Not melting plastic  

Extruder tension?

Have you checked the extruder tension screw (left side)? If you examine the ejected filament, you should see some marks from the hobbed gear. If not, your tension might be too low. Are you hearing any clicking noises?

It's helpful to install a small magnetized extruder visualizer so you can see the extruder motor motion.

Extruder tension setting

I am not seeing any marks on the removed filament at all. I will try increasing the tension on the screw.

What kind of small magnetized extruder visualizer do you have in mind. This is my first time hearing of such a thing?

Just something simple that attaches with a magnet to show extruder motor movement

There are all sorts of clever designs to pick from. Here's my favorite:

RE: Not melting plastic

just paint half the end face of the shaft of the extruder stepper with a pen so you can see better how it rotates. no need to print anything to stick there.

do you get filament out the nozzle if you (carefully) manually keep pushing it down into the heated hotend? are your extruder gears aligned as per the instructions?

Blocked hot end

Update: I increased the tension a very little and now the filament has definite marks from  the hobbed gear.

The filament auto-loads smoothly and effortlessly

I can see the hobbed gear turning as it tries to feed filament into the hot end and then just grinding on the filament without the filament moving.  What is possibly happening is that the filament is hitting some kind of internal blockage (guessing) and just plain stopping before it gets to the hot part of the nozzle and melting.

Has anyone taken apart the nozzle and checked to see what is in the hot end? I found a small, round, black piece of plastic debris in my kit, not obvious as to where it would go, and could not find the "accupuncture" pin  described in the assembly manual anywhere in my kit. It is included in a separate tube? If so I didn't get one. I really don't want to have to disassemble the entire extruder to find what is inside the hot end.

During calibration, I unfortunately had the extruder tip grind into the heated bed  for a few seconds before I could power off, and it ground away the black plastic and reveal copper traces underneath. I am hoping that this hasn't affected my heated bed. The exact placement of the Super Pinda sensor seems a little too critical. I used the thickness of a cable tie as a starting point for distance but it failed calibration at that height, so I lowered it incrementally.

I have to say that the construction of the entire unit was difficult and meticulous but doable, taking 4 days and stopping when I got tired enough to make mistakes late at night. This is not my first kit project; I have made many, many of them and the instructions are top notch with a few English errors along the way, which is understandable. The assembly manual could benefit from more close up photos of the expected results as the printed manual has a contrast problem showing black on black parts, so I used the online manual on my laptop to see the higher resolution photos as recommended. After construction, everything works as it is supposed to, except it does not print.

The manual suggests that the tension will need to be adjusted, and so it was.

Now to start by removing the brass extruder tip and seeing what blockage exists inside.

-Brett

Extruder temperature

I painted the end of the extruder shaft and it now gives a very nice presentation on the rotation of the extruder motor. Thanks!

Now I suspected that the temperature of the heated end was at fault, and I checked with both an IR reading pyrometer, and a direct coupled digital cooking thermometer (accurate to within 1 degree). No mater what the filament setting, the tip will not go over 179 degrees C.

I think this is the problem. The hot end is not HOT! 

After changing to filament type PC which has a hot end temp of 275 degrees C., I pre-heated and check tip temperature again. Still stuck at 179 degrees C. As far as I know, not hot enough to melt PLA plastic. I am able to touch the hot end with my finger tip and it warm but does not melt my finger, either.

This is very confusing.  What would cause a hot end to get warm but not hot? Perhaps the thermistor is bad? The control panel says that it has reached temperature, but when I did an IR (non-touch) test on the heated bed which is saying it is at 60 degrees C, it is 50 degrees C.

So perhaps come temperature conversion is off? Is there an adjustment in the software for correcting temperatures?

-Brett

that sounds strange. your thermometer does go higher than 179°C? can you set and reach temps below that?

if there's an actual issue with the hotend temp, you should try contacting prusa for help

Hot end temp

Yes, my IR pyrometer goes to 2000 degrees, and my digital contact thermometer goes to 700. So, they are accurate and with in a few degrees of each other.

I will contact Prusa and see if the controller is kaput!

Thanks!

-Brett

Blocked Hot End

After a thorough and exhaustive conversation with the most excellent Prusa customer service team, we discovered a blockage in the cold tube part of the hot end assembly, and a new one is being shipped. These things happen in manufacturing. I am suitably impressed by the overwhelming competence of Prusa's support system, and just how intelligent they are.

Will update once I receive new hot end assembly and can print. I am looking forward to a long and happy printing ability with my new Mk3+.