A call for help/ideas to develop the Heated Bed

Oh and an after thought.

I wonder which is the better conductor of heat....

Acrylic

or Glass

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Necessity hopefully becomes the absentee parent of successfully invented children.

Glass is a much better conductor than acrylic and most plastics but a lot worse than any metal.

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[www.hydraraptor.blogspot.com]

Glass can also handle much higher temperatures without melting, deforming, or releasing dangerous gasses. On the other hand, you can use a jewelry or glass bead making torch to heat up sections of the glass hot enough to melt it and embed the nichrome wire directly into the glass, so it does not need electrical insulation or mechanical protection. You must pre-heat the whole glass before doing this, of course, otherwise the thermal stress of spot heating from room temp will shatter the glass. Slow cooling (preferably DAYS) will anneal all the stresses out.

With embedded wire on the bottom side, the top side should still be optically flat. No the bottom can be insulated with almost anything, including wool felt or cotton fabric.

Mike

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Team Open Air
Blog Team Open Air
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The only problem I see with a high insulation bed is that lots of heat will be wasted out of the bottom. As long as the bottom side is insulated better than the top, we should be fine. We probably don't need any bottom insulation with an aluminum bed, but we will need quite a bit with a glass bed.

On the issue of multiplexing: If your work area is a rectangle shape you can turn on all the rows and all the columns that you need at the same time. You won't need the extra power per zone required to heat each row one at a time.

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
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I think you have that the wrong way around......

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Necessity hopefully becomes the absentee parent of successfully invented children.

Aluminium bleeds heat like a stuck pig. I guess this is why it is used as the premier heatsink material.

Insulation is good no insulation is bad.

Actualy both benefit noticeably from insulation. There is perhaps an argumnet to putting a silicone mat over the parts of the top surface that are'nt being extruded onto, so as to save energy.

I dont fancy messing with embeding nichrome in glass plate. It would melt my kitchen table cloth...... <|

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Necessity hopefully becomes the absentee parent of successfully invented children.

The best heatsink material is diamond, but with obvious cost issues.

Was considering a grid of IR LEDs to heat the bed, but I doubt you'll get the power output or efficiency.

Now that is interesting.

I never had Diamond down as a conductor of anything.

It's amazing what you find out discussing things with like minded folk.

I guess this has always been because it looks glassy, I have assumed wrongly that its other properties were similar.

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Necessity hopefully becomes the absentee parent of successfully invented children.

IR and glass is a bit counterintuitive too.

Having recently been reading up on making CO2 laser tubes, It turns out that glass isn't the best window material for IR.

I can see how your thinking though.

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Necessity hopefully becomes the absentee parent of successfully invented children.

Diamond is a good thernal conductor because it is all carbon-carbon bonds. With the incredible strength of the carbon-carbon bound, the thermal energy in one atom is quickly shared with the four neighbors it is connected to. And then they share with the 16 (or is it only 12 new) neighbors. And so the energy is rapidly and efficiently spread. But in the pure state they are excellent insulators. Blue diamonds are semiconductors, and there is some research on using diamond instead of silicon for semiconductor electronics. If it moves the operating temperature up far enough, imagine an uncooled Venus rover based on diamond semiconductor electronics!

Mike

... you should google for Graphit and Graphene - not so expensive as diamond, but with similar very interesting properties ...

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Viktor
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The Wikipedia page for diamonds did point out that the carbon-carbon bonds in diamonds are weaker then any other form, including graphite. The strength comes because it is the only form with 3D bonding structure. Accept for carbon nano-tubes, of course.

Now back to finding a good insulator to under the PCB board for the zone heater, if I can get the resistance high enough to get the power consumption down!

Mike

Sure

Your are of course all correct and wired in in this respect.

I unfortunately have always been fiscally opposed and unhinged and associated diamond with a fiscal obsession.

So have ultimately treated it with the contempt I thought it deserved.......

Until the statement that suggested it had something to offer beyond fiscal obsession.

Diamonds are a girls best friend, yeah right, gold digger, go get a life....

Diamonds have scientificaly valid properties and physical properties I had not appreciated.

Who do I have to fuck for them........

Yeah I am that sad, but happy...

Death before dishonor, boys and girls, every time.

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Necessity hopefully becomes the absentee parent of successfully invented children.

Oh sorry did I forget to mention.

It doesn't have to be mine.....

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Necessity hopefully becomes the absentee parent of successfully invented children.

A few thoughts:

I am planning to use clay to make my plate. It is a good thermal insulator once fired, and it does not move much with the temperature. Liquid clay can be spread very evenly on a surface and makes a nice flat working plane.
Plus, if I add some thin sand in it, I suppose the surface will be textured so that the plastic will stick to it.
I plan to buy some Buff clay. I will let you know what happens.

Another idea:
Corners are problem, since heat dissipates faster in these regions. I see two ways to avoid that. The first is to round the corners in the design of the parts. The second is to print a separate pillar just next to the corner, so that it keeps the heat.

Last one:
Instead of heating the whole volume to keep the heat, one could use an IR lamp to heat the surface of the part only.


If only I had a set of Mendel parts, I could test it... :p

Edited 1 time(s). Last edit at 04/09/2010 12:35PM by Lionel.

After making the test heating coil from PCB and having the length come out half of what I expected, I was concerned where I went wrong in the math. I reviewed my numbers and found the problem. I had initially assumed a 12 inch by 12 inch heated bed, because that is what I would like t design for in my Open Air project. But when I started laying out the board, I was realizing that the PCB Fab-in-a-box is limited to letter sized paper, so 8 by 8. That reduces the heating coil to 1 inch by 1 inch. Had I made the test piece to the original dimensions, it would have been 50% longer lines with 50% more lines, effectively doubling the length to near the 79 inches I predicted. So my only error was not following my own assumptions!

I have reconsidered the power drain, and scaled the numbers back to 100W peak. This means that the resistance must now increase to 11.5 ohms. For the 1 inch by 1 inch heating element, that means 0.0062 lines and spaces. If I expand to the goal of 1.5 by 1.5, the width increases by the square root of the area, so 9.2 mills for the traces and spaces.

The process I am using has a max resolution of 0.006 lines, and I have yet to achieve that, so I think the next test piece will by 1.5 by 1.5 zones. I may not get a chance to make the next batch until next week, as tomorrow I am drive cross-state to work on the rental trailer my sister is renting from me. Over due for installing the tie downs so it won't blow away in the wind.

I think maybe a 5x5 zone array of 1.5 inch sized zone might be possible. I will try designing in the ATTiny and MOSFETs to make it functional, then print it onto the transfer paper here at work, where the printers are higher resolution.

Mike

rocket_scientist, did you actually test the circuit you made? If yes, do you have results to report (other than your discovery of shorter than expected trace length)?

There is something I am wondering about based on your messages. I certainly understand that we are talking about multiplexing zones. But I get the impression that your plan is to PWM a voltage supply rather than a current source. I'm speculating that this is why you are concerned about achieving a specific impedance for the serpentine etch.

However, to measure temperature the approach is to measure the voltage across the serpentine etch when a known current is applied.

If the PWM was modulating a current source (instead of a voltage) supply then we can measure the voltage across the zone when it is "on" and derive the temperature. In addition, we wouldn't care so much about achieving a particular resistance. In other words, the length and width of the serpentine etch could be simply driven by the area allocated to the zone and the width of etch that is easy to manufacture.

I have been assuming PWM of a current source. Are we thinking about this differently?

TC

Lionel Wrote:
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> Another idea:
> Corners are problem, since heat dissipates faster
> in these regions. I see two ways to avoid that.
> The first is to round the corners in the design of
> the parts. The second is to print a separate
> pillar just next to the corner, so that it keeps
> the heat.

So for that the best way is to go with the heated room, as commercial machines. Now we have the heated bed... and maybe we can even go a bit further and stay half of heated bed and heated room.

I think we could add a 20mm (or more) barrier on limits of the heated bed, a barrier on Z axis direction.

I think that having a relative high room temperature is good for avoid warping and get a "more strong piece", as layers may stick better if the piece is hot.

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New cutting edge RepRap electronics, ARM 32 bits @ 100MHz runs RepRap @ 725mm/s:

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Isolate the bottom and the corners with vacuum-plates like
this.

[www.porextherm.com]

Erich

TC Wrote:
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> rocket_scientist, did you actually test the
> circuit you made? If yes, do you have results to
> report (other than your discovery of shorter than
> expected trace length)?
>

I still have not found a way to repair the first test piece. There are a minimum of 2 complete breaks and many partials that will likely become fuses when full current is applied, so there are no test results to report back yet.

> There is something I am wondering about based on
> your messages. I certainly understand that we are
> talking about multiplexing zones. But I get the
> impression that your plan is to PWM a voltage
> supply rather than a current source. I'm
> speculating that this is why you are concerned
> about achieving a specific impedance for the
> serpentine etch.


I do not know a simple circuit for constant current, so I was going for the simple circuit to provide constant voltage. Namely a high side MOSFET to +12V and a low side MOSFET to ground. I wanted to measure the current going into the low side MOSFET, assume constant on resistance, and try to measure the copper's resistance that way. If you have a simple constant current source circuit that will work on 1-2Amps, I am willing to look at at. And constant current would come closer to letting the copper's thermal change in resistance self regulate. But still need to add external regulation to get it stable enough.

Mike

Edited 1 time(s). Last edit at 07/07/2010 02:53PM by rocket_scientist.

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Team Open Air
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Personally I would be advocating PWM control (you only need to switch a single side, take your pick).

Controlling power, so long as you work out worst case cold/hot and size your switch for that then it should be fine. (Max Current is when cold, Max Voltage is supply)

Define yourself an off period and measure the resistance during it. With the heating element being one arm of a potential divider.

Of course do some math to make sure that the remaining on period is enough to get the power into the heater circuit you need.

Sounds more trivial than it is described this way, but you probably get the idea.

KISS.

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Necessity hopefully becomes the absentee parent of successfully invented children.

I don't have a specific circuit to suggest. The Wikipedia page on current sources is a good introduction however.

I was trying to make two points:

1) we rarely use current sources in our designs so we all automatically think about controlling voltages. In this case, PWM of a voltage supply to regulate power applied to a resistor based on thermal feedback. That's fine. But we can also use current sources, and even PWM them.

2) in this case, we want to measure the temperature by measuring resistance. The typical way to measure a resistance is to apply a known current and measure the voltage.

So, it seemed to me that we could do both by PWM control of a current source (measure the voltage when the current is on to determine temperature).

One of my concerns is that the etch needs to be wide enough that it is easy to manufacture in the first place, and won't get damaged by high currents in the second place. I was getting the impression that the resistance of the serpentine etch was being constrained by the use of a 12V supply and that use of a modulated current source might help.

I really haven't gone beyond these simple minded thoughts.

TC

TC,
I had not been thinking about it that way. Yes, if we set a constant current source, it would be easier to measure the serpentine resistance. Unfortunately, we get no benefit from constant current over constant voltage. In my original design, the cheap 400W power supplies I bought put out about 12Amps per 12Volt circuit. Working within those limits, you get around the same resistance. 12 volts squared, divided by one full row of resistance equals 100 watts, and that means 0.7 ohms for all 8 columns in parallel, or 5.6 ohms for a single one. 12 amps squared times one full row of resistance 1.44 ohms for all 8 in parallel, or 11.5 ohms for each zone.

To make the zone easy and reliable, the traces need to be fat, which means the length is short and the resistance is quite low. To get 100 watts peak heating to warm up quickly to 120C, we need lots of current to keep the resistance down. If I shop around for a cheap, but much higher power computer switching power supply, I can find them up to 38 amps on the +12V. That is probably 2 or 3 separate supplies being counted as one, but probably safe to gang them together. Using a peak 32 amps on 8 zones at once gives 4 amps per zone. And 1/8th of the peak power of 100W is 12.5W. S now we have 12.5W = 4A²*R or 12.5/16 = 0.78 ohms per zone.

going back a page or two, my math was :
Resistance is L*R(t)/(W*thickness). 1/2 ounce copper is 0.0007" thick. We want 0.78 = 6.788E-4 ohm mills*L/(W*0.7), or 0.78*0.7*W = 6.788E-4*L. Substituting L = 1.0E6/W we get 0.546W = 6.788E2/W or W2 = 678.8/0.546 = 1243 mills2. W = 35 mills, and L = 1.0E6/35 = 28.57 inches. Now that is for the 1.5 inch by 1.5 inch square. This is quite doable, if the MOSFETs and power supply can handle the current. We also get a voltage drop across the heating element of 4amp*0.78ohm = 3.12, which is directly readable by the microcontroller.

Constant current sources look like more work than constant line voltage sources. I will have to keep researching for an efficient way. The simple ones just drop all the rest of the line voltage across the regulator, and at 35 amps and 12V, that is going to be a LOT of waste heat!

Mike

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If the right solution is to PWM a voltage supply across the zones then use of a current mirror MOSFET might be worth investigating. These devices have a second source/drain junction that will sink a fraction of the current on the primary source/drain junction (a fixed ratio like 1:250th).

Connecting this mirror pin to a known resistance and measuring the voltage across it would let us to calculate the resistance of the zone.

It would probably be necessary to measure the voltage across the zone so that the temperature effects of the source-drain voltage of the MOSFETs can be eliminated.

TC

KISS

Keep It Simple.....

(usually often also means cheap and accesible)

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Necessity hopefully becomes the absentee parent of successfully invented children.

aka47 - It wasn't clear what prompted your KISS comment. If there is something specific you find complex it would helpful to know what it is, and hopefully, what you think is a simpler approach.

TC

TC

Using the supply as is without adding current limiting etc is simple.

Using readily available switching elements (Bipolar, FET, MOSFET etc) that are understood by many is simple.

Adding current limiting and using cutting edge componets is arguably unsimple.

Don't get me wrong. I am a huge fan of technology and new technology in particular and am equaly inclined to pick the sexiest route before the pragmatic one.

I am also a huge fan of Occams Raisor. (Probably spelt wrong)

Hope this helps

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Necessity hopefully becomes the absentee parent of successfully invented children.

I certainly appreciate the keep it simple approach. However, I believe that simplicity is easier to recognize after exploration of alternatives.

A current mirror MOSFET is hardly cutting edge or rare. Use of such a MOSFET would likely simplify the measurement of current which is necessary to determine the resistance if PWM of a voltage supply is used (measure V, measure I, calculate R). We need to determine R so that we can calculate the temperature.

RE: Simplicity, you might enjoy reading John Maeda's "The Laws of Simplicity".

Ever heard of Triz?

You might enjoy reading about that also. The basic pricipal is to identify contradictions in a design and find solutions to them as a methodology for design innovation.

In this case, measure the resistance without a controlled current.

TC

I'd be happy to post a diagram of how simple it gets but the forum does'nt appear to want to allow me to attach the diagram I drew. (server reset errors)

1 Transistor/Switch Element.
1 Low wattage sensing resistor.
1 Heating element.

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Necessity hopefully becomes the absentee parent of successfully invented children.

I have started a fresh thread for the mutli-zone heater discussion, so this thread can return to it previous topic.

Mike