Slow getting to 110C bed target on 10" i3v

[JimG]

The AC SSR's I use for controlling nichrome heaters drop around 0.6V across the switched terminals when conducting. They are spec'ed at around 1.6V drop, but in my applications I only see around 0.6V.

When dealing with mains voltages this is not a big deal. I will be curious to know what kind of voltage drop you see across your SSR? The relay, although a little noisy, at least does not drop much voltage when it is switching.

In AC heater circuits I have seen far more SSRs fail in a shorted condition than an open condition. It would be a lot better if they failed open, but Murphy's Law is at work, I guess.

Jim

[rco3]

If your heater bed isn't in contact with the glass plate, then you are depending on heat transfer across a layer of air which is a great insulator. Glass is a poor conductor of heat, too (0.96 W/m-K; air is 0.02, copper is 400 W/m-K), so heating one region of the glass plate does little to heat the adjacent regions. So what you really need is contact between PCB and glass all around the glass plate. Remember, that thermistor tells you what the temperature of the PCB is in that specific area of the PCB; the solid copper layer should do a pretty good job of distributing the heat, but you need to get that heat from the PCB to the glass evenly as well.

When I assembled my 10" i3V, I noticed some warp in the heater PCB and corrected it by gently flexing it. I think that ideally, you want the PCB to be convex up so that the glass plate sits on a very slight hump in the middle, and then you use the clips to pull the edges into contact as well. The middle hump (or ridge, in my case) keeps some pressure against the middle to maintain contact (not too much, you'll flex the glass). I find that clips on all four sides are usually needed to get good adhesion near the edges, too. The LAST thing you want is for ANYTHING to be between the glass plate and the PCB. You might try a very small amount of heatsink grease to fill very small gaps, but that's a poor substitute for direct contact. It's probably more useful as a visual to see where the glass isn't touching. Where you have good contact, the grease should be squished thin enough to see the PCB through it. Where all you see is grease… need better contact.

The nice thing about this PCB / glass bed scheme is that you can replace the glass when it breaks. Otherwise, I'd be tempted to simply glue nichrome directly to the glass.

--Rob

[JimG]

Makes me wonder how much more effective, if any, a mirrored surface might be? I like the idea of clips on all 4 sides. Think I will give that a try.

Thanks.

Jim

[JimG]

Glass is actually not quite as bad a thermal conductor as I thought. While metals like copper are hundreds of times more conductive, glass is more conductive than water, wood, and even salmon

There does not seem to be much difference, conductivity-wise, between ordinary plate glass and borosilicate glass. The thermal expansion of the boro, however, is only around 1/3 that of plate glass.

Glass is not quite as conductive as a thin sheet of rock, like slate, would be. But it is pretty close.

Jim

[AbuMaia]

glass is more conductive than water, wood, and even salmon

Have you been cooking on your print bed? :-P

[JimG]

Have you been cooking on your print bed? :-P

Now, there's an idea ....

Jim

[Steven57]

For what it's worth I used 12gauge for power and I used pipe wrap insulation (yellow fiberglass) under the glass. Cost about $5 at Lowe's/HD and three piece fit perfectly under the 10" bed. And there's plenty left over to share!

[TechMasterJoe]

i tested and i see .6103v drop and have sense built a mosfet board based on
http://www.newark.com/international-...-7p/dp/28W1167
this will be a ground switching setup with a real mosfet driver feeding Gate voltage + opto-isolated
apx RDS will be under .0008ohm (test board .000691)
this i think will get the volt drop to stay under 0.02v at the switch
2 mosfets in parallel each rated for 260amps, 300amps pulsed
this will come with 10ga wire apx 3in long soldered on with XT60 connectors as i have tested them and see less then .01v drop at 30amps
i plan on making a few if anyone wants one PM me
test done with 200mhz Tektronix oscilloscope, Fluke 87-5 + i 410 amp clamp
I will include tested RDS with finished part and a thermal photo under 35amp load
test board is 1in by 1.15in 1.5oz copper PCB with 10ga wire soldered on temp on mosfet cases is 6.3c above ambient without a fan under 45amp static load
apx cost for each board will be around 11$ for 15$ i can include an extra 6ft of 10ga silicone wire (842 strands) + apx 3$ for shipping US only for now
this will cover just about any hotbed install i think

hey if anyone is bored hook your hotbed to a audio amp and place a magnetic on on the bed (best to take the glass off so it don't get broken)
it plays music (act's like a voice coll)

i just did some numbers based on the wire i have 10ga silicone ultra flex and yes it's true 10ga but even true 10ga will net a 2.5~3.5% loss in only 6 feet
i can cut this in half if i drop down to 8ga but price might go up a buck

[JimG]

Very nice solution.

Jim

[dacb]

Here is a data point... On my 10" MF, when it was in the garage, and ambient was > 20 degree C, I could reach 110 without a lot of struggle. Below that, it was constant on and problematic although I was well below the rated PSU wattage. I don't print a lot of ABS anymore so it isn't much of an issue and the printer is inside where 22 C is pretty close to the norm. Heats to PLA 70 deg by the time the extruder is hot. I like that ABL lets the bed warm thoroughly while probing. My sensor is mounted below the heater PCB per the directions. I use a cardboard spacer which doesn't reflect heat upward as I'd like. I tried (briefly) a silicon potholder and found it the same as cardboard. So now we use the potholder in the kitchen. I've wondered: How do Al beds do? I occasionally come across a slice of Al that would work but I think about adhesion. The sensor below doesn't really reflect what is actually going on at the glass surface. Particularly hardware store glass like I run. I always figured a drop in thermal transmission through the glass so the extra time lets the temperature penetrate and as glass is an insulator this can take some time vs. what the probe reads.

IIRC, some, like maybe clough42, don't use a heated bed for PLA? I also recall 200mm bridges from his masterpiece, so I aspire.

Not a 12in so YMMV.