Hi Everyone,

I'm an "emerging" prop maker, and I'm happy to be here as I have no doubt that this resource will become an invaluable one as time goes on. What brings me here today is the need to build a vacuum form heating element. I've spent weeks researching and learning about the available materials that make it "Go" but the area I've fallen terribly short in is electronics.

Confront me with wood, metal, earth, rock, fire, water, or Christmas party crashing terrorists and I have little problem, but I just do not possess the necessary mental capacity for electrical problem solving & theory. Like most laymen say when the topic comes up, the eyes glaze over and go cross. But I'm trying none-the-less! However, I know when I'm licked. Given how my brain works, there's too much to learn to work UP to the point where I can start to learn how to intelligently and efficiently "math out" my own heating element. So I very humbly throw myself at the mercy of AAC and it's users.


The Project: 2' x 2' Nich heating array.

Needs: Effectively heat plastics, acrylics, etc, up to 1/8" thick in under 2 minutes, to the point where they can be vac formed. And doing it as on-the-cheap as possible. (Who didn't see that one coming?)

Areas of struggle: 1) Method of mounting Hot nichrome to ceramic board. 2) Choosing correct wire gauge for my power source & length of run, then wiring the elements together.

Items chosen thus far:
1) Power source: 3amp 130v Variac - http://www.amazon.com/Variac-Variab...ie=UTF8&qid=1423686608&sr=1-1&keywords=variac (my price range)

2) Expensive Ceramic backer boards, 1900*f working temp. (Chosen being I can butt my wire right up to it, eliminating excessive sag and the need for more complicated mounting solutions)


Plan:
So far as I've been able to decipher: you throw some gator clips on each end of some nichrome and the resistance of the wire makes heat as you increase voltage. The more wire used, the more power needed to reach your desired temp, and vise versa. What I cannot figure out for the life of me is the gauge to power ratio and the length of *coil*.

Below are some ideas I've been considering, but I find myself in a paradox: I can't figure out total length of coil until I know my gauge and power limitations & I don't know what my gauge and power limitations are until a length of coil is chosen. (And I was trying to keep this as simple as humanly possible.)

(Ideally I think I'd like 115" of wire, but that's straight uncoiled wire.)



REFERENCE Material:





Questions:

Has anyone had a better idea than anything I've laid out?

I've learned that the thicker the wire, the more resistance & heat there will be, so what's the right gauge wire to get the most length out of my chosen power source?

If I stretch the wire further so there are less Turns Per Inch, will I have the needed power to heat longer coils?

I'm concerned about mounting methods, shorting out, reducing current flow, and finally failing fasteners due to heat. Would a cotter pin be sufficient in holding the Nich to the ceramic boards with minimal issue, similar to what's shown in the photos?

What method of attachment do you think is used in the photos?

Are there specialty connectors used in the photos to connect the Nichrome to the power source?



Lastly,

Thank you, Very, very much .
I am extremely appreciative of any assistance offered. I am certainly at a loss, and there are more questions here than I had originally wanted there to be. But it is imperative that this build be completed.

 

Wow! You have a lot of misconceptions!

First, (and let's get this out of the way) nobody designs in more parts than necessary just because it's not his money.

Second, the larger the diameter of the wire, the less resistance, the higher the current, and the more heat per second.

Third, the standard way to fit a nichrome heater is to stretch the coils out.

Fourth, nichrome coils are held by ceramic doughnuts. Go rape a dead clothes dryer and find some for free.

To be continued...

 

Now for thermodynamics. The problem is to heat a certain amount of mass by x number of degrees in y number of minutes. Every mass has a specific heat...B.T.U.s per pound-degree of change. I don't expect you to tell me the specific heat of polyethylene (or whatever you're heating).

Heat is not the same as temperature. Heat is more like a mass and temperature is more like an altitude. With a 3 amp Variac and a 120 volt power line, you can muster 360 watts. That's 360 watt seconds per second. Heat is 3.413 B.T.U.s per watt hour. You can make 1229 B.T.U.s per hour. Temperature depends on how big of a thing you're heating.

You have 2 minutes. That's 41 B.T.U.s
That's enough to heat 1 pound (of water) 41 degrees Fahrenheit.
I don't even think you're in the ball park.

Name the substance, weigh the substance, and call me back.

ps, you'd be a lot cheaper to use a 10 amp dimmer on a 120 volt rated coil.

 

Are there specialty connectors used in the photos to connect the Nichrome to the power source?

.

You need Fibreglass or silicone insulated wire at the element termination points. Often known as appliance wire.
Max.

 

You need Fibreglass or silicone insulated wire at the element termination points. Often known as appliance wire.
Max.

I have some asbestos coated wire that I scrounged from an old kitchen stove. Anybody want to phone Hazmat?

 

google up quartz heating elements. Not a cheaper solution, but it doesn't stretch and sag like the wire. would be easier to install, easier to repair and easily as effective as ni-chrome.
rock wool - would be a good cheap insulation material. check out welding supply shops for more good insulation materials

 

First, (and let's get this out of the way) nobody designs in more parts than necessary just because it's not his money.


ps, you'd be a lot cheaper to use a 10 amp dimmer on a 120 volt rated coil.

So I could buy a normal dimmer wall switch, lop the end off an extension cord and hook it up that way, and get more power with some adjustability? That sounds great.

Wow, lots to take in. thanks for the replies.

Let's jump right in.

1 Not sure where I gave you the idea for the first thing, but didn't mean to come anywhere near it.

2. so it's the same basic idea save for some technical data, bigger wire = more heat?

3 I know you stretch the coils but, I meant stretch them as far as I can while still being recognizable as a heating coil*.

4. I'm trying to avoid ceramic mounting hardware. At first I loved the idea and wanted to go all out with it, but it's pricey, and you still need mounting hardware to fasten IT down, so it's parts on top of parts. Which is what I meant by "complicated mounting solutions" in my original post and why I'm so keen on a wire tie or pins.

You need Fibreglass or silicone insulated wire at the element termination points. Often known as appliance wire.
Max.

Thank you! By termination points you mean, appliance wire to connect all the elements together, then to my power source or dimmer switch. Right?

I assume that wrapping the end around a bolt an cinching it down is an adequate connection. Correct?

google up quartz heating elements. Not a cheaper solution, but it doesn't stretch and sag like the wire. would be easier to install, easier to repair and easily as effective as ni-chrome.
rock wool - would be a good cheap insulation material. check out welding supply shops for more good insulation materials

I will look into the heating element, thank you.

And I actually am going with a stone wool, the boards I referred to above are actually stone wool compressed into a hard board. And ironically! I was on my way to the local AGL when I pulled out of the way to let a fire truck through, and it went straight into AGL with 4-5 others. This was two after some propane tank explosions and two employees were injured. So, yeah, I went the other way.



I'm going to try and simplify further.

What gauge wire should I buy for a 115" COIL, powered by my amazon contraption or this new and ingenious (thank you very much for that by the way) dimmer switch?

How difficult is the dimmer switch approach?

 

Ah! You're back.

That first bit was about your statement: "And doing it as on-the-cheap as possible." That always insults me to some degree. Ancient quote from somebody famous: "Anybody can fix a $3 problem for $3, a good engineer can do it for 50 cents." Hence, we discard the $64 Variac and use a dimmer.

http://www.amazon.com/E-Age-50-220V...cp_2_FB98?ie=UTF8&refRID=0EN2YCPCQ9MGPQSV9D87

http://www.amazon.com/Lutron-S600H-...?s=lamps-light&ie=UTF8&qid=1423803396&sr=1-20

http://www.amazon.com/MLCS-9410-20-...cp_5_502A?ie=UTF8&refRID=05SSJFN0PC02KP2EWN4T

Next, nichrome won't solder. You have to connect it with a bolt of some kind...unless you can spot-weld.

Third, nichrome doesn't care if it's coiled or how much it's coiled, as long as it can get rid of its heat before it melts.

4th, Wiring a dimmer is as dumb as you can get. Two wires, no polarity. The dimmer is in series with the heater.

5th, I don't know what size. Sounds like time for Google.

6th, How you going to keep the heat even? Cancel that. You have a pretty nice layout. What's wrong that you're looking for answers?

7th, Have you thought of attaching the vacuum under the heating chamber so you don't have to move the floppy, stretchy, hot mess?

8th, Go to a wooden leg store and look at their machine. They do really crazy shapes!

 

A clothes dryer element is 5000 watts at 240 volts. 1250 watts for a 120 volt circuit.
What if you cut it in half and had two 2500 watt coils at 120 volts?
Too much power.
What if you left it alone and just gave it 120 volts?
1250 watts. 10.4 amps. Very doable

Why am I talking about clothes dryer elements? Because the last time I bought one, it cost about $6. Then there is the idea that you will run them way under their limit, so they will survive...unless you bottle them up and let them get white hot.

Do you have a thermostat in mind?

* Edited because of Pilko

 

Ah! You're back.

That first bit was about your statement: "And doing it as on-the-cheap as possible." That always insults me to some degree. Ancient quote from somebody famous: "Anybody can fix a $3 problem for $3, a good engineer can do it for 50 cents." Hence, we discard the $64 Variac and use a dimmer.

I get the impression we'd get along well. (If I had a clue about electronics) I like this quote very much. While I'm in no way going to claim I am that person, it's the reason I'm going through this process. It's just not 'my area" and I don't know, what I don't know. Ya know?

I didn't want to put that in there believe it or not. But it's there in the same capacity as the warning sticker that tells you "The blade of this knife is sharp." A disclaimer of sorts so no one went through the effort, time, and trouble of finding a wonderful solution that never could have been an option to begin with. When you put that in there, people automatically get more creative, too. And that's when the best results happen, or certainly the most interesting.

Thank you! The dimmers are great. Thanks for laying out options. I'm going to show my appalling lack of knowledge here though, the two wires with irrelevant polarity are what come OUT of the dimmer and go to the elements in a series, how does power come in? I'd pictured an extension cord sliced open with the -/+ going to the corresponding locations on the dimmer. I will also look up images of this.

I had considered welding the wire to whatever rig I cooked up, but I just like bolts more. It seems better thought out to me for some reason. I can change it later, swap out my wires, that kind of thing.

About coil, I didn't think 2 or 3 turns in one inch would heat as quickly as say 8 TPI, so I was trying to zone in on what a good rule of thumb is. Come to think of it, I should have just asked that way.

5. Agreed- I will try to find a functioning calculator. To use the nichrome calculator, I need the amperage & volts (up to 120) of the switch I go with correct?

6. Thank you! I used instinct to come up with the idea, then math to make it real.

7. I sure did! This idea I got from plans on instructables dot com and threw in some of my own modifications. The oven will be at the top of my structure, then the platen below the oven, and finally the vacuum right below the that. The plastics frame (mild steel) will travel up and down guides at each corner, secured with a modified quick clamp while it's cookin'. The plastic can never go anywhere you don't want it to, and takes ...a second...probably to drop onto your work piece. It's going to be a free standing machine.

I will, definitely, post pictures of it upon completion. Given this new information you've shared I am reinvigorated with enthusiasm & hopeful of a good outcome again.

8. Sailed well clear over my head. Table leg molds?

The dryer elements sound great but like you said there are only 2.

I was not planning on a thermostat. The oven won't be sealed. it's 5 sided, open bottom. Temps & cook time will vary daily. f anything, I'll create a scale for the dimmer dial that can pop on and off for each kind of plastic, then draw lines on it using a laser thermometer on the plastic itself for a ballpark idea.

Unless I 'm missing the point of the thermostat altogether.


I didn't think I'd have this much fun talking about electrical stuff. Thank you for the surprise.

 

forgot #6, there's always someone that knows better or has a better idea or understanding of the materials ad how to best apply it. So I was looking for a thumbs up or down pretty much.

 

8) Legs for humans...prosthetics.

Only 2 dryer elements? Who said you can't use 4?
I merely think 2 will do the job. (Read my signature.)
Also read the electrical limits paragraph.

One of your limits is that 20 amp circuits are standard for house wiring. Electrical code says you must never design for more than 80% of that...16 amps...1,920 watts.
If that doesn't work, you are going to need a better outlet, like a clothes dryer outlet. That's why you should go look at a wooden leg shop. They have the machines and the experience.

Are you going to pre-heat this and then add plastic or are you going to attach the piece, rev up the heater, and stop when it sags?

A thermostat can be a control mechanism or merely a disaster limiter...a safety device. If you don't install some kind of safety, this is a serious hazard that must be watched at all times.

Grainger sells a vacuum foot switch so you can control the vacuum nicely with one foot.

 

Nichrome wire calculator.

http://www.jacobs-online.biz/nichrome/NichromeCalc.html

 

That calculator seems to come up with, "You can't get there from here."
The answers I'm coming up with are like .016 inches in diameter and 418 watts for (5) 21 inch pieces in series.
Not even close.
Wrong manufacturer?
One thing seems certain, you can't figure that the temperature of the nichrome has anything to do with the temperature in the box. Go play with it.

 

@ #12 If you supply a 5000W, 240V element with 120V you will produce 1250W at 10.4A.

 

@ #12 If you supply a 5000W, 240V element with 120V you will produce 1250W at 10.4A.

You caught me in a mistake.
That means this rig will run on one clothes dryer element at 10.4 amps and 1250 watts. Very doable.

 

16 feet , 19 awg , 1734W , 14,45A.

 

Obviously I couldn't get that to work. How did you get it to go longer than 35 inches???
Well, now it works.
Working for 16 amps, the limit for U.S. outlets, and the 125 volts I get at my house:

88.75 inches of 22 gauge is real close to melting at 2500 F
142 inches of 20 gauge @ 1860 F
181.25 inches of 19 gauge @ 1660 F
222 inches of 18 gauge @1480 F
284 inches of 17 gauge @1280 F
and that's about the end of it.

I'll bet the mfg uses 3 hundred inches of 18 gauge nichrome for a clothes dryer and coils it to get it down to 4 feet long and stretchy.

 

My preference is for reliability, so I would use the fattest wire, 17 gauge at 284 inches. To knock that down to 105 inches, you could wind:
3 turns per inch at .286" mean diameter
4 turns per inch at .21" mean diameter
5 turns per inch at .172" mean diameter, which is less than 3/16th of an inch, so I don't think you can accomplish that.

You don't really wind them at 3 turns per inch. You wind them at .286" mean diameter and stretch it out as you mount it.

 

@ #12, Your 17awg at 284" looks like a good, reliable combo.