Plastic Pooling in Extruder Barrel

I've just posted this on my blog but I'd wanted to copy it here in order to get some feedback. I built a new extruder barrel and made it relatively easy to disassemble. I heated it up, pushed HDPE through, let it cool down and took it apart. Here's what I found.



These images show the motor end and nozzle end, respectively. On the motor end, there's a small amount of pooling. I'm guessing the gap is due to the shape of the drill bit. The nozzle end has quite a bit more pooling. The gap there is caused by the shape of the acorn nut.

What I'm wondering is, does this create extra resistance for the filament? If so, is there any way to prevent it? My first thought is to fill in the gaps with JB Weld. Maybe even turn the nozzle cavity into a taper.

What do you think?

My guess (I'm not a polymer chemist but I play one on TV) is that the pooling shown won't have a big effect on the amount of pressure need to run the extruder. With vicous fluids resistance to flow is dependent on cross sectional size: small size = high resistance the relative size of the nozzle hole vs. the size of the pooled areas suggests that most of the resistance in the extruder is at the very end. Also, somewhat paradoxically, if the pooled material were to cause resistance the best response would be to increase the amount (diameter) of the pooling. The motor end pooling probably does represent the results of back pressure on the filament, tuning the feed rate/temp/extrusion rate equation seems to be the best way to deal with that. Overall I'd say don't sweat the load on this one.

I think the pooling at the top end may be a problem because the filament is not at full temperature until it gets inside the barrel. The instructions specify a bottoming tap to be used so there should not be a gap at the top end for the plastic to well into.

I didn't have a bottoming tap so I turned down the end of the thread and counter bored the PTFE so I had a snug fit see [hydraraptor.blogspot.com]

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[www.hydraraptor.blogspot.com]

I ended up using the heater barrel and a pair of vicegrips to do a cheap-and-dirty bottom tap. I also put a hose clamp on it to keep it from slipping. The motor end now has sandpaper wrapped around it to keep that end from slipping. Apparently, PTFE is quite slippery. Who knew? :-)

Good solutions to the two problems but I wonder how they occurred. Looks like the clamp hole is a lot bigger than the PTFE rod and for some reason the thread is not as tight as it should be. Could this be an imperial / metric conversion problem?

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[www.hydraraptor.blogspot.com]

Not sure about the clamp hole. I'm guessing it's a combination of measurement system mismatch and moulding process.

The thread on the other end seem perfectly fine when everything's powered done and cold. When the extruder's running, though, the heater barrel slowly slips out.

It's amazing how much pressure that little gear motor can exert. I'd have everything tightened up and what I thought was perfectly solid and the motor would just push it apart.

Zach and I were testing the new electronics and we observed a pattern where the heater element had some lag time and the heater on/off was fighting with the motor timing. This was with the latest software as of 1 week ago.

you could watch the temp fluctuate as much as 25c... and the motor would stop pushing while it cooled off/etc.

There is for sure some combo of issues: temp of heater, temp of material as it touches the metal head, lag time it takes for heater to heat the material, and the motor pressure.

Anyway, it didn't seem simple to me, I was watching the temps go all over the place while motor was trying to react

Doesn't surprise me. The extruder is a fairly complex component with all sorts of interesting feedback loops, equilibria and interdependencies. If I remember correctly, the study of heat propagation under fluctuating temperatures led to the development of Fourier analysis. And they weren't even dealing with a motor pushing plastic filament through it.

Well simply comparing the thermistor resistance with a target value and switching the heater on or off only gives a fluctuation of about +-3C on my system so it doesn't need to be complicated.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
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> Well simply comparing the thermistor resistance
> with a target value and switching the heater on or
> off only gives a fluctuation of about +-3C on my
> system so it doesn't need to be complicated.


Not all of us have the same level of skill. In this one case, were were adjusting the tightness of the two sides of the extruder housing... as it was binding the material.

Steve DeGroof picture implies there are at times a molten chamber and what you are doing is pushing into liquid... depends on the temps you set and the material. I'm sure there are tons of ways to get it wrong

nophead,

yeah, thats the method we were using. of course the fluctuations were from watching the host software which polls stuff every 30 seconds. i only got about 2 hours of extruder time in, so it may be a bug or something. i'll have more time to check it out when i get back.

FYI...

Playing around with the extruder exerciser, I'm running into some similar problems to what Steve originally posted.

In my case, material "mushroomed" inside the extruder tip and I was no longer able to melt it. I have a fluke IR Thermometer and it was clear to me that the temps were much lower at the head than at the main part of the shaft... short of removing the tip and cutting off the mushroomed material... jam wasn't going to clear. I also couldn't pull it back given the geometry

This is my first attempt at extruding, so learning and sharing.

Perhaps some heatsink compound in the acorn nut thread would keep it hotter.

Have you put the thermistor on the nut or the barrel?

I am on the older design so have not run across these problems yet but I intend to "upgrade" in the future so I am keen to see the pitfalls.

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[www.hydraraptor.blogspot.com]

OK, I wasn't going to post this because a) I'm not sure it really helped and b) it's a little weird. On the other hand, I'm getting a decent extrusion and it doesn't seem to be doing any harm, so here goes:

1. found a component lead that was the same diameter as the nozzle
2. cut a piece of filament, longer than the heater barrel
3. heated the component lead and stuck it in the end of the filament
4. stuck the filament into the chuck of a Dremel
5. used a screwdriver tip to turn the end of the filament to a point
6. threaded the filament through the heater barrel
7. wrapped the end of the barrel with PTFE tape, overlapping the end of the barrel
8. put a blob of JB Weld on the end of the filament
9. put the acorn nut on, threading the lead through the nozzle
10. waited patiently for the JB Weld to set
11. removed the acorn nut
12. pulled the lead out through the nozzle and heated the acorn nut to remove the filament

What I ended up with is a nozzle that tapers to a point and fits snuggly on the barrel. The JB Weld seems to conduct heat fairly well.

I purchased the molded parts from the reprap store recently and have found that there are several variations in dimentions from what would be expected. most notably at this time is the diameter of the hole for the ptfe material . havent decided how I'm gonna deal with it but may use this sandpaper idea myself .

The sandpaper eventually gave out. I was pushing it pretty hard, though. I ended up running a couple screws through the extruder body and PTFE (on either side of the hole). That kept it in place.

I'm thinking that it might be handy to have a groove cut near the top of the PTFE barrel, with a matching ridge inside the clamp. Give it a solid mechanical connection rather than just relying on friction.

I agree with the groove design, I keep pushing mine out too. The higher temp materials present a problem, and heat seems to make things worse.

Steve, what's the latest to share, you have been a great resource!

The two screws seem to hold everything in place. What I'm planning on doing is, once I have a working Darwin, one of my first projects will be to make a new extruder. I want to put a ridge on the inside of the clamp and then cut a matching groove in the PTFE. I've also noticed a bit of sagging of the extruder body at high temperature, so I'm thinking some support struts might be needed.

Right now, I'm waiting for Zach to get some plastic part kits in stock. In the meantime, I've been learning how to use AOI by going through the part models and trying to make each one single-head printable.

BDolge Wrote:
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> My guess (I'm not a polymer chemist but I play one
> on TV) is that the pooling shown won't have a big
> effect on the amount of pressure need to run the
> extruder.

No chemist myself either, but my guess is the same principles apply here as to resistance in blood vessels (MD myself).
As the area of the pipe changes with the square of the radius, even relatively small changes may have significant effects. I

Edited 2 time(s). Last edit at 05/13/2008 06:16AM by bjathr.

bjather,

Not a nice equation but it makes sense. I've had so many issues with the extruder it's ridicules.

Demented