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Heated Bed Design Basic Questions not answered on the wiki (that I could see)

Posted by corry 
Heated Bed Design Basic Questions not answered on the wiki (that I could see)
March 12, 2015 02:53AM
Kapton vs Silicone: Pros/Cons? Wiki pretty much just has a picture of the kapton heater and says its similar to silicone....
Obviously kapton is thinner, and claimed to have a higher tensile strength, chemical resistance, etc....lots of things that probably don't matter much in 3d printing.
Silicone is cheaper, and has slightly higher thermal operating range, but that second part like the chemical/radiation resistance of kapton, really probably doesn't matter in 3d printing.

What I'm most interested in is heating efficiency. Being thinner means the insulation of the kapton heater has a smaller thermal mass, so it seems it has a slight leg up over silicone in terms of performance, but I can't find any proof of that, and the silicone may have better thermal characteristics.

Thermistor placement: Designing your own printer for your first printer probably isn't the smartest thing one can do, but what's life without a little trial by fire? smiling smiley Obviously having fun, but ignorant of a single simple problem. Where is the thermistor placed on these? Is it on the back side basically just to protect the heater from overheating? Are all heated bed temps for printing based on this, and not on a second one placed closer to the surface? Again, does it matter? What if the heater is insulated, does that change any of those popular numbers? I'm pretty sure those tables are just starting points precisely because of the differences in printer designs, but I haven't seen much talk about that, so it seems like that's the temp they want the bed to be at the place where the plastic hits the bed. Please correct me smiling smiley

When I get a little further along in the project and have some real pictures to show I'll post a lot more, like all the specific goals I'm going for. Right now I've been jokingly referring to it as the anti-rostock since the rostock seemed to be about being cheap/simple to build. Mine is neither smiling smiley Hoping to keep some of the other goals from it though, like speed, while adding other stuff as well (The complexity wasn't intentional!). Even if it doesn't work well, I'll post it to let some other person who suddenly finds time to work on something with similar ideas not to do what I've done smiling smiley
The main difference that matters, but only if your bed moves in the Y axis, is that kapton heaters are lower mass than the silicone heaters. If it has to move fast, lower mass is better.

One slight difference is that the thermistor is usually embedded in the silicone heater (and you often don't know anything about it) which means if one of both wires get broken you'll have a hard time replacing it. The kapton heaters usually have a separate thermistor that can be placed where you want and can be replaced easily with a part that has known specifications.
Re: Heated Bed Design Basic Questions not answered on the wiki (that I could see)
March 12, 2015 12:20PM
Precise temperature readings is not a virtue of current 3d printers neither is truly needed, fluctuations up to 5 degrees are within acceptable range, when you measure the hotend temperature, you are reading the temperature of the aluminum block not the brass nozzle or the stainless steel barrel (you assume that are the same or close but most likely they are not).

With beds it's the same, if you put the sensor in the center the temperature at the edges is less, so I usually put them somewhere in between so still even when center will be hotter and edges will be cooler the temperature is a bit more even. Put in on top is better but then you will have a bump against the glass so usually goes at the bottom (anyway the top of the glass will never be at the temperature your thermistor is reading) you can use a thermometer and read actual temperature and then either adjust in the slicer or changing the table in firmware (I prefer adjust in the slicer) but then again a cold draft can cool down the glass and it will take longer to heat up again than the heater itself.

What you need to look for on a 3d printer heating system is more of efficiency and reliability than accuracy, which one will heat up faster, stay heated longer and uses less power, and like the digital dentist mentioned "mass" if is going to be set in a moving axis.

Out of the two you mentioned I will pick the silicone one.

What we really need is an embedded heating element in a glass sheet, but that for now it's out of reach.

Edited 1 time(s). Last edit at 03/12/2015 12:31PM by ggherbaz.
Re: Heated Bed Design Basic Questions not answered on the wiki (that I could see)
March 12, 2015 01:29PM
The new MK3 heated aluminum bed is a good combination of light weight, good heat distribution, and does work with an inductive bed leveling probe. It also has mounting holes for 3 point leveling.

I have one on a Pursa I3 and it works well. There is also a larger heated aluminum bed, but it requires 18 volts.

Here are some tips for heated bed setup:

Get the new MK3 aluminum heated bed.
Put the thermistor near the bottom center with kapton tape.
Insulate the bottom of the bed.
Use #12 gauge wire from the power supply to the heat bed terminals on the printer control board.

My bed is configured as listed above with kapton tape, and it reaches a surface temperature of 100C+, which is necessary to print ABS without warping.

Edited 1 time(s). Last edit at 03/13/2015 07:08AM by normw.
Re: Heated Bed Design Basic Questions not answered on the wiki (that I could see)
March 16, 2015 07:13PM
Thanks guys, I ended up going with the Kapton one just based on availability from Omega. I replaced the thermistor in the hot end (e3d) with a screw in type, so I had an extra one to use anyhow. I figure I'll use that to protect the heater itself, as most designs use, but since I'm going for way over complicated, I think I'm going to grab an IR thermometer. I'll see if I can get an FOV and mounting location to cover a large area of the bed for bed temp readings. With the heater temp, and the bed temp, I should be able to set up a PID control loop to keep the bed temp as close as possible to optimal. Did I mention lots of unnecessary complexity? smiling smiley Yes, I'm very very very bored! Got lots of other stuff too, like I just ordered a honeywell FS01 micro load cell to use for "leveling" (calibration really since its a delta). So much more. This is what happens when engineers get too much time on their hands.
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