Jump to content
Ultimaker Community of 3D Printing Experts
Sign in to follow this  
kris

Idea on underextrusion

Recommended Posts

Hi everyone,

I have got an idea that I would like to share here, again on the topic of underextrusion. I did not think it all the way through but figured it makes sense to share it anyway.

I remember that my underextrusion started in the middle of a print and from what I have understood and read on the forum this is what happens mostly.

Part of the extruder (Part Nr. 1309-Z2P-B) is PTFE. I also recall that PTFE, or Teflon, has a very large coefficient of thermal expansion.

So I downloaded the files and did a quick FE-analysis of the piece under temperature.

I am attaching the results for everyone to examine. I guess teflon is used to reduce friction. But to me it looks as if the deformation might cause a wedge shaped filament. The feeder might have serious trouble pushing that material down the nozzle, especially if it cools partially.

Regards,

Kris

picture1.jpg

picture2.jpg

picture3.jpg

 

Share this post


Link to post
Share on other sites

Interesting FE mode I have a coupler of questions:

How did you measure the force the spring exerts on the coupler?

What is the temperature of the coupler at the spring/coupler interface?

Did you detect any deformations at the coupler heated nozzle interface?

Thanks for the post!

 

Share this post


Link to post
Share on other sites

Hi Aaron,

All in all my boundary conditions are very simple for the first analysis. I didn't measure the force from the spring. I simply assumed that it holds down the coupler perfectly. I also used a uniform temperature distribution on the coupler.

I just redid the analysis with the major components of the extruder included (and a contact algorithm betwwen those components). In addition to expanding on the top, the PTFE coupler will expand inwards further down. Here it has nowhere to go because it is held together by the metallic nut. The channel diameter for the filament will decrease from 3.2mm to 3.0mm.

I cannot say if this has any influence on the performance of the extruder. But if the filament assumes the form of the expanded coupler, it might experience a form fit with the coupler. This could potentially be a problem during retraction because the feeder will not be able to pull out the front part of the filament. Pressure on the nozzle with be reduced, but the behaviour of the filament might be very unpredictable after that.

Regards,

Kris

picture4.jpg

picture5.jpg

 

Share this post


Link to post
Share on other sites

I had to take apart my nozzle assembly yesterday so I did some measuring of the components.

The outside diameter of the thick part of the teflon piece is about 11.1 mm when the nozzle has been at 260C for some time compared to 11.0-11.05 mm when the nozzle assembly is taken apart.

The temperature of the teflon piece is about 50 C when the nozzle is at 260 C.

I can not see any major gradient in the temperature in the teflon piece with my thermal camera, but the resolution is a bit low for this kind of measurements.

The force of the spring is about 1.5kg when compressed as when assembled, measured using kitchen scale.

The lower part of the teflon has a tight fit in on my machine, so if it has to expand it can not increase in diameter.

 

Share this post


Link to post
Share on other sites

Interesting simulation, I think PLA is pretty solid still at 50C, at 60C though it can deform a little. Can you show a temperature plot and not a displacement plot? I see what you mean about the pumping action, sort of like chewing gum.

 

Share this post


Link to post
Share on other sites

Can you not heat up the Hot end then push the filament through by end?

Surely this would indicate is the Teflon is the source of the problem?

 

I am not really sure what you mean. If I heat up the nozzle I can also push through the filament manually.

 

Interesting simulation, I think PLA is pretty solid still at 50C, at 60C though it can deform a little. Can you show a temperature plot and not a displacement plot? I see what you mean about the pumping action, sort of like chewing gum.

 

I have so far only applied a constant temperature to the entire setup, because it is easier to simulate. The heat is however only generated by the nozzle. I will try to redo the calculation to see what the temperature gradient is.

 

I had to take apart my nozzle assembly yesterday so I did some measuring of the components.

The outside diameter of the thick part of the teflon piece is about 11.1 mm when the nozzle has been at 260C for some time compared to 11.0-11.05 mm when the nozzle assembly is taken apart.

The temperature of the teflon piece is about 50 C when the nozzle is at 260 C.

I can not see any major gradient in the temperature in the teflon piece with my thermal camera, but the resolution is a bit low for this kind of measurements.

The force of the spring is about 1.5kg when compressed as when assembled, measured using kitchen scale.

The lower part of the teflon has a tight fit in on my machine, so if it has to expand it can not increase in diameter.

 

Regarding your last comment; the teflon part does have a tight fit and the casing prevents it from expanding outwards. But as the simulation has shown, the inner diameter will decrease instead. The piece will expand inwards. To me that seems even worse.

 

Share this post


Link to post
Share on other sites

You might be onto something here.

I took apart my nozzle assembly to change nozzle and with the new nozzle I suddenly had some extrusion problems which I have never seen before.

Since I used the same components (apart from the nozzle) and the same filament as before I could narrow it down quite quickly.

The symptoms were that the filament loaded okay but when I started to print it did not extrude well.

Pushing the filament through the nozzle by hand was also harder than it used to be.

It turns out that the teflon piece on my machine has such a tight fit that the spring was not able to push it all the way down to where it meets the upper part of the nozzle.

When there is a gap between the teflon piece and the nozzle part, half-molten plastic will collect there and increase friction. It gets particularly bad after retraction.

The molten plastic also seems to catch on to the filament when retracting, possibly pulling the teflon piece even further up.

I took everything apart again, cleaned the components and grinded a few hundreds of a millimeter of the outside of the teflon piece to make the fit less tight.

Now it prints just like when it was brand new again :smile:

I think it is really crucial that the fit of the teflon piece is not more tight that that the spring always is strong enough to push it firmly against the upper part of the nozzle.

I imagine that if the teflon piece change diameter due to the heat near the nozzle it can possibly grab on to the filament on retraction and then not get back to it's original position due to molten plastic entering the space.

ultimaker 2 nozzle teflon space

2014 06 07 3650

 

Share this post


Link to post
Share on other sites

Hi Anders,

Your post made me think: The teflon piece will expand due to the increase in temperature. It is constrained by the metallic bushing, the only direction in which it can move is upwards. By doing so it has to push against the spring. But due to the thermal expansion it will probably generate a force which exceeds the force generated by the spring. A gap will form, as you have described with your first picture. I will see if appropriate boundary conditions in my FE model will let me show this. I think if we would take a video of a freshly installed teflon piece and raise the temperature, we would also be able to detect the movement under a time lapse.

Regards,

Kris

 

Share this post


Link to post
Share on other sites

Got some results from the calculations. Actually very interesting ones. I now added the appropriate heat transfer properties so the model should behave pretty accurate.

My analysis does not take into account the rear cooling fan, so the temperature on the top of the PTFE fitting is too high. But still, here is what I got. First temperature plots (in Kelvin):

picture_t1.jpg

The PTFE bushing will expand inwards and upwards, leaving a little gap between the brass nozzle and the bushing itself (as Anders described above).

picture_t2.jpg

I do not know what the gap does over time. But I tried doing a time lapse video of the PTFE bushing while heating up. That didn't work, but when examining the photos I noticed some marks that look as if the bushing has moved several times in vertical direction. It also looked as if the bushing moved up slightly, but very hard to tell.

picture_t3.jpg

And this would be my current theory on what happens after multiple retractions:

picture_t4.jpg

The inner diameter of the bushing decreases from 3.2mm to 3.0mm due to thermal expansion.

Material will accumulated between the nozzle and the bushing. Retraction behaviour will become very unpredictable. Maybe the filament is even torn off above the interface. If air is pulled into that gap, that air will get pushed out the nozzle at some point. I noticed that when I had underextrusion, the material would come out very uneven. The same amount of material will leave the nozzle, but it will be mixed with air (or at least voids).

So my best guess so far is that underextrusion is mainly caused by the nozzle itself in combination with retraction. Performance will deteriorate over time due to the material accumulation. This also explains why the problem can occur anytime and only a complete clean will help.

A possible countermeasure could be to increase the hole in the bushing to account for the thermal expansion, i.e. 3.4mm. I would even try this if I had a spare one. Maybe we could loose the spring as well (see v3 below)?

picture_t5.jpg

 

Share this post


Link to post
Share on other sites

Here are a few results for a tightly fitted PTFE bushing. Not sure if it would work. Thermal expansion would most likely create a larger force upon the remaining structure compared to the used spring. If the structure can handle it, there is no problem. If not, the wall thickness of the bushing could be reduced.

p1.jpg

To account for thermal expansion the inner diameter has been increase to 3.4mm .

p2.jpg

The results; the bushing does not overlap the brass fitting anymore. The hope is that resistance for the filament, especially under retraction, is reduced.

p3.jpg

 

Share this post


Link to post
Share on other sites

Kris, your posts on this topic are very interesting read, thank you!

My analysis does not take into account the rear cooling fan, so the temperature on the top of the PTFE fitting is too high.

Do you have any idea whether the error is anything significant or not? Let me refer to a couple of posts in other threads:

http://umforum.ultimaker.com/index.php?/topic/3803-ultimaker-2-dual-extrusion/page-2&do=findComment&comment=53318

http://umforum.ultimaker.com/index.php?/topic/3803-ultimaker-2-dual-extrusion/page-2&do=findComment&comment=55376

http://umforum.ultimaker.com/index.php?/topic/6084-um2-bowden-block-cooling-improvement/

It would be very interesting to know how much the temperature charts would change in case of:

  1. Dual extruder
     
  2. Black-anodized hot end aluminum parts
     
  3. More powerful fan and/or changing of its flow

As I have built for my UM2 a kind of a heated chamber, the ambient temperature could easily get to 50+C, I'm very interested in ways to mitigate the problems arising from deformation of the PTFE part. I see my teflon isolator coupler having changed its form, although only slightly.

 

Share this post


Link to post
Share on other sites

I think convection is not doing much to cool the PTFE fitting. A heat sink that leads away heat from the part is far more effective I guess. Maybe using heat transfer paste between the aluminium parts of the extruder helps a bit? Or adding another small heat sink at the front.

Another stupid idea; Could the flow of the middle fan be reversed? At the moment it blows warm air from the heat sink onto the PTFE fitting.

I think the PTFE fitting is getting bigger and bigger because it gets filled with PLA/ABS which will prevent it from contracting back to its original diameter. And it will work its way up through its metal casing. The spring will not be able to push it back. A gap will form between the fitting and the nozzle.

Yes, black anodization might help as well. As so often, a combination of improvements might give us the results we are looking for. The simple things can be tried first.

Any chance the guys from UM could try this? Maybe they already have :-P

 

Share this post


Link to post
Share on other sites

I think convection is not doing much to cool the PTFE fitting. A heat sink that leads away heat from the part is far more effective I guess. Maybe using heat transfer paste between the aluminium parts of the extruder helps a bit? Or adding another small heat sink at the front.

Another stupid idea; Could the flow of the middle fan be reversed? At the moment it blows warm air from the heat sink onto the PTFE fitting.

I agree; convection is probably not doing a lot to cool the PTFE fitting - but there are not too many options for keeping it cooler. At least if we want to avoid making the whole print head bigger and essentially redesign the whole thing. Heat sink on PTFE wouldn't easily help due to its high internal heat resistance.

The idea of reversing the air flow of the middle fan sounds actually promising! Another thought... As the walls of the hot end isolator (the hollow bolt) are quite thin, what if there was a heat sink which would snap into the holes around its rim - and the heat dissipator of the heat sink would be on the place of the (currently) missing extruder?

 

Share this post


Link to post
Share on other sites

@3poro: adding additional heat sinks would probably help to reduce the temperature. The effect could easily be measured. Using the space for the second extruder for the heat sink is a good idea. Does anybody know if UM is trying something like this already?

@EldRick: I noticed your post. Do you know if there is a measurable effect?

 

Share this post


Link to post
Share on other sites

That E3D hotend looks great. There are a few things that stand out:

1.There is very little material connecting the actual hotend with the upper part that meets the bowden tube. Thus heat flow to the upper part will be limited.

2. The upper section has additional large cooling fins

3. All metal means that there is limited and uniform thermal expansion throughout the design

 

Share this post


Link to post
Share on other sites

@EldRick: I noticed your post. Do you know if there is a measurable effect?

 

I can't think of any reasonably easy way to measure the net effect. I have an IR pyrometer, but the teflon block is small and well-hidden behind the fan shroud.

However, if UM chose to put a fan there, it's probably necessary for cooling the cold side of the hotend, and more airflow on the block is going to be better than less. How much better, I have no idea.

 

Share this post


Link to post
Share on other sites

That E3D hotend looks great. There are a few things that stand out:

1.There is very little material connecting the actual hotend with the upper part that meets the bowden tube. Thus heat flow to the upper part will be limited.

2. The upper section has additional large cooling fins

3. All metal means that there is limited and uniform thermal expansion throughout the design

 

Think it's one of the best hotends on the market. I'm just not that keen on modifying a brand new UM2 -_-

 

Share this post


Link to post
Share on other sites

Think it's one of the best hotends on the market. I'm just not that keen on modifying a brand new UM2 -_-

 

me neither :???:

My hope is that existing parts can me modified and optimized to overcome the problem. Apart from under extrusion I am very happy with the UM2.

 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

Announcements

  • Our picks

    • Architect Design Contest | Vehicles.
      We're open for entries! - Design and submit your 3D designs of architectural entourage - vehicles - for a chance to win a large filament pack. Presenting an idea, an architectural design or something as big as an urban project isn't easy. A scaled model can really help to get your idea across.
        • Like
      • 24 replies
    • What The DfAM?
      I'm Steve Cox, an experienced engineer familiar with 3D printing. I wanted to share some DfAM guidelines with this community to help and make stronger parts.
      I'm also an Autodesk Certified Instructor for Fusion 360, so many of the images in ...
        • Thanks
        • Like
      • 22 replies
×

Important Information

Welcome to the Ultimaker Community of 3D printing experts. Visit the following links to read more about our Terms of Use or our Privacy Policy. Thank you!