the PID autotune, write down the values, then re-upload your Sprinter firmware if you wanted.]]> NewPerfection General Fri, 17 Aug 2012 23:50:17 -0400 https://reprap.org/forum/read.php?1,115865,148885#msg-148885 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,148885#msg-148885 peddiparth General Fri, 17 Aug 2012 23:29:42 -0400 https://reprap.org/forum/read.php?1,115865,148884#msg-148884 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,148884#msg-148884 peddiparth General Fri, 17 Aug 2012 23:28:02 -0400 https://reprap.org/forum/read.php?1,115865,148872#msg-148872 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,148872#msg-148872 -------------------------------------------------------
> Cameron, what is your temperature range when you
> set it to (say) 185C?
> Does your hot end stay at 185 exactly, or does it
> overheat and overcool giving you a temperature
> range?

Using PID control, the overshoot when heating to 200C is only about 2 degrees. After which it maintains 200C +0-1 degrees. So the range is only 199 to 200C during printing, and a max of 202C when first warming up, when I print at 200C.]]> NewPerfection General Fri, 17 Aug 2012 22:35:10 -0400 https://reprap.org/forum/read.php?1,115865,148867#msg-148867 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,148867#msg-148867 Does your hot end stay at 185 exactly, or does it overheat and overcool giving you a temperature range?]]> peddiparth General Fri, 17 Aug 2012 22:30:34 -0400 https://reprap.org/forum/read.php?1,115865,148839#msg-148839 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,148839#msg-148839 NewPerfection General Fri, 17 Aug 2012 20:35:26 -0400 https://reprap.org/forum/read.php?1,115865,148816#msg-148816 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,148816#msg-148816
I am generously using kapton tape around the copper block and aluminum clad 5Ohm resistor, so convection cooling is negligible.
I am going to try dropping the external resistors one at a time and retest. I just wanted to know if there are any other variables I am not looking at in Sprinter configuration.]]> peddiparth General Fri, 17 Aug 2012 18:01:20 -0400 https://reprap.org/forum/read.php?1,115865,148005#msg-148005 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,148005#msg-148005
I don't understand why it needs a second thermistor?]]> nophead General Mon, 13 Aug 2012 17:46:53 -0400 https://reprap.org/forum/read.php?1,115865,147996#msg-147996 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,147996#msg-147996
I think if the heating core was tapped for M6,
then it could be located nearer the nozzle, and
only one M6 locking nut would be needed against
the top; with maybe a serrated washer to lock
against effects of highs/lows in temperatures.

Also, a more balanced core around the nozzle
would make for more even heat distribution with
a second thermistor.]]> Scrachbuilder General Mon, 13 Aug 2012 16:15:14 -0400 https://reprap.org/forum/read.php?1,115865,147908#msg-147908 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,147908#msg-147908 thejollygrimreaper General Mon, 13 Aug 2012 06:45:21 -0400 https://reprap.org/forum/read.php?1,115865,147674#msg-147674 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,147674#msg-147674 I have ordered 5ohm 10watt aluminum encased resistors just in case of this situation.
PWM does not seem to compensate for the actual current flowing thru the resistor causing the overheat issue. Oh well.]]> peddiparth General Sat, 11 Aug 2012 18:27:31 -0400 https://reprap.org/forum/read.php?1,115865,147671#msg-147671 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,147671#msg-147671 nophead General Sat, 11 Aug 2012 17:23:27 -0400 https://reprap.org/forum/read.php?1,115865,147668#msg-147668 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,147668#msg-147668 aplavins General Sat, 11 Aug 2012 17:14:15 -0400 https://reprap.org/forum/read.php?1,115865,147661#msg-147661 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,147661#msg-147661
The first version's design issue was that it was massive. I did not even try heating it up as I used a bolt coupler for Aluminum block. I fixed the issue by switching to copper CPU heatsink.
The second version's problem was that the brass screw was 2.5" long. It was melting too much plastic. Also, I am using a 1Ohm aluminum encased 5Watt resistor. This hotend would heat up to 225 before stopping altogether when set to 185 for PLA.
I had the problem with plastic oozing from the threads of the acorn nut, but I fixed it with high temp gasket silicone.

The resistor now completely failed and is showing 1M ohm resistance.

Does using a 1 Ohm resistor screw up anything in sprinter causing the resistor to heat up too much? Is there anything i can change in Sprinter settings?
I am thinking of trying 1" brass this time with the same length ptfe.


Regards,]]> peddiparth General Sat, 11 Aug 2012 16:47:45 -0400 https://reprap.org/forum/read.php?1,115865,122926#msg-122926 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,122926#msg-122926
I have made the new parts now and went with aluminium for the top section in the end to act as a slight heatsink.

I also moved the heater block down to the very tip to try to help create the optimum temp gradient.

Need to silicone the heater resistor and thermistor back into the block and wait for it to set then will give it a go...

If it doesn't work well then I have plenty of aluminium and brass and stainless already, and some PTFE and tiny tiny (scary scary) drills on the way...


George]]> gsport General Wed, 07 Mar 2012 18:51:12 -0500 https://reprap.org/forum/read.php?1,115865,122891#msg-122891 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,122891#msg-122891 Yes, I think nylon would be OK for the top section.


ttsalo,
PTFE works if it is totally enclosed inside PEEK like the Makergear hot and the J-head, and it dispenses with the need for a heatskink and a fan making the extruder lighter. This is what I am trialling at the moment: -


]]> nophead General Wed, 07 Mar 2012 15:49:46 -0500 https://reprap.org/forum/read.php?1,115865,122880#msg-122880 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,122880#msg-122880 -------------------------------------------------------
> 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]
> xtruder.html
>
> 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.

I see. The V9 extruder has a pretty big aluminum base plate to which the upper end bolts into and a small fan, so that does the heatsink's job. This still sounds like a better idea than using PTFE at all after reading about the durability issues of PTFE at these temperatures and pressures.]]> ttsalo General Wed, 07 Mar 2012 15:27:01 -0500 https://reprap.org/forum/read.php?1,115865,122873#msg-122873 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,122873#msg-122873
I had been under the impression that PTFE was fairly vital in getting the smoothness of movement and low friction we need, but if not then it opens up the possibilities.

It also seems desirable to have as few junctions as possible as these are a point of potential weakness and leakage.

I think I am perhaps over thinking it due to the problems I had with the flawed extruder design I was sent with the kit, having not had any successful prints yet I really just want to make progress now. Until my new bar of PTFE arrives I think I will just bodge this one back together (modifying to eliminate the flaw). Do you think that I can get away with nylon 6 for the top 15mm or so of the barrel (mostly inside the extruder head) as that is all I have available to repair it right now?

George]]> gsport General Wed, 07 Mar 2012 15:00:27 -0500 https://reprap.org/forum/read.php?1,115865,122871#msg-122871 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,122871#msg-122871 nophead General Wed, 07 Mar 2012 14:50:42 -0500 https://reprap.org/forum/read.php?1,115865,122869#msg-122869 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,122869#msg-122869
you might have to allow for a little bit of stretching, but it should give you a fairly good idea.

we need to put all this stuff on the wiki with good diagrams and calculations.]]> se5a General Wed, 07 Mar 2012 14:25:06 -0500 https://reprap.org/forum/read.php?1,115865,122868#msg-122868 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,122868#msg-122868 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.]]> nophead General Wed, 07 Mar 2012 14:12:09 -0500 https://reprap.org/forum/read.php?1,115865,122861#msg-122861 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,122861#msg-122861 -------------------------------------------------------
> 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.]]> ttsalo General Wed, 07 Mar 2012 13:42:25 -0500 https://reprap.org/forum/read.php?1,115865,122856#msg-122856 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,122856#msg-122856 -------------------------------------------------------
> 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]]> nophead General Wed, 07 Mar 2012 13:08:47 -0500 https://reprap.org/forum/read.php?1,115865,122837#msg-122837 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,122837#msg-122837
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 General Wed, 07 Mar 2012 11:21:59 -0500 https://reprap.org/forum/read.php?1,115865,122821#msg-122821 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,122821#msg-122821 nophead General Wed, 07 Mar 2012 09:32:04 -0500 https://reprap.org/forum/read.php?1,115865,122817#msg-122817 Re: Hotend theory https://reprap.org/forum/read.php?1,115865,122817#msg-122817
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.]]> nophead General Wed, 07 Mar 2012 09:19:43 -0500