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Need some advice - new oven

 
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farmertom



Joined: 07 Sep 2014
Posts: 3
Location: Taft, Tennessee

PostPosted: Tue Sep 16, 2014 6:37 pm    Post subject: Need some advice - new oven Reply with quote

Hi guys
I am new to this so be gentle with me. Smile . I am looking to build a vacuum forming machine to form parts for rc airplanes. I am thinking of building a 12 x 36 inch table/overhead heater. That is where I need your advice. I will be heating .020 to .040 petg plastic. I am thinking of using Nichrome wire for the heater and Durablanket for insulation. The substrate will be 3/4 inch plywood. I need some advice on how many loops to use and help with how to wire it. I have looked at the oven calc. but I really can't figure it out. Or you guys with experience, how would you build this oven, use different hardware? Drawing, pictures, links, all help me.
Thanks again. Very Happy
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Hot_Wheels
Newbie


Joined: 19 Sep 2014
Posts: 22

PostPosted: Thu Sep 25, 2014 11:05 pm    Post subject: Reply with quote

I'm stuck at the same phase as far as trying to build a oven. I called one supplier he gave me some numbers based on my working area size but he was not able to advise me the type of wire, gauge or length. What oven calculator link where you able to find? I was trying the one on armor..something but the link is no longer valid.

Thanks
Rick
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jdougn
Guru


Joined: 18 Mar 2009
Posts: 714
Location: Louisville KY area

PostPosted: Fri Sep 26, 2014 6:24 am    Post subject: Reply with quote

Hey guys, things are a little slow on here at times but it you don't mind waiting sometimes it'll get picked up. I'm not the best at oven design but might be able to help get you started. I run a 2'x4' modified protoform that I make fender flares for jeeps on.

The ever popular ProtoForm vac former uses 9600 watts for a 2' x 4' oven. Calculates out to 8.3 watts per square inch. I would suggest that is a good place to start the calculations for your oven. Or, if you go to their website they also sell oven kits. http://build-stuff.com/hardware/heating-elements/

After calculating the watts per square inch now you'll need to take that info to the Oven Calculator. It's found in a sticky at the top of the vac forming forum.
http://www.tk560.com/ovencalculations.html

Gives the option of comparing 4 different oven designs so don't let that be confusing. The Oven Calculator will indicate exactly how much coiled nichrome you'll need per segment to build an oven to your input specs. However, it is up to you to determine if the specs will work with your situation. For example, it will not tell you if the design you entered will overload the circuits in your house. That is up to you, or a qualified electrician.

It will take some work to figure out how your oven should be designed electrically. If it helps, here is a really good build thread that includes the construction of a protoform oven. http://www.tk560.com/phpBB2/viewtopic.php?t=1162&postdays=0&postorder=asc&start=0

Hope this information helps.
Doug N
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DougN - Just in case you're wondering, I got my "rating" legitimately... by posting aimless drivel, useless advice, and pointless questions.
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kayaker43
Expert


Joined: 13 Jun 2007
Posts: 171

PostPosted: Sun Oct 12, 2014 7:28 pm    Post subject: Reply with quote

Sadly the watts per square inch doesn't mean much by itself. I have used ovens that ran 12 watts per inch that seemed to put out half the heat of the Hobby-Vac that uses closer to 7 watts. The difference is the infra-red wavelength which is a function of not only the wire specs but also the thermal mass, heat transfer rate and general design of the whole oven area. Its a complicated interaction that's not predictable by the numbers. Fortunately even if its way off it sort of works but with a lot less efficiency.

It takes so much testing to optimize an oven but then the tuning is all off if you change one thing or substitute materials. Compound that by the fact everyone wants a different size built in a different way and there will never be a simple "calculator" that does a really good job. All you can do is start somewhere and modify from there until it gets the job done,.. or copy/buy an optimized unit.

The industry standard for commercial machines for many years was 15 watts per inch because they were so poorly designed. In the end you can melt anything if you throw enough energy at it.
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Doug Walsh
www.build-stuff.com
Hobby-Vac and Proto-Form machine plans

Also other plans books and videos for people who like to build things
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spektr
Master


Joined: 07 Jan 2008
Posts: 425

PostPosted: Sat Oct 25, 2014 10:48 pm    Post subject: Reply with quote

oh please people. they want to heat .040 petg.... if you were really pressed you could do small canopy parts with a harbor freight heat gun..... a 2 x 4 machine with 4 watts sq inch heavcily throttled will run all day long. Yes the IR spectrum makes a difference, but in my world, petf, abs and styrene are all close enuf on the absorbtion scale that it really is invisable in the process. a simple low wattage oven will be fine
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