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cod
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Joined: 05 Nov 2006
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Location: San Francisco
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 Posted: Tue Feb 06, 2007 2:29 pm Post subject: infra-red reflectivity |
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In my ongoing compulsion to maximize every little detail of my machine's performance I am looking into ways to get the most performance out of my oven.
In previous posts it was mention by the Doc that aluminum was superior to steel for reflecting infrared. Why is this so? I would think that the most efficient reflector would actually be as close to a mirror-like surface as possible. Somewhere, either here or in one of the many tomes on the subject of vacuforming, there was mention of the reflective walls of an oven creating a "hall of mirrors" type of effect, where the IR would be 'fooled' by reflective surfaces , ricocheting multiple times off walls that would essentially create a bigger oven via reflection.
if this is so, it seems to me that the best utilization of this effect would be(in order of effectiveness):
a true optical mirror surface(good luck finding a fire-rated mirror)
a diffuse surface metal(steel etc.)
a white surface(painted hardibacker)
I'm not up on how IR rays react to these various surfaces in comparison to visible light, but assuming they follow similar rules.
whaddya think?
also, wondering if silver paint would reflect less or more IR than white
cod
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tubachris85x
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Joined: 15 Jan 2007
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Location: Ft. Lauderdale, Florida
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| Posted: Tue Feb 06, 2007 2:58 pm Post subject: |
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Are you talking about an oven usin IR bulbs? I have planned on using that kind of oven for my machine, but I couldnt identify what kind of bulbs they were at the hardware stores. As far as reflection, I dont see why something like that reflective heat resistant tape wouldnt work. I have no idea about a silver spray paint, becuase Ive found that the silver sprays tend to be very dull, and wont reflect much of anything. In my oppinion, I think you should just stick with the metallic reflective stuff, cause you may just wast time with a spray paint. White though would have more reflection then the silver most definatly.
-tubachris
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drcrash
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Joined: 04 Sep 2006
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Location: Austin, Texas
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| Posted: Tue Feb 06, 2007 4:14 pm Post subject: Re: infra-red reflectivity |
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| cod wrote: | | I would think that the most efficient reflector would actually be as close to a mirror-like surface as possible. |
You're confusing two different reflectivity issues here---what fraction of light something reflects vs whether it reflects that light coherently so that you can see an image in the reflection.
A flat (matte) white wall reflects more light than a typical household mirror. If I remember correctly, a stark white wall reflects 90 percent of the light that hits it, and a mirror only reflects something like 75 percent of the light that hits it. A typical mirror is actually light gray---a very, very shiny light gray.
Somewhere, either here or in one of the many tomes on the subject of vacuforming, there was mention of the reflective walls of an oven creating a "hall of mirrors" type of effect, where the IR would be 'fooled' by reflective surfaces , ricocheting multiple times off walls that would essentially create a bigger oven via reflection.[/quote]
You only need shiny ("specularly" reflective) walls for the side walls, which need to reflect IR that's traveling from the back wall toward the plastic, rather than scattering it. For the back wall, its okay if the surface is reflective but matte, i.e., scatters the light. In fact, it's probably marginally better for the back wall to be matte, but it won't matter much.
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also, wondering if silver paint would reflect less or more IR than white
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In general, silver surfaces do not reflect as much visible light as white ones, so white is better in terms of sheer reflectance.
In this case, that may not be true, because at least some of the "silver" oven paints have aluminum dust in them. Aluminum dust may be more reflective in the IR range than whatever the white stuff is in the white paint, even though it's grayer in the visible light range.
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Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com |
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cod
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| Posted: Tue Feb 06, 2007 4:15 pm Post subject: |
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test(can't delete?)
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Last edited by cod on Tue Feb 06, 2007 4:25 pm; edited 2 times in total |
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drcrash
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| Posted: Tue Feb 06, 2007 4:20 pm Post subject: |
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| tubachris85x wrote: | | Are you talking about an oven usin IR bulbs? I have planned on using that kind of oven for my machine, but I couldnt identify what kind of bulbs they were at the hardware stores. |
He's just talking about regular ovens using heating elements like nichrome wire coils or calrod bars. Those heating elements mostly radiate infrared, which is absorbed and converted to actual heat by the plastic. (Or food, whatever.)
You could make an oven that used heat lamps to heat plastic, but that tends to get expensive. The bulbs are not cheap, and you need to buy heat-resistant sockets for them, too.
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Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com |
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cod
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| Posted: Tue Feb 06, 2007 4:22 pm Post subject: |
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| tubachris85x wrote: | Are you talking about an oven usin IR bulbs? I have planned on using that kind of oven for my machine, but I couldnt identify what kind of bulbs they were at the hardware stores.
-tubachris |
I am referring to any kind of IR, either generated from a bulb or a coil(bulbs contain a small coil of wire basically)
Heat lamp bulbs is what you are looking for if you go to the hardware store. I began my project thinking I would use a bank of bulbs, but I don't think it would have worked as well as my nichrome coil setup- not as even-heating for a 2x2 oven.
Bulbs are an interesting example of what I was querying, since they can have an inner surface that is highly relfective. the PAR-style of bulb is especially made to increase the focus of heat by using a lens and reflection design, I believe. I have a 250 watt red heat lamp bulb in my bathroom. It cost 5 $. I would need ten of them to equal my nichrome wire circuit wattage. This would cost nearly the same, and would force you to design the 10 bulbs to be spread over 2x2 feet . I suppose it still might be worth pursuing, but not for me Very Happy
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drcrash
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| Posted: Tue Feb 06, 2007 4:24 pm Post subject: Re: infra-red reflectivity |
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| cod wrote: | | In my ongoing compulsion to maximize every little detail of my machine's performance I am looking into ways to get the most performance out of my oven |
If you make your oven too hot, it won't work well at all, at least not for thick plastic. It takes a few minutes per millimeter for the heat to soak through the plastic, and if you heat the surface too hot too fast, it will scorch before the inside is cooked. (For the same reason you cook big food items like turkeys on a low heat for a long time---if you don't, they'll burn on the outside before they're done on the inside.)
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Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com |
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tubachris85x
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| Posted: Tue Feb 06, 2007 6:47 pm Post subject: |
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Since I will be using this method, can someone point me in the right direction as to where to find these kind of bulbs? I looked at home depot and lowes, but none of the workers there seemed to know what I was talking about. As far as the elctrical, I can do my self, its just a matter of findning the right bulbs. Thanks
-tubachris
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dman
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Joined: 09 Oct 2006
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| Posted: Tue Feb 06, 2007 8:10 pm Post subject: |
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| Try an auto paint supply store. They sell infra red bulbs. A little pricey though. |
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cod
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tubachris85x
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| Posted: Wed Feb 07, 2007 5:23 am Post subject: |
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Whew, that is expensive! I may have to reconsider and re plan my oven then...
-tubachris
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cod
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| Posted: Wed Feb 07, 2007 9:18 am Post subject: |
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| tubachris85x wrote: | Whew, that is expensive! I may have to reconsider and re plan my oven then...
-tubachris |
not to mention you will be blasted with very bright light and need sunglasses 
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tubachris85x
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| Posted: Wed Feb 07, 2007 10:20 am Post subject: |
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| cod wrote: | | tubachris85x wrote: | Whew, that is expensive! I may have to reconsider and re plan my oven then...
-tubachris |
not to mention you will be blasted with very bright light and need sunglasses |
Well, m initial design for it was to be just another box, similar to my table, with the halogen bulbs lined in 3 rows of 3, evenly. I would just place the frame over it, and put a cover w/ handle that would rest over the frame thats holding the plastic sheet. The box would belined with that reflective heat resistant tape as well. I would go for the nichrome wire oven, but Im trying to avoid having to order supplies online unless I really need to. The reason is that I dont have a credit card so everything I do get online would have to be payed for through my parents, so im trying to build this strictly from things at either home depot, lowes and where ever I can get it locally.
-tubachris
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cod
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| Posted: Wed Feb 07, 2007 11:20 am Post subject: |
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| tubachris85x wrote: | The reason is that I dont have a credit card so everything I do get online would have to be payed for through my parents, so im trying to build this strictly from things at either home depot, lowes and where ever I can get it locally.
-tubachris |
ah, yes, the parents. wouldnt want dad to start engineering my oven either
I think you can find everything you need EXCEPT for the nichrome wire locally. I couldnt find that or the ceramic posts locally, but everything else I did. Try appliance/oven supply/parts store ?
cod
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cod
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| Posted: Wed Feb 07, 2007 11:33 am Post subject: Re: infra-red reflectivity |
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| drcrash wrote: | | cod wrote: | | In my ongoing compulsion to maximize every little detail of my machine's performance I am looking into ways to get the most performance out of my oven |
If you make your oven too hot, it won't work well at all, at least not for thick plastic. It takes a few minutes per millimeter for the heat to soak through the plastic, and if you heat the surface too hot too fast, it will scorch before the inside is cooked. (For the same reason you cook big food items like turkeys on a low heat for a long time---if you don't, they'll burn on the outside before they're done on the inside.) |
thanks for the info. I was under the impression that IR heats from the inside-out, like microwave ovens. The turkey analogy makes sense tho.
As far as maximizing oven performance, and despite our 'vice'-president's proclamation that 'energy conservation is an out-dated concept', I think I will strive for a maximally efficient oven, then maybe incorporate some dimmers to dial it down - you wouldn't believe what we pay here in Bay area for a kill-o-what
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