Slow getting to 110C bed target on 10" i3v

[printbus]

One comment on the cloth idea - I'd only do that if I know the bed can (eventually) reach the temperature I want without it. If it can't, the heater may not be able to maintain the bed temperature when you remove the cloth.

Seeing temperature characteristics change when the clips are moved doesn't really surprise me. We all know these heater boards aren't all that flat or rigid, and changing the clip locations likely shifts where the heat bed is touching the glass and where it isn't. If you happen to end up with even a very small gap between the thermistor area of the heat bed and the glass, that area of the heater circuit board will heat up faster than if the board is pressed against the glass and drawing heat into the glass.

[usarmyaircav]

so after changing the wires from the power supply to the relay my bed heated to 110c in 8 minutes. I still need to make a small hole where the thermistor is in the cork but much better!

[JimG]

OK, reporting back with the results of the great wire swap of 2014.

Before starting the exchange, I measured voltages at the PSU and at the solder joints on the heated bed while the system was in its "stock" configuration: 12.06V at the PSU; 11.24V at the heated bed. Total power loss from wire resistance around 13% (but remember I already had pretty heavy 16AWG wire for everything except the heater board leads).

I completely removed the 18AWG leads from the bed by unsoldering them. Next time, I would NOT do this (see more below). I soldered new 14AWG lead wires directly to the solder pads on the underside of the heater. I ran the 12V side of the heater circuit from the bed to the relay board, and from the relay board to the PSU. I ran the ground side of the circuit directly from the heater to the PSU. All wires now 14AWG.

Result: 11.59V at the heater. Approx. 7.5% loss of power due to wire resistance.

I don't know yet how this will translate into warm up times for 110C ABS printing. Along the way I also discovered that the heater is very slightly warped: the top surface is very slightly concave across the X direction. It is most noticeable on "my" end of the bed. I built a flexible shim from printer paper and pushed it in between the plywood and the underside of the heater. This was just enough to bring the heater up into contact with the glass, but not enough to interfere with bed leveling. Paper? Well, my cardboard insulation hasn't caught fire yet so I figured why not.

If I did this again, I would proceed along my original mental plan, i.e., leave short pieces of the original 18AWG leads in place, and solder the new 14AWG leads to the remnant stubs of the 18AWG. But I do a lot of soldering on PCB's, and the 14AWG wire is VERY flexible, so I felt confident and decided to do a complete leadectomy. Mistake.

The physical thickness of the 14AWG poses some difficulty in securing the wires to the plywood bed because the little holes that Colin provided for a wire tie are too close together for 14AWG. So I had to unsolder the new 14AWG leads and point them toward each other rather than 90 degrees to the back of the heater. Even doing this left a wire bunch that was too thick for the small spacing between holes in the plywood. I eventually got it all sorted out and got the wires secured well, but it took a bit of doing.

Did I mention the part about pulling the solder pad off of the PCB? Yup, applied a little too much heat on one of the new leads. I had to scrape length of solder resist off of the main trace on the board, clean and tin the now-exposed copper, and resolder the new lead.

Did I mention the part about how well 14AWG wire conducts heat away from the tip of your soldering iron? If you attempt this, dig around and find a nice heavy tip for your iron. The little tips I use for most PCB work didn't stand a chance!

Jim

[old man emu]

I found that my heater pad was warped in a similar way to yours. I forget whether I did anything about it or not since I figured that I was printing on a glass sheet anyway and that would be flat as it appears to be. I didn't consider any effect that lack of contact with the glass would have. I was more concerned about getting a level print surface.

One thing I would consider is attaching the thermistor to the glass plate to obtain a more accurate reading of the temperature at the actual print surface. That could be difficult as it is hard to find a space between the heater pad and glass.

How have you secured your thermistor to the heater pad? There was a tip in relation to fitting the extruder thermistor of setting it in place in is hole with a blob of auto muffler putty. I wonder if that would work with the heater pad and be better that kapon tape.

Don't forget that going from 18AWG to 14AWG is like going from a fire hose to a garden hose. There's a mighty big constriction to flow. I think you did the right thing by struggling with the 14AWG all the way to the pad.

Old Man Emu

[JimG]

I applied some of that white silicone heat sink grease to the underside of the heater, then plopped the thermistor in and on top of the grease. Taped it down with kapton.

The other thing I did was glue a very fine wire thermocouple to the underside of the bed so that I can independently monitor its temp. Just to keep the thermistor honest.

I found that the thermocouple reading and thermistor readings never vary by more than 4C. This would be very crude for a lot of applications, but I think is just fine for this one.

Jim

[Stigern]

One comment on the cloth idea - I'd only do that if I know the bed can (eventually) reach the temperature I want without it. If it can't, the heater may not be able to maintain the bed temperature when you remove the cloth.

Seeing temperature characteristics change when the clips are moved doesn't really surprise me. We all know these heater boards aren't all that flat or rigid, and changing the clip locations likely shifts where the heat bed is touching the glass and where it isn't. If you happen to end up with even a very small gap between the thermistor area of the heat bed and the glass, that area of the heater circuit board will heat up faster than if the board is pressed against the glass and drawing heat into the glass.

Yeah, the only reason I use a cloth is for faster heatup and evenly heating the glass plate. As for clips, I have added 3-4 more clips, some smaller ones.

[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

[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!