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Extruder barrel questions

Posted by kurtis 
Extruder barrel questions
September 01, 2010 01:47PM
I'm having trouble getting ABS to extrude and I'm trying to figure out why. I suspect that it is a heat distribution problem. I will get a little bit of ABS to ooze out once in a while, and sometimes get it to extrude a little, but most of the time nothing comes out no matter how hard I push the filament. From what I've read, the problem might be that the filament is getting soft too high up in the barrel, and when it is forced down, it is not hard enough to act as a plunger, but instead just "squishes" down, fills the barrel, and plugs it.

I'm also getting large fluctuations in my temperature reading. The thermistor appears to be contacting the barrel, and seems well insulated (a few wraps of ceramic cloth insulation and kapton).

I'm noticing that as soon as the plastic comes out the the nozzle, it hardens. Instead of coming out in a straight line, it curls right at the nozzle exit. I suspect this is because the nozzle is not hot enough.

By the way, I'm using a Mendel (actually, an Isaac) with Wade's Extruder.

Now my real question:

I think one of the main causes for my problems is the barrel length. I've noticed that on the Reprap wiki, it calls for the barrel to be made from 36mm (or 32mm) brass rod. My barrel is about 50mm long. Could this be causing my problems? I'm looking to purchase more barrels and nozzles, possibly from [www.reprapstores.com], but looking at the photos, these look to be about 50mm long as well.

Is there a difference between Mendel and Makerbot barrel lengths? Do I need to cut the barrel shorter? How critical is the barrel length?

As I understand, keeping the heated zone small is better, so a short barrel is ideal. Also, keeping the heat near the nozzle seems to be better, so perhaps a heat sink (a larger washer?) where the brass meets the PTFE/PEEK insulator would help the top of the barrel from getting too hot and softening the filament to soon.

I plan to also order the hybrid thermal barrier to prevent the PTFE swelling issue. I may also try the new "TZ" barrel with the machined out centre section. Anyone care to share their experience with these?

Edited 1 time(s). Last edit at 09/01/2010 02:08PM by kurtis.
Re: Extruder barrel questions
September 01, 2010 02:25PM
Kurtis -

There is a lot of inaccurate information about hot-ends floating around. If you get your hot end parts from MakerGear, you will have a durable and totally reliable hot-end. And, if you do have any problems, support is available though the MakerGear IRC channel and the MakerGear Google Group.

Rick
Re: Extruder barrel questions
September 01, 2010 02:56PM
Thanks Rick. Are those compatible with Mendel? I see the barrels are 50mm in length. That's what I have now, and I feel like it might be too long, based on the build instructions on the Reprap wiki. This is my main concern with my current setup.

The Heater Core does look interesting though. It just screws on to the stock barrel? Looks clean and simple, but I imagine it wouldnt be quite as efficient as having the heating element directly on the barrel.

There are so many designs that change constantly. It is hard to know what will work and what doesn't. It would help if I know why my current setup isn't working...

I measure my ABS filament diameter to be about 2.8mm, while my barel ID is about 3.1mm. Could this large gap be giving me problems as well?

Edited 1 time(s). Last edit at 09/01/2010 02:56PM by kurtis.
Re: Extruder barrel questions
September 01, 2010 03:06PM
Kurtis -

Our parts work just fine on a Mendel. We've helped hundreds of botters with their printing issues. You might want to drop-in the MakerGear IRC and chat with some of the people using our stuff. You will not have heat distribution issues and related hot-end type problems with our parts.

Rick
Re: Extruder barrel questions
September 02, 2010 05:11AM
Quote

As I understand, keeping the heated zone small is better, so a short barrel is ideal.

A shorter barrel doesn't make the heated zone shorter. Also, having melted too much filament inside the barrel doesn't make it stuck as long as everything else is right.

You might want to have a look on wether your barrel gets heated properly. Having a temperature readout in the right range doesn't always mean the inside of this barrel actually has this temperature. Try additional 15 deg. Get things better? If yes, you should have a look at how tight the temperature sensor fits to the barrel. Having it loose or closer to the nichrome coil than to the barrel isn't ideal.

Filament diameter varies somewhere between 2.7 and 3.3 mm depending on the manufacturer. All of these diameters work.

Another reason for the filament becoming stuck is an uneven bore on the inside. Is the transition from the PTFE part to the brass part smooth? Having the bore in the PTFE a bit smaller than in the brass part is fine, but not the other way around.

Quote
Rick
There is a lot of inaccurate information about hot-ends floating around.

One of these inaccuracies is probably, commercial hot ends would be more reliable than others.


Generation 7 Electronics Teacup Firmware RepRap DIY
     
Re: Extruder barrel questions
September 02, 2010 05:32AM
Temperatures in the barrel also depend on the material, because they have different heat conductivities which contributes, together with several other factors, to different temperature gradients.
Re: Extruder barrel questions
September 02, 2010 09:54AM
Thanks Markus. I have tried different temperatures, and feel tham I'm in the right range. At one point the ABS was coming out brown, so it was starting to burn. Over the whole range of temperatures, I never really got it extruding properly. It would just curl right at the nozzle exit, then jam.

My temp sensor is contacting the nozzle head, and is away from the nichrome wire.

I am getting a bit of resistance at the interface between the brass and the PTFE, so that could be a source of the problem.

I plan to buy another barrel with a few different nozzles with various diameters to try. Starting with a new, clean barrel and thermal barrier might help.

So, there isn't much difference between the long 50mm barrel and the shorter 35ish mm barrel?
Re: Extruder barrel questions
September 02, 2010 10:29AM
@Traumflug - what do you mean by your last comment?
Re: Extruder barrel questions
September 02, 2010 02:46PM
To prevent PTFE bulge, just get a 1/2" copper pipe connector. It fits snugly around the insulator (I actually had to hammer the insulator in with a soft-faced hammer). That will keep the PTFE from bulging and should cost less than $1 at any hardware store. You might need to file down the stops if it has them on the inside.


Cameron

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Re: Extruder barrel questions
September 03, 2010 10:09AM
I got a more direct answer to the barrel length question here: [forums.reprap.org]

So, it appears that shorter is better, but not strictly necessary. I think my current barrel is a mess inside with old, charred plastic, and the transition from PTFE to brass isn't quite smooth, so completely new hot end might be the answer.

I think I'll try out a bit of everything while I'm at it: The regular and "TZ" barrels, the removable heater, different nozzle diameters, the hybrid thermal barrier, etc. I'll try to remember to post my results; whether success or failure.
Re: Extruder barrel questions
September 03, 2010 01:53PM
Want to try a glass nozzle, too? No need for PTFE or drilling. Just wind the nichrome around the tip, connect a thermocouple, and clamp the other end to your extruder base. More detailed instructions at wiki:Glass Nozzles

Mike
Re: Extruder barrel questions
September 03, 2010 03:17PM
Kurtis, just to chime in here, I think having good temperature measurements is key. It sounds like you're having problems with your thermistor; see if you have any loose connections anywhere. I can't speak for thermistor circuits, but I usually get less than 1 deg C of noise with a thermocouple; any more and it's usually an indication of a bad connection somewhere, or a short between the heater coils and the thermocouple. A bad temp measurement means you'll either be overloading the mechanism due to undercooked filament, or burning out your PTFE with too high a temperature. I used to extrude ABS at 240 deg C, and the PTFE went bad at 260 deg C, so there wasn't much wiggle room there. These days I use PLA at 200 deg C, which is much easier. I'm experimenting with Mike's glass hot ends (thanks Mike!), but don't have any results yet.

I use very simple hot ends, just a brass nozzle/barrel about 45 mm long screwed into PTFE. I oversize the brass ID as well, 4 mm ID seems to reduce the friction. Most important seems to be a short nozzle tip length; the 0.5 mm section of my nozzle is only 0.8 mm long.

A good test is to run the extruder up to operating temperature, and with the extruder stepper turned off (mine is off by default until I send an extrude command) try spinning the large gear by hand to pump the plastic through. You should be able to turn it fairly easily by hand, and see plastic pumping out the nozzle. Curving is normal - all my extruders do that. PLA definitely drools when it's up to temp, I think ABS is better, but you still get a bit of ooze/drool.

If in doubt, back off the idler springs to limit the amount of force on your heater assembly; better to let the pinch wheel slip a bit than destroy your hot end. Also, 4 M4 washers on each idler bolt, between the idler block and the main block will make reloading filament into the extruder much easier.

Hope that helps. Email me at wbortz at gmail dot com if you've got more questions.

Wade
Re: Extruder barrel questions
September 03, 2010 03:32PM
Hi Wade, thanks for the input.

Now that I look at it, my nozzle tip is fairly long (about 2mm), and may be another source of my problems. I can see how that could cool the plastic as it is trying to extrude and cause problems. It is also possible that I'm having problems with my thermistor as well. I'll check it out. Part of the trouble is that I didn't build or purchase this extruder myself, so I'm not completely familiar with where the parts came from and how it was put together.

I've got so many possible problems with the current setup, I'll start fresh with a new hot end and make sure everything is smooth, clean, and connected properly right form the startro
I'm currently not using the idler springs, but will probably add them soon.

Thanks again.
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