Assembled printers - cannot get a good print, turns out to be bad hotend thermistors on arrival
Long time Prusa owner here - just purchased two CoreOnes assembled for work, one directly from Prusa Research, the second one from MatterHackers. Both of them out of the shipping box had no firmware installed, I had to install the firmware before I can get it to work. Both of them had bad thermistors for the hot end, which lead to a lot of head banging and troubleshooting before I could figure out why I couldn't get a successful print off the new machines.
(BTW, this post isn't meant to shame / cast blame - but I think Prusa got a bad batch of thermistors in and there are other people may be affected and maybe I can save someone a lot of grief).
On my first printer, I could print ABS, but prints in PLA and PETG would fail. The print won't stick, or the first few layers will stick and then it would come off the bed. I thought at first it was bed cleaning and bed adhesion, but after a few weeks of trying, I wasn't getting ANY successful prints in PETG and about 50-50 on PLA.
Then one day as I was troubleshooting the printer I noticed that after a reboot and cooldown (tried a cold pull to see if the nozzle was stuck) the hotend still read 292 deg C. At first I thought it was a firmware bug of some sort, and I started filming for a bug report... only to find that the hot end was at room temperature. The hot end thermistor had failed in such a way that it was reading 292 deg C at room temperature. I replaced the thermistor and suddenly, viola - prints were coming out great.
I took delivery of my second printer - and could not get a single good print off of it. Knowing the issue I ran into (and since it too was missing firmware), I grabbed a thermocouple and data logger from my lab and recorded the hot end temperature running the same G-Code file.
In the plot, the blue line is the bad printer from Matterhackers, the orange line is the Prusa Research printer that I fixed by replacing the hotend thermistor. The first plateau is 170 deg C during the probing cycle, then the part is printed at 230 deg C for PLA. On both printers, a K-type thermocouple was wedged into the front facet of the hot end under the silicone sock.
The bad hotend thermistor was consistently 20-30 degrees under the setpoint. This meant:
1) I had a hard time with bed levelling; a lot of times when it's cleaning it would report a dirty nozzle because the nozzle was only 140, 150 deg C.
2) The print would print for a few layers, and then detach off the bed because I was barely printing at 170 deg C.
To Prusa's credit, they are shipping me new replacement thermistors. But it surely looks like a bad batch of thermistor made it past QC and into printers into the field, and if someone else has the same issues... well, this is one thing to check.
-=- Terence