postcure oven

Sorry I just found a great thred in the archives! Sorry

Which thread would that be?

I’m half way through building mine - any tips?

cheers!

Kit

I post cured a 9’2" board for 8 hours at 60 deg C.

Used foil backed builders insulation 4" thick for walls and top, 8" on the concrete floor. A couple of bar heaters 4" off the bottom, the board on tressles 36" above and a fan in the box to circulate the hot air. Used thermocouples on a cheap multi tester and got great results…

I just did a serch for oven and all the info I could want was lurking in the archives! Although some questions could still be answered, like is there a time frame for the post cure? Like it has to be done 48hrs after the lam? Would there be an advantage to doing it more than once like after the lam, and then again after hotcoat?

Thanks

Rather than make this another “Geez the archives are good” thread.

Could you post links to the specific threads that were helpful to you.

And maybe what you have done/have found/are doing?

Maybe even photos.

Cheers!

http://www.swaylocks.com/forum/gforum.cgi?do=post_view_flat;post=208082;page=1;mh=-1;;sb=post_latest_reply;so=ASC

http://www.swaylocks.com/forum/gforum.cgi?post=191812;search_string=oven;#191812

Those links helped me alot, and sure I can find them for you!

My photoes are still on the way as I have not started yet, and hence the question on the “how long after the lam job can I wait to do the post cure?” In the archives there seem to be a concensus to not do it too early but is that the same as saying that you can wait 2-3 weeks or more?

I’m sorry I deleted my first post but I regretted posting it after doing some serching in the good archives.

How is your oven coming along KKsurf? Any tips? Photos?

Thanks

I’m pretty sure that you can post cure right after the hot coat hardens. Here’s a pic of my crude homemade fire hazard.

poor man’s post cure (PMPC): put the board in a couple of black trash bags, tape it closed and set it in the sun. Works fairly well…

I havn’t started glueing it all together yet -

but it’s going to be:

EPS sheets made into a box.

Electric blanket layed in the bottom to provide “safer” heat, not much chance of overheating or fire(they are for indoor use afterall), and the temperature will have a nice even slow ramp-up.

Fan stolen from old computer to give a little air circulation, keep it all evenly warm.

Thermostat to switch blanket on/off to maintain constant temp.

The only bit i’m struggling with is where to find a cheapo themostat…

The cheapest one that does what I want is NZ$90.

You can get all sorts of really cheap fixed-temp ones, but the problem with those is that the temperature has to drop about 15 C before the thing resets and the warming starts again.

So, the average temperature in the box will be lower than you actually want, or you will have to get one that switches off at a higher temp than you want… which is dodgy, don’t want meltage/warpage/explodage!

I WILL find a way to do it cheaper!

Kit

Hi guys,

Thanks for that. I reckon you’ll find this one useful too:

http://www.swaylocks.com/forum/gforum.cgi?post=266010

The ovens I have built in the past have been for drying timber, so you’ll appreciate that a 7’ x 2’ x 2’ oven is a whole lot different to a 12’ x 3.5’ x 1.5’ oven!

I am planning for my next oven to be a folder and lightweight. When not in use just fold it up and put it away. Should be startin g on it in the next month or so - expect to see photos and description up on here.

As for a thermostat you could do what I do and use a thermal circuit breaker/switch. Since these are normally +/-5C of a set temperature placement within the “oven” can be critical. If you can’t find a 50C switch you might need to place it somewhere that the heat accumulates. For example a 60C switch mounted on the “roof” of the oven when you are using warm air might give you 50C where the board actually sits. Another alternative is to use a warm mass (like an electric blanket) and a normally open thermostat switch (again, set rating) wired up to an exhaust fan. The warm mass generates heat. When things get too hot the fan sucks air out until things are cool enough. Both methods (and hybrids of the two) work for people. You can probably even use a thermal switch from a car if you fiddle with it enough (you’ll need to figure how to adjust it down to 40C-50C).

Cheers,

Hi again All,

Thought I’d take a stab at the temperature thing, after some digging around here and elsewhere. BTW the Post cure epoxy thread is relevant to this one, too.

First - what’s the ideal temperature and allowable range for our oven?

Various threads on Sways suggest ideal temperatures ranging from about 43ºC to 60ºC (110ºF to 140ºF). In fact, the ideal temperature should fall somewhere between a minimum and maximum dictated by the epoxy and foam we are using.

First the easy one - epoxy. Although most epoxies in use here will cure at ambient temperatures, given sufficient time, the whole point of post-cure is to go above ambient temperature to cure faster and result in greater strength. As such heating to over regular ambient temperature suggests the further we go over 30ºC (86ºF) the better - as long as we do not exceed the recommended maximum for that epoxy. In practice that’s unlikely to happen, since we will exceed the maximum allowable temperature for EPS before then.

Now for EPS :slight_smile: There are some interesting temperature facts bandied around on Sways. One tells us that EPS will weaken/soften at 65.6ºC (150ºF) and another tells us that EPS will “melt” at 85.0ºC (185ºF). So I went out looked up the physical properties of polystyrene… One of the interesting things is polystyrene doesn’t quickly transition between states like many other polymers, so there is no clear-cut point where it liquefies, for example.

Properties:

  • Vicat B 90.0ºC (194ºF). Vicat B is the temperature at which a specimen can be penetrated to a depth of 1mm by a flat-ended needle that has a 1mm diameter end.

  • Glass temperature 95º.0C (203ºF). Glass temperature is the temperature below which molecules have relatively little mobility.

  • Melting point 240.0ºC (464ºF). Melting point is the temperature at which a solid shifts to liquid form.

Note that these properties do not specifically refer to polystyrene in EPS form. In the EPS form it has a high proportion of gas and Vicat type properties will appear to be somewhat lower. This might account for the reference on another thread here to EPS “melting at 185ºF” (85.0ºC)… Or that may have been a softening property similar to Vicat A or Vicat B.

In any case this information about EPS suggests that staying well below 90ºC, or even 80ºC would be a very good move. Especially considering the extended amount of time for which we post-cure these structures.

Based on all this I’d suggest that the safe optimal temperature range for curing is around 40ºC to 60ºC (104ºF to 140ºF), with an ideal target temperature of 50ºC (122ºF). Obviously those could be pushed up or down, depending on how adventurous/conservative you are.

On the practical side. A typical 60ºC thermal switch (AU$4.45) will cut the power at 60ºC and restore power at 40ºC. That can be used to modify an extension chord/power adaptor/etc to cut out at those ranges, with the switch/sensor being on a “rail” so that height can be adjusted for fine tuning heat sensitivity (a cheap thermometer would come in handy for tuning it). This represents a cheap option that should keep your oven cooking your boards in a happy, safe post-cure temperature range. It’s how I plan to heat mine… For now ;D

Thoughts? Comments? “Bug-offs”?

Great!!!

The only reason I was trying to avoid using the cheap thermostats was so I didn’t risk having the temperature up as high as 60 C at anytime…

But with the astoundingly detailed info you have just provided, my concerns have been put to rest!

Thats exactly the figures i’ve been trying to find. - believable, relevant ones too :slight_smile:

The 60 deg C opening, reset at 45 (approx) deg ones are cheap-as-chips.

As long as that temperature is safe, which your info supports, then I’ll be using one of those.

Of course the others would be better, but not worth the money at this stage.

I feel that adequate air circulation will minimise hot spots, meaning that sensor placement within the box will not be as critical, but care will be taken anyway.

My resin supplier told me 40 - 50 deg C for 8 hours for my resin/hardener combo - I suggest anyone unsure should have a conversation with the tech guys of whatever resin company they use.

The company I use manufactures so many resins and hardeners (huge selection), and each one has different cure recommendations… definitely depends on what your using, especially speed of hardener.

Usually, slower hardeners need hotter/longer post-cures.

awesome, cheers for the great info!

Kit

Glad it was of use :slight_smile: It’s something I needed to look into for my own oven and though I’d share on this thread, since other people seem to be in the same boatd as me.

I agree the full-blown thermostats would be a little better, but for our purposes I think the thermoswitches will be fine… If I ever see a cheap thermostat somewhere I’ll grab it :smiley:

I also reckon you are right about air circulation. But I’d like to add that a direct (radiant) heating method might have some real risks in this setup, as oppsed to indirect (convection) heating. Direct heat is likely to casue hot spots and cool spots in the oven, with all kinds of possible associated issues.

So the name of the game is really even heating. We could use the thermal switch with an electric blanket to good effect. Use the switch as a control for a powerboard, place it on top of the board and proceed as normal. With full-blown “ovens” a simple height adjustment mechanism should allow for pretty accurate control. Personally I think convection heating needs a little room around the board and can benefit greatly (in terms of even air temp in the oven) from a simple fan on the “roof” of the oven to ciurculate the air.

And yeah, it’s important to make sure your temperature range suits your resin/hardener. If they are available consulting with the manufacturer’s tech guys will be a great move.

Thanks for the interesting conversation!

Im a bit of a greenie so Im planning to make a solar heated box (good old FL sunshine…my dashboard gets to 180F in no time)…

a bit of savings on the electric bill doesnt hurt either. Thinking greenhouses…

Good stuff gents.

Great stuff here!

My setup is going to be with EPS sheets and a hairdryer and some sort of temp control, perhaps just a thermometer with an alarm for warning and then just not leave it alone, this would also minimize fire hazard + the epoxy I use only need to cure for about 2hrs at 50C

Re. the even heat in the “oven” I was/am contemplating creating a double wall inside the oven where the hairdryer is connected. In the “wall” I will either just make a gap at both ends for the air to flow through or perhaps make multiple small holes in the sheet for circulation and then monitor the temp in the big compartment. I think something like that would be necesarry when using a hairdryer for even heat, and I dont have an electric blanket which sounds perfect!

Some Stuff that was posted by guys like Greg Loehr and Bert Burger when they were still active might be found on this thread. Things like epoxy softening and boards popping loose, maintaining vacuum while post curing, etc. Worth a look.

http://www.swaylocks.com/forum/gforum.cgi?post=248408

craftee,

Unless you want to sit and watch the thing I’d suggest using a 60ºC or 70ºC “normally open” thermal switch wired to some kind of exhaust fan. This will switch the fan on when the temperature gets too high, removing warm air from the box. I’d be inclined to take air from the top through a pipe running down the side of the box (prevents it from leaking warm air when you don’t want it to). All that should cost you less than AU$10.

Moksa,

Sounds good! I reckon that the main thing is that the hairdryer isn’t blowing directly onto the board. You could achieve the same thing a number of ways, I think… A “floating” peice of sheet steel would do the job nicely too… Or one of those “diffusers” that go onto hairdryers. You might find a thermometer will cost you more than a thermal switch, if you have to buy one :smiley: I reckon it’s the kind of setup that you can build on over time. I wouldn’t leave it unattended until it’s automated… And probably not if it uses a hairdryer :smiley:

John,

Thanks for the link! The biggest thing I got out of it was the time between jell and post-cure. I HAD planned on only waiting a day or so. Now I’ll make it at least three, probably a week.

Sheet and a half of foil backed foam from HD to build the box, 20 watt light bulb, $15 thermostat. Test it first so you don’t torch it or over heat.

BTW, it is hard to make a really good box all squared up. Curvey things are easier.