Hi Temp Extruder Barrel

OK

First off I think I need to say that the current design etc is great for what it is intended for and probably does'nt need much if any modification whatever.

Having been doing a bunch of thinking re improving the thermal capabilities of the current to make it more suitable for higher temperature plastics (Maybe even PTFE....)

I have some ideas to contribute and offer up for shooting down.

Objectives.

1. Have a barrel which can produce a higher temperature than the current cheaply whilst still using the PTFE bits upwards of the existing.

2. Use plain uncoated Nichrome heating wire, (Cheep)

3. Improve (may not be possible) thermal/power efficiency, and reduce fire/burn hazards when operated at high temperature.

Trade off's in the current design.

A. Electrically insulating the heating element from the hot part of the barrel whilst making a good thermal contact. (We are using PTFE coated Nichrome, not great thermal conductivity wrapped into the thread of the threaded barrel, improved surface contact and have contacts at each end which get hot but are conducting heat away from the barrel.)

B. The hot barrel is in contact with the PTFE part of the barrel limiting the upper temperature that the hot barrel can be run at before there will be a loss of mechanical properties to the PTFE. (Not a problem for its original purpose of CAPA extrusion due to low working temperature, and is a simple mechanical design)

C. The dome nut being made from conductive material acts as a heat sink and basically gets as hot probably as the barell (I don't know really about this one I haven't got one it is supposition)

A suggested solution or bunch of them to above (some bits of this may challenge kitchen table levels of productive capability)

i. Replace the Dome nut with a Stainless Steel Dome nut (poorer conductivity, keeps the barrel warm and the workpiece cooler, less of a burn risk)

ii. Make a replacement barrel having two heat zones, A hot end nearest the dome nut followed by a thermal break (stainless steel studding connector with a thermal break insert between both barrel segments) followed by a cool end nearest the original spec PTFE barrel. If the cool barrel is made from aluminum with two sizes of aluminum washer stacked on it to from cooling fins this may be enough.

iii. Rework the hot barrel. make the hot barrel also from threaded drilled aluminium instead of the current brass. Anodise the central threaded portion only using Boric Acid to form a v thin insulating layer like that used to make electrolytic capacitors. Place two thin grooves lengthwise at the dome nut end. one to receive and crimp one of the Heating element leads (using uninsulated nichrome) the other to receive one of the thermistor leads using the same method of fixing. Note that using this method the Hot barrel becomes the ground circuit for both the thermistor and the Heating element at the Dome nut end. (better thermal conductivity.

iv. Make conections to the hot barrell at the cooler end of the hot barrell for common Gnd.

v. As ever coat the hot barrel with your favorite thermally and electrically insulating compound and insulate.


Overall this (if the anodization thing flies) will mean that the hottest end of the hot barrel will be the dome nut end. Thermal contact with the heating element will be improved.

Thermal coupling between thermistor barrel and heater at the dome nut end will be as good as it can ever get (metal to metal contact).

The middle section of the hot barrel will go from its coolest furthest away from the dome nut to hotter around the heating element to it's hottest towards the dome nut end. Where it is needed.

The PTFE Barrel section will be protected by the cool finned aluminum barrel segment.

Rest of the extruder borrows from all the brilliant thinking so far.

Thoughts for what they are worth.

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

Just a note...my understanding is that you cannot melt PTFE because it breaks down into some nasty shit and kills you. Drastic over-estimation? Anyone? Give me some feedback on that presumption. I'm to lazy to look it up. Just what I heard.

Demented

I have been thinking alot about improvements on the thermal end of the extruder and have one good idea that I would like to toss out there.

What about taking apart a cheap soldering iron and using the heating elements out of that. Made to go up to high temps and sustain them. Way higher than we need. Also, easier than fiddling with Nichrome wire for those--I raise my hand!--who haven't done so before.

Demented

Demented sir

Soldering Iron element looks like a viable suggestion.

I too have been musing on that one and can see two potential downsides one of which probably isn't.

1. Most soldering iron heater cartridges are mains powered. (Although I did poses once a 12 v soldering iron I got from a model railway shop)

2. in binding your cartridge to your print head your element would be making contact along one side only and the barrel would have a hotter side and a colder side. Heating of your polymer would be uneven and could produce some interesting results (although this one would have to be tried to be sure)

Overall I haven't really come to any firm conclusions on it, this is one to try later for me if other routes fail. (I just want to play with anodizing I think and any old excuse will do)

Further to my original posting here it could be useful to have the heating element wound with a shorter pitch towards the hot end of the barrel and a longer pitch towards the colder end of the hot section to further enhance different temperature zones. A polymer preheat zone if you like followed by a more intense make it liquid (ish) zone just prior to extrusion.

I think this one would be interesting to experiment with and monitor whether it made any difference to extrusion rates etc by measuring drive motor current and rate of production of filament.

It could be simulated by having several close wound heaters spaced along the hot barrel interspersed with thermocouples. With each heater being modulated to give approximations of hot zones.

Umm on PTFE my understanding is that it has a liquid state but this is of fairly high viscosity.

On a scale of Mechanically sound solid to Mechanically unsound and very thick barely liquid there will be a point at which for a given temperature the mechanical properties of the PTFE become insufficient for our needs. ie the threaded barrel will have reduced grip/purchase.

I think of it as like Jelly, at a certain temperature ie freezing It would be considered a solid with solid type sound mechanical properties. At room temperature it looks solid (but wobbles a bit) but is arguably mechanically unsound in comparison to how sound it was at freezing. (I know, this is a very poor explanation, and is a touch contrived to explain how I see it, sorry)

BTW DC, Well done with the RepStrap.

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

I presume the PTFE is to provide a form of thermal insulation from the support bracket. We have used "Gun Gum" as an alternative to JB weld out in the wilds of East London. It's an exhaust sealant mentioned in an earlier thread. As an exhaust sealant it is rated over 1000degC. Our next intention is to mount the heater barrel in a cylindrical mould and form a plug of Gun Gum on the end the same diameter as the PTFE. PTFE is a bit pricey and we have taken to using the remainder for sliding inserts.
This will enable the heater to reach temperatures suitable for most thermoplastics.

The F in PTFE is fluorine which is not nice stuff when liberated in combustion. A bit like the C in PVC.

It occurs to me that if the screw drive extended into the barrel, possibly resulting in a longer barrel, that this might exert higher pressure and prevent the extruder mechanism from melting. The problem is drilling a feed groove next to the hole for the screw, the only sane machining option is to cut a slot along the side of the barrel (and fill it with a wire or encase in an outer tube).

could one of these be fiddled with... they run on batteries - [www.thinkgeek.com]

deadgenome Wrote:
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> could one of these be fiddled with... they run on
> batteries -
> [www.thinkgeek.com]

I have one of those (although I must say, it is pretty useless). They seem to work by creating a spark between two electrodes- you can often clearly see it. So I would think that the heat generated is incidental, and highly localized.

Isn't nickel-chromium wire fairly readily available from toasters and such? Granted, heating elements seem like they would be easier to work with.

Kyle

Ni chrome is indeed very readily and reasonably inexpensively available even with out cannibalizing toasters (although that is for free if the toaster is goosed).

I don't think the issue is really the heating element/method. More how do we adapt a very successful design to be able to handle thermoplastics that have a higher melting point.

For the purposes of discussion we can consider the plastic as being melted if we have reduced it's viscosity to a point where we can extrude it for FDM purposes.

Given this Nichrome is more than adequate for the heater for any Thermoplastic I have ever encountered. (In fact Pottery Kilns use Nichrome Heating elements and a book I have on knife making uses a kiln and Nichrome heating elements for heat treating steel)

Cheers

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

I used to be a member of an amature rocketry group and we used uncoated nicrome wound round small threaded ceramic tubes as igniters... worked pretty well and could be reused quite a few times. Given that these had to put up with the heat coming out the back of a rocket engine, something similar might work pretty well for an extruder nozzle.

Andy sir

I've learned the hard way that reducing a thermoplastic's viscosity isn't enough. You must increase the heat until adhesion is obtained between layers. That is higher than the initial viscosity change point. My plastic melts and extrudes but doesn't adhere. Pain in the ass.

Demented

Thanks for the tip I will bear that in mind.

Anyone know much about ultrasonic welding of plastics ??

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

My welding instructor does. Says the machines cost a bit. Don't know why though...eh.

Demented

I was just wondering if there was any mileage in using Ultrasonic Welding for higher temperature plastics.

Along the lines of reducing the filament diameter to the point where heating and extruding were no longer necessary, Basically dispensing filament and welding it in place ultrasonically.

The whole feed mech could be a touch lighter. actual layer thickness could be governed more by the distance from head to work piece.

The head could be made out of the same threaded rod maybe (different diameter bore) and dome nut but use a ring tranasducer of piezo ceramic material to drive it ultrasonically. I haven't thought this one through yet it is just a freewheel.

The costs as I see them are in the Power drive for the transducer and the transducer itself.

No heating issues just different power levels (to induce heating) to the weld site for differing plastics.

What do you guys think ??

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

I must start by saying I know nothing about the subject, but I think you have to clamp the two items in between a transducer and an anvil. Once you get past the first layer I think you have a problem that there is nothing solid underneath.

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[www.hydraraptor.blogspot.com]

... another idea: - instead of ultrasonic welding you could maybe try with inductive welding?

When you have metallic/ferromagnetic particles in your FDM- or dispenser-paste or coat the topmost slice with a thin layer of iron-dust, then you can apply a high-frequent EM-field, so the embedded metallic particles start to rotate and vibrate.

The vibration of the particles heats only the area around your microantenna, so it should be possibe to apply a small HF-antenna around the needle-tip and heat/weld only the area under/around the extruder-tip ...

Viktor

You could lay the plastic down in an unheated state, then cook it with a concentrated beam of something, laser or maser perhaps.

Actually, if you can turn down the temperature fast enough on the existing extruder design, you've perhaps solved the issue of a support material.

Yup I agree

Turning down heat quickly after fusing is good, thats why I was thinking along the ultrasonic welding lines. (Fraction of a second cycle times according to Wikipedia)

Transducer + Anvil + Pressure.

Hmmmm as the filament being fused onto the existing mass is v thin (0.1 mm + or should that be ++) then I was rather hoping that the existing mass )That which has already been deposited and fused/welded) would act as enough of an anvil to suffice.

Wikipedia suggested a max depth of 25mm before the effect was lost. (Or have I misread this, eminently possible and highly probable)

Transducer, Thought about this some more and thought filament diameter bored smooth (definitely on outside) tube for inner barrel with outer sleeve (dome nut on end) driven by transducer. With the Transducer driven sleeve anchored to the same tool head piece as the inner dispensing tube.

Pressure I was hoping for that which was the space between dome nut and work piece being just smaller than the filament diameter. ergo squeeze = pressure.

With the Transducer driven Dome Nut and Sleeve stamping up and down on the filament ultrasonically and welding/fusing it to that which had already been laid down before it.

God alone knows how anyone would get anywhere near trialling/experimenting with this one, it is intriguing all the same though.

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

Actually...what I meant is you could lay down filament and intentionally not cook it enough AS the support material.

Those horrible packages that you can't get shit out of are put together with ultrasonic welding. There is--I think--like only one or two companies that really makes machines like that. Watched a documentary on it. They not only had an anvil and a transducer but also a horn to funnel and mold the waves into focus. Very complicated. Also, as pointed out above, you need to clamp them.

Demented