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building a table; got a few questions left

 
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burley
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Joined: 09 Aug 2006
Posts: 8

PostPosted: Fri Aug 11, 2006 3:14 pm    Post subject: building a table; got a few questions left Reply with quote

Since I’ve decided to build my own vacuform table, I’ve been doing quite a lot of reading about vacuforming and the building of a table.
The Tk560 website, the tutorial and last but not least this forum all were a great source of information and in effect a great help.

I also ordered the vacuumformers bible by Thurston James but have not received it jet Sad

A lot of my questions were already answered in this thread, THANKS!

However I still have some left, every noob always do’s Wink


What do's the minimum wattage per square inch (in the oven) need to be, to make the material (HIPS) warm enough to perform a good pull?
I figure that the larger the table, the higher the output must be. Is there a formula of some sort?

At the bottom of the oven calculator page (ovencalculator) you can set the number of loops.
Is there a relation between the number of loops and the size of the surface of the table? (or does “more is better” aply?)

Is there a minimum number of loops?

Should the height of the oven grow/shrink with the size of it?

Quite a few questions I know, I’ll be sure to think of another 2 or so Wink
Anyway, thanks for your time and interest!
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crashmann
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Joined: 27 Sep 2005
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PostPosted: Sat Aug 12, 2006 1:16 am    Post subject: Reply with quote

Hi Burley, and welcome to the platen (instead of board Razz)!

The watts per inch calculation determines the amount of heat output per square inch. With my 28" x 36" oven base, I get a value of 2.381 watts / square inch. A thick sheet of .125 HIPS heats up to forming temperature in about 6 minutes, and thinner plastic heats up faster. So, based on this performance, I would say 2.3 W/sq" is a good number to shoot for. A lower number would take longer to heat up the plastic, and a higher number would shorten the heating time.

For the number of loops, basically you're just trying to ensure you have even coverage on the oven base. You don't want any gaping empty areas. For my table, I have 6.5 loops, and it works just fine. With a smaller table, you could have fewer loops, and a larger table would probably require more loops.

I believe Jim's 28" x 28" oven has sides that are 4" tall. Since my table is larger, and therefore has the potential to have the plastic sag more, I made my sides 6" tall. Again, if you go bigger, go a little taller. Now this is negated if you decide to do an over-under design like Harley Guy or BlindSquirrel Smile

You'll need to think of more challenging questions next time Smile

Charlie
TI-386
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burley
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Joined: 09 Aug 2006
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PostPosted: Mon Aug 14, 2006 3:29 am    Post subject: Reply with quote

Thanks for your answers!

I'll go thinking on some other, more difficult questions then Wink
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burley
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PostPosted: Mon Aug 14, 2006 9:05 am    Post subject: Reply with quote

So if for instance I was to build a really large table (effective 60" * 40") and would use 3 seperate heating circuits of 24" * 52" would 6.5 loops per circuit be enough, or could I go lower?

If I calculate the number of loops per sq" from your table I get 0.0064 loops/sq". Using that number on the table mentioned above, I would get 24 loops in total. Or about 8 loops per circuit.

Is this the way to go and can I calculate the number of loops like this? Or is this not the correct way?


You say
Quote:
has the potential to have the plastic sag more


Does this mean, that in daily practice the sagging is something one does not has to worry about that much?
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crashmann
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PostPosted: Mon Aug 14, 2006 2:41 pm    Post subject: Reply with quote

Hmm, I need to ask you some questions for clarification...

You want to build a large vac table, 60" x 40" Are you creating a flip flop style table or an over under rig?

If you're doing an over under, I think you could size the oven to 60" x 40" I don't believe you need to make the oven larger than the sheet being formed. you would have to confer with Harley Guy or Blind Squirrel for their expert opinion.

If you're doing a flip flop design, the oven base should be sized 66" x 46"
(based upon my guesstimation of scaling up from our smaller ovens)

With a larger base, you would need to go around with more loops to provide enough coverage to heat the plastic effectively. Again, using oneragistic analysis (or guesstimating), I would think designing with 10 loops and 12 segments would be sufficient. The oven calculator makes this pretty picture:


Then you'll need to split the loops across three separate 15 amp 110 volt circuits? (Is that the goal regarding your three separate heating circuits?) This is where the heating element calculator doesn't work.

If you split the length of stretched out nichrome wire by 3, then calculate power output based on that...

For the height of the oven, I think you would need at least 8" maybe 10" That's going to be a lot of drooping plastic!

I'll have to think about this some more when I get home. Brain hurts... Razz

Charlie
TI-386
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burley
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PostPosted: Mon Aug 14, 2006 3:14 pm    Post subject: Reply with quote

We are planning on a flip flop design.

For size of the oven I'm using 72" * 52" right now, and I set the height at 8" (was thinking about 10). I would split the oven in 3 segments of 24"* 52" with on each a seperate oven circuit.

Overhere we are using 230 volts, and I was planning to use 8 amps. Giving it 8 * 230 = 1840 Watts or 1,47 W/sq"".

But looking at the W/sq" it seems a bit low, so perhaps I should go for 14 amps, giving it a wattage of 2.5 W/sq"


I'm sorry about your brain Wink If it's any constalation, mine hurts to Smile
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burley
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PostPosted: Tue Aug 22, 2006 10:19 am    Post subject: Reply with quote

Finally, I've received my vac-formers bible Smile


Anyway, what do you think about the different oven options I drawed.

The loops along the edges are all 1" apart, the other loops are 2" apart.
The size of the oven table will approx be 40"x 84"

Please give me some feedback and tell me what you think; what I should do different, etc, etc.


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crashmann
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PostPosted: Wed Aug 23, 2006 12:04 am    Post subject: Reply with quote

Wow! Those are some pretty pictures!

They look like psychadelic eyeball illusion pictures (it also helps that I'm listening to Pink Floyd as I look at them right now Very Happy ) I really like the fist picture. It reminds me of some crazy patterns on our furniture from the 1970's Shocked

You're building an enormous table. I assume that you are going to put the hinge on the long side, so that only 40" is going to be flipped up and over? Can you divulge what you plan on pulling on your table?!?

Your wiring layouts look good to me. Although it may be a little more design work, you may want to consider spreading the coils three inches apart in the middle. I think there will be a great amount of heat in the center, and as the plastic sags, it will be closer to the coils, thus accelerating the heating. You may want 12" or 14" sides on the oven to ensure the whole sheet gets heated without the center touching the coils.

Disclaimer (the lawyer in my brain made me type this) My comments above are based on my experience with my vac table which is significantly smaller than what you are designing. I stongly recommend checking with an electrician to verify your wiring plan and power calculations.

Also, after seeing the rigs by Harley Guy and Blind Squirrel, you should seriously consider going with an over under design. Flipping a huge sheet of plastic is going to be awkward, and terribly disappointing if it gets boogered up during the flip. Plus, you'll need two people just to handle the oven lid, two (separate) people to flip the plastic frame.

Where are you going to store these huge sheets of plastic?

I recently had a delivery from GE Polymershapes that came in a tractor trailer! As you can see, my neighborhood is not designed to accomodate large trucks like this!


This is the first time it came on a skid with via palette jack:

(Ok, so the actual shipment didn't really warrant a huge tractor trailer, but I was very impressed to see the big truck in front of my house. Plus, he had Harley Davidson motorcycles in the trailer too!)

Normally, my plastic is delivered in a cardboard box by the FedEx guy


Every delivery for your shop would have to come as a freight shipment, and there's an extra charge for using a lift gate (unless you have a loading dock).

So, these are some logistical things to be aware of as you're designing the big rig.

I hope this helps, and definitely keep us posted with your progress!

Charlie
TI-386

Do you see what you have done Jim?!? People around the world are building monster rigs!!!
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burley
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PostPosted: Wed Aug 23, 2006 3:32 am    Post subject: Reply with quote

That's a big truck! Very Happy

Your assumption is correct, the hinges will be on the long side, and the 40" side is going to be flipped.

In your opinion is there a way to really know which sort of wiring is the best way to go, or is it just trial and error? OF course the ovencalculator helps a lot, but in my mind it still is a little vague Wink

In our current design we have oven walls of 10" but thats still under discussion, so perhaps we will make them a few inches taller. We are also still talking about an over/under design, but we feel that making a holding frame of this size, which is easy to move up and down is more difficult then a flip/flop design.

Unless of course we would be working with pneumatics, but that's a subject I'm completely unfamiliar with.

About the plastic, we figure this is actually cheaper then using smaller sized sheets, since the company we order from does not have to recut them. Also we have the room to storage them Smile Hadn't thought about the shipping costs, that's a good one! I'll look into that.


Edit:
I've cooked up some more "psychadelic eyeball illusion pictures" Wink This time I used your suggestions, and the loops in the center are 3" apart, however this means that the number of loops are reduced significantly. Doesn't this mean the oven creates less heat?

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jegner
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PostPosted: Wed Aug 23, 2006 6:27 am    Post subject: Reply with quote

Wow!

How big are you making your machine? I know Thurston James made a big 48"x48" machine, a flip-flop but as others have posted, if you are going larger than that, your machine is going to be huge, and the logistics of operation are going to become a factor. Plus, at 40 inches, you are wasting plastic. 48x48 is 1/2 a sheet of plastic. An economy of scale comes into play with your supplier, so standard sizes are encouraged. If this is your first machine, may I suggest you build a scaled model version, something smaller so you can 'learn' on. If you made a 24x24 machine, you could always resell it when you don't need it anymore.

I just wonder what you could possibly need such a large machine for?
Shocked

As for th oven coils, I'd suggest the fewer the better. Heat transmission is the key. Those edges and corners are going to be the cold spots, and the center is going to get really hot.
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Stomper
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Location: Woodburn, Oregon USA

PostPosted: Wed Aug 23, 2006 7:02 am    Post subject: Reply with quote

That is one abitious project!! You could practically vac-form small people with it... you aren't going to get someone to lay down on that to vac-form them are you? Wink
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crashmann
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PostPosted: Wed Aug 23, 2006 12:13 pm    Post subject: Reply with quote

Ultimately, I don't know of any way to determine which wiring method would work the best, except to wire it up and try it out Smile

I still like the square spirals (but only because they are so mesmerizing Shocked )

In theory, either layout should work fine. They both have the wide spacing to help reduce the heat concentration in the middle. The number of coils is not really a factor, ultimately, it's the amount of current flowing through the circuit generating heat. As long as the watts / square inch is close to 2.3, then you should have all the heat you need.

Charlie
TI-386
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burley
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Joined: 09 Aug 2006
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PostPosted: Thu Aug 24, 2006 3:48 am    Post subject: Reply with quote

Many thanks to all for your comments Smile

Jegner do you advice on using even less loops in the center and perhaps more along the edges of the circuits which I currently have in my drawing? Also where I live sheets are sold with the measurements of 40" x 80", therefore we are using these measurements for our vactable.

I was wondering if I could combine two (leftover) pieces of nichrome wire, or is this going to be a problem?

Does any of you use an electrically operated vacuum valve to activate the vacuumtank/pump, as mentioned by Thurston James? If not, I was wondering why not, since it looks like a good idea.

Also, do any of you guys use something similair to the Payne variable temp control units mentioned on the infraredheaters.com website?


PS. I'm not planning on creating a wall decoration like the one Jabba the Hut had, but who knows Wink
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jegner
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PostPosted: Thu Aug 24, 2006 7:38 am    Post subject: Reply with quote

burley wrote:

Jegner do you advice on using even less loops in the center and perhaps more along the edges of the circuits which I currently have in my drawing? Also where I live sheets are sold with the measurements of 40" x 80", therefore we are using these measurements for our vactable.


Thurston James talkes about outsides edge and the corners being the hardest part to heat, and in his book he talks about basically putting 1/2 of your heating output into the outer edge. If you look at his oven coil diagram, you can see what I mean. I think there is some logic to this, in experience, and I think using the fewest number of segments just helps simplify the machine. The nichrome wires do not have to be one long piece of wire, but they don't splice well. You would need to join two sections at a tapping post where your power lines are attaching.

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maul_security



Joined: 23 Feb 2011
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Location: franklin, tn

PostPosted: Wed Feb 23, 2011 10:41 am    Post subject: Reply with quote

I know this is a really old post, but I am considering a big vac table build and doing research to try to learn from those who have gone before. I want to build my table 48" x 48". It looks like a few people have tried to go even bigger than that. Does anyone know if they succeeded or have websites? I would love to pick their brains.
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