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Thread: BUILD - PLA burnout furnace, the controller part

  1. #1

    BUILD - PLA burnout furnace, the controller part

    The start is over on this other thread...

    http://www.alloyavenue.com/vb/showth...t-Be-THAT-Easy

    I want a PLA burnout furnace, separate to the furnace I am building for melting metal. The use of a cooker ring is what I want to investigate.

    For burning out PLA, a controlled burn which rises in temperature over time is important, so a PID controller seems appropriate. I have a "block" from China I used previously in a ramp/soak oven for reflow soldering, but it proved to be tricky to program, no data logging features and not that accurate.

    So I have started down the road of building my own.

    Design intent:
    Plug in an SD card with a file of time & temperature.
    Controller reads the SD card (in fact won't work if it's missing) and loads set points.
    The user starts running the profile.
    Cooker ring controlled via opto-isolated output on the microcontroller.
    The user can interrupt the profile at any time with adjustments to the next setpoint.
    The user can stop the profile at any time (unplugging the electric would do the same, this is just a bit more elegant).

    Basic prototype setup:


    User Interface:


    I'm will use PID software here:
    http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-introduction"]

    Challenges:
    Safe to use.
    Where to position the thermocouple for optimum readings.
    Response of the cooker ring over time.
    Using Kaowool without any refractory will lead to airbourne particles.
    Longevity of a cooker ring.

    Updates will be posted here as I progress. Play along, suggestions and comments welcomed.
    Last edited by negativ3; 09-18-2017 at 10:33 AM.

  2. #2
    looks pretty cool, that would be handy for a filament extruder more than the burnout furnace. I usually just stick my plaster mold in a bonfire overnight and pour first thing in the morning.

  3. #3
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    If you put the burner element anywhere other than the bottom you can use the furnace to do wax recovery for lost wax investments. The idea is that you melt the wax out at a low temperature prior to the burnout. The wax can often be reused.

  4. #4
    Quote Originally Posted by 3v0 View Post
    The idea is that you melt the wax out at a low temperature prior to the burnout. The wax can often be reused.
    Really????? Here is how you dewax lost shell. Hard and fast or watch your shell crack. Wax still gets collected if you don't torch it directly.
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  5. #5
    Quote Originally Posted by negativ3 View Post
    ......I'm will use PID software here:

    Challenges:
    Safe to use.
    Where to position the thermocouple for optimum readings.
    Response of the cooker ring over time.
    Using Kaowool without any refractory will lead to airbourne particles.
    Longevity of a cooker ring.

    Updates will be posted here as I progress. Play along, suggestions and comments welcomed.
    The Kaowool is easily stabilized with the use of rigidizer (colloidal silica). The oven can be made safe from burn and shock hazard. The rest depends primarily on the answers to the questions below.


    What are the approximate internal dimensions of the burn out furnace?
    What is the maximum operating temperature?
    What is the maximum allowable temperature deviation from set point and within the furnace?

    Best,
    Kelly
    My furnace build ----- I toil and fettle then foam turns to metal!

  6. #6
    Quote Originally Posted by kcoffield View Post
    The Kaowool is easily stabilized with the use of rigidizer (colloidal silica). The oven can be made safe from burn and shock hazard. The rest depends primarily on the answers to the questions below.


    What are the approximate internal dimensions of the burn out furnace?
    What is the maximum operating temperature?
    What is the maximum allowable temperature deviation from set point and within the furnace?

    Best,
    Kelly
    Thanks Kelly, now investigating colloidal silica.

    Answers to your questions...

    What are the approximate internal dimensions of the burn out furnace?
    6" ID x 8" Height at the moment.

    What is the maximum operating temperature?
    750 C, 1380 F.

    What is the maximum allowable temperature deviation from set point and within the furnace?
    Not considered at the moment however getting accuracy and repeat-ability are what I'm after for consistent results.
    I would estimate that crossing set points within 10 C at the correct time would be acceptable.

  7. #7
    Senior Member chubbyjp77's Avatar
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    Negativ3 I like your signature, but never tell me the odds.

  8. #8
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    Quote Originally Posted by jagboy69 View Post
    Really????? Here is how you dewax lost shell. Hard and fast or watch your shell crack. Wax still gets collected if you don't torch it directly.
    I was thinking of traditional lost wax rather than ceramic shell. But are you sure heating a shell to the melting temperature of wax and soaking would crack the shell. It would be much less brutal in terms of temperature differentials then what I imagine the weed burner creates.

    After reading your post I watched all of the first 3 parts. Good stuff. Regarding the switch plate it was so close to working with sand casting that it should have easily filled properly if it had been thickened.

    Making the ceramic shell once the wax if finished is a lot of work compared to pouring investment. Kudos

  9. #9
    Quote Originally Posted by negativ3 View Post
    What are the approximate internal dimensions of the burn out furnace?
    6" ID x 8" Height at the moment.
    What is the maximum operating temperature? 750 C, 1380 F.
    What is the maximum allowable temperature deviation from set point and within the furnace?
    Not considered at the moment however getting accuracy and repeat-ability are what I'm after for consistent results.
    I would estimate that crossing set points within 10 C at the correct time would be acceptable.

    6” ID by 8” tall is small since my reference point is my larger furnace which is 14”ID by 34” tall with the electric heating insert installed.

    Your temperature expectations are modest for most NiCr type resistive elements, but probably helpful for using the stove heating element and sufficient for burn out. The small volume will likely make for smaller temperature gradients. The roof of the furnace is a likely candidate for placement of k-type thermocouple but you may need a stainless steel plate between the heating element and crucible to distribute the radiant heat. 10C hysteresis may be a bit ambitious depending upon the set point but double that may be in range.

    If it’s just burn out, do you really care if there was say a ~100F gradient from floor to roof? Considering typical burnout cycles, I don’t think it’s a problem. If you have stove elements and they are more or less free, may as well give them a try but winding your own resistive elements is pretty simple and the wire is available from most kiln/pottery supply stores. My coil winder is also in that previous link I posted and there's not much to it. You might consider downloading and studying this primer.

    http://www.hi-tempproducts.com/pdf/the-kanthal-furnace-mini-handbook.pdf

    FWIW, my small electric furnace has coils I wound mounted in the lateral walls, is 10” ID by 18” tall inside dimensions, and near as I can tell is about 100F cooler at the base than it is at the roof at 2000F. Since I’m just melting aluminum with it, it’s not material, but in my larger furnace for heat treating it is, or at least can be.

    Best,
    Kelly
    Last edited by kcoffield; 09-18-2017 at 12:31 PM. Reason: Font
    My furnace build ----- I toil and fettle then foam turns to metal!

  10. #10
    Thanks 3vo. The problem is wax expands when it gets warm and shell does not expand. This causes the wax to crack shells. A few of us are working on a better solution to dewax for the home gamer. After most of the wax is out, the piece goes into a kiln for a more complete burnout. An autoclave is the answer, but they are not cheap.
    WWW.TheHomeFoundry.org
    Visit me: WWW.HandcraftedLanterns.com
    "Sometimes by losing a battle you find a new way to win the war"
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