Glass nozzles for extruders

But the tube looks to be about 50mm long. You want the transition zone to be ~5mm so you would need to put the heatsink close to t the bottom.

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[www.hydraraptor.blogspot.com]

Oh... yes... let's see. I would be expecting a fairly linear temperature gradient along the tube. Assuming that the hot end is at 260C and the base is at 40 C, then the temperature should drop by 4.4 degrees C per millimeter. 5 mm up the tube the temperature will be 240 C, which is still hot.

My main concern about a heat sink would be that it would draw a lot of power from the heater. But maybe glass is a good enough insulator that it's not the case. Let's see...

The cross-sectional area of a 6mm tube is 21 mm^2, and pyrex has a heat conductivity of 1 W/mK. So only 0.1 watts of heat should be conducting up the glass tube in this circumstance, with no heat sink, which is pretty minimal. Given that, perhaps my assumption of a linear temperature gradient is not correct? Perhaps much more heat will be lost to convection than will actually conduct up the tube. I'll look at that when I get home and can check my convection formula.

Anyway, it's the most pessimistic case, so I'll assume it's okay for now.

If I add a heat sink 20 mm up the tube, let's say the temperature there is 40 C, and the hot end is still at 260 C. The temperature gradient is 11 degrees C per millimeter. Then 0.24 watts conducts up the glass, which should be relatively easy for a heat sink at 40 C to dissipate. 5 mm up from the heater, the temperature will be 205 C. 1 cm up from the heater, the temperature will be 150 C. That might be a short enough transition zone?

If I move the heat sink to just 1 cm up from the heater, the heat sink will probably be getting pretty warm. I'll consider three cases. For a heat sink at 40 C, the temperature gradient will be 22 C/mm, which means 0.47 watts. For 60 C, it's 20 C/mm, so 0.43 watts. For 80 C, 18 C/mm, and 0.38 watts. (I think it should be easy for a heat sink at 80 C to dissipate 0.38 watts easily, so it's unlikely to get this hot).

The worst case is the last one. 5 mm up the nozzle the temperature is already just 170 C. And the additional power draw is minimal.

Okay, I'm convinced. A heat sink on the tube is a good idea.

... as long as the temperature gradient doesn't break the glass.

The best extruder I have made uses stainless steel with a transition zone of about 6mm. SS is about 17 times better conductor than glass, but being stronger, can be thinner walled (0.5mm). Still glass should lose less energy as it isn't 17 times thicker.

The amount of energy lost by my stainless steel one is actually not significant. It use about half the power used by my Mendel extruder because it has a smaller heater block that is well insulated. Perhaps with the glass one you can have an even shorter transition.

Another thing that makes a big difference is having a taper in the transition zone. Not sure how you would do that in glass, but perhaps you can make the transition zone short enough that it isn't needed.

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[www.hydraraptor.blogspot.com]

How do you propose connecting the heater block to the glass? You need to make good thermal contact.

I still like just wrapping the nichrome wire around the glass for heating.

Nophead,
thermal stress is a critical concern with glass, but the borosilicate has ore strength and a lower coefficient of thermal expansion. While making them, I have one end glowing orange to yellow hot (working temp is around 1250C) and 5cm away I am holding it in my hand at 37C, so a gradient 24C per mm, 5 times the gradient that Jacob is working with. According to Camglassblowing, the first critical temperature for boro is 150C, below that thermal fracturing is not a concern. They list 500C as the point where you have to be careful of thermal shock, and 820C as the point at which it can begin to soften. So I think we can place the cooling fin(s) as close as we want with ABS. I will try another build with a less than1 cm heater, and a copper washer just beyond that to see if it decreases the force needed to push the filament in.

Jacob,
have you tried pushing filament into the 6mm thin wall tubing yet? Mine just a hair to thick to fit.

Mike

Mike - I haven't yet, because all my glass stuff is at home and all my reprap stuff is at the university. I'll bring them in next week.

I'm glad that the borosilicate will be able to bear the temperature gradient. It was something that I wasn't sure of, even though I know that it's made to support steep gradients.

Nichrome should work fine too, and it'd be a bit more compact. It's just a bit annoying to replace when something goes wrong and the extruder needs to be taken apart.

Greg Frost - I plan to apply thermal paste, and use something along the lines of a split-ring clamp. I hope that should make good thermal contact.

Nophead - a taper in the transition zone - you mean that the barrel gets gradually wider? That would be my goal with a PTFE inner lining, although I'm just guessing with that one.

Edited 1 time(s). Last edit at 09/11/2010 12:41AM by jbayless.

Yes if it tapers wider at the transition a plug can't stick to the sides, it works like a mould draft.

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[www.hydraraptor.blogspot.com]

Shame there is no easy way to imprint a spiral onto the outside of the tip to wind the uninsulated nichrome into.

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Necessity hopefully becomes the absentee parent of successfully invented children.

This article talks about all the ways they make threads in glass:

[www.allbusiness.com]

You may be right about no "easy way" though.

Looks a bit tough for my garage setup. Yes, I REALLY wish that a spiral could be cut in the glass for the nichrome. Then, the out tube of glass would easily fit over the wire. Same with making a slot for the thermocouple. I have been playing with that all day. Mostly created thermal stress cracks using cold tools, and warping the shape of the tip.

A small amount of expansion at the top of the heating zone can be done with some glass blowing. I will try to add that tot he list of short heating zone vs long. I think short is best for brass, but the very insulating properties of glass that we want to separate the hot zone from the cold with one single material will also make it hard to heat the plastic quickly to the plastic point. I think for glass a LONG heating zone will give the bet results, but there may be a trade off between to long to avoid plugging and too short to avoid low temps in the orifice.

Mike

Hmm it should be possible to wind/fuse a thin filament of glass onto the outside of the tube making a spiral on it rather indented into it.

It may be possible to make anchor points (little loops) at the same time.

I don't fancy your chances of maintaining dimensional stability though while doing it. Unless perhpas the filament was fine enough.

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Necessity hopefully becomes the absentee parent of successfully invented children.

No, the nichrome wire burns away before the glass even gets soft enough. And if you heat the glass up high enough to kepe it warm for a few seconds, it is so soft that the pull of the wire bends the whole end. I am looking into using tungsten wire, but it is 20 times more conductive then nichrome, so must either use a very thin wire, long length, or switch to high current low voltage heater (40A at 0.5V?). I have also been thinking embedding steel, or perhaps tungsten wire bits to guide the nichrome into either spirals or lengthwise zig-zags. The way if I can seal the whole thing in a second glass tube, there is no Kapton tape to melt.

Mike

Umm I was'nt suggesting casting the nichrome into the glass rod.

I was suggesting fusing a spiral (Glass filament) onto the glass tube with some anchor points that you could wind uninsulated nichrome onto after it had cooled. Then secure the nichrome and wire take offs with the anchor points you have made from glass.

Winding insulated nichrome onto the glass tube just in case it shorts is compounding the fact that the glass is a poor conductor. Glass plus insulator, bad.

Creating a glass tube that you can wind nichrome on to and it won't short but make good contact with the glass was the aim. Glass in direct contact with nichrome, better.

Edited 5 time(s). Last edit at 09/12/2010 07:09PM by aka47.

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Necessity hopefully becomes the absentee parent of successfully invented children.

Ah, I completely missed the point, aka47, thanks for explaining in detail. Yes, an additive spiral of glass is the easiest of the four or five methods I was thinking of. My only current problem is that no one seems to sell borosilicate stringers, very thin glass rods. 3mm seems to be the smallest, and that is too coarse for making the spiral. I will have to work on stretching and thinning the 3mm down about 0.5mm. That way the walls of the spiral will only be two or three times as high and thick as the wire, and a tight spiral can be possible. With practice, of course!. This sounds promising, so I will try it as soon as I can.

Once again, if a tape and insulation free nichrome and thermocouple installation can be done, then a glass cover for protection and thermal insulation can be added on top.

Mike

rocket_scientist Wrote:
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> Once again, if a tape and insulation free
> nichrome and thermocouple installation can be
> done, then a glass cover for protection and
> thermal insulation can be added on top.

Have you considered something like Ceramic glue?

I've been looking into this myself to cover nichrome on standard metal extruder nozzles. The one I found that seems most promising is the Resbond 940 series. One of the things that pointed me in this direction is that it is used in many heating elements to cover nichrome wire elements, and has a max temp in the range of 2000F-3000F (all higher than we're going) depending on which one you use.

They're not cheap though. Sure you won't use a lot, but you'd want to make a large number to even the cost out, otherwise it'll be hugely expensive - way more than the nozzles.

Note: Some of them are thermally conductive, some not so much. Also some are more electrically conductive than others, but most are insulating. I would however suggest that if you do go down this route, you try and get something that has a very similar thermal expansion gradient to the glass. That way, the glue will hopefully expand at the same rate as the glass, and not cause it to shatter.

would you need borosilicate stringers? I suspect that such small strings would not have a problem with thermal stress, so any stringer would do.

Once you put stringers on though, you wont be able to insulate with an 8mm ID glass tube.

Edited 1 time(s). Last edit at 09/13/2010 06:32AM by Greg Frost.

Yes, I just ordered some larger outside tubing.

As to using stringers of soft glass mixed with boro, they strongly recommend against mixing even slightly different Coefficient of Expansion glass types in fusing and slumping. They talk about doing test runs even within a single brand in case a small difference not mentioned in the spec causes a problem. But it is possible that with the much small stringers that we can put COE 104 stringers on COE 33 borosilicate glass.

I have only briefly looked at the high temp adhesives from Mcmaster-Carr because they get real pricey. If people start deciding they want to switch their brass/PTFE hot ends for glass nozzles in large numbers, and we don't get a commercial supplier of simple glass nozzles with cheaper prices for greater quantaties of more precise nozzles and orifices, then I will see about buying a bucket of the expensive stuff, and knocking out several dozen fully complete heater sections.

Mike

Speaking of commercial suppliers, today I biked down to CanSci glass in Richmond. They very generously offered to make me some free extruder samples to test out. The quality is really fantastic:




It looks like they've used a similar technique to Mike's; first creating a taper and then using a diamond file or something to grind down the tip to the desired diameter.

Edited 3 time(s). Last edit at 09/13/2010 05:37PM by jbayless.

We have Cotronics Resbond 907 Ceramic Adhesive available in small quantities on the MakerGear HeatCore page. It should work fine for this application.

Thanks for pointing that out, Rick. Work won't let me complete the checkout process, so I will order from home. I will pick up a few color variations in ABS, and some PLA and HDPE to see how they work in the glass nozzles compared to ABS. I have also been thinking bonding together a few pieces of different intrinsic colors and run it through a glass nozzle with easy view between the heater wires an see how the plastic melts and flows at the constriction near the tip. With the USB microscope we might get a good view of the small scale effects.

Jacob, they DO look great! By the photo, I am guessing the left one is nearly down 0.1mm opening, the middle one somewhere around 0.25, and the one on the right is 0.5mm. Those seem to be the popular sizes. I am interested in hearing how they due, and how expensive they will be.

Mike

Edited 1 time(s). Last edit at 09/13/2010 07:32PM by rocket_scientist.

My cheap plastic Staples ruler might not be too trustworthy, but those are probably good guesses. I can't find my nice precision ruler anywhere. Yeah, they haven't determined a price yet, so I'm also curious.

New experiments. A fresh shipment of new glass rods has added some new possibilities. Even larger outside tube can fit over the 8mm nozzle with some stuff on the outside. 7mm thin wall to test later for passing the 3mm filament better than the 6 which is just too tight a fit. And capillary rod with a really tiny whole down the center.

I thought first of using the capillary rod to surround the thermocouple wire, but it is too thin for that. In fact, it just barely fits the nichrome wire. So I started to experiment with the suggestion of spiral wires spaced with glass bumps. It mostly works, but still has the problem that the nichrome wire is not well bonded to the glass.

You can also see that I eventually caused too much thermal stress by reheating to add more bumps while protecting the wire. Once I had everything wound, I tried to insert it into the new, large over tube. But the bumps are of varying sizes and some were to high to get the tube over. Why trying to fit it, the tip cracked from thermal stress. More work is clearly needed before something usable will come this way.


But the spiral of the wire gave me an idea. I cut a short section of capillary wire and a long section of nichrome. With some more practice needed, I could draw some fairly long covered pieces before the wire would get too hot in one spot and melt. I made several partial nozzles and tried adding spirals of glass covered nichrome wire. None were great, but I think one may work.

A mottly assortment assortment, but then it is hard to both thin the glass without burning through the wire and then heat the nozzle and wire glass just enough to bond without making the nozzle bend in every direction, or melt through during the second phase.


This one ended up prettier than most, but the nozzle end melted off so finding enough of the wire to make an electrical connection will be tough. But the spirals are getting better.


This one does not look as pretty as the previous, but it has the advantage that both ends of the wire are exposed and easy to attach. I used 4 gauge, so even the short length is 7 ohms, which is doable.

I still have not added the thermocouple to the heating wire. But the current stuff is clear enough to see the plastic melting and flowing inside. I have other work to do this weekend, so it will be awhile before more results are posted.

Mike

I stayed up late last night and ran a test on the good one. There is no wire wrapped around near the tip, so it took a LONG time and way over powered to get enough heat to the tip to melt the plastic and produce a strand. But once the pre-melted plastic reached the tip, it flowed fairly well. After getting the tip unstuck from some projection, the strand came out long, thin, and straight. Yank testing shows it is quite strong. I was not ay my most observant, but it seems fairly smooth. I just checked the width with the pocket microscope, and the strand is 0.6mm thick, which is probably only slightly wider then the nozzle orifice.


EDIT to avoid triple posting on my own thread!

Rick, MakeGear was both fast and efficient. I have 3 pots of the high temp adhesive, and a new idea on how to use them, an ABS filament color assortment that I want to fuse together into a short length of many colors to watch it flow through a clear glass nozzle, and a pound each of PLA and HPDE so I can answer questions about how they work in glass.

Currently I am experimenting with encasing the nichrome in glass, stretching it very thin and melting it onto the nozzle, which should improve heat transfer when I use a thicker wire that will require a longer length and tighter spiral. The tight spiral will have more cross-sectional area transferring heat to the glass then plastic. The first one is great for matching the melt process since the wires are so widely spaced, but poor for fast, even heating. But even the much tighter spiral I intend to make soon will still have trouble with even heating.

So it is time to compromise. From a fluid dynamics viewpoint, the single material from cold mount to heater to nozzle is best, and that requires the smooth inside lines, strength, and insulating qualities of glass. But from a thermodynamics viewpoint, winding thin strands of wire, spaced out along a insulator like glass in order to get the heat in to the plastic is not working well. In that respect, putting metal inside the heater zone and nozzle would work best. Now, from the comments about returning to enamel power resistors in a block of aluminium, I have come up with a compromise of my own.

I will use the high temp adhesive to wind the spiral of nichrome wire, then use a little more to bond a piece of copper tubing, intentionally oversized, and crimped down its length to press against the nichrome and glass and wrap all the way down to the nozzle tip. That way the copper will carry the heat everywhere, and fairly evenly too, but still not be as bulky as the power resistors and aluminium blocks. The crimp may even provide a tunnel for the thermocouple wires. I still want to make a hole in the side of the nozzle, put the tip of the thermocouple in and weld if shut with glass, then apply the first layer of adhesive and spiral the nichrome around the thermocouple wire sticking straight out. Then, once that has set, add the second layer, slide a pre-shaped piece of copper tuning over, tighten the crimp and let it set (or heat to set the adhesive). Then, I should have an even if not fast heating, control the temperature of the plastic not the glass or metal, have clean, smooth flowing lines for the filament and melt to compress and exit through the nozzle, and a slim, solid package that should be very stable and reliable. That is the hope anyway.

Now off the fix rental properties.
Mike

Edited 1 time(s). Last edit at 09/17/2010 02:04PM by rocket_scientist.

Couple of notes on your glass nozzle project. First off a word of caution, flame working borosilicate tubing can be very dangerous, particularly to the one thing that's hard to replace, your eyeballs. The sodium flare given off by working boro is enough to damage your eyes without protection. Get yourself some filtered glasses. Clear boro isnt quite as bad as colored, so you can use the cheaper filters for working "soft" glass. Find these at glass supply places such as Delphi creativity, wale apparatus, arrow springs, and many others. The other benefit is that with the flare removed you can see heat in the glass much better and tell when it is able to be moved. Welding shade 3 or 4 can be subbed in a pinch, and are better than nothing, but won't give you the correct sodium spectrum notch that helps gauge heat.
Working boro in the way that you do has other consequences, namely that you are not annealing your pieces (annealing in the flame does not give the same results) , and leaving them with high stress levels. You are then going to heat them and physically stress them by using them as an extruder nozzle. This is dangerous. The stresses built in can cause the part to shatter suddenly and without warning. Propper treatment of boro glass involves holding it at ~1050F for a time dependant on thickness, and then slowly reducing temperature in a kiln over sever hours.
When working with scientific glasswares a technique often used is the glass lathe. The work is held between a pair of chucks and spun slowly and evenly (which is the key to all glass work). You can move the chucks closer together or further apart to draw or bulge the glass. Although you don't want to know what the real one costs, a stepper, and a couple of bearings, perhaps some old drill chucks and you could be in business. Makes things far more repeatable.
Most glassworkers use propane/oxy by the way. I have a specific glass working torch that I use which is designed for oxy/propane. Mapp isn't worth the added cost, but it does work for "soft" glass on it's own with no oxy. Beginners kits are sold this way for beading.

#3 instead of #5 clip ones, because they were out of #5 and she said only people who spend all day at their torches need #5. And yes, it does make it easier to see the glass temp. I use oxy-acetylene because I already had a used torch. I have looked at hard-glass torches, and they start a a few hundred bucks and go up. One I priced out including pre-mixer, stand, 3 basic interchangeable tips, oxygen and propane spark arresters, propane safe fuel and oxygen lines, propane safe fuel and oxygen regulators, and it was about $500 without tanks or fuel! Yes, oxy-acetylene is more expensive and overly hot, but cheaper in the short term. Other tools start adding up quickly too.

I have insulating batting, which helps some with the annealing. Once I make parts with insulated wires or high temperature adhesives, proper annealing becomes impossible anyway. For the simple nozzles, I try a quick and dirty anneal by evenly heating the whole piece, then wrapping it in thermal insulation. But you are correct, the best is kiln controlled annealing for hours or days for all glass parts. I would also love a glass lathe, and had thought that steppers would do quite well for my needs. Also, triple or quad flames all pointed at one spot and evenly spaces around the circle help with the same problem.

I this becomes yet another very expensive hobby, or if it becomes mainstream reprap extruder tech and nobody else want to touch it, I will probably get the propane-oxy torch, some marvers, buy or make a kiln, etc. But for now, I am changing the designs radically as I think of new ideas, and the temperature we normally work at does is low enough to rarely stimulate the built in thermal stress. Most parts crack in my hands, not after delivery.

Mike

You just hadn't mentioned safety gear before, I feel it's important that others not familiar with this stuff know the right precautions to take if they decide to try it for themselves. Don't mean to come off as a safety Nazi.
I have the torch, kiln, and a good stock of boro, as well as a couple lathes/spindles that I could press into some use. Not set up at the moment as it's been too hot to have in the shop until recently.
You can pick up graphite rod from mcmaster pretty cheap, chuck it in a drill and use some sandpaper on it and you can make a cheap easy flaring tool. They have block and sheet too. Graphite will let you shape the glass longer before getting too cold like metal parts will.

Would it be possible to cast an aluminium block around the nozzle? Google suggests that the melting points make it possible, but I don't know about the thermal stresses involved or if aluminium's thermal expansion would be enough to cause problems.

If it can be done, then you would have a heater block with excellent contact with the nozzle and which could be placed right at the tip.

For heaters, I'm currently looking into flexible films, eg [www.minco.com]

Koko,
Nice to hear that there is an expert here. I have been learning as I go, and making more mistakes than progress some times. The current process to make a simple nozzle is so easy that anyone with a MAPP torch can make one, so if I settle for something that simple, I am thinking of trying to make a video of the process so anyone can make their own.

Andrew, yes, borosilicate can easily handle molten aluminum. I am even wondering about using a glass nozzle to extrude aluminum. But the second part is the real killer . The coefficient of thermal expansion of borosilicate is 33, and aluminum 248. They would not co-exist well without something flexible in between. However, clamp on or clip on heat exchangers with some flex to them are possible, just not a solid casting that fits only at aluminum's melt temperature.

Mike

Greg Frost could not source a block of aluminum for a block heater, so made one from a stack of 3mm aluminum rectangles.
Here's a picture he posted of his heater.
[reprap.org]

This got me thinking. here is what i came up with:
[reprap.org]

The nozzle would be drilled directly into the bottom plate using a countersink bit, so no need for a lathe! (in fact there's a perfect dremel bit for this)

A flange would be easier for novices to form than an accurate glass nozzle. The flange would hold the block onto the glass so we wouldn't need long bolts like with many PEEK designs, and the pressure of the filament would (hopefully) seal any gaps. If it needs more sealing, maybe PTFE thread seal tape?

Lastly, the aluminum would be in direct contact with the filament for greater heat transfer.

any suggestions?

edit: i was wrong. corrected

Edited 3 time(s). Last edit at 11/02/2010 04:45PM by Buback.