Digital Hotbox

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Nezwin
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Digital Hotbox

#1 Post by Nezwin » Wed Oct 01, 2014 6:14 pm

Recently having moved from QLD to NSW, I've come to realise I'm going to need some kind of hotbox for curing glues & resin. In QLD the sun & ambient temperature just about cut it, here in NSW, particularly in winter, technology needs to step in where nature fails. I'm not going to say "I'm the first to do this" (as I'm sure there are plenty of others out there doing the same) but most hotboxes I've seen utilise the 100 watt bulb solution to raise the container temperature. This one uses a cheap $15 heater operating through a $35 ($50 inc. shipping - http://www.lightinthebox.com/max-30a-di ... om=sysmail) controller unit. The idea originally came from trying to pull a kitchen oven apart, then speaking with my electrically minded brother who set something similar up for his homebrew.

Additional items other than the heater & controller are the extension lead ($5, from memory), electrical lead hooks (for keeping the lead down - $2?), 4 caster wheels ($8 each but only because they had only 3 of the cheap ones), a few boxes of screws, some terminal blocks and a few short pieces of wire (which I already had).
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The frame is standard shelving angle (I'm not going to lie, this wasn't cheap), providing 2.4m length by 600mm width by 600mm height. The walls are 7mm ply ($50ish a sheet where I am, cheaper in urban centres) with a layer of 15mm insultation board glued internally ($15ish a sheet, I think).
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The ply lid has warped but that's an easy fix with some more support struts over the ends. I finished the wiring this weekend and it's functioning, albeit in an ugly & unsafe fashion. I need to get the controlling unit & wiring nicely tucked away in a box, rather than stuck to the side with tape...
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Here's the unit in action -
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Reading 20.8 deg. C, I've got it set for 50 deg. C so that it will turn off at that point, then turn back on at 45 deg. C (although these parameters are completely adjustable, within a fixed range).

Now, i know what you're all thinking - "Why would you build a hotbox that crazy big?". Well, I'll usually glue up a bow & laminate a glass lam at the same time, so I needed some extra width. The other dimensions just seemed to all slot together, so why not? It doesn't have to be this big but for someone who might be gluing multiple bows at once (Yeoman) it's about right, and you could probably even put an extra shelf in too. Plus, it gives me scope to move into some aeronautical & automotive projects I've been putting off.

In terms of construction cost, I think it is a bit more expensive than the electric bulb solution, but with a maximum 15 watt heater operating far more efficiently than 4 100 watt bulbs, savings will be made on electricity and general usage. The bulbs will have to be on a while to crank up heat, this fella controls himself. I also read rumours that the specific bulbs used are getting harder and harder to source, so in future this option may be something more available to most people.

Anyway, please excuse the shoddy build, but maybe it'll be a chance for me to get something completed and show you all what I can actually do...

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Stickbow Hunter
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Re: Digital Hotbox

#2 Post by Stickbow Hunter » Wed Oct 01, 2014 7:04 pm

Looks a good setup mate. Do you think the heater end of the box may get quite a bit hotter then the other?
Nezwin wrote:but with a maximum 15 watt heater operating far more efficiently than 4 100 watt bulbs,
I think you would have needed a lot more than four 100 watt bulbs for your hotbox to get up to temp.

Is the heater only rated as 15watts?

Jeff

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Nezwin
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Re: Digital Hotbox

#3 Post by Nezwin » Wed Oct 01, 2014 8:11 pm

Stickbow Hunter wrote:Looks a good setup mate. Do you think the heater end of the box may get quite a bit hotter then the other?

Is the heater only rated as 15watts?

Jeff
Cheers, Jeff.

Mental slip there with the 15 watt heater - I installed an additional resistor in the control switch to reduce heating drain to a maximum of 15 amps. Aim is to keep the fan on full, to circulate the heat throughout the box while keeping the element itself from getting to more than half of it's maximum temperature (full rating is 30 amps). I had images of a cheap little heater like that melting & catching fire enclosed in a box, so I wanted to take the edge off of it. It's rated at 2000 watt, but should be operating a little over 1000 watt with the adjustments. Regardless, I'll be keeping a close eye on it the first few times I cycle it to maximum temperature - it was $15 for a reason.

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yeoman
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Re: Digital Hotbox

#4 Post by yeoman » Wed Oct 01, 2014 8:42 pm

Interesting setup. I may plagiarise some of the features.

When I was younger I had a fan heater that wasn't particularly effective for heating the room I was in. So I'd sit right in front of it and it'd shut off. Since then, I'd assumed little fan heaters had their own thermostats to prevent them getting too hot. I guess mine wasn't cheap enough.

Instead of a resistor, why not just put the heater on at a lower setting? Does it not have independent heat/fan controls? Either way, a clever mechanism for combining heating and air distribution.

Personally I probably would have run the power cord on the outside of the box. But that's just me.

Where did you get the insulation board? I couldn't find it on the websites of the big retailers like Bunnings or Masters, and the cost of foil backed insulation surprised me quite a bit.

For a while I've thought he lightbulb idea was a bit inefficient. I mean, a lightbulb that gave off heat was inefficient because so much of the energy consumed was given off as heat. Conversely, used as a heat source it's still inefficient as a lot of the energy consumed is given off as light!

I've been toying with the idea of using infrared ceramic lamps, such as for incubators and reptile tanks. However I'm deficient in knowledge on such matters so I'm hesitant to actually laying down dosh to try it out in case it doesn't work. I mean, seeing as it heats the objects rather than the volume, how effective is a thermostat going to be?

Anyone else have alternatives to the lightbulb idea for heating their heat boxes?

Dave
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Re: Digital Hotbox

#5 Post by bigbob » Thu Oct 02, 2014 8:56 am

looks like a good job you've done there Nez. Before I finished my box, I actually bought 2 of those controllers one after another and 'blew' both trying to wire them in. I have reservations about the durability of the heater confined to the box due to long exposure to heat. My box has 10 globes which initially were incandescents but now are halogen and 75w. From constant exposure to the heat some globe fixtures, I think they are a batten mount have disintegrated and no longer will hold the bayonet fitting. hopefully they will still be available at an electrical supplier. My temperate control is managed simply by having a thermometer with the probe mounted through the lid, and 'cracking the lid to regulate the temp. Not ideal and patently a little inefficient in terms of wasted energy, but I'm ok with it.
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Re: Digital Hotbox

#6 Post by Nezwin » Thu Oct 02, 2014 9:03 am

yeoman wrote:When I was younger I had a fan heater that wasn't particularly effective for heating the room I was in. So I'd sit right in front of it and it'd shut off. Since then, I'd assumed little fan heaters had their own thermostats to prevent them getting too hot. I guess mine wasn't cheap enough.
This one has a little switch on the bottom so that if it falls over, it turns off. I toyed with the idea of bypassing it, but figured it's probably not a bad safety feature. I couldn't identify a thermostat in the guts of the little thing - there was a control circuit, a power circuit & emergency cut off (the bottom switch) and that was about it.
yeoman wrote:Instead of a resistor, why not just put the heater on at a lower setting? Does it not have independent heat/fan controls? Either way, a clever mechanism for combining heating and air distribution.
I actually spent a bit of time thinking about this last night and I think I've made a bit of a mistake. When I was a kid, my dads welder had a really sensitive feed dial (a potentiometer) on it, so we hooked up a resistor & decreased the sensitivity to it, giving us finer control over the wire feed. I figured that, if I did the same, it would decrease the load on the heating element - this only has 'off', 'fan only', 'one element on + fan' and 'two elements on + fan' settings. I wanted both elements on, but not at full draw, so I installed the resistor on the output from the switch to the internal power circuit (I have a huge amount of left over electrical stuff from LED blinkers & a digital distance measurer for the car that I built years ago). I think the mistake I made here was that the welder worked off of a potentiometer, where as this is a switch - an entirely different beast that won't respond the same at all. Either way, the heater is working still, so I've not broken it, but whether it achieves the desired outcome is yet to be seen.
yeoman wrote:Personally I probably would have run the power cord on the outside of the box. But that's just me.
Good idea! Sometimes you get so engrossed in a vision for something you don't see those simple ideas that would improve it.
yeoman wrote:Where did you get the insulation board? I couldn't find it on the websites of the big retailers like Bunnings or Masters, and the cost of foil backed insulation surprised me quite a bit.
Agreed, I was pretty shocked myself - I was looking at some huge prices to have a few sheets trucked up from Melbourne, and it was difficult to source locally. A friend from work, who is an ex-builder of sorts, knew a guy in Wagga who does installations and had 3 sheets spare that he was able to drop off here fairly cheaply. I've a little left still, but not nearly enough to do another box this size I'm afraid. I saw that Bunnings do an insulation roll, which would serve a similar purpose, at about $50ish. If you do manage to get some insulation board, make sure you cut it with an electric saw of some kind - the sheet for the lid I cut with a knife & the edges are all kinds of ugly whereas the rest (Where the edges are hidden by the support trusses) I cut with the table saw and it came out far, far neater.
yeoman wrote:I've been toying with the idea of using infrared ceramic lamps, such as for incubators and reptile tanks. However I'm deficient in knowledge on such matters so I'm hesitant to actually laying down dosh to try it out in case it doesn't work. I mean, seeing as it heats the objects rather than the volume, how effective is a thermostat going to be?
I had a look at infrared lamps but they're not cheap and it struck me that it's more a 'soft' heat than what I might want. My dad did a lot of laminate work back in the day, and he built an autoclave (it's just a pressurised oven, very easy to make) from an old kitchen oven, a retired pressure tank & his compressor, which is where I got the idea of using a thermostat, a box and a heater. I've placed the sensor at the far end of the container so that it picks up the low end of the heat differential through the box (although I'm hoping the fan limits that range), I'll adjust the thermostat setting until I'm happy it's at the right spot. The composites expert I source my resin from have given me a temperature of 70 deg. C as a mid-range value for curing, while currently I've been using the ambient temperature in direct light and it's taken a lot longer to set the resin than the spec sheets state. I'll start with 60 deg. C and see if the box catches fire or the heater melts before cranking up to 70 deg. C
bigbob wrote:looks like a good job you've done there Nez.
Thankyou!
bigbob wrote:Before I finished my box, I actually bought 2 of those controllers one after another and 'blew' both trying to wire them in.
The wiring wasn't as straightforward as I might have thought, but it wasn't too bad. I thought it was kind of odd that I had to split the power coming in (it would've been easy for the company building the unit to have done that internally) but I figured it out eventually. I had originally fed my neutral wire into the earth, which tripped the breaker for the house, but after I linked the neutrals to one another it has worked a treat.
bigbob wrote:I have reservations about the durability of the heater confined to the box due to long exposure to heat.
Understandable, and I've got my own reservations too, but we'll just have to see how it turns out. The heater was only $15, so even if I have to replace just that part of the system, it's not a huge loss. I'm wondering if a few fans + an electric oil heater might work better.
bigbob wrote:My temperate control is managed simply by having a thermometer with the probe mounted through the lid, and 'cracking the lid to regulate the temp. Not ideal and patently a little inefficient in terms of wasted energy, but I'm ok with it.
If it ain't broke, don't fix it! And your finished output is certainly not an issue, so you must be doing something right!

Neil

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Re: Digital Hotbox

#7 Post by rodlonq » Thu Oct 02, 2014 12:19 pm

yeoman wrote: For a while I've thought he lightbulb idea was a bit inefficient. I mean, a lightbulb that gave off heat was inefficient because so much of the energy consumed was given off as heat. Conversely, used as a heat source it's still inefficient as a lot of the energy consumed is given off as light!
Dave
Dave, I think the idea behind the reflective lining is that the light bounces around until it hits something with low reflectance/high absorptance, i.e. the form and bow within (think solar water heater). The only energy not being used will be that escaping via thermal conduction through the insulation. I laughed the day a fella at Bunnings tried to sell me energy efficient low power LEDs because he thought the heat box would get hotter than with old school tungsten filament globes. :biggrin: .

Good idea Nezwin. It will be interesting to see if your little heater stands up to the test. 2000 W should certainly be enough to heat even a large box like yours, especially decent insulation. Perhaps if you find the temperature not uniform enough, you could try to inject the heated air from the heater into a plenum or manifold in the base of the box.

Cheers... Rod

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Re: Digital Hotbox

#8 Post by yeoman » Sat Oct 04, 2014 7:04 pm

So have you had a chance to set it going for an epic trial run yet? If so, how'd it go?
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Re: Digital Hotbox

#9 Post by Nezwin » Sun Oct 05, 2014 5:23 pm

Afternoon Dave,

I set it going this afternoon after I'd fixed a couple of extra struts to each end of the lid - they took must of the warping out but there's still a 1/4"-1/2" gap there which I think has made a pretty major impact. I placed a few rail line offcuts (potential anvils?) on either end but no matter what I do, there's still a slight gap. Over time, with a few heating/cooling cycles, I'm hoping most of this warping will work its way out, which should improve efficiency.

After about half an hour of being switched on it'd risen from around 25 deg. C ambient to 44-46 deg. C, where it stayed and hasn't gone any higher, even with the heater on full blast. I turned it off & let it cool, removed the extra resistor I'd installed at the heater switch, then went again. Half an hour later, same story - 45ish deg. C. It's still good enough for using, however, as CG Composites (Brisbane based composites company where I source my resin & glass) have said "70 deg. C is optimum & will cure in an hour or so, but anything over 30 deg. C and you'll be fine". This was further reinforced by a local glider manufacturer who only cures just over 30 deg. C without any issues, I think it just takes a bit longer but impacts the mechanical strength very little. My previous experience with composites has had strict curing temperatures & pressures for items requiring high strength tolerances while items, such as a guitar & case, were cured at english room temperature, which is significantly lower than Australian for the most part. As an aside, I found that when made during the summer, the guitars had a different tone to those made in winter, which I put down to the curing cycle.

Anyway, I'm left with a passable hotbox I believe, which has quite a fancy digital thermometer on the side which isn't strictly needed. If I could go back & do things differently I would...

- Do away with the shelving angle. It wasn't necessary in the end, the timber worked fine. (originally the box was going to be made from mild steel but I ended up just using timber & it was fine)
- Make sure the lid is built far stronger than just a sheet of ply & a few struts - a full frame would be needed, or much thicker ply
- Don't bodge the caster wheels, which is pretty simple
- Don't cut the foilboard with a knife, use the tablesaw (I did this for almost all of it in the end)
- Maybe use a more powerful heater?
- Run the power lead on the outside of the box

I've currently got it on with a glass lam & a spotted gum/black bean riser gluing up. I'll leave it going a few more hours and see how they turned out, although I'm pretty confident it'll be all good. The box might not have got up to the 50-60 deg. C I'd been hoping for, but 45 deg. C is still going to speed up curing times to just a few hours or so.

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Re: Digital Hotbox

#10 Post by yeoman » Mon Oct 06, 2014 6:19 am

Thanks for the update.

Somewhat unfortunate you didn't get the results you were after, but some valuable lessons learned regardless.
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Re: Digital Hotbox

#11 Post by yeoman » Wed Oct 08, 2014 8:06 pm

So, are you planning on making any changes or leaving it as is? Tried to cure any epoxy yet?

I've been looking online for cheap fan heaters. They're all touting a built-in thermostat as a great selling point, and unless my eyes deceive me it seems that there's a dial on the (stage) left of your little heater. Are you sure the heater wasn't regulating itself somehow?
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Re: Digital Hotbox

#12 Post by rodlonq » Thu Oct 09, 2014 7:15 am

Neil,

Don't write off your fancy thermostat yet, you may come across a different heat source that it ill be more suitable to control with eg. I found an old electric water bed heater I was going to try but it turned out it was cactus, which is what it was on the dump :lol:

I reckon Dave is on the right track, perhaps your heater is controlling the temp to 45C max.

Cheers... Rod

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Re: Digital Hotbox

#13 Post by Nezwin » Thu Oct 09, 2014 3:07 pm

Thanks for the interest, guys. A little bit of encouragement goes a long way for some of us.

You could indeed be right about the internal thermostat, Dave, I'll take a look over the weekend, although I didn't see anything which might've indicated a thermostat when I had it open. To be honest, I had originally planned to extract the element from the heater & mount it (with the fan) to a small frame, wired directly into the power. This could well be the direction to go in, perhaps. I've been pondering it a little too (between a hectic work schedule!), and while it may have been reading 45 deg. C at the sensor, this is still the opposite end to the heater & adjacent to the gap in the lid, so the temperature gradient could well have brought the rest of the box 5 deg. C or more higher.

With the first run I cured a glass lam & a riser layup, both with epoxy (same product, one thickened into a glue, the other used as low viscosity resin). The riser has come up nicely and I've no doubt there won't be any failure in the glue joints. The glass lam was a little more telling in that I was able to easily release it the following morning - I usually wait a few days, just to be sure. The resin seemed different than previously... Hard to put my finger on it, but with previous attempts (cured at ambient) after 24hrs a fingernail impression could probably be left in the resin with full curing not really being achieved for a few days. I'm confident that, even with only a few hours at 45 deg. C then left in the box overnight, the resin was 95+% cured - certainly within a range I'd be happy to grind immediately. Without a doubt, I'll be using the box in future, even if it doesn't quite meet full expectation as it currently is.

Incidentally, I've sort of promised to put a pre-tensioned glass lam buildalong together over the weekend for a Brazilian guy who can't source Bo-Tuff. I'll have some more experience getting the box to work as a result of that and I'll post the results.

P.S. I left the lengths of rail over the ends of the lid since the weekend and the warping has lessened considerably. This could well improve the efficiency a fair bit on the second round of trials.

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Re: Digital Hotbox

#14 Post by Nezwin » Sun Oct 12, 2014 12:37 pm

Dave,

I took a better look inside the heater yesterday and removed any unnecessary parts of the internal circuitry.
heater gutted.jpg
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I couldn't find anything that suggested a thermostat until I got up real close to the earth end of the heating elements and found this -
thermostat.jpg
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It's way to small for my big fingers to be playing around with, but I think the copper part is a thermostat switch and the resistor controls the temperature at which it activates. I'm not going to go messing around with it for now, due to aforementioned fingers & other things to do, but changing that resistor could make a big difference, I think. The heating element itself is very thin though, so I'm nervous about making too great a change.

By removing the front cover I found it heated up a lot quicker, but still didn't get much over 45 deg. C. Still, it's doing a great job, all things considered. The glass lay up I did for the buildalong cam out wonderfully, probably one of the best yet.
new lam finished.jpg
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Similarly, the risers I glued at the same time have come out perfect. I worked them both this morning with no problems, which indicates the epoxy has cured to a high enough degree to be considered finished, and I only applied the heat for about 3hrs then left them in overnight.

If you're thinking of heading down this path for curing glues, I'd recommend it. I'm not sure how essential the digital thermostat is but the system as a whole is working wonderfully for me.

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Re: Digital Hotbox

#15 Post by yeoman » Sun Oct 12, 2014 1:16 pm

Thanks for the update.

Three hours at 45 C plus cool down time did it for you, huh? Good result!

As it happens, I built a hotbox yesterday. Not knowing how effective yours was at ~45 C, I went down the heat lamp route. I got six ceramic light fittings and six halogen 'basking' lamps from the local reptiliary, some wire, a plug, and one of these:

http://www.jaycar.com.au/productView.asp?ID=ST3823

It's essentially a thermostat with a broad operating temperature. It shuts off at 70 or 71, and switches back on at 58. They come in other measures, and I may end up getting one rated for 60 C. At the going price, experimenting comes cheap. Anyway, I had the father in law (an electrician) help me wire it all up.

The box doesn't yet have insulation, but I tested the circuit by turning it on and holding a lighter below the aluminium plate I bolted the circuit breaker to. It switched off in short order, and I blew on it to cool it off and it flicked back on again in due course.

Mine's big enough to do four or five bows at once. Backed bows that is...not glass laminated bows with enormous forms.

Personally, I think I would have preferred the fan heater option, but never mind.

Looking forward to seeing more of that buildalong of yours.
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Re: Digital Hotbox

#16 Post by Nezwin » Mon Oct 13, 2014 6:24 pm

Looks like a good set up, I'll be interested in how well it works. I'd looked at one of those little thermostat switches, but went with the adjustable unit instead. Good luck with it!

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