A lot of heat is lost from the large flange on the top of the heater barrel and transfers by convection to the heatsink above and by conduction through the bolts.
When I stripped it down I noticed the bolts had loosened.
The PTFE had shrunk lengthways and expanded in diameter and was no longer making a good compression seal.
Although no plastic had escaped it had leaked under the PTFE.
I think it was leaking so slowly that it oxidized where it met the air and went hard, stopping further flow. I hadn't run it very long so it may have escaped eventually.
So PTFE is obviously no good in compression at these temperatures. I replaced it with PEEK, which is a shame because it is about ten times more expensive.
I also replaced the aluminium flange with an M8 x 25mm steel washer insulated from the barrel by a PEEK collar.
Here are the parts, the PEEK section is drilled in situ to get perfect alignment: -
And here it is assembled: -
I put some PTFE plumbing tape over the hot end of the PEEK before pushing it into the aluminium in an attempt to improve the seal.
The heatsink now runs at 80°C without the fan. I would have liked it to be lower but as long as it is below the glass transition of the filament and the clamp it should be OK. I tried insulating the bolts with PEEK washers but that only dropped the temperature by 5°C, so not really worth the effort.
After one heat cycle I noticed the bolts were not as tight as they should be so it looks like PEEK creeps a bit as well. Perhaps glass filled would be better.
It is disappointingly complex with lots of machined parts but it does work very well. The heater power has dropped to 50% from about 80-90% with the fan. ABS filament extrudes manually very easily and even HDPE only requires moderate force. I think actually having the long heatsink preheating the filament to just below the glass transition is probably a benefit.
It is too early to say whether this design will be reliable but other than the PEEK section leaking there isn't anything likely to fail. I don't mind making things that are difficult to make provided I only have to do it once.
I also have a much simpler design in mind that should achieve the same short transition zone.
I wonder how much the PEEK simpler washers would help if there wasn't a specialized PEEK collar.
ReplyDeleteSo I know that lots of us are sending you lots of suggestions without trying them ourselves, but being as you currently have a setup that could test this material pretty easily I would suggest that you try a generally poor engineering material. Chalk. I know that may sound a little crazy, but is it dirt cheap, a pretty good insulator(much better than stainless) but awfully brittle as we all know. But I have to wonder if it wouldn't make a great replacement for the PTFE section in the transition zone. You could even use it to insulate the screws, however, the PTFE is definitely easier to machine.
ReplyDeleteIt would be really cool to see if plain old chalk would do the job. If not, then I have some more suggestions, but maybe I should try them myself instead of pushing all this work onto you Nop. Thanks so much for your wonderful work in this project.
Anon,
ReplyDeleteI guess not much because the PEEK collar helps in three ways. It insulates the washer so is is at a lower temperature than the flange was, about 120C. So less heat is lost from it by convection and less through conduction of the bolts. It also insulates the now much smaller flange from convection.
geo01005,
I don't have any chalk so I would have to buy some. Do you know if it can be machined? I would imagine if I tried to drill it down the middle it would just crumble away.
geo01005,
ReplyDeleteMy wife has some chalk so I had a play with it. It was tapered so I tried turning it straight on the lathe. It fell to bits. A straight stick might be easier to turn. I can drill it by simply twisting a drill bit in my fingers.
It is so soft that I think that the molten plastic would erode it away as it went past.
I can get the same heat flow as the PEEK with a section of glass tube taken from an 1.25" fuse, OD 5.8mm ID 3.5mm. But to make a high temperature seal I would have to cement it with Cerastil or similar so buying a bit of PEEK rod is probably easier / cheaper. I can get 10 x 250mm for < £5. Enough to make several extruders.
Years ago I worked on a fdm2000 and had to strip the head down for maintanance, it was a similar design to yours with a melt chamber having a close fitting bush (ptfe it looked like) for the fillament and screw in tips of different sizes, it had pid temp control, the support tip had a solenoid to lift and lower it to clear it when building the part.
ReplyDeleteRS do a machinable ceremic but it is expensive.
Hope I am not teaching granny to suck eggs, I am new here.
I have never seen a commercial FDM machine so any insight is welcome, thanks.
ReplyDeleteThe fdm2000 extrusion head had a small epicyclic geared motor driving two pinch rollers (knurled)
ReplyDeleteto push the 2mm (approx) fillament, the melt chamber was a tube (looked like mild steel)about 13 to 15 mm bore(from memory) a screw cap fitted to the top with a bush (white and looked like ptfe) this was a close fit to the fillament, also a tip screwed into the bottom of the tube,this had a hexagon for removal about 25mm af, the thread into the tube must have been around 18mm, the heater band fitted around the tube with insulation around it, the principle was I think that the fillament being pushed in at the top caused the molten polymer to extrude from the tip, the tips were of different sizes with very small holes,(.1, .2) the fillament came in vacuum bags as abs absorbs moisture, the amount of oooze indicated moisture. please remember this is all from memory, the part and support head were mounted close together and appeared to be the same apart from the lifting solenoid on the support head, the heads were maintained at different temps by pid and the whole build chamber was kept at 80 degrees (i think) this helped reduce warping, if I can help further then don't hesitate to ask.
Interesting that there is a relatively large chamber of molten plastic. That would melt the incoming filament quickly while presenting little resistance to it.
ReplyDeleteAny sign of an outlet valve or a pressure sensor, or do you think the only flow control was via the filament feed?
No outlet valve or sensor, I suppose they could be getting the load on the motor as feedback, when I had to remove the filament the end was tapered to a point, the drive motors were very small but powerful and had to work in a hot environment but I never had one fail, I suppose that the filament caused pressurising of the chamber and the melt pool allowed for stabalising of the polymer.
ReplyDeleteWhen I put a new roll of filament in, it had to be fed by hand into the head and just by pushing by hand caused extrusion from the nozzle, the new machines have a canister with a chip embedded and automatic loading.
I think I still have an experimental head I built 10 years ago that used fishing line, if I can find it I will post images if you wish, it was bolted onto a cnc machine, we also built one that used glue sticks that worked really well.
Yes pictures would be interesting please.
ReplyDeleteI think I will build a heater block with a cavity in it to see if reduces the force required to extrude.
I have been trying to reduce the amount of molten plastic for better control but perhaps more is better for easier feed.
Less force required to feed allows the drive to go faster for the same power so it may actually improve the control response.
Hope it works, I think reducing molten material means having higher temperature because it has to melt the polymer in a shorter time, ok when running but in pause mode soon degrades the polymer, I may be talking rubbish here but I wonder if there is a relationship between relevant sizes of filament (piston) melt pool and nozzle as in hydraulics,maybe someone with a brain could help us here.
ReplyDeletewill have a rummage through the shed to try and find the fishing line head
I have been reading up on the reprap project for a few weeks now and wondered if anyone has tried using cement instead of teflon for the insulating cylinder? The thermal conductivity for cement should be roughly 0.30 W/K.m, very similar to teflons' 0.23. Cement would be much better in the compression of your design. The only drawback I can see is the increased friction of cement, so maybe have a thin liner of teflon?
ReplyDeleteJust some random ramblings of someone just getting involved in this...
-germanicus
Hi germanicus,
ReplyDeleteConcrete has been tried by Vik Olliver but not in this configuration. Drywall with a Teflon liner has also been talked about but not sure if anybody has actually tried it. Google for reprap and concrete or drywall for more details.
I think it would definitely need a slippery liner, so PEEK is a lot simpler and not too expensive when bought as 10mm rod.
Ironically fire cement is also used to fasten the heater to the barrel, so what you are proposing as an insulator is already being used as a conductor!
I have been thinking about light bulbs, with tungsten filaments. I don't know what sort of oxidation rate there would be at the lower temperatures used here, but certainly it's not hot enough for the tungsten to burn ! The 12 volt range would be most useful, and I suspect around 5 watts ought to be ample, unless using thick extrude nozzle and high rate.
ReplyDeleteThe major advantages would be ready availability, and ability to place the element a minimal distance from the melt chamber.
Graham Daniel.
I'm recently desiging an extruder head for the Fab@Home in a school project. I've done some research and went to an institute were they also used a FDM 2000. I took some pictures on wich you can see how the machine works. How can i post these pictures...?
ReplyDeleteHello Bouke,
ReplyDeleteI use Google Picasa Web Albums to host all my photos. Blogger automatically puts them there when I blog a picture.
flikr.com is another free picture hosting site.
That is an option. I thought it would be nice to add them to this discussion on the subject. For the occasion I created a new blog were you can find the pictures:
ReplyDeletehttp://stage-bouke.blogspot.com/
enjoy
I must say Nophead that your blog is very rich on very useable info...
Bouke,
ReplyDeleteNo I don't think there is anyway to add pictures to blogger comments.
Thanks for the pictures, very interesting. The head looks very heavy, was it a moving head or moving table machine?
Did you acertain whether the motors were DC motors, steppers or servos? They look like BLDC servos with gear boxes to me.
The head moved X and Y, the table Z.
ReplyDeleteAt the time I didn't ask about the type of motor (stupid.
The head on the F@H uses steppers, they'll also be used on my design. The software is already designed for steppers ans I've got the stepperdrivers.
Something totaly different: have you ever tried plain gasket material as a thermal barier? This can resist high temperatures probably better than PEEK (glas @ 140 C)
No I not familiar with gasket material. Is it a good thermal insulator? Would it have a porous surface rather than smooth?
ReplyDeleteI too was initially worried by the low Tg for PEEK, but it appears to be a myth. The Metalab guy's ignored my worry and tried it and reported it worked, so I tried it and also found no hint of it losing strength at 240C.
Yes, most gaskets are a bit porous because they usually consist of compressed fibers. Most hardware stores sell it and it is very easy to cut, a bit like leather. You could stack a couple of layers to get some thickness.
ReplyDeleteWhat about your first experiment with PEEK, didn't it deform? Or am i confused with PTFE?
Also, you use the PEEK by compressing it between flanges. Do you think it is useable as a coupling flange. Without bolts going all the way trough it. So no metal contact between hot and cold parts, just the PEEK in between. (kind of hard to explain without drawing...)
PTFE compresses and bulges so it is impossible to keep it in compression.
ReplyDeletePEEK on the other hand seems to have far less shrinkage and no noticeable bulging. I added some spring washers to take up any slack and this extruder has proved to be very reliable appart from one blocked nozzle incident.
On one of my subsequent designs I tapped the PEEK and screwed it in and that works as well, requiring no compression. Andy Hall tapped the other end and made an extruder without any external support. The PEEK forms the structural element as well as the insulator.
And you have probably seen my subsequent design that uses a stainless steel pipe both as the insulator and the structural support.
Bouke
ReplyDeletewell done with the pictures of the fdm head, its been so long since I used it that the pictures bring it all back, I am pretty sure that the motors are not steppers, they were smooth running but with a large gear reduction, any chance you could put a picture up with an object to show the scale
I'm sorry, I don't think I'll ever come near that machine again...the pictures you saw are all I've got.
ReplyDeleteIf you want pictures I would try to get some of a Maxum. This machine uses, what I call, a planair motor for the X and Y movement. You could describe it as a stepper folded open in two directions. The head is pulled to the 'steppersurface' by magnetic force. The surface consists of a grid of magnetic areas wich control the movement of the head similair to a lineair motor. Very fast and accurate. The only disadvatage: with a powerfailure the head crashes down trough your workpiece... :(
Turns out that a soldering iron gives you everything you need to build a heater, also the electrical insulation:
ReplyDeletehttp://boukewerner.blogspot.com/2009/03/cheap-insolution.html
I just read Nopheads experience wirh soldering irons so I edited my text a little bit...
ReplyDeleteI'll stop using your blog as a forum now.
See you at the buildersblog