I just got a 200mmx200mm prusa 3i clone pcb heater bed from china and it si bent something like 4~5mmside to side. I would have expected these to be flat? or is such a degree of warping OK? http://www.aliexpress.com/snapshot/7722831927.html?orderId=75667151900721
Annoying, but pretty common. At that price you get what you pay for. (In fact, at that price, you may have bigger issues than warping – please check the trace resistance to make sure it isn’t dangerously far out of spec!)
The standard solution for these cheap PCB heaters is to bolt it to something flat and rigid. (An aluminum sheet heat spreader is typically a good idea in any case, to minimize hot spots.) I’ve heard people propose flattening them in an oven, but I don’t know if that sounds like a good idea.
What is the spec? or where would I find it? I assume some sort of ohm reading? I have opened a dispute with the seller, for myself I expected it to be reasonably flat.
There’s a range and some disagreement. http://reprap.org/wiki/PCB_Heatbed#MK2b_Dual_Power_Technical_Details
What you should do is figure out how much current your setup can safely provide (PSU, fuses, connectors de-rated 20% for adjacent wires carrying high current, wiring gauge) and measure the heatbed resistance and make sure it won’t draw more current than that. Don’t forget that your RAMPS (or whatever) power supply connector sees even more current draw than your HBP connector, unless you have a SSR or board with separate power inputs.
Soooooo many people have burned up heatbed wiring and connectors over the years…
I am using an ATX 520w computer PSU. NB This is all a test bed to try some monitoring / tests on and not going on an actual printer. So if its stuffed it is no biggee, more a learning experience. However I will chase a refund to see what happens ie there is a “satisfaction guarantee” Best I can find so far is 1.2ohm, but no real tolerance ie +/- I will check it when I get home.
12v/1.2ohm=10A
That’s probably about the most current you can safely put through a RAMPS connector, so I wouldn’t go any lower resistance than that without soldering 16ga wiring directly to the board for the HBP and PSU. Higher resistances are safer but weaker heaters.
Being made of Laminate FR4, this bend is common with almost all Mk2b heat beds.
You can clear that bend by manually bending from the opposite side. You have to do slowly and carefully, so mostly you can eliminate the bend. I have already done that.
As Ryan suggested, even after that if you use Aluminum sheet, you will almost get a flat bed.
In this case, you can go for MK3 Aluminum heat bed.
You can use an oven to flatten it, I do it at work all the time. The typical glass transition temperature of fr4 is 140-160C. The problem is that you need two equally flat surfaces to sandwich it that can withstand the heat without warping. You can do lower temps too, 110C or so, but it will take longer.
These types of heatbeds will never be perfectly flat, thats why it is better to put an aluminum plate, or glass above it. On mine I use a 1mm thick aluminum sheet as a heatspreader, then some cheap glass on top.
Don’t be tempted to buy a mk3 heated bed, the flatness of those are terrible.
The idea is that the glass is stiffer, so when you clamp it on, the pcb bends instead of the glass. A cheap aluminum plate won’t do because they’re not going to be flat.
1.2 ohm is good, if your meter is correct. Any lower and you risk drawing too much current. Any higher and the bed might not heat up fast enough or to the right temps.
I have a similar bed, I just clipped a piece of glass to the top of it and it’s good to go. Super flat and I can just remove the glass plate, throw on another one and get back to printing while the previous plate cools and I can remove the prints later.
@Stephanie_A
great advice.
Ah great advice on the oven method I didnt know about that, thanks. Anyway it is 1.5ohm which seemed a bit too high, and the seller has agreed to a refund so all good. I think for the real thing I’ll get a silicon pad type and use a machined/ground flat 4 or 5mm aliminium plate? I can get the ground / surfaced ali bed 300 x 300mm made in NZ for about $30~40US ( I have some quotes). I am told 1watt per sq cm so 900watts be enough grunt? I was also thinking of going 240volts AC as the printer is the box / enclosed type and I can fit door trips for safety/isolation and a RCCD.
Normal power range is more like 0.5w/cm^2… That’s plenty to hit ABS temps. A 900w bed is nuts. That much power will flex the bed plate whenever it kicks on.
Whether mains voltage heaters are reasonable/safe depends on how the bed moves. Delta: totally fine, as long as you’re competent at wiring. Mendel/i3: hell no, don’t even think about it. Too many flex cycles on the wiring unless you seriously engineer cable chains and flex-rated wiring and strain relief.
The electrics doesnt worry me but the flex does, no it wont be a 3i type.
What he’s saying is that 900 watts won’t heat up the aluminum evenly. It will get too hot, too fast. There is a sweet spot.