Print bed issues....  

Re: Print bed issues....

It sounds like you have bad PID settings. Doing the PID calibration should help.

Re: Print bed issues....

Thanks, I tried that and it is better but I still get the odd occasion when the hot bed takes too long to heat up and I get the 'Thermal runaway' message again. Temperature doesn't seem to fluctuate like it did before, but I still can't preheat to 100 for printing on ABS - get to around 60-65 degrees (eventually) and then I get the error message

I am hoping that this will be resolved with the new 2.5 print bed upgrade!

Chris

Re: Print bed issues....

Sounds very much as though you have connector problems ("high" resistance joint).

Please check out the heated bed connector at the RAMBo end as a matter of urgency. You could end up with a fried connector.

There may be a bad solder joint on the heated bed. Also check that out.

The other possibility is that your 12V supply rail is a little low - this can be adjusted without issue on the PSU. An output voltage of up to 13.5V is fine.

Peter

Re: Print bed issues....

Thanks for this - I did wonder about this, but have been using the printer without problems for some time (including with ABS) - I did inspect the soldered connections on the heat bed carefully and they looked alright but I guess checking the resistance at the Rambo plug end wouldn't hurt - and perhaps getting a new lead if necessary?

What has thrown me is that this has just started happening - I thought it might have been the heat bed thermistor failing and replaced this last week, but I guess the bed movement might have damaged the 12v power lead?

Not sure what the resistance of the heat bed should be though, any idea? I have seen a post that suggests 0.8 Ohms? I could hook up a multi meter and see how much current it is drawing (be helpful to know what the wattage of the heat bed actually is, might ask Prusa)

I saw this thread

https://shop.prusa3d.com/forum/original-prusa-i3-mk2-czech-%C4%8Cesky--f22/board-smoking-and-heated-bed-resistance-low-t3742.html

and wondered if it was worth trying the controller mentioned towards the end - as it was only 5 quid I have ordered one, but my heat bed resistance is high if anything?

Regards

Chris

Re: Print bed issues....

The resistance of the heated bed is between 0.8 and 1 Ohm; not really measurable with a DMM (the leads will have a similar resistance).

It is most likely that the connectors have moved over time; maybe corrosion or dust causing a higher resistance connection.

Remove the heated bed connector and inspect inside the socket and the plug. Look for any blackening.

Replace the connector; remove and replace a couple more times and ensure it is fully homed.

With the cover open, heat the bed to 60 degrees and see if you can feel any heat around the connector (and underneath).

Peter

Re: Print bed issues....

Yes, I take your point - I have had the connector in and out of the Rambo quite a few times when changing the bed thermistor - I'll check tonight to see if I have pushed it all the way home, or it gets warm?

Thank you

Chris

Re: Print bed issues....

Looks like my connector in the Rambo is fine - I ran the heat bed up to 60 degrees for an hour and the plug in the Rambo was cold, and on inspection it looks like new (it is actually a Molex plug - http://www.molex.com/molex/products/datasheet.jsp?part=active/0395300002_TERMINAL_BLOCKS.xml )

I guess now I will start looking at the PSU voltage and perhaps the current the heat bed is drawing - when I checked the head bed the centre (where the thermistor is located) it felt cooler than the area around the outside - I am wondering if I have damaged the tracks in the centre of the bed...

I tried to get the temperature to 75 degrees and it very slowly got there, I then tried 100 and after a while got the 'Thermal runaway' message again - temp might have gone up by one or two degrees perhaps 🙁

One thing that has occurred to me is that if the upgrade to 2.5 means having to print ABS parts I am going to have a problem...

Chris

Re: Print bed issues....

I had an issue with tbe heat bed cable. It was beginning to fracture due to excessive flexing. The nylon support filament had become unplugged at the heat bed end. And the cable was overheating and making the insulation melt... regards Joan

I hate mobile phones...

Re: Print bed issues....

Joan is using her phone again... 😉

Yes, the cable can also partially break inside the sheathing (happened to me with the Mk1).

The bed has zonal heating so the corners should be hotter than the centre.

Peter

Re: Print bed issues....

You noticed!

🙁

Joan...

Re: Print bed issues....

Thank you Joan and Peter (and everyone else!)

Next I am going to have a look at the cable - measure the voltage and current drawn, and have copied these posts to Prusa to see if they can advise.

Apparently the 2.5 upgrade does not include any printed parts, just the STL files so you have to print them yourself - although when I ordered the upgrade I thought it specified what colour of parts?? Fortunately I did buy a spool of 'safety orange' ABS with the printer, unfortunately (at the moment)
I cannot print using ABS!

... I am starting to feel like I am spending more time 'repairing' the printer than using it 🙁

Chris

Re: Print bed issues....

First thing you need to know is that you don't need to print replacement parts in ABS.

Everything I have printed in this regards has been with PLA.

Peter

Re: Print bed issues....

Including the extruder parts?

I was told PLA would deform with the heat?

In any event having paid for a spool of 'Prusa orange' I guess it's just as well to use it <g>

Thanks for the info about 'zonal heating' - I didn't know that, one less thing to worry about!

Chris

Re: Print bed issues....

Including the extruder parts?

Even the fan nozzle which is the closest thing to the heat block.

Peter

Re: Print bed issues....

Measured 12.7 v at the Rambo socket and 12.5 v at the heat bed connector - the impedance across the heat bed plug was 1.1 ohms

Not sure what to try next, there is continuity from the plug to the bed for both circuits, but I would get this even if half the cable strands
were broken, as long as one was still intact?

Only way I can think of checking this out is to replace the two circuits - perhaps with silicon meter wire which is very flexible and hopefully can
take a pretty high current. Can't measure the current draw as my meter only goes up to 10 amps unfortunately.

Guess I could try increasing the PSU voltage to (perhaps) 13.5 volts? Not sure how to do this easily without disconnecting all the cables and
removing it from the printer 🙁

Perhaps I could cut an inspection window in the side of the printed cover, not sure exactly where though

No response from my email to Prusa yet...

Chris

Re: Print bed issues....

I think I would replace the cables.

I used really heavy duty speaker cables, but anything capable of at least 15 Amps will be OK. Just make sure your RAMBo connection is good.

Peter

Re: Print bed issues....

Ok, I guess that's a fairly simple task, and will rule that out of the equation - I am sure I can find some suitable cable.

Thanks for all the advice

Chris

Re: Print bed issues....

I have now replaced the heater cables but this did not make any difference 🙁

I did notice that there were a few blemishes on the PEI sheet where I have removed prints, and as I had a replacement
I decided to replace it and discovered what looks like a break in the bed tracks just to the right of the centre - initially
I thought this was above the thermistor location but it was on the other side.

The area is probably the size of half a grain of rice (or less) but this will have an effect I would guess.

Think I have reached a bit of an impasse, as if this is the cause I am not going to replace the heat bed when I have an order
in for the upgrade to 2.5 - which includes a new heat bed - so perhaps it is better to fit this when it arrived and see if this
resolves the problem?

Chris

Re: Print bed issues....

That is very surprising. I think I can recall only one other user with damaged bed tracks; basically, it just doesn't happen.

Sorry to have sent you down the wrong path, but in this instance and as you suggest, it is probably better that you wait for the upgrade.

Peter