Maxtemp Error at higher temps only  

Maxtemp Error at higher temps only

I've never seen the Maxtemp Error until today. Accompanied by a loud beeping of the LCD box that can only be shut down by turning of the 240V AC supply.

I observed what was happening but do not understand why, maybe you can help.

This is the second time I am printing at 270C nozzle temp. The first time, there were problems with the nozzle temp falling once the fan came on in layer 2, and it was a vary small calibration set of two cylinders, printing at temperatures falling from 170C to 180C or something like that. The bed temp was probably 65C /55C as per standard PLA Slic3r settings. No problems occurred during a few of those prints starting with 270C.

Now I'm at the stage where I'm actually printing what I bought the printer for: A 200mm high surfboard fin, 9.2mm wide at the base and about 110mm along Y axis. If you have tried anything like it, you will know that is somewhat tricky.

I have had a fairly successful print using PLA, and now it's time for Polycarbonate, Polymaker PC-Max.

I read throught available info material and decided to set the nozzle temperature to 270C for first layer and 265C for other layers; and the bed temp. to 110C for all layers.

The first print failed, the second attempt got going due to sheer dumb luck and is looking good at z20.6.

Here is the problem description:
At foirst attempt, bed was pre-heated to 110C to check with IR thermometer what is actually happening. It's close to the glass transition temp of 113C but it turned out that the central area where the contact with the bed will occur only gets 98C to 104C hot.
The I started the print from SD card and the nozzle gradually heated up to the desired 270C, going up one deg C at a time, while the bed was already 110C. When the temperature had finaly risen to 270C, the extruder started to move, as it should, and continued to move and do the calibration sequence, although a loud warning tone was emanating from the LCD box.
Once the calibration was done, the print head just stopped where it would normally start the test strip extrusion, and only then was the Maxtem Error shown on the display.
Just after the beeping started, the nozzle temp was shown as about 300C/0C and bed temp as something like 108C/0C. Both temperatures kept falling gradually as they would when power has been cut.
I had to turn power off to reset the printer.

On second attempt, bed and nozzle started cold-ish, and nozzle reached 270C long before bed got to 110C. Nozzle temp remained steady around 270c, fluctuating by 2C maximum.
Once the bed temp reached 110C, it took another minute or so before the printer started to move the extruder.

The moment the extruder started, the nozzle temp started to fluctuate erratically, jumping up and down by 10's and 20's of degC, clearly erroneously reported, because it could not possibly change so fast. This continued until the calibration of all 9 points was finished and the printing began. The erroneous temperature reports never got over 300C, so the Maxtemp Error was not triggered.

As son as the actual printing began, the temperature display showed believable numbers without erratic fluctuations. It was something like 260C, gradually rising until 270C was reached, then fell to 265C as programmed at the second layer. Now at z23.3, still no sign of the slightest erratic nozzle temp jumps.

So I think it is not likely due to extruder movements wiggling some cables the wrong way, but something else.

Thanks for reading this far, any ideas what causes this?

Cheers, Mik