OK, call me lazy.
I’ve always meant to build another one of these. And always meant to share how I built my last one. But the usual things happen… Groundswell, etc…
Reading the weekend’s posts I see some people have been having trouble because they don’t have one. So I figured I have a stab at kickstarting everyone on making their own.
The one I made was for rapid curing of wood for making bows. The StickBow site has an article called The Heatbox which shows a heatbox very similar to the one I made.
The principles should be the same for epoxy curing, tho… With a couple of minor differences.
My last box was a box built as follows
20mm x 20mm frame.
Frame coverred with thin MDF (3mm, I think).
Lined inside of box with silver insulation.
Rather than a hinged lid I just had a lid that could completely lift off and on. 3mm MDF is prett light
There are two main things you need to keep in mind:
You are working with mains electricity! If you are not an electrician you should do what I did (invite a mate who IS around for a barbie
This thing will contain heat! That means it’s a potential fire risk, if built improperly. Make sure nothing flammable is going to get hot.
Regarding the fire risk, this depends on how hot you are going to let it get. Thankfully the thermostat/thermoswitch lets you control exactly how warm it gets. Mine operated in the range 30˚C-45˚C. So the roofing quality silver insulation sheeting was perfect. And kept the heat in nicely (minimising the “lights on” time).
Some other notes:
There’s a whole stack of ways you can heat the thing and lights may not be ideal for your application. It worked nicely for me drying timber is all.
I also mounted a couple of PC fans in mine to ensure adequate airflow (which accelerates timber drying). That’s probably not necessary for boardmaking.
Make sure you make your box big enough! I discovered the hard way that expanding the box CAN be a pain in the butt (rewiring, etc).
If you want to use it as an “autoclave” make sure you leave plenty of room for your bag, give consideration to HOW you heat it (direct heat probably won’t be the go) and make sure you consider the amount of heat your bag can take!
Total cost for me to build was under AU$50. Salvage yards are a wonderful thing.
Anyway, these are just semi-organised thoughts. Hope it kickstarts things. I’ll add any additional info that springs to mind as it does so.
Sabs has a good system for a hotbox…
From memory its a box the size of a board, made out of insulation foam? i cant remember… at one end has a cheapie fan forced heater to provide flow over the board…
Can’t remember the details, maybe capn sabs will fill us in…
If you’re not doing many boards, a simple card board box lined with aluminum foil with a hole for a hairdryer is fine. It doesn’t have to be very complicated to work, I still use the same one from about a year ago. You could also use aluminum backed insulation and make your own that would probably hold up better. The hair dryer method works extremely well, heat and circulation in one step, pretty cheap too. Hope that helps.
RR epoxy low tech cure.
An electric blanket wrapped around your board and placed in a board bag then zipped up. Three hours later done. It worked for me. The board doesn’t surf all that great but it seems to be strong. Just my luck my first board will around to humble me for a long time.
I am surprised that there seems to be nothing in that article about the % moisture that is the goal. That number is critical for wood that is to be bent when building a boat, I can’t imagine the same is not true for wood to be used in a bow that will be flexed over and over. Kiln-dried lumber is typically at 6-8%, air-dried at 12-14%. For boats, air-dried wood at a slightly higher % is considered best. Since bow wood is not restrained by fasteners, I’m guessing that the ideal % is lower. Much smaller cross-sections may also make a difference. I also guess that the strength of epoxy is not impaired by accelerating the process in this way unless one used truly radical temps.
I’d be careful with heat guns, especially if the process is to be left unattended for any length of time. They have been known to produce sparks, especially on HIGH setting. I’d also be aware of the characteristics of the material used to construct the hot box and the peak temps that whatever you choose as a heater will produce. I’m not sure what is used in modern MDF, but historically, engineered wood products make extensive use of urea-formaldehyde, which could produce some unpleasant outgassing when first heated. Good ventilation would seem to be in order (of course, you already have this, right?). I’d also check the flame and smoke propagation rating of whatever is used for insulation, it varies quite a bit. For most small and hobby builders the heat-box will probably not be very far removed from the resin storage. A fire could be a little inconvenient…
Black plastic trash bags, taped tight around a newly-laminated epoxy board. I just put it out in the yard for a post cure. My little digital thermometer inside the bag indicated - on this particular day - that while it was 73F outside, inside the bag was 95…couple hours like that and I’m all set
Yeah, you see how simple the basic formula is. Lots of ways to skin this cat and no need at all to fork out $1,000s or $10,000s! Best to focus on is heat control, tho. And I still reckon for board making indirect heat will be best (warm air blower of some kind). Since my next box will be for boards it’ll use an indirect heat feed.
I have some concerns about some of the easy methods posted - more from a fire risk perspective than anything Heard of some real disasters here. but then, I am used to running these boxes constantly over a week or two, so that could be the difference.
Sounds like you, also, come from a non-boardmaking background. Yep, ventilation is crucial. And I’m glad to see someone else who sees the fire and electricity risks the same as I do.
Yeah, that article doesn’t mention final MC for a good reason… It varies! That heatbox is part of the equation. When making bows we aim for ambient MC where the bow will be used. Generally we cut the timber, seal the ends, let it sit in a shady place with a constant temperature for a week, then into the hotbox for 7-14 days (depending) and then let sit for a week watching for MC stabilisation. on an interesting side-note you can order final MC from kilns these days! And they prefer to send wood to it’s destination at ambient MC if they can (overdrying costs them more).
Ingenious. Not sure I could use it here in summer, given we hit 40C temps regularly. That thing’d get hot enough to melt the plastic.
True about hot plastic…
I guess one thing to clarify is that there are 2 ways to use heat with your epoxy:
You can wait for the resin to cure, then post-cure heat cycle it for higher temp. final stability;
Or, you can put your still-wet epoxy work into a heat box for high-temperature curing.
My process is obviously a post-cure deal, while there are others who use heat boxes as if they were UV lights for indoor UV poly curing…either way will work, either way will help ultimate heat-resistance of the finished product. I believe that the post-cure cycling needs to be done (just like with subsequent epoxy coats) within 24 hours or so to have a true chemical or cross-linking aspect. And make sure either your board is vented (open) or the core is relatively temperature-stable
Doug, if its summer and 80*+…no bags needed Just put the board out in the yard in the sun within 24 hours of spreading on the epoxy layer…