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heating alternatives

Discussion in 'General Electronics Discussion' started by donkey, Sep 10, 2013.

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  1. donkey

    donkey

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    Feb 26, 2011
    hey all. As you know I am currently in a project with another member who has been kind enough to offer his time to get a 3d printer going. The only thing I want to know is if anyone can offer a good option on heatbeds.
    Nichrome wire is where we are leading but thought I would at least ask before jumping down that path because its the part of the machine that makes or breaks a print.
    what is needed is a good even heat distribution, adjustable temps, super quick warm up time and low amps if possible. I started looking at PTC ceramics but found that if I went from one plastic to another I would also have to change the heatbed as the temp was controlled by itself not by an external factor (like current applied)
     
  2. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

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    Aug 27, 2013
    Hrmm, I have not played with 3D printers yet, only casually observed; however, I have some experience with heating things :) A lot depends on budget, space and configuration of the machine. Most of the 3D printers I have seen have a stationary bed which can make the task easier. While not "cheap", aluminum plate is a GREAT conductor of heat. While not particularly "fancy", a common kitchen range element is dirt cheap and easy to work with. Put the two together with a few DS18B20s, a SSR and a uController and you should have a pretty amazing bed. NOTE: most range elements are designed for 240Vac; however, they will work just fine with 120Vac @ less than half of their design power (still a gracious plenty for your purposes.)

    Fish
     
  3. duke37

    duke37

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    Jan 9, 2011
    Or quarter the power.
     
  4. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    449
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    Aug 27, 2013
    Yes, 1/4 the power:

    Code:
    E = IR  --> I = E/R
    P = IE --> P = E^2/R
    
    Therefore for E1 = 1/2E with R = Constant ==> P1 = 1/4 P
    
    
    What I could not remember was the change in resistance with respect to temperature, but I have gone back and checked that:

    For temperatures:
    @ 1000F (538C) the increase is less than 9%
    @ 2000F (1093C) the increase is less than 12%

    If design power is 1800W @ 240V, this implies a heated resistance of 32 ohms, a current of 7.5A and a cold resistance of less than 36 ohms. Assuming 34 ohms is a worst case scenario @ 120V, the current would be 3.53A. Assuming the design temperature limit is ~750F (400C) this would suggest ~18ga Nichrome wire which should heat to ~350F @ 120V 3.66A (~420W).

    So, I was imprecise in stating the power would be less than half, I should have said the power would be less than 1/4; none of this fundamentally changes my assertion that a 240V range element used @ 120V will provide a cheap and easy heat source for a 3D printer bed:) (Nor did you imply that it did ;-) )

    The main advantages to stove elements over DIY nichrome wire solutions are: 1) Mature/Reliable/Durable product design 2) Cheap 3) Relative ease of control 4) Mains powered. But, as I stated, I have only been a casual observer of the 3D printers, there may well be better solutions available.

    Fish
     
  5. donkey

    donkey

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    Feb 26, 2011
    they have a heated bed, and its only 12volt but its a 10 amp draw. I know that is only to warm it up and once it reaches temp that will cut down significantly.
    The problem is upsizing. I want to make one bigger than the standard 200mm square and finding a way to evenly distribute heat is what is perplexing me. I remember from my cooking days (or burning days in this story) that when you saw a cooked omelette from a electrical stove top there was a definitive ring. so it wasn't evenly heated.
    also from understanding how the nichrome wire heats if I make it too long than resistance will mean not the whole wire gets heated.
    My ideal would be 500mm square if that helps. I was thinking of making a makeshift inductive heat element. the "pan"/bed can then be swapped out for cooling and the machine would still be semi cool to the touch. Then I realised with all the steel going through the machine I would probable melt half of it.
     
  6. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    449
    103
    Aug 27, 2013
    donkey,

    First, to be clear, you do mean 500mm x 500mm, not 500mm^2 ?

    The key to even heating in both your table and cooking is to increase the thermal mass between your heating element and your work/food. Consider this: if you heat one spot on a piece of aluminum foil, do the edges get warm? The answer is generally "no". If you take a 500mm x 500mm x 25mm piece of plate aluminum and heat even one corner the entire plate will enter a thermal cycle where an equilibrium temperature above ambient is reached for the entire plate. In better quality cooking utensils the bottoms are "layered" or, more correctly, "laminated". The Lamination process usually involves different metals, typically aluminum, copper and steel. If you were to use 4 pieces of 5mm aluminum plate with 3 pieces of 1mm copper sheet between them the resulting thermal mass would heat considerably more evenly than the single 25mm aluminum plate, even if the heat source was not ideally distributed.

    I do not remember if Oz uses 220V mains or 120V/240V mains, but in either case the use of a range element is dramatically easier than DIY nichrome wire. In addition to my electronics hobby, I am a home brewer. I began heating my boil kettle with propane, but have migrated to electric. I built a "stove top" for my boil kettle using 4 * 6in 1800w range elements for my 21in diameter boil kettle. To ensure even heating I use this pot: http://www.ebay.com/itm/Precise-Hea.../251122819149?pt=Cookware&hash=item3a7816204d . Sadly there is not going to be an "off-the-shelf" heat bed readily available for you; however, if you will laminate alternating plates of aluminum and sheets of copper you should be able to fashion a heat bed that provides very even heating. I would think for your purposes a single 8in range element would work very well.

    If cost/complication is not an obstacle, and you want truly even heating, you could make your bed from two pieces of aluminum plate with copper tube sandwiched between them. In this scenario you would heat water or oil in a flow through heater like this: http://www.ebay.com/itm/Spa-Heater-...t=LH_DefaultDomain_0&var=&hash=item19d34e2d90 and use a pump to circulate the liquid through the copper pipe. As long as you don't need a bed temperature greater than ~90C you can use water, a much simpler/less messy solution than oil.

    All of my suggestions have been >> 120W but use mains power rather than a low voltage supply. In your OP you stated you wanted: 1) Even Heating 2) Adjustable Temps 3) Super Quick warm-up time. 4) Low Amperage. I cannot see a way to achieve all of these goals from a low voltage supply. Ultimately (as pointed out above) P = IE. Since even heating is a function of thermal mass & thermal conductivity to achieve any particular temperature different than ambient requires a fixed amount of power + thermal losses over the time period in which the thermal mass is heated, AND because you wanted "super quick warm-up time" with low amperage, I cannot see a different solution than using mains Voltages. As heating and mains voltages are inherently dangerous, using existing, mature, mass produced, engineered products seems like the best course to follow.

    The only two obstacles I see are 1) cost of the bed 2) Skill Set // Access to the tools required to make the bed. Aluminum plate is not terribly expensive, but copper sheet is a bit pricey. I would suggest looking for copper sheet in the form of "flashing" made for the roofing/building industry; while still driven by the price of copper, it is typically cheaper from building suppliers than other sources. As far as Skill Set // Access to tools, I assume from the nature of your project that you have both. Copper and aluminum can be cut/drilled using common wood working tools.

    As far as inductive heating goes, I love the concept, but that is a pretty complicated path to start down. I have investigated inductive heating casually, but I have never been able to get the cost/reward ratio to align properly. Perhaps in another decade we will have switches that make it more cost effective, but for now there are just too many compromises for too little reward at too high a cost. It is a great choice in highly specific industrial situations, but the range of effective application is still pretty narrow.

    Good Luck!

    Fish
     
  7. donkey

    donkey

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    Feb 26, 2011
    fish4fun I have been a way for a while and its good to see some new blood on here. especially one with new insights. great help so far.
    I have considered a myriad of alternatives but they mostly rely on a manufacturer making it for me. I think for home brewing Nichrome will be the best bet for now as everything else adds way too much to the design and makes a simple heatbed into another complete setup that is way too time consuming.
    I might try your concept of aluminium cookware though as a transferable print area, will post results when I get this dang thing working.
     
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