Welcome! Log In Create A New Profile


Induction Heated Nozzle

Posted by aka47 
Induction Heated Nozzle
August 11, 2010 10:15AM
It should be possible to heat a short nozzle/barrel using an induction coil.

Think of it as a transfromer with the primary being a switched induction coil and the secondary being a single turn short circuit.

The Barrel/Nozzle would be the secondary.

Anyone got any thoughts or ideas on this ??

Necessity hopefully becomes the absentee parent of successfully invented children.
Re: Induction Heated Nozzle
August 11, 2010 01:14PM
I've got two questions...

1. Has anybody done the math as to what kinds of frequencies and currents would be required?
2. What's the advantage compared with using a heating element? I mean I can probably think of a few; for example, you could replace the barrel without having to replace the heating element, making the parts more interchangeable. But what's the main advantage?

Edited 1 time(s). Last edit at 08/11/2010 01:15PM by jbayless.
Re: Induction Heated Nozzle
August 11, 2010 02:12PM
I expect It's not really worth the effort, cost and complexity using induction or any non-direct heating.

Why not just use an Aluminium heated 'bobbin' coil you can screw-on the heater tube, so it could be removed if required.

Direct heating with a resistor, element or wire is always going to be more straightforward especially for DIY projects.

What would be really good would be a way to swap out hot-ends easily so you can change nozzle size without lots of hassle.
Re: Induction Heated Nozzle
August 11, 2010 05:31PM
It's been proposed several times. One hacker went so far as to built a circuit and demonstrated the heating work, though, I don't think he actually tried attaching it to an extruder (I may be mistaken on this part.. google might give you some good search results if you search for reprap inductive heating?). I believe Dr. Boyer's comments to this were in effect -- Nichrome works.. why change it; in addition, he noted high power, hi frequency signals running near sensitive microcontrollers and devices can create problems and issues missing if you were to just use the low frequency DC resistive heating setup.

I also think others also noted that it would be "really cool". I think I'd concur with that opinion even if I also agree with Dr. Boyer that in practical terms, it advantages seem narrow and focused, while having greater disadvantages for general purpose use, as compared to a resistive heating setup.
Re: Induction Heated Nozzle
August 11, 2010 06:19PM
All very good answers/thoughts and I think somewhere back in the mists of forums I have probably said similar.

If it were simply a comparison of heating method my thoughts would probably agree completely.

Switching noise etc is a good reason not to rush into anything and the control is more complex than for pure resistive heaters. Although not so much as to be prohibitive (PWM control is PWM control whether you switch a tuned circuit inductive heater or you switch a resistive heating element). SMPSU technology is pretty much ubiquitous and well understood. The circuit will be pretty similar to fly back SMPSU technology.

So there are some things to iron out, problems to discover and solutions to find. (Thats part of the fun is'nt it ??)

OK what are some benefits:-

1. The nozzle can be made much smaller and therefore have a much lower thermal mass. (Benefits quicker response, lower wattage and a smaller melt zone)

2. The nozzle can be almost completely (except the tip) contained within the insulator/barrel. (Benefits reduced heat loss and lower power consumption, the surrounding mechanical parts can be made from plastics with a lower melting temperature)

3. The heater/nozzle Barrel assembly can be made smaller than with a heater block. (Benefits more print heads in a smaller space each independently controlled, reduced or eliminated cross thermal cross conduction to ease multi material printing)

4. The Heater/Nozzle Barrel assembly can be made with simpler tools and reduced mess coupled with being mechanically simpler giving greater reliability.

5. The induction coil can run at much lower temperatures than the current heater assembly making interconnection and construction much easier and simpler. (high temp magnet wire is relatively inexpensive especially compared to nichrome)

6. No Nichrome required, cheaper construction from more widely available parts (although power resistor based heater blocks are pretty common and cheap)

So I am coming round to the possibility that the benefits are potentially there but not necessarily where you might see them when first looking.

But is this enough to make it desirable to experiment with and prototype solutions ?????

Necessity hopefully becomes the absentee parent of successfully invented children.
Re: Induction Heated Nozzle
August 11, 2010 06:22PM
I did some experimentation with induction heating a while back: [forums.reprap.org] and [forums.reprap.org].

The advantage of Induction heating is that the RF coil does not have to touch the item heated, and that the item heated can get quite a bit hotter than the RF coil. In order for induction heating to be a 'killer ap', it would have to allow a simpler extruder head structure somehow. If you could have a simple copper wire with a few turns over a simple insulator (glass sleeve, kapton tape, air gap???) with the extruder head in the middle, it might be enough to make up for the complexity of the rest of the circuit.

I was not able to get something like this to work well enough to be a 'nichrome killer'. My copper wire (high temperature magnet wire) got hot enough to smoke. Maybe someone else could find a better "sweet spot" in configuration space than I did (I only tried a few configurations and frequencies). I was probably providing way more power than necessary to the extruder barrel.

Re: Induction Heated Nozzle
August 11, 2010 06:27PM

Did you current limit your primary, and what was your nozzle material (brass, steel, stainless ??) ??

Necessity hopefully becomes the absentee parent of successfully invented children.
Re: Induction Heated Nozzle
August 11, 2010 06:55PM

1. The nozzle can be made much smaller and therefore have a much lower thermal mass. (Benefits quicker response, lower wattage and a smaller melt zone)

Fast response and low thermal mass make it harder to control. I have always used relatively high thermal mass and all I need is bang-bang control to make it oscillate about +/-1 or 2 C measured at the thermistor. RepRap and Makerbot moved to low thermal mass with nichrome and Kapton. That requires PID control and everyday there is a problem with it in the Makerbot forums. Wrong tuning parameters, temperature always hanging low, or oscillating, etc. It heats up much faster, e.g. only ~15s, whereas mine takes about 10 times longer. But Makerbot owners need to wait a few minutes before extruding for the plastic to get hot, otherwise it jams the extruder. I don't have to wait, so it is a case of the tortoise and the hare.

The average power is only about 12W, so insignificant in the total power consumption, which is insignificant compared to the price of plastic

So a classic case of KISS. I have a resistor, a block of aluminium, a thermistor and about 5 lines of code in my firmware, a page of Python in my host. It always works, no tuning, no tables, no jams, no failures, and my extruder is actually ready to extrude quicker. Why do anything more complicated?

Re: Induction Heated Nozzle
August 11, 2010 07:03PM
Hmm interesting, just read the links.

Looks like I am v.late to the party.

I was thinking along the lines of 1 amp 12v through 24 turns of primary giving something like 0.5v at 24 amps in the secondary (nozzle).

Which is pretty much what you were doing in your experiment.

For combined Nozzle and Melt chamber (better conduction between components and lower resistance path for those high currents) I was thinking along the lines of making it from an M6 x 16mm Set Screw. Materials to be experimented with until we got one we liked.

Initial thoughts on the magnet wire were 0.5mm sqr (rated at over 1 amp)

Overall aiming for 12watts of heating capability.

The Primary I was thinking of winding onto a PEEK extruder barrel/insulator. over the top of the area that the combined melt chamber/nozzle was screwed into. The Peek rod being about 30mm long and 10mm diameter with a 6*0.8mm indent turned into the hot end of the barrel to help locate the windings. All snug and close up, but well insulated by the PEEK.

That is pretty much as far as I had got with the thinking before creating the topic. All based on some simple rule of thumb electrical transformer theory.

The idea for the PEEK and PTFE Barrell is an adaptation of this one.


Edited 1 time(s). Last edit at 08/11/2010 07:10PM by aka47.

Necessity hopefully becomes the absentee parent of successfully invented children.
Re: Induction Heated Nozzle
August 11, 2010 07:23PM
Hurro nophead.

Your quite right of course, but the fun and cool is in messing with technology.

Necessity hopefully becomes the absentee parent of successfully invented children.
Re: Induction Heated Nozzle
August 11, 2010 09:09PM
It might actually allow for more simply-constructed extruders, if it's done well. But it comes at a cost of more electrical complexity; for example, there might need to be power capacitors to balance the inductance of the circuit.
Re: Induction Heated Nozzle
August 12, 2010 04:19AM

I was thinking along the lines of 1 amp 12v through 24 turns of primary giving something like 0.5v at 24 amps in the secondary (nozzle).

Initial thoughts on the magnet wire were 0.5mm sqr (rated at over 1 amp)

0.5V and 24A means you expect your brass nozzle to have resistance of 0.02R. But the wall of the barrel is relatively thick compared to wire and much wider, and brass is a pretty good conductor, so the resistance of the single turn will be much less than that. So you need to exploit the skin effect by using a high frequency. The problem is though that the skin effect will also occur in your primary and it is a lot longer, so significant heat is produced in it as well.

I think you need to use Litz wire for the primary, but then it gets bulky.

The electronics is usually a pair of push pull MOSFETs driving rectified mains into the coil via a capacitor, with an oscillator chip driving their gates to ensure none overlapping on times and taking care of the high side level translation.

If you only start with 12V, rather than a few hundred, then your losses are a lot more because power is I2R. Not only in the MOSFETs. but also in the primary. I.e. for 10 times the number of turns the resistance goes up 10 fold, the current down ten fold, so power loss in the primary and the MOSFETs is 10 times less.

Re: Induction Heated Nozzle
August 12, 2010 09:41AM
yes those were fag packet sketches and it needs real numbers putting on it.

steel is a poorer conductor than brass but has better megnetic properties.

I need to make a couple of nozzles and meter out the resistance.

As the nozzle is a tube it will present as two equal resistances in parallel. if i place a meter lead at each side of it. The actual resistance of the secondary loop then will be twice what is measured.

resistance in the secondary will increase with increase in temperature too.

idealy we would match the number of turns in the primary to what is needed to give 12 wats in the secondary. given the characteristic of the material.

The losses you describe are more pronounced if working in a non resonant mode. creating a resonant tank circuit encourages higher voltages to run in the tank circuit. A single drive transisgtor can be used too to give a simpler circuit. (kids on swings)

your primary coil inductance and hence your required tank circuit capacitance will be directly influenced by the nozzle material. How much so i am not sure yet.

All in all the design is complex. drive is only marginaly more complex than a resistive drive ( will use micro pwm signal just the same) but mechanicaly is more compact and easier to construct to a fixed recipie.

varying the recipie though will give non optimal results.

Necessity hopefully becomes the absentee parent of successfully invented children.
Re: Induction Heated Nozzle
August 12, 2010 09:56AM
something else i just remembered.

induction coils of whatever type generaly need to be secured. they have a habit of reacting to the forces they are applying and attempting to mive around.

Motor coils etc are usualy dip soaked in a high temp thermoset resin and then baked.

painting on high temp varnish would help lock the coil in place with our design.

looking at the coil you experimented with. the coil was loose and the diameter was larger than i was thinking of. reducing the diameter reduces copper losses in the primary. tidyly packing turns onto an insitu former or bobbin helps too.

in industrial apps the coil is actualy uninsulated air spaced copper rod or pipe. but the power levels and temperatures in use are such that anyother way would not work.

Necessity hopefully becomes the absentee parent of successfully invented children.
Re: Induction Heated Nozzle
August 12, 2010 10:07AM
I may be wrong, but I don't think you will see much resonance with a big fat shorted turn in the middle of your coil. I also think resonance won't help you with primary coil losses. For a given heating current, you have a primary current defined by the turns ratio.

Re: Induction Heated Nozzle
August 12, 2010 12:13PM
That all depends on when you are driving it and by how much.


When cold the resistance for the secondary circuit will be at it's lowest and we will be designing to deliver as close to a full 12 Watts into this as we can. So our turns ration is calculated to convert 12v at around 1 amp (12w) into what ever is the best match to get maximum heating effect out of our brass heater circuit, which just happens to the the secondary in our air cored transformer.

So in cold mode I agree with your statement. The secondary will damp (take out all the energy) the tank circuit to the point it doesn't do anything. It may even introduce a loss.

When hot (or heating) and not running in a resonant mode the resistance of the secondary circuit will have risen and we will no longer be able to drive it at the full wattage we could when cold. There will be a drive surplus. Due to the fact that the increase in temperature has un-matched our matched components.

Here is where the resonant tank circuit makes the difference. Storing some of that drive surplus and recycling it in addition to the next drive pulse.

The currents and voltages circulating in a tank circuit can be quite different to the supply purely because it is an energy storage circuit. We are adding sips of surplus energy together. (Kids on swings)

The voltage in the primary tank circuit will increase and because we are working with a transformer the voltage in the secondary will increase proportionately until at whatever temperature (think resistance) the secondary is running at we are still dumping the full 12w into it. ie it again damps the resonance by taking all of the energy out.

I have purposely ignored a bunch of other losses here to illustrate the action of the resonant components.

This is as I understand it all from my readings etc.

On top of this auto-matching of energy supply we will also be reducing drive once the desired temperature has been achieved.

It guess it really needs some real experimentation though to see if the practice matches up to the principles as they have been understood.

From a construction point of view it is mechanically very simple, from a design perspective it is horribly complex.

Necessity hopefully becomes the absentee parent of successfully invented children.
Re: Induction Heated Nozzle
August 12, 2010 01:30PM
Here's an image by aka47:

On the other hand, my thought is that a rather high-resistance secondary would be ideal, so that we can avoid high frequencies. So almost the entire nozzle could be made of a non-conductor like glass or high-temperature plastic, with a thin ring of stainless steel embedded inside to absorb the eddy currents.
Re: Induction Heated Nozzle
August 26, 2010 06:28AM
Re: Induction Heated Nozzle
October 13, 2010 01:32PM
By the way, Fluxtrol offers some good training videos introducing the principles of induction heating.


I contacted them about having a thin-walled stainless steel ring inside a glass tube. Here's my message:

Dear Fluxtrol,

I am a senior engineering student at the university of British Columbia, and leading a student design team to develop an improved 3D printer. I'd like to use inductive heating, but I haven't learned about it at university, and I'm in need of some expert advice. My question is probably a bit unusual, but maybe you will find it an interesting problem.

So in our 3D printer, we build a model up one layer at a time by melting a plastic rod and extruding it through a fine nozzle. Normally the plastic is pushed into a resistively-heated metal tube, with a fine hole drilled at the end. However, it is important that the plastic transitions from solid to liquid over as short a distance as possible, and so a metal tube is not ideal - its high temperature conductivity makes it hard to make a steep thermal gradient.

I am investigating the use of a glass tube instead. We can easily draw it to a fine nozzle at one end, and take advantage of the low thermal conductivity of glass to achieve a steep temperature gradient. The problem is the resistive heater: It is difficult to transfer heat from the outside of the tube to the inside, because of the thermal resistance. A high heat flow is needed, so the temperature of the heating element must become exceptionally high - wrapping a heating element around the outside poses a problem. The thermal resistivity of the glass therefore both an attraction and a problem: it's good for maintaining a steep temperature gradient axially along the tube, but terrible for allowing heat to flow radially into the tube center from the outside.

That's why I would like to use an inductive heating system. I picture slipping a thin ring of resistive metal, such as stainless steel, into the glass nozzle. Then it can be wirelessly heated by a copper coil wound onto the glass nozzle. The ring will in direct contact with the plastic, and the thermal resistance of the glass will now only be an advantage - insulating the heater radially and axially.

The plastic only needs to be heated to 260 degrees Celsius to melt, which will require four to eight watts, depending on the flow rate. It's a 3 mm diameter plastic filament, so the metal ring should have an outer diameter of no more than 3.2 mm and as small a wall thickness as possible. We would like to drive the coil from a 12 volt AC power source, and the frequency should not be so high that it requires expensive electronics.

Beyond that, I am not sure about the details required. If I want a stainless ring of only 0.1 to 0.2 mm wall thickness, do I need to be concerned about the skin depth? Will I have a very low "K" factor unless I use a high frequency? Do you have any concerns about my design that I should be aware of?

Thank you very much for your help!


And here is their response:


Hi Jacob,

Thank you for your interest in Fluxtrol Inc.

Unfortunately, use of 12 V at 60 Hz will result in almost 0% efficiency for this type of load. The desired frequency range for the part you are trying to heat is 500 - 1000 kHz. In this frequency range, this could be a very nice application of induction heating and efficiencies should be in excess of 80%. For such a low power , the circuitry could be relatively simple and inexpensive. I would suggest you recruit some EE's to work on the project with you.
Re: Induction Heated Nozzle
October 13, 2010 02:26PM
Jacob, great work!

Getting inductive coupling to work at this kind of frequency probably means a variable frequency switching power supply, rather like the switching supply inside our computers. I believe that the power MOSFETS we use for heater control can be switched at these frequencies. By finding the most efficient coupling frequency, you then control the temperature by increasing (or decreasing, not sure which is best) of the 50% duty cycle of the MOSFET. As the frequency goes above optimal, the coupling efficiency drops, and the inductive heating element cools. I believe that as the frequency gets higher. the inductive load of the drive coil prevents the MOSFET from passing very much current, so the total power dissipated by the heater drive system stays low and efficient. This then becomes a frequency controlled cycle, rather than a PWM controlled cycle, but otherwise the same hardware and software. The ATMegas, running at 20Mhz, should be able to generate square waves up 10Mhz (big jumps in frequency near the end, though) and external circuits considerably higher.

This definitely bears looking into!


copper tubing and bolt heater

AC-AC controlle dinduction heating

Edited 1 time(s). Last edit at 10/13/2010 02:35PM by rocket_scientist.
Re: Induction Heated Nozzle
October 13, 2010 08:13PM
Sorry to double post, but I just realized I have a question. When inductively heating a piece of stainless steel tubing in the end of the glass nozzle, won't it be difficult to measure the temperature? Whether we use a thermistor or a thermocouple, the device and it's wiring will still be within the magnetic field of the inductive heater, and that may swamp the measurement values. The thermistor or thermocouple wires will be directly heated by the high frequency magnetic field, and worse, will generate eddy currents that are much larger than the current needed to measure the temperature. Or does the coupling efficiency go down with the diameter of the metal? So that a narrow gauge thermocouple will receive very little direct heating or eddy current and still have the millivolt output virtually unaffected by the induction coil?

We may have to use infrared non-contact temperature readings to control the induction heater. Since glass is opaque to far infrared and transparent to near infrared, a non-contact IR thermometer should be able to read the temperature of the glass (if based on far IR) or better yet the temperature of the plastic and metal inside the glass (if based on near IR) and use that to provide feedback control of the induction heater.

Re: Induction Heated Nozzle
October 13, 2010 09:51PM
Hey Mike,

That's right, we can't use a thermocouple or a thermistor - although the reason I was thinking of was that having a heat source inside the insulating glass and a sensor outside the glass would be very bad, because there'd be long delays between the input and sensor output.

I see two ways to resolve this: Use a metal with a high temperature coefficient as the heating element, and measure changes in its resistance (quite difficult I think). Or use an IR thermometer. Use of the glass nozzle opens up near-spectrum IR thermometry as a very good option, because we'll get temperature readings from the inside of the nozzle, rather than the outside, so the sensor response will be very fast.

This is all becoming quite exciting. =)
Re: Induction Heated Nozzle
October 14, 2010 12:26AM
Just the words "thermometer" and "glass" got me thinking about an actual spirit thermometer. Maybe there is a way to use an optical switch like our endstops, clamped to a thermometer so that when the liquid in the thermometer rises it will block the light and trigger the opto. the switch would be far away from the actual heating zone.

of course, it would be much harder to actually implement than it was to type. and it probably look ugly and hacked together.

Edited 1 time(s). Last edit at 10/14/2010 12:34AM by Buback.
Re: Induction Heated Nozzle
October 14, 2010 02:17AM
Yes... that would probably be tough. winking smiley

I hope that a 1 MHz coil running several amps will not emit too much electromagnetic radiation and interfere with the other electronics.
Re: Induction Heated Nozzle
October 14, 2010 09:15PM
I have been looking for an IR sensor chip that could be directly connected to the controlling micro. The few that I found did not go up to 250C. And the prices were looking kind of high. I also looked for a complete digital thermometer that had a USB connection. Hundreds of dollars! It looks like the cheapest way to go is to buy a Harbor Freight digital IR thermometer and take it apart and find the analog signal that it converts to the digital display. The cheapest one, $10, half the cost of the cheapest single chip sensor I could find, only goes up to 110C. But the $20 and $25 ones go much higher, up to 520C. So I think I will try one of those to see how easy it is to 'hack'. Also see what it reads on a glass nozzle. All the ones I have found talk about sensitivity ranges of 5 to 17 microns. In that range, glass is opaque, so we would be reading the temperature of the glass, not the plastic. But it makes sense. Below 1000C, the amount of black body radiation in the 1-4 micron range is extremely small, and nearly flat. It would take a chilled detector, or a good light/dark chopper to read anything in that range, and lots of signal processing to guess at wavelength peak which is the best way to measure temperature. The cheap meters only read total energy, and assume an emissivity of 0.95. That means that they can easily be thrown off by reflected sunlight or bright lights.

Using a non-contact thermometer solves one of the problems of trying to get something inside the glass without messing up the smooth lines and surfaces that make building a nozzle from glass so attractive in the first place. Putting the stainless steel sleeve inside may still mess things up, but it puts the heat were it needs to be, inside the glass. This may be an interesting experiment when I get the time. I will need an oscillator setup to to drive the induction heater. And I need to experiment with that just to find a good frequency and power supply voltage to get the S.S. hot enough.

Re: Induction Heated Nozzle
October 15, 2010 01:45AM
Hey Mike,

I double-checked and got the same numbers as you do. I used this as the absorption spectrum data of borosilicate glass:

And calculated the peak radiation wavelength using Wein's Law. For a heater element at 260 C, the peak wavelength is 5.4 micrometres.

So yeah, seems like you're spot on.

In that case, IR sensing might not be the best possible solution, although still probably a good one. Going "sensorless" and using the temperature coefficient of the metal ring might be better after all... Although it would take some very intelligent electrical design, it probably wouldn't be very expensive to reproduce.

Edited 2 time(s). Last edit at 10/15/2010 01:51AM by jbayless.
Re: Induction Heated Nozzle
October 15, 2010 03:59AM
... maybe you can use lowtemp melting alloys ("Roses metal" @98°C, "Fields/Woods metal" @71°C) and so lower needed temperature, so you can go away with 'conventional' temp-sensors embedded in the glass?

Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]
Re: Induction Heated Nozzle
October 15, 2010 03:07PM
actually, we are going in the opposite direction. We are not trying to melt the Stainless Steel, but simply heat it to a constant temperature to melt the plastic. ABS is what is setting the limits. And a conventional temperature measurement in an induction heater is likely to be self heated more than contact heated, rendering it useless. Thus IR non-contact thermometers working from a distance outside the magnetic fields.

Re: Induction Heated Nozzle
October 15, 2010 03:35PM
... another crasy idea - what's with an embedded glass-fiber, that changes some optical properties with temperature?

Maybe an interference-sensor, that changes reflected light from a laserdiode with thermal elongating of the light path?

Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]
Re: Induction Heated Nozzle
October 15, 2010 03:38PM
If the temperature sensor is small and thin enough, and the frequency is low enough, it might be magnetically transparent. But there's still the problem of getting leads out from inside the nozzle, unless you're measuring on the surface, in which case there's a large resistance between the heater and the sensor, which is bad.

Although induction heating in a glass nozzle would be a very elegant way of melting metal for the purposes of extrusion. A higher power would be needed, but if the frequency is very high, it could be melted in small amounts without as much of a problem of thermal conduction through the melting metal.

Rather than a 3mm rod, a very fine wire would be needed.

I wonder what metals would be able to melt inside a glass nozzle, before the nozzle itself melts?
Sorry, only registered users may post in this forum.

Click here to login