Glass nozzles for extruders

I like the idea. I wonder if you can buy very small pre-made test tubes with a flange on the top (something with an ID approaching 3mm. Then you would only need to cut off the closed end. A significant potential problem with this design is leakiness. I'm not sure ptfe tape will help. Molten plastic seems to find a way to leak if it can. Also you want the tip of the extruder to protrude a bit so that the surround does not interfere with the build piece. You could achieve that with some fancy file work I suppose.

yeah you are probably right. there would need to be some sort of gasket material to prevent leakage. Would high temp solder work?

The smallest test tubes I can find are 10mm diameter. Way too large.
I'm not sure the rim on a test tube is large enough anyway. I would like something with a 3-5 mm flange to give a large mating area for a gasket. I can't seem to find anything that fits the bill.

If rocket_scientist or someone with borosilicate would be kind enough to send me a tube with a flange as described, I would build and document an extruder of this type (for science!).

No, the solder has no elasticity, it will deform once with thermal change and forever after be too small. The best gasket material would be the same thing used for cars. Cork board or fiber board would both easily work in these temperature ranges. Also O-rings. McMaster-Carr lists silicone and PTFE O-rings with high enough temperature ratings.

I have been puzzling over how to make an extension to the nozzle tip of aluminum without using a lathe, and I still am drawing a blank. Adding the stack of aluminum plates with built in heater resistors to evenly heat the plastic though the glass sounds great. But so far I have not made or seen a good glass-metal nozzle transition, and I fear it will be so inaccurate that we will once again have a step in the flow causing plugging.


There is another way to attach the aluminum plates to the tip of the glass, and that is to repeat the compression fitting some are using for the open end. I you buy a copper ferrule to fit the 8mm glass tubing, and put tapered holes on two adjoining layers of the plating, then bolting them together will compress the ferrule and clamp onto the glass, no need for even the bottom flange. That way, with a full compression fitting as the mount at one end, and the aluminum plates compressing the ferrule onto the glass at the other, there would be no need for hot work at all. Just buy a length of 8mm thick wall ($1.70 per m), cut off the length you want, clamp and go. I still thing the glass nozzle orifice will end up working better, but I still have more development and testing work to do.

Mike

ok i'm on my lunch break so no time to model. here's a napkin drawing.
[reprap.org]
(edit: link works now)

before, i was thinking taped holes to hold the block together, but with the compression fitting i would need a bolt and nut like Greg Frost has. If you have the tools, trapped nuts would be better. (edit: actually you could just tap the bottom layer, duh)
I also decided to have the orifice screw into the bottom plate so that the bolts don't interfere with builds. I think this should be just as easy to make with a countersink bit.

the ferrule would probably have to be down one layer as the force might deform the top plate instead of the ferrule. you could use steel for the top two plates and aluminum for the rest, but that's another expansion variable you'd have to take into account.

Abrading the glass would probably give greater holding power, but i've read that that can weaken the glass.

other thoughts:
-what material for the screw in orifice? can pilot light orifices be used?
-what about embedding nichrome between layers?
-Using a coil of nichrome as the ferrule?
-someone mentioned film heating elements on the forum somewhere. it would be prefect here, placed between the layers.
- what about making a ferrule at home from a copper wire formed into a ring?

Edited 4 time(s). Last edit at 09/22/2010 06:12PM by Buback.

buback, the link on you post did not work, but I found you image in the wiki recent changes page. Here is a fresh link:




Using nuts on the bolts to hold the stack together will make it stronger than taping threads into aluminum, but will also extend the bottom of the stack and cause more of a clearance problem for the nozzle tip to extend past. Taping threads into the bottom layer and keeping the bolts just long enough to thread all the way through will give the most clearance for the nozzle tip and build piece.

Yes, two steel plates to hold the ferrule would be better. The difference in thermal expansion should not be a major problem.

You can safely abrade the glass using sand blasting or glass etching techniques, but both require tools, supplies, skills outside the typical backyard shed. I have tried the compression fitting on one of my nozzles, and it provides considerable grip without needing and roughing up of the glass surface. You should be able to clamp on the aluminum plates without doing anything to the glass, too.


Mike

Edited 1 time(s). Last edit at 09/22/2010 08:13PM by rocket_scientist.

Wacky idea for those with a drill press. Take a small block of aluminum drill a hole through it as close as you can to the size of the tubing. Make the hole relatively close to the edge of the block. Saw through the side of the hole, making the slot parallel to the axis of the hole. Then drill through the slot, perpendicular to the slot, with a tap drill. Open one side with a clear drill. Tap the other side. Now you have a clamp on block. A little thermal paste, and a little screw. teamnar.blogspot.com bottom of the first entry. I use a similar method to clamp the resistor in my design, if you'd like something like this give me some accurate dimensions (preferably mic or caliper) and I'll throw something together. The block could be wrapped with nichrome if you like the wire.

rocket_scientist:
I assume you have been heating the tips and stretching them to create a nozzle? Is it possible to instead create a kind of mold that you heat up and press the tube into? Or maybe you just need to get the glass red hot and shape it like marble makers do with those spoon looking things, except use a specially shaped tool instead. I saw earlier discussion about trying to wrap the heating wire inside a glass nozzle. IRRC, the result was that the nicrome wire burned away before the layers of glass fused. The melting point for nichrome and borosilicate are apparently very close, so maybe it won't make a difference. I was just thinking maybe it would help to keep the wire out of direct contact with the torch flame - maybe the wire is oxidizing in the flame as opposed to just melting away. Pressing the wire into hot glass as opposed to melting a layer over top might help depending on what really caused the wire to burn up.

I don't know how you might keep the wire from shorting out or (if it really is that close to melting the wire) from stretching and changing resistance.

If the sketched up clamp worked though, that would probably be better since it lets you choose between wire or power resistor heaters and metal-to-plastic contact will more evenly heat the plastic at the melt zone. The higher thermal mass should make temperature swings less significant too. Plus you wouldn't have to replace the whole hot end just because the heater broke/wore out. Embeded heaters might have the advantage on multi-print head setups though, since they could be more tightly spaced and lighter.

Marble molds are usually made of graphite (mine are). The problem with shoving the glass in is that the wall thickness in the smooshed region will be dependent on the amount of glass that is hot enough to move, and this is difficult to judge and control. Drawing the point out and cutting it off, then grinding back as has been done is going to be the best way to get a smooth tapering bore to a tiny opening.

I have been thinking about the glass barrel design a bit. I'm thinking of trying something using two shaft set collars, a PTFE washer, and a segment of brass tube with nozzle built in that is just long enough to hold the heater and clamp onto with a shaft set (M6 threads cut into the bore of course).

The idea is to use two 6mm bore shaft set collars to clamp the brass bits and glass together with a PTFE washer creating the seal. I'm hoping to use 3mm stainless steel screws to pull the two shaft sets together. Perhaps the screws won't transfer enough heat to the shaft set that clamps to the glass for the different COE's to matter. A lip formed on the bottom of the glass might be needed to keep the shaft set from sliding off.

There might be a way to build the heater into the lower shaft set and further shorten the brass section, but without parts on hand I can't try anything yet. Another, and maybe less expensive option for those with better tools and some scrap blocks, would be to replace the lower shaft set with a power resistor block.

Since this is not a one-piece design, it will probably suffer from the same plug issues as on a normal hot end, but at least the melt zone should be shorter than the typical setup. I think good temperature control and even heat distribution will be tricky unless metal parts are integrated somehow. I'm sure you can calculate the temperature difference between the glass surface and the plastic, but this seems like something that would be temperature and time dependent. Although it probably works well enough for a 5-10 minute test, I don't think it would be as easy as setting your temperature 20+ degrees hotter for the duration of a 3 hour print.

New results.

I fired up the glass wrapped nozzle and just for fun, I tried to hold a piece of perf-board underneath and move it back and forth to build up a small block. Nothing stuck, but I also saw that the blunt tip was rubbing against the fresh, soft plastic against the bottom of the glass and causing it to stick more to the ground glass tip than to the perf-board. I think that the drawn tips which are much narrower at the tip will be better than the blunt nosed.

Also, even with the wire embedded into a thin glass stringer and fused to the glass nozzle, it still seems to change temperature very slowly, and does not get hot enough at the tip. It takes some time before the plastic flowing through the nozzle gets hot enough to flow well. I think I am going to have to work on getting the wire even closer to the actual tip. And it looks even more critical that I find a way to enclose the whole heater section in copper tubing to help the heat flow better.

Mike

Mike,
I dont think adhesion should be an issue, Just put some greasy lubricant on the ground glass and the plastic will not stick to it and it will stick to the bed instead.

I finally got around to having a play with one of the glass nozzles you sent me. See this:
[reprap.org]

As I suspected, PLA is a bit sticky in the glass. Im looking to see if I can source a PTFE sleeve to go between the glass and the filament above the heater block.

is PTFE available in paper like sheets? If so it could be rolled up into a tube and inserted into the glass.
perhaps a test tube would work well with a Ptfe insert. I had written off test tubes because they were way to wide.

Is there any type of adhesive ptfe spray sold to consumers?

Greg, it might be best to use the 1/8" /ID, 1/4"OD PTFE tubing from mcmaster-carr to provide the sleeving. It will require a larger diameter glass. When I get a chance, I will try to build one. The next problem is that the inside diameter will transition from 3.2mm PTFE to 6 or 7 mm glass interior diameter. That will make it hard to heat the plastic until it hits the tip and widens out. I am not sure if that will end up with a net gain or lose in friction.

Mike

This looks ideal with the current nozzles: [www.mcmaster.com]
Has anyone ordered from mcmaster-carr from from Australia? I wonder how long deliveries would take and how much they would charge.

Greg Frost Wrote:
-------------------------------------------------------
> This looks ideal with the current nozzles:
> [www.mcmaster.com]
> Has anyone ordered from mcmaster-carr from from
> Australia? I wonder how long deliveries would take
> and how much they would charge.


Brace for disappointment, McMaster doesn't (or is very resistant to) ship outside the US.

I searched for sleeving and STILL missed that one! Yes, it looks like it should work quite well. Hopefully we can stretch or flare it at the top end to keep if from being pushed through along with the PLA. Let me wait until I get a few other things to order from them to spread out the shipping cost and I will give it a try. I now have my own supply of PLA, so I can make a comparison test. Possibly even the same glass nozzle before and after!

Mike

Two random thoughts: (may or may not be useful, and I have a thesis to submit before I can start tinkering)

The glass nozzle looks very much like a chemical eye dropper (often sold as Pasteur pipettes). I couldn't quickly find a good example or supply, but am pretty sure I've used ones in borosilicate glass with a short tip and ~0.5mm opening.

A possibility for inductive heating, spreading, or conducting the heat inside is to plate part of the nozzle with metal. Silver using the recipe at [blog.modernmechanix.com] might work, and stuck very well to the test tubes we tried it on.

Edited 2 time(s). Last edit at 10/18/2010 12:56PM by crlfe.

I tried looking for pipettes as well, but they were all expensive. They also need to widen out to 3mm for currently available filament. If you could find them without the gradients, that would probably make them cheaper, as well.
They will be perfect, if you can find them.

I'll have to wait and prod a chemist friend to get exact specs and samples to test. However they should be dirt cheap: the closest match I found on my quick search was the Chase 63A14673-010, which runs $75 per 1000.

Edit: Not to say that it's a particularly good match. However something 10x the price is still peanuts.

Edited 1 time(s). Last edit at 10/18/2010 01:49PM by crlfe.

If you are looking to order in quantity, you can just have something custom made. I got a quote from a company called chemglass a couple weeks ago:

Tapered, Borosilicate Type Glass Tubes, 6mm OD x 3mm ID x 60mm Overall Length, Will Have Tapered End With a 0.4mm ID +/- 0.2mm, Other End Will Be Open and Fire Polished
it would be $3.50 each for 200+
On the other hand, they quoted $96 for a sample of 3 pieces minimum

I had not thought of chemically plating silver on the inside of the glass. That would maintain the efficient shape of the glass. And once a good layer of silver has been deposited, you can switch to electroplating copper onto the silver to get a less expensive thick layer to carry all the heating current. That would make the inductive heating approach work well for everything accept monitoring the temperature, and we are current wondering if that can be done with non-contact IR thermometer, so that would complete the system. Still seems overly complex to me, but I will see about giving it a shot when I get the chance.

I have looked for IR sensors, and they seem to be a bit expensive. But if I buy a whole unit, with digital read out from Harbor Freight, then I can get something that will cover the whole temp range needed for $25. I will hopefully be posting a hack into the HF thermometer to get either an analog or digital signal to the control electronics posted soon.

Mike

I don't think copper will work. Copper has a too low resistance to be heated by induction. No resistance no heat...

Klaus

That's a good point, unless the film is thin enough to offset. And I think silver is an even better conductor.

Not sure whether the plating would carry enough heat from an external coil to the tip and interior of the nozzle to be worth it.

I'll leave a few experiments on my TODO list, but it's probably not worth anyone else's time.

Induction heating can heat good conductors, you just have to use a high frequency to exploit the skin effect. That means all the current flows in a very thin layer at the surface regardless of how thick the material is. At a few tens of kHz the skin effect dominates.

---

[www.hydraraptor.blogspot.com]

Good point nophead! I remember most of the diagrams should copper tubing, not copper wire, to make up the 20 turn primary. Since the same frequency occurs there, it would also need as much skin as it can get without being heavy.

I still think that what ever conductor is deposited on the inside of the glass must be thick to carry the thermal load, even if the skin effect makes the effective electrical resistance high. It will also need to be thick to keep from peeling off the glass due to differential thermal expansion.

The next question is, is it possible to make a SPIRAL pattern of silver, and run one end out the tip and the other out the big end and plate it with enough copper to do straight resistance heating?

Mike

Not only is there the skin effect, there's also the fact that you can just pump a ton of current through the copper until it heats up. Having a small number of turns on the primary winding will induce a small voltage in the secondary, and for a low-resistance secondary, that should result in an extremely high current.

I'm stil working through the math. I hope to soon have a closed-form expression for the power in the secondary, and the voltage across the primary, as a function of all the relevant parameters.

Could you do induction as you are planning with a high resistance primary driven at high frequency?

eg: Would you gain any effect making the primary out of nichrome, thereby having the option to heat the glass on the inside AND the outside?

Just thinking it might be possible to drive this with DC to get the glass close to temp/warm, then drive it with high frequency AC to keep it there using induction heating.

I've not tried this, so it may not even be possible. Was just a thought.

Well, I'm hoping to avoid heating the glass directly, in order to achieve a sharper hot-cold transition zone...

Driving it with DC on nichrome is basically what we are doing now. If I could get the wire embedded in the inside of the glass tube, I would be happily promoting that. It would be a great way to do it with glass. But I have not yet figured out how to embed the nichrome wire in the glass without burning up the nichrome in the torch flame, let alone putting it on the inside without adding ridges to increase the friction of the plastic going through. Nichrome is an alloy of nickel, chrome, and copper so I doubt it can be electroplated on. I think we will have to stick with copper electroplated on for the smoothest induction heated element.

Mike