Building A Hot Box??

Discussion in 'Boatbuilding' started by CatBuilder, Feb 6, 2011.

  1. CatBuilder

    CatBuilder Previous Member

    I need to maintain 170 degrees of uniform heat in a box. Too much heat and you release toxic amounts of cyanide gas. Too little heat and the foam won't bend.

    Any ideas or designs for a good box that maintains 170F?

    I currently have the box, which is 8ft wide, 4ft deep and 2ft tall. It's got a 1500 watt quartz heater in it. It's not insulated and I tried using a hot water heater contact thermostat to control the temperature. I also have a candy/deep fry thermometer to check temps inside. The rack is a series of wires/cables strung across the thing at a reasonable height. The heater is at the very bottom of the box, facing up.

    Foam is supposed to be cooked at 170F for 25 mins per sheet in order to become soft enough to bend.

    I have the following results:

    *See a good 165 right above the heater on the rack using the thermometer.
    *See 160 or so a foam panel's width over away from the heater.
    *Tapers off to 120 at the edge of the box farthest from the heater.

    *Put foam in the 120F area for a pre-heat. It warms, but doesn't bend, as expected.
    *Put foam in the 165F area directly above the quartz heater and after 25 mins, the foam turned brown, releasing hydrogen cyanide. I wasn't dumb enough to stick around for the release this time. This indicates a temperature above 180F, the max temperature for heating this foam.

    I need to get the box to be a nice, even 170F with no scorching of the foam. Scorching not only releasees enough hydrogen cyanide to make you seriously ill, but also will not bond properly to the fiberglass and will make an unreliable core.

    Any ideas on how to achieve an even, predictable 170F? :confused:
  2. hoytedow
    Joined: Sep 2009
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    hoytedow Wood Butcher

    Put a fan in there too for convection and better heat distribution. Use multiple meat probe thermometers placed strategically in diverse places on the box.

    Attached Files:

  3. CatBuilder

    CatBuilder Previous Member

    Meat thermometers are a great idea! Can stick those in and still read them with the box closed.

    Do you think I need insulation too?

    The box is made from 3/4" CDX on the bottom, sides and back. The top and front-opening door are made from 1/2" MDO chip board.
  4. Poida
    Joined: Apr 2006
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    Poida Senior Member

    Since I know nothing I will put in my couploe of cents worth.

    A fan would be a good idea, getting a fan that will allow that kind of heat through it might be a struggle.

    The entry of the hot air should enter at the bottom of the box but a plate placed over the entry hole allowing an inch gap around the periphery.

    A return pipe from the box needs to go back into the air intake I would suggest a venturi. This basically an aconomiser, so you are not losing heated air but recycling it.

    Keeping it at exactly 170 degrees? Thermostats have a differential ie. they will switch with generally a 5 degree difference. If they switch off at 170 they will not switch on until 165. The difference will also depend on the quality of the thermostat but you may have to consider a more expensive unit .

    Insulating the box means that heat is not dissapating through the walls of the box and cooling the air by doing that and recycling the heated air makes for better uniformity of heat.

    How you control the heater will depend on what sort of heater you are using.

    But as I said I know nothing about thermo whatnots but just my thoughts.
  5. tunnels

    tunnels Previous Member

    What are you making ???
    why do you need to bend sheets ?
    If you heat thats hot and let cool it goes harder than before !! and sometimes if your are a bit slow getting it to where you want it to be they buckle and go out of shape and you lost a whole sheet of core !! Had to bend 2 inch foam and our resident know it all told and tryed to show and ended up lossing almost a container load of foam .
    There are other ways of doing it !.
    How thick are your sheets?? and what sort of a bend are you wanting ? Is it just a curve and what sort of radus? Or a cone shape ( smaller at one end and bigger at the other ??
  6. CatBuilder

    CatBuilder Previous Member

    Thanks. Poida, I read yours without questions, so I am not directly responding. Makes sense.


    I am building this.

    I'm heating 25mm (1") A600 Core Cell. The manufacturer's method is this:

    0) Build hot box
    1) Pre-heat foam to 60C (140F) for 25 minutes
    2) Heat foam to 75C (167F) for 25 minutes
    3) Move foam quickly to mold, place, bend and screw from behind to battens

    The shape is a standard hull shape of a performance cruising catamaran (ie: 11:1 L/B ratio, moderate rocker, etc...)

    See the actual hull here (picture it with the above sistership to understand the curve of this hull).


    Also, here is a video of someone doing the same thing with 1/2" (12mm) Core Cell using a heat gun:
  7. rberrey
    Joined: Oct 2010
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    rberrey Senior Member

    Cat one of the designers at Bateau might be able to give you some advice on an oven, he always responds to core questions. Email Evan , search in the core section of the forum to click on his contact info. rick
  8. tunnels

    tunnels Previous Member

    so you using plain sheets ?
    Perfirated ,with holes ?
    Thought of using a differant type of sheet with cuts at 90 degress ?
    they do bend a little easyer and load better bonding specialy in the lower parts ?
    The racing yachts in Korea we slit the foam with a fine narrow saw blade about 2/3s of the way through the core and put the slit side inside and the uncut side out side . Had cuts about 50mm apart (VARIED AS TO HOW MUCH CURVE YOU NEEDED ! CLOSER MORE CURVE!!, WIDER LESS CURVE!!,) and as many as it takes to bend the sheet and stayed fair without any flat spots !!. If you have a lot to do use a shaft with multi fine blades at the right spacing set up and to do a wide panel . Works a tread the cut on the inside closes as you form the curve and if you vac bag the inner glass the cut fills 100% with resin so it ends up really strong .
    Once we sorted our system it was plain sailing . Had 8 boats to make and very short a time limit so the quicker and easyer the better for what we had to do . Tryed the heat things was to time consuming and unpredictable . The cutting was fool proof and fast . We even worked out a radial cut for the bow sections with a very tight radius and that just fell into place with very little effort .
    Took me about 2 hours to work out how many cuts we needed and how deep they needed to be , plus the spacing and where in the sheet the most bending took place and where and how etc etc ,once everyone understood the system and why we had to do it this way it was full steam ahead and we rocketed though the core stage on each and every boat .
    While working on the second boat we pre-cut enough for these special radial cut sheets( port and stb ) for the next 4 boats and they were stored away to save time rather than setting up for a single boat lot each time . :p:confused::D

    I have some pictures some where ! will have to go searching . Computer had a massive melt down a while back and everything is not where it should be after the repairs were done !.
  9. AndrewK
    Joined: Mar 2007
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    AndrewK Senior Member

    CatBuilder, as mentioned before to get a good result with thermoforming you need a solid mold, be vacuum bagging and a second pair of hands. With a batten mold you get a better result (fairer surface) with using smaller blocks of foam and kerf cuts for the topsides and strip planking in the bilge.

    But if you want to give it a go you have to insulate the box, fiberglass wool or styrene will do. You must have fans, thermostats and lots of power around 4800 W, (two 2400W fan forced elements).
  10. hoytedow
    Joined: Sep 2009
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    hoytedow Wood Butcher

    Attached Files:

  11. CatBuilder

    CatBuilder Previous Member

    Hmm... looks like cuts are suggested by many here.

    I was trying to avoid that to produce a better part with less resin usage and a uniform, perfect core.

    I guess I'll give it another try heating on the female form and see where that goes. If not, I will have cuts as my backup plan.

  12. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

    Last time I needed hotbox I used 100mm house isolation plates and car heater. It took ~8h to warm a barrel of epoxy to 65C.

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  13. War Whoop
    Joined: Jun 2003
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    War Whoop Senior Member

    You need to insulate the Oven, Polyisocyanurate boards from home depot will do and the Meat thermometers are a good idea,I would use a forced air propane heater.
  14. rberrey
    Joined: Oct 2010
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    rberrey Senior Member

    I forgot there was one more reason other than price I did,nt go with the corecell, something about an oven. I thought about using a salamander, this would give heat and forced air. You might have to build a rack with wire mesh and use two salamanders , one for each side, or top and bottem. The trick is keeping an even temp, you could hook up two exaust fans in the box for quick cooling as the heat sorce woud be outside the box and can be cut off with the flick of a switch. rick

  15. Submarine Tom

    Submarine Tom Previous Member

    I would build a recirculating, fanned oven with the heater external. Like a low speed wind tunnel. Create lots of turbulance for ample mixing resulting in uniform temperature. Well insulated with a variable thermostat, not an on/off one, upstream of the oven box.

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