PLA Heat Creep - SOLVED

Help with printing with specific plastic filaments.
KelVarnson
Posts: 65
Joined: Thu Dec 03, 2015 10:03 pm

PLA Heat Creep - SOLVED

Post by KelVarnson » Fri Apr 01, 2016 2:45 pm

At least I think I have it solved.

Through my extensive trials and errors over the last few weeks, I have learned that adding more cooling air to the heat sink is not a reliable solution to the well-known PLA heat creep issue. The thermal interface between the heatsink and the metal filament tube is very poor, apparently by design. I can only speculate that the heat sink is intended for some other purpose than to actually cool the filament itself. In fact adding a second blower to the heat sink did nothing to reduce the incidence of heat creep in my machine. I even tried increasing the surface area of the fins by using slotted washers with thermal compound, and that actually made the problem much worse, I suspect by picking up additional heat rising from the heater block.

After careful study of the drawings for the heat sink and the filament tube, I decided that my best hope was to directly cool the filament itself, to prevent it from swelling above the brass insert and getting jammed. I took the stock Lulzbot extruder body and added a 20mm x 20mm x 8mm fan to the front. The fan blows into a cavity that feeds four longitudinal vents, equally distributed around the filament path. The vents are essentially slots that run parallel to the filament path. I added a little recession in the bottom to allow the air to flow past the top of the heatsink, and I also rotated and expanded the "D" detail at the bottom of the extruder body 180 degrees so that it ends up venting out through the slot in the mounting plate. The result is that the air flows along the entire filament path, and vents out of both the top and bottom of the extruder.

I have since been able to print several 4+ hour PLA prints with PVA support, something I had been unable to do for quite a while. The fan I selected has a low enough profile so that two of them will fit facing each other on the Dual Head V2. I have only installed one so far, but there is clearly room for the other, and I don't think they will fight too much for air, as I don't think a whole lot of airflow is required along the filament to cool it enough to prevent heat creep.

I've attached stp and stl files for the part. I will attach photos shortly, I don't have any good pics yet because the one that I printed in ABS is installed in my printer. I have since printed two more in PLA with PVA support (support is necessary for the fine details of this part). The PVA is being dissolved right now, I will post photos when that is done. It's probably easier to visualize the details of this modification by looking at the stp or stl files, it might be kind of hard to see in photos.

The fan I used can be found at Digi Key, and I wired it right in parallel with the 5V micro blowers that are blowing on the heat sinks:

http://www.digikey.com/product-detail/e ... ND/2757793

It is mounted to the front of the extruder using 2-56 screws.
Attachments
Extruder Body, Vented, 2016-04-01 01.stp
(1.66 MiB) Downloaded 290 times
Extruder Body, Vented, 2016-04-01 01.stl
(394.52 KiB) Downloaded 291 times

KelVarnson
Posts: 65
Joined: Thu Dec 03, 2015 10:03 pm

Re: PLA Heat Creep - SOLVED

Post by KelVarnson » Sat Apr 02, 2016 4:56 pm

Here's some photos of my modifications to the stock extruder. First, a side-by-side showing the front of the stock extruder vs. the modified one:
Front Side By Side.JPG
A close-up of the cavity shows the internal structure of the vented filament path. It's a little hard to see, but it is basically a hollow column with four longitudinal slots, equally-spaced around the filament. The filament path has the same I.D. as it does in the stock extruder. The fan cavity wraps all the way around the back and left side of the column, hopefully providing airflow "somewhat" equally to all four flutes. These details require support when printing in order to turn out well:
Front Cavity.JPG
Looking at the bottom of the stock (top) and modified extruder bodies, you can see the four flutes along the filament path. You can also see the depression that I added to allow airflow past the top of the heatsink, and that I enlarged the "D" shaped detail and rotated it 180 degrees. The plan is that the air comes down through the four flutes, gets past the top of the heatsink via the little channel, and then exits through the flat side of the "D", which is oriented over the slot in the aluminum mounting plate. Before I added these details, the airflow out of the bottom was completely blocked, and heat creep was still occurring:
Bottom Side By Side.JPG
This photo is a little rough, but it shows the four flutes from the top of the filament path. It's a good idea to use something to clear the flutes out after printing. I used a coping saw blade, but I haven't done that on this specimen yet, so you can see that the lower right flute is slightly blocked. These are fairly fine details, although they seemed to print out pretty well here at 0.25mm layer height in PLA with PVA support:
Flutes, Top, 2.JPG
Finally, here's a pic of the fan just sitting in place on the front, not screwed down. The Sunon fan only has three screw holes, I put four on the extruder because I wasn't sure what the best orientation would be. By the way, I made the decision to have the air blowing IN to the cavity, rather than drawing out. This was just based on gut feel more than anything, and also the thought that if any filament grinding does occur, it will be blowing air out of the top of the extruder body, perhaps reducing the likelihood of debris clogging these tiny little air channels. Also, I didn't want to draw heat UP from the hot area below the extruder body:
Fan, Front.JPG
Before this, I had gotten to a point where I was barely able to print at all with PLA due to heat creep. It didn't start out that bad at first, but it got progressively worse. I am not sure what to attribute that to, perhaps buildup on the inside of the metal filament tube reducing its ability to conduct heat away from the PLA? I don't think it was due to a decline in the micro blower performance, because I added a second micro blower in an attempt to alleviate the problem, and it had no effect whatsoever. It had become clear to me that my best shot was to cool the filament directly. My first pass had the flutes over the entire length, but was not venting out of the bottom, and as I said above, that did not work. Adding the channel on the bottom has resulted in 100% success, at least so far. I used the unscientific method of holding a piece of vinyl tubing up to my ear and using the other end to sample airflow out of the top and the bottom of the extruder, mounted in the machine, and the flow seemed quite robust. So I think it is working as intended.

Thanks for looking. I welcome all comments and suggestions.

kcchen_00
Posts: 1695
Joined: Thu Feb 19, 2015 12:42 am

Re: PLA Heat Creep - SOLVED

Post by kcchen_00 » Sat Apr 02, 2016 8:47 pm

That's an interesting mod. Where does the airflow exit once in the hotend?

KelVarnson
Posts: 65
Joined: Thu Dec 03, 2015 10:03 pm

Re: PLA Heat Creep - SOLVED

Post by KelVarnson » Sat Apr 02, 2016 10:18 pm

To make sure I understand the question, I might need to fully understand the terminology used around here (I'm kinda new at this). I understand "hot end" to mean the heater block and the nozzle. Do I have that right?

The air coming out of the bottom of the extruder body exits through the open side of the slot on the aluminum plate that the heatsink slides into and that the extruder body mounts to. There shouldn't be any appreciable amount of air flowing down into the filament tube and into the heater block and nozzle, if that is what you mean. That path is effectively blocked by the filament.

Do you have a concern that the exiting air could influence the print? I just want to make sure that I understand the question.

kcchen_00
Posts: 1695
Joined: Thu Feb 19, 2015 12:42 am

Re: PLA Heat Creep - SOLVED

Post by kcchen_00 » Sun Apr 03, 2016 11:48 am

The hotend comprises of the heatsink, heat block and nozzle.

I'm just wondering about the flutes you've made along the side of the filament. I assume they serve to cool the filament and provide airflow into the hotend assembly. Once in the hotend assembly, where does the air flow? If there's no exit, how does new / cool air circulate?

Just re-read your reply...

So the flutes only serve to cool the filament at the exit of the extruder body, and flows out the aluminum plate slot... Essentially, "super cooling" the filament to combat heat creep in the hotend.

Interesting design.

KelVarnson
Posts: 65
Joined: Thu Dec 03, 2015 10:03 pm

Re: PLA Heat Creep - SOLVED

Post by KelVarnson » Sun Apr 03, 2016 2:03 pm

Thank you for the clarification of the terminology, and the compliment. Yes, the air flows along the filament only when it is in the extruder body region, but it should be cooling it along that entire length.

I am counting on the paths of least resistance being the top and bottom exits of the flutes, making it impossible to build up enough pressure to force any air into the hot end. And as I said, I have unscientifically verified that there is appreciable airflow from both top and bottom exits.

I'd like to think that this approach will work 100% of the time while printing PLA, but the only way to be sure is to get significantly more prints through it, and/or for others to give it a shot and share their results.

killacycle
Posts: 38
Joined: Mon Apr 04, 2016 2:14 pm

Re: PLA Heat Creep - SOLVED

Post by killacycle » Mon Apr 04, 2016 2:31 pm

Eva (my wife) and I have had real problems with glow-in-the-dark PLA and other filled PLA jamming (grinding).
After months of occasional failed parts, I think we finally have figured it out.
We too used a 20x20mm fan, but as a substitute for the tiny blower on the finned section of the hotend.
We installed this nifty new duct and 20mm fan and we haven't had a jam since. Works great. Awesome prints.
We posted the duct I designed on Thingiverse here:
http://www.thingiverse.com/thing:1461335
(We bought our 20x20mm fan on Ebay for ~$3 which is one third the Digikey cost.)

The problem seemed to be that the PLA filament partially melted and swelled a bit just before it entered the heated section, while it was still in the finned section of the hotend. The filament would then grind (strip) during retraction. The E3D hotend doen't do this, and it has a similar cooler design, but uses a 30x30mm fan because the finned section is longer. I used a 20x20mm fan and designed a duct to fit the shorter Hexagon hotend. So far, it works like a champ.

Bill D.

KelVarnson
Posts: 65
Joined: Thu Dec 03, 2015 10:03 pm

Re: PLA Heat Creep - SOLVED

Post by KelVarnson » Mon Apr 04, 2016 4:20 pm

The part you wrote about it jamming during retraction from the heat sink area is particularly interesting to me, because of what I just experienced today. We started seeing skipping/grinding on a print again, even with the fan installed (the subject of this thread). But, it wasn't a 100% failure like I had seen before, it did manage to keep printing.

Before, there was obvious swelling of the filament, with a distinct "step" left in the removed filament, where it jammed against the top of the brass bushing.

So I may try your duct and fan idea, thank you.

killacycle
Posts: 38
Joined: Mon Apr 04, 2016 2:14 pm

Re: PLA Heat Creep - SOLVED

Post by killacycle » Mon Apr 04, 2016 4:43 pm

We often found that we had to extract the jammed filament out of the extruder with a pair of pliers, carefully leveraging against the plastic extuder frame. This led us to discover the bulged/swelled filament, just before the melted end. In turn, we noticed the jams were mostly happening during retraction.

We print with Inova and PLA mostly, and we had no jamming problems with Inova. We have been hunting down this PLA jamming problem for months.

I designed and printed the small duct and put it on the printer, and it was like flipping a switch. Problem solved.

The new Taz6 comes a 30mm (or 40mm?) hotend muffin fan and a duct similar to mine. They dumped the small blower, likely because it caused some cooling problems (and cost a bit extra for that tiny blower.) There are a number of 30mm muffin fan Taz hotend retrofits on Thingiverse. I think my design and yours are the only ones that uses a 20mm muffin fan. I figured that the 20mm fan seemed to deliver the CFM needed and tucked out of the way nicely under the mounting plate.

Bill D.

KelVarnson
Posts: 65
Joined: Thu Dec 03, 2015 10:03 pm

Re: PLA Heat Creep - SOLVED

Post by KelVarnson » Tue Apr 05, 2016 10:11 am

Thanks for the detailed info, Bill, I appreciate the response. Yesterday's failure was particularly disappointing for me, because it had been working much better than it had in a long time.

Virtually all of my PLA failures had exhibited the swelling and the "step" I described, which was an indentation from the brass bushing. Once in a while I would get one where the filament had to be pulled with pliers, exactly as you described, but most times the filament would still retract, but then would jam again as soon as it got to the "step". So I would say generally that most of our failures did not involve retraction. By the way, I typically use around 16mm for retraction when switching between heads.

I'm glad you had good results with the larger heatsink fan. I may try it, but I am a little skeptical, because a.) adding a second blower did not fix the problem for me, and b.) looking at the drawings for the heatsink and the filament tube it looks like the designers were trying to LIMIT thermal conduction between those two parts. I really do think that it's possible that I am not fully understanding their intent with this setup, if anyone thinks they can clarify this for me, please step up.

Since most if not all of my failures involve swelling of the filament in the area right above the bushing, and I have had at least partial success with my redesign, I may change it to focus even more airflow on that particular area. For instance, I am thinking maybe removing the flutes that exit out of the top, and increasing the clearance in the area right above the top of the heatsink. What would be really slick is if I could use the same fan to accomplish this while simultaneously cooling the heatsink, but looking at the assembly this seems like kind of a stretch.

I am glad you found a solution to your issues. Thanks for sharing the information.

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