Heated bed question

Hi Everyone. I am returning to a project from over a year ago I had to abandon due to RL. Last I was working on my Prusa (I guess you call it an i2 now) I had the PCB Heated Bed MK1 with glass mounted to it with clips and the thermistor taped to the edge of the top surface of the glass using kapton tape.

My goal was to print with ABS (which I never accomplished since I didn't have a working fan, nor a bed that would heat to 110C).

I finally got myself an external temp sensor and I found that when the thermistor was reporting 89C that it was fairly accurate - where it was located. But the center of the PCB was closer 111C and the temp varied between that and 89 close to the edges.

So my questions:

1. Is this temp variation from center to edges common? If so does a 20 deg swing from the center to the edges surprise anyone?
2. Should I move my thermistor? If so, mounting it on the bottom of the PCB might result in a higher temp reading - should I mount in the center or 1/2 way to edge, or all the way to edge (all referring to bottom of PC?
3. What is a good way to mount the thermistor to the bottom if going that way - some kind of epoxy? Or something like Arctic Alumina?
4. Am I overthinking all of this? Is the temp variation I'm speaking of when printing ABS even that important?

Quote
ifrisbie
Hi Everyone. I am returning to a project from over a year ago I had to abandon due to RL. Last I was working on my Prusa (I guess you call it an i2 now) I had the PCB Heated Bed MK1 with glass mounted to it with clips and the thermistor taped to the edge of the top surface of the glass using kapton tape.

My goal was to print with ABS (which I never accomplished since I didn't have a working fan, nor a bed that would heat to 110C).

I finally got myself an external temp sensor and I found that when the thermistor was reporting 89C that it was fairly accurate - where it was located. But the center of the PCB was closer 111C and the temp varied between that and 89 close to the edges.

So my questions:

1. Is this temp variation from center to edges common? If so does a 20 deg swing from the center to the edges surprise anyone?
2. Should I move my thermistor? If so, mounting it on the bottom of the PCB might result in a higher temp reading - should I mount in the center or 1/2 way to edge, or all the way to edge (all referring to bottom of PC?
3. What is a good way to mount the thermistor to the bottom if going that way - some kind of epoxy? Or something like Arctic Alumina?
4. Am I overthinking all of this? Is the temp variation I'm speaking of when printing ABS even that important?

1 I have as much as 10 degrees drop from center to edges (very far edge) I had close to 20 but I added cardboard between the heater and the bed plate so I have glass-heater-cardboard-bed plate.
2. I keep mine mounted in the center board has a nice hole through to get close to the glass
3 I mounted mine with RTV copper (high temp automotive gasket maker)
4 maybe a little but ABS is a little picky skirts are your friend when printing larger objects to help keep from corner lift.

Edited 1 time(s). Last edit at 08/28/2015 01:52PM by jasonsaffle13@gmail.com.

Put a piece of cardboard under the heater to help insulate the bed. Also, while you're heating the bed, place a piece of cardboard on top so that it helps reflect some of that heat back down into the glass. I also have my thermistor mounted in the middle of the bed but mine is with kapton tape, just make sure it can't get yanked off by the bed movement.

Edited 1 time(s). Last edit at 08/28/2015 02:16PM by widespreaddeadhead.

Quote
[email protected]
1 I have as much as 10 degrees drop from center to edges (very far edge) I had close to 20 but I added cardboard between the heater and the bed plate so I have glass-heater-cardboard-bed plate.
2. I keep mine mounted in the center board has a nice hole through to get close to the glass
3 I mounted mine with RTV copper (high temp automotive gasket maker)
4 maybe a little but ABS is a little picky skirts are your friend when printing larger objects to help keep from corner lift.

I don't have the MK2 version with a hole in the center and even if I did my therm isn't the type to be mounted like that (I think). I've read about the cardboard - I'll give that a go - thanks to both of you for the hints.