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Macko
Joined: 04 Sep 2013 Posts: 2 Location: British Columbia, Canada
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Posted: Thu Oct 24, 2013 5:53 pm Post subject: 3x4 rig build |
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First I would like to thank you this board and everyone who shared their knowledge, ideas, all kind of information. I have been a silent reader for a long time reading, planning for several months trying to wrap my head around some of the problems I was facing while designing this 3'x4' vacuum former.
My rig is based on the Proto-Form machine. The plan is great it helps me through a lot of details that already worked out, though I am modifying it - as many other did before me.
Here is some data about my machine:
I will be using 6"x24" elements with 22awg nichrome coils.
In all there will be 12 elements grouped to four zones.
With the needed 14400 watts/ ~65.5 amps/220V I am way over the 40 amps limit - but I don't want to use more than an 8awg wire for the needed circuit.
So I divided the machine in two parts. From the electrical point of view it will be two identical 2'x3' rig. I have installed two 40amps circuits from the main fuse box and ending in two 50amp receptacles. From here I am using two 10' welder cable to connect the machine of several reasons.
- I don't like the idea to permanently wire the machine to the wall.
- I can always move the machine to another location.
- I can use the two welder cables to connect to each other and take my arc welder around/outside.
- I dont't want to play snake wrangler with much thicker cable than the 8awg.
Both the LUG with the fuses and the metal cabinet for the contactors will be doubled for the two separated circuits.
I also installed a separate 20amps circuit only for all the other 110V equipment the machine will use and this will also be connected wit an extra cable.
I don't mind that I am using three cables to connect the machine because it still gives me more flexibility than one beast of a cable that I can only use for this machine.
I am about 1/3 into the build (well that's what I am telling to myself anyway).
I have some (...many) questions though, but here is one if someone has some thought to share about it:
Instead of the copper jumpers (in the Proto-Form plan) that connects the the elements can I just use the 8ga TGGT wire? For me these seem more protected so there is less chance for them to touch the metal or anything else. Or If I am using something like the copper jumpers I would like to stick it through a silicon tubing before I bend it so it touch-protected a bit more. Just an idea.
So thank you for all the shared info on all the threads...
M. |
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spektr Master
Joined: 07 Jan 2008 Posts: 425
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Posted: Fri Oct 25, 2013 7:55 am Post subject: |
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I reallly dont understand the need to have that much wattage. The watts /sq inch are astronomical and the resulting dwell times will be so short they will be very difficult to control. I would use a much smaller heat source , increase the dwell time and have a much better shot at success. |
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Macko
Joined: 04 Sep 2013 Posts: 2 Location: British Columbia, Canada
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Posted: Sat Oct 26, 2013 4:15 pm Post subject: |
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Quote: | I reallly dont understand the need to have that much wattage.... |
I would be most happy if could cut some of the power need believe me.
Corect me if I am wrong, this is my calculation:
One element (6"x24") uses ~5,97 coiled inchs of 22awg nichrome wire adjusted to 42 Ohm. That's ~5.45 amps / 220V / 1200 watts / 4098.215 BTU and 8.33 watts/sq.in.
These are all values from already built machines based on the Proto-Form plan.
In total I need 12 element for the 3'x4' oven so the total comes to 14400 watts, About 65.4 amps at 220V.
From "jdougn"'s thread I've found the following info:
"It is probably more reasonable to suggest that a general use 220v oven may have a watt density from somewhere around 4 w/in2 to around 8 w/in2. (w/in2 = watts per square inch)
Four watts per sq inch tends towards the cool side but works with certain plastics and vac former setups that Spektr has used. Of course, 8 w/in2 may be towards the hot side but works with many of the plastics commonly used in vac formers. If 8 w/in2 is used then other variables of the vac former setup may need to be considered to efficiently heat certain types or thicknesses of plastic. "
I tend to find the same data in other sites/threads that is why I calculated so.
I will need in time to use heavy sheets - maybe close to a 1/4" for some large armor pieces so I don't want to under power the machine.
I would like to know if I went too far somewhere in my calculation... that would be nice. Why is 8 w/in2 so astronomical... how much power would you use for a 3'x4' oven? |
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Fredo Master
Joined: 21 Mar 2007 Posts: 251 Location: Kingdom of Nye Area 51
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Posted: Sun Oct 27, 2013 11:43 am Post subject: |
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Macko,.. Here's a tech manual from salamander that may be of some interest to you. http://www.infraredheaters.com/pdfs/techman.pdf Gave me a better understanding of whats needed in the way of IR heat to form plastic. _________________ If someone has a gun and is trying to kill you,
it would be reasonable to shoot back with your own gun...The Dalai Lama..Seattle 2001 |
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spektr Master
Joined: 07 Jan 2008 Posts: 425
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Posted: Mon Oct 28, 2013 7:23 am Post subject: |
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Heres the rub on power calculations, and the reason I use a longer dwell time than some.
These comments are for single sided vens, dual sided heating is different.
Sheet thickness is a buggaboo for a lot of "hobbyist" thermoformers. 1/4 inch ABS is formable when done single sided IF you can get the core temp into the forming window without nuking the first side surface. If you waltz in with the heater you are building, you will stand a high probability of running the abs sheet first side thru the melt window prior to getting sheet core temps into the forming window. This is because the conductivity of the plastic itself limits heat flow within the sheet.
Remember the 3 factors for heat transfer.... Wattage, distance and TIME.
Now its nice to calculate how much energy you need to dump into a sheet in 1 minute to form it, an industry standard measurement I might add, BUT it isnt especially usefull for us....... We lack the controls to move stuff that fast and have different requirements... We need to form accurately in equipment we can build... Consequently, our recipies can work fine by toning down the BTU imput and stretching out the dwell time.
I was doing 1/4 inch Lexan Windshields for race cars on a 9 minute heat cycle. I backed off the heat, upped the time and guess what, they were flawless.
Theres nothing in this for me to argue with you, so I wont... BUT I've done this a long time, and you will find that throttling back the heat and extending the duration will make the process so much easier.... |
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jdougn Guru
Joined: 18 Mar 2009 Posts: 714 Location: Louisville KY area
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Posted: Thu Oct 31, 2013 9:21 pm Post subject: |
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Fully agree with Spektr. You can use way less heat. When I started trying 1/4" ABS on my protoform vac former single sided heat wouldn't do it. Or at least a nichrome/hardibacker oven wouldn't. The protoform was modified to use two ovens (still nichrome & hardibacker) and it'll comfortable run 1/4" ABS on 6-8 minute heat cycles depending on the sheet. The 1/4" (0.236" General Purpose Grade ABS from Spartech) is what I make all my stuff out of now. http://www.napierprecisionproducts.com
Doug _________________ 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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spektr Master
Joined: 07 Jan 2008 Posts: 425
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Posted: Fri Nov 01, 2013 8:18 am Post subject: |
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the quote you lifted from Doug was before I convinved him that the wattage numbers he was using were actually part of his problem forming parts correctly. I seldom find a need to be above 4.5 watts/sq in.....
Scott. |
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Fredo Master
Joined: 21 Mar 2007 Posts: 251 Location: Kingdom of Nye Area 51
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Posted: Fri Nov 01, 2013 9:46 am Post subject: |
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Is that 4.5 watts for each side for a total of 9 watts?? Am I correct in assuming the total power requirements don't change that much, its just split between two sides... I'm not forgetting time and distance... I understand anytime you double the distance it cuts radiation in half..
In my day, I did a lot of pic taking with film. One day it dawned on me, a vacuum former is like my camera... f stops=[power control], shutter speed, =[ time], focus= distance, film =plastic... A rather simplistic way of looking at it, but it worked for me... _________________ If someone has a gun and is trying to kill you,
it would be reasonable to shoot back with your own gun...The Dalai Lama..Seattle 2001 |
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kayaker43 Expert
Joined: 13 Jun 2007 Posts: 175
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Posted: Tue Sep 09, 2014 9:04 am Post subject: |
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Its funny how the industry standard for decades was 15 watts per square inch for thermoforming. You will see that figure published in all old books and catalogs. That excessive wattage allowed custom machine builders to throw way too much heat into a design while completely ignoring infra-red wavelength and other good design practices. Then they added $1000 worth of controls to turn it down until it sort of worked. No R&D needed that way. build it, adjust it and deliver it. It was expensive and inefficient.
Now we are coming to realize the importance of matching IR output to the materials absorption characteristics. This allows ovens to heat as aggressively with half the wattage they used to use. More energy penetrates into the sheet instead of relying on conduction from the surface.
The tuning is sensitive and the target window is small. That's why I have been reluctant to stop selling oven kits. With my kits I can control the tuning but as soon as someone substitutes materials or makes even tiny changes, the tuning is lost. A hardibacker or or ceramic tile copy will not work exactly the same as a Proto-Form element despite looking the same and having the exact same resistance or wattage. With flip frame bottom ovens you are also utilizing convection but with top ovens the IR output matters much more.
You will always have perimeter heat losses from the clamp frame at a fairly constant rate, reducing the overall heat output too much results in cold edges and corners and a greater need for zoning. If you can heat more aggressively (with good penetration), then edge losses are minimized. Complaints of too much heat are usually attributable to missing the IR target window or using water sensitive plastics like lexan or ABS. The surface bubbling from water expansion looks a lot like heat blistering but its not.
I won't debate with the local experts on every forum because they provide a valuable service by helping others and there is always more than one way to do anything. I'll just say that critical comparisons to the Proto-Form oven design always seem to come from those who have never actually used one or those who have copied them but made changes that altered performance.
Fortunately for the homebuilder there is a wide range of acceptability with oven design. just get it close and it softens plastic to some degree and still gets the job done. Don't confuse that with getting it right. For some users fast cycle time and overall energy usage really matters. _________________ 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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