Bed Thermal Runaway  

RE: Bed Thermal Runaway

Hi Sean,

The message Bed Thermal Runaway, indicades that there is something wrong with the normal way of heating and maintaining the temperatuur.

The cables can be broken, or the heatbed thermistor has a problem. Is the heatbed thermistor still attached to your heatbed? The drop of temperature you mentioned could indicate that the thermistor is not connected to the bed, as is should be.

Let me know if this helps you

Frans

RE: Bed Thermal Runaway

@franskrau

The heatbed thermistor is still attached to the bed. I just checked the connect and it is still secure. 

RE: Bed Thermal Runaway

Does it happen when the part cooling fan kicks in.  If yes then try and move your part away from the center of the bed.

I print almost exclusively ABS with sometimes bed temps at 115 celcius depending on the part.  To solve the thermal runaway issue i had to bump up my PSU voltage to 13.3V and insulated the bottom of my bed.

RE: Bed Thermal Runaway

@lee-g

So I was wrong. It's not instant, but was a gradual drop. My cooling fan kicks in on the second layer and yes once it kicks in I noticed that the bed temp starts to decrease. I tried doing the same print, but moved it closer to the front edge of the bed and it still happened. 

RE: Bed Thermal Runaway

I was quit curious about your issue, so I tested my m2 right away 🙂 And I have the same result. Heated  up to 85 degrees, placed the printhead in the centre of the heatbed and z=3 mm, and turned on the partcooling fan max rpm. And I also have a steady "drop" of  10 degrees. So looks like this is "normal".

Maybe a workaround to lower the rpm of your partcooling to 70%?

 And as suggested, you should check the PSU voltage. I tuned it also a long time ago. There is enough info about "how to" in this forum.

Let us know how you proceed

Frans

RE: Bed Thermal Runaway

@franskrau

The design of the fanshroud is not optimal. It blows too much air directly onto the heated bed. You can either optimize the fanshroud or reduce the airflow. Maybe you can find a better design on thingiverse.com.

RE: Bed Thermal Runaway

I'll try turning up my Psu voltage and try reducing the partcooling. Thank you!! I'll let you know how it turns out.

RE: Bed Thermal Runaway

I agree with Karl-Herbert about changing the fan shroud. 

All official fan shrouds/part cooling ducts from Prusa are not great and are easy to improve upon.  I have tried them all from the MK2 onwards and have always changed them to something different with improved results.  The latest fan shroud is the worst and just creates a huge down wash of air with not really any direction.   

RE: Bed Thermal Runaway

@sean-h

Try something like this and adapt it to the MK2/S: https://www.thingiverse.com/thing:3390107

RE: Bed Thermal Runaway

Tried to upload my design but it won't let me.

RE: Bed Thermal Runaway

Zipped model

RE: Bed Thermal Runaway

Update: Tried reducing the fan speed down to 70% with the same issue. Regarding the voltage, yes I've found a few guides on the forums, but if I'm being honest I don't trust myself enough to actually mess with the psu and increasing the voltage, nor do I have the tools to measure. For the fan duct would I be able to use PLA to print it rather then ABS? I can't print any materials it seems that go over 70 degrees. PLA has been the only thing I've been successful on. 

RE: Bed Thermal Runaway

You could try it and see how it holds up.  It might let you print PETG or hold up long enough for you to be able to print a PETG version of it.  My shroud will allow you to reduce fan speeds.  You could try half the default speeds and see if it stops your temp drop.  The thing to do is run the minimum amount of fan without sacrificing print quality. 

What country are you in?

Strongly suggest getting a cheap multi meter so you can up the PSU voltage.  Will make a big difference.  Insulating the bottom of the bed also helps significantly.

RE: Bed Thermal Runaway

@lee-g

I'm in the United States. Do you know off the top of your head what a safe PSU voltage would be to up it too? I really know little to nothing about checking voltages and upping it aside from what I saw in the other forum posts. I can also look to see what I can find on insulating the bed as well. Thanks.

RE: Bed Thermal Runaway

Thats a shame.  If you were UK i could've posted you an ABS version.  

Maybe try insulating the bed first as it might just be enough.  I used single corrugated card board as i had some at home to use.  I will likely get shouted at now for using cardboard but in reality the temps never get anywhere near the ignition point of cardboard and my printer is in a safe location so if the printer did go up in flames it would not threaten my house or belongings.  The risk with using cardboard is if there is an electrical fault then you can get very hot localised heat that could ignite the cardboard so although i am using it then i would not necessarily recommend it.

I am running my PSU at 13.4V but have heard of others going higher.  Even if you took it to 13V you will probably find that is enough and if it is not you can always take it a little higher.

RE: Bed Thermal Runaway

@lee-g

To isolate the bed i would prefer material like this one: https://imgbin.com/png/BHQZ0dz1/aluminium-foil-multi-layer-insulation-thermal-insulation-building-insulation-png

RE: Bed Thermal Runaway

Thanks Karl that is certainly the type of material that should be used.

My problem is I always look for the cheap option.  I have no problem spending money if required but if there is a cheap option I will almost always take it.  My cardboard and tin foil works very well so it's staying for now at least.

Anyone else should use something like what Karl has recommended.