Sota heat bed problem

[geoffs]

I've been printing semi-successfully with PLA and I'm again trying ABS - it's been a while since I used the printer.
Trying to set bed temperature to 110C but it just wont get there, crawls to a stop at about 105C

I'm running 12V supply and I have the thermistor type set to 4. It's fairly cold here (3.2C) but the room is heated to about 18C.

I seem to recall some complaints that the resistance of the Sota heatbed was too high but can't find a reference to this.

I do have MkII heatbed that I could install but I like the blue of the Sota - it matches the rest of the machine

Any suggestions short of running off 24V?

[zoominbc]

Your supply probably can't drive the heater. You may think about using a separate 12V supply to drive the heater alone, some supplies allow adjustment and you might be able to set it to 13v or so . I would also try the max temp you can get to anyway, ABS will probably work for you.

[geoffs]

The supply is 12V/30A so should have enough current to drive both the heatbed and the extruder.

I measure the resistance of the heatbed at 1.4 ohms which with 12V gives about 100W. The jumpers are set on the heatbed to the 12V/130W setting (there is a 100W setting) so I'm thinking there's a problem with the heatbed traces somehow.

I think I need to measure the actual temperature of the heatbed rather than the one it's reporting. My laser pointer style temperature gauge has problems getting a constant temperature reading which is always higher than expected.

I did make a modification last night in putting a couple of layers of corrugated cardboard between the heatbed and the aluminium plate. It did make some difference to the speed that the bed heated up but still short of the temperature I had set.

I have a ramps 1.4 board that I managed to pop a FET on. I might modify it so I can run it from 24V. It might mean I change the resistor in the hotend (j-head) so I'll need to check that.

[geoffs]

As part of a rebuild, I've changed a couple of the solder links on the Lava board and it's heating up much more quickly than it did.

I now have a problem that I'm guessing is related to the change I made in that the temperature is reporting low. Luckily I have an infra-red thermometer I was using and stopped the heat up before any damage was done. Reported temperature was 47 degrees but measured with the infra-red is was 100 degrees.
Thermistor measures 10K ohms at 24 degrees and Marlin is set to type 4, 10K heat bed thermistor.

Question then is does anyone have a circuit diagram of the blue Lava board? There are some components on the board that look like surface mount transistors but I don't know if these are only used when the bard is configured for the Ord Quatum printer. I'm hoping it's not the thermistor as it's a surface mount device that will take me some time to get a replacement for.

EDIT: Traced out the circuit for the thermistor and it seems it's not connected to anything other than the two connecting wires. Time to order a new thermistor.

[geoffs]

I found a thermistor (Semtec 104GT-2 100K) which I bought as a spare for my J-Head hotend so I wired it up and taped it to the bottom of the hot bed and reconfigured Marlin.
Same issue, the temperature that Marlin reports is a lot lower than I get my hand held, infra-red thermometer.
One thought I had was that the thermistor is mounted on the bottom of the heat bed PCB but the heating element is on the top, perhaps the fibreglass is providing enough insulation to make a difference?
I have an unused reprap Mk2 heat bed PCB so I sat that on top of the Lava hotbed (not connected to anything)and after a few minutes, the infra-red thermometer was reporting the same as Marlin - within a few degrees.
If I put a sheet of glass on the heat bed, after 10 minutes the infra-red temperature is 10-15 degree lower than that reported by Marlin.

The easiest solution is probably to just put in the Mk2 but it doesn't have the nice blue LEDS There's also the problem of sourcing some of the same style standoffs as used on the Lava bed.

[fulg]

It is unfortunate but the LAVA bed is dead hardware, I would not waste any more time with it. There is a thread here that says the thermistor on it corresponds to table #4 in Marlin.

I currently have an MK3-ALU board from reprap.me with a surface-mount thermistor, because the aluminium bed spreads the heat evenly and the thermistor cannot fall off. I also miss the LEDs from my MK2B but given the choice, I prefer the safety over LEDs! I find it hard to trust a heated element to a piece of Kapton tape, which is the usual recommendation for mounting thermistors.

As far as mounting to the Y carriage, I haven't seen the standoffs for the LAVA bed. I use M3x25mm bolts with silicone tubing (not springs), with a nut below the PCB screwed tightly so the bolt cannot turn. Then on the underside of the Y carriage, I have locknuts with printed inserts to make bed leveling adjustments without a screwdriver and a wrench. These inserts were the very second thing I printed (the first was a leaning calibration cube!), with an EZStruder ziptied to the X carriage, because I hadn't found a proper mount yet!

Cheers,
Ben.

[geoffs]

The mounts/standoffs on the Lava board are swaged/crimped (not sure) onto the PCB and don’t protrude above the print surface which screws/bolts will.
A bit more testing shows that if I'm patient and wait for the surface of the bed to be at the required temoerature via the infra-red thermometer, then the termperature is maintained quite accurately. I'm trying to source a piece of 3mm glass to replace the current 4 or 5 mm I'm using.
As soon as I get some suitable terminals to connect the Mk2, i'll give it a try.

Geoff

[fulg]

Right. I don't really mind the protruding bolts as they are lower than the glass surface I use. If I was printing directly on the aluminium, I would countersink the holes in the alu plate and keep the printing surface flush, I haven't bothered to do this yet.

For the glass I've found it cheap from a seller on eBay, and it was very well packaged to survive a trip from China to Canada via standard mail, I was impressed. I can get them locally but at five times the price, it is worth taking the risk with international shipping...

Cheers,
Ben.

[geoffs]

Hi Ben,
I might just invest in an aluminium heatbed. I need to order some parts for my X-Carve and the local source sells 3d printer stuff as well, combined shipping keeps the price down a bit.
I've considered borosilcate glass but didn't like the chances of getting it intact from eBay. I have found a local (eBay) seller that guarantees to get the glass to you in one piece or to keep sending it until it does. 3 times the price of the seller you listed though.
Another issue I have is that the weather is starting to get colder so that effects heat up time. Not quite cold enough to justify lighting the fire though.

Geoff

[fulg]

To help with the heat-up time, I glued a cork backing (from reprap.me) on the underside of the MK3-ALU PCB, and when pre-heating, I use a piece of corrugated cardboard on top of the heated bed to try and minimize the heat loss (just don't forget to remove it before starting a print!). I haven't measured if these measures make a real difference. I can get to 60C in 2 or 3 minutes, and 90C in 5 or 6 minutes. The cardboard plate is required to get past 100C in a reasonable amount of time.

I'm on 24V, but I haven't done the 24V-on-12V-terminals trick yet, not sure the MOSFETs on my Azteeg X3 can take it. So the heat up time is the same as if I was on 12V.

Cheers,
Ben.