Jon's 3D Printing and RPI Adventure: Heater and Thermistor Replacement

3D printers are very much a Rosane Rosanna Dana sort of thing. If it isn't something, its another. And one fix often leads to another problems or cascade of problems.

In this case, I updated my CR10 V2 printer to the Micro Swiss NG direct drive and all metal hot end. This cool new Direct Drive sits right on top of the hotend, extruder, and gears are at a 45 degree angle to the printhead which lets things get closer together and means that the hot filament path is very short and has no bowden tube in it. This is great for soft materials like TPU. The previous version (which I put into my old Ender3) had a 2 inch piece of bowden between the extruder gears and the hot end. This one just goes extruder and metal all the way.

What happened was this: I put the thing together (see the instructions Here) and the finished product had my control and fan wires in slightly different place than they had been with the origininal head. There was also the problem (though I didn't know at the time that it was a problem) that the wires didn't have anything to be secured to. No convenient place to put a wire tie to secure the wires in a safe way.

Wires just sort of hanging out

This didn't appear to be a bad thing but it left the wires hanging in such a way that they were very susceptible to repeated motion stress. So after just a month or so of printing the constant back and forth right near the print head caused the small wires of the thermistor to break. And suddenly my machine didn't know what the printer head temperature was and it shut itself off.

Crap.

So, I see I have to replace the thermistor and wires and since they are sold in pairs with the heater element, I may as well replace them to. What I really need to do is fix the core problem of securing the wires near the extruder such that they don't just go and break themselves again.

My first approach at this was to start designing a wire drag chain with secure locations at both sides for the X axis control wires. I spent a month or more on this and finally decided that the entire idea was wrong and that I should just have a good attach point built into the cowling that holds the extruder fans and install new wires and new wire cover. Now THAT appears to have worked.

So I try running it again and I almost immediately get a bad nozzle clog. I can't clear it with any of the normal means. The clog is like, up inside the new NG extruder. DAMN. How am I going to clean that? I heat the thing up and then take off the few easy parts, that includes the fan case and the nozzle and the little heat ring dooker that goes from the extruder body to the hotend. It turns out that the little heat stop unit that goes from heaterblock to the extruder has a blockage. Since the job of this thing is to cool things off between the heatblock and the extruder, if there is a solid piece of previously melted plastic in there, it isn't going to come out by trying to force more filament through or by raising the temperature. The only way to get it out is to pull it out of the machine and take it to my work bench and heat it up with a little torch and push the blockage out. This isn't difficult to do, though you do want to be careful not to make things so hot that you char the plastic. When I read a bit about this it said to use a heatgun. I have one of those and must remember to use it next time. Easier to control the heat than with my little propane torch.

Ok. So I get the clog out.

The other thing was that I did an email exchange with the Micro Swiss guys (they have great customer technical support) and was advised to pretty much do what I had done but also to make sure that my retraction settings on my build were set to something less than 1mm. Retraction is what the printer does when it has to jump over an empty space in the model. You don't want it dragging threads of plastic across, so you need to pull back on the filament to stop the pressure. When you use a bowden tube, you have a long ways from the extruder pushing and the hotend stopping. So you build up a long length of pressure and it takes a lot of retraction to stop the threading. I had my retraction set to 6mm. I thought I had changed it, but I had missed a few of my build templates. So I was retracting the hot filament right out of the hotend and into that little throat thing and the molten filmanet was cooling quickly there and making that terrible blockage. So don't do that.

And now I do more prints. At this point I was still working on perfecting the cable chain that I eventually abandoned. Everything look screwy. Everything looked rough and lousy. Had I not cleared out the blockage? I tried doing some prints with TPU. TPU is very soft and sticky and can often grab hold of little clogs and push them out. The TPU certainly seemed to be flowing well, but the prints still kept coming out all rough and lousy. What is UP?

An Older smooth one and the new rough (bad) one

A Rough PETG output. Ugly

I decided that I had to get serious. Have a plan. I made a list of things to do to completely clear out the entire filament flow path. Perhaps I have something caught in the depths of the Extruder and I just have to take it apart and clean it. I even made a spreadsheet of the things I would do and how I would do them. But it just seems like the flow was working so well. I was getting terrible threading though, with all filaments.

The internets suggested that to stop threading I could try a lower temperature.

So I got a threading test tower (just a few fingers sticking up in the air) and used the Curo advanced settings to change the temperature on each segment of the tower as it went up. I tried this on PETG and PLA and got OK results with sort of low temperatures. Then I tried it on TPU. TPU temp is supposed to be 215C. I swept from 215 to 195. 20 degrees of sweep. And I was still getting terrible threading.

TPU Temperature Sweep

Was everything just too hot?

And then it struck me. I had put in a new heater element and a new Thermistor. I really didn't know if the new thermistor was the same as the old one or what sorts of tollerances there may be on different thermistors. I bought a cheap Multi-meter that had a little wire temperature probe and I pulled off the nozzle and stuck the temperature sensor up there and did a test.

I found that my actual temperature was 20C higher than the temperature I was setting. Holy cow, no wonder everything looked like shit. I tried just sweeping even lower. From 195 to 180. Now things were looking better. I wasn't getting that Rough surface effect, but I was still getting stringing. Almost like I didn't have retraction turned on at all. I looked at my settings and though I had retraction set to 1, I did NOT have it enabled. dammit.

I enabled retraction, ran at the artificial low settings, and Bam, things suddenly looked OK.

I was thinking about just making a paper table that told me to do things like "If you want to run at 215, set you temperature to 193", but that was not very satisfying. So I figure out how to put a new thermistor temperature table into the Merlin firmware and recompiled and reloaded the entire thing. Only took me a day and wouldn't have taken that long except for that you typo that almost melted my printer during testing. (if you want to do this, there are plenty of tutorials on the web. Or ask me to write one JUST FOR YOU. I would it because I would be so glad that someone read my blog.).

And so, there you have it. I think I now have a working printer that has the right temperature, has a great direct drive extruder, has new pretty blue cable stocking, and has a customer cable attachment shroud that looks great. See the pictures.