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Over/under table and oven design
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Mattax
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PostPosted: Sun Oct 01, 2006 8:46 pm    Post subject: Over/under table and oven design Reply with quote

Hi all,

I am finally getting around to working on my oven and frame holder part of the over/under vac table. I have found both from trial and error and from reading on here that some things I was tryting will not work and others that will.

Here is what I have for the oven box:

slightly larger than 24 x 24 and 12 inches deep/tall stainless steel oven box.

Plastic holding frame has changed from an earlier design that I needed toggle clamps to lock the frame, to: two 1-1/2 inch aluminum extrusion frames mounted one on top of the other and held together with the locking clamps I purchased earlier this year from McMaster Carr. I will insert link later here.

I like Harleyguy's air cylinder design and want to incorporate it into mine as I will be pulling ABS, but cannot find the air cylinder he is using. Can you help me out?

I need the cylinders and mounts before I can order the framing to hold it.

The oven box will undergo one change due to height of 12 inches. That will be to lower the niachrome wiring down closer to the plastic frame holder which will stop at the bottom of the oven - as of now will stop directly into the bottom of the oven and plastic will be 1-1/2 inches below that- see frame holder design above. However, I am looking into making this frame slightly larger than the oven box outer dimensions so that the plastic can get higher to the oven.

Thought for all: would a 1 inch square frame be adequate to hold the plastic vs 1-1/2 inch? If so, a simple redesign could work.

So based on Harleyguy's seperate plate with ceramic mounts and niachrome wiring design, this should allow me to lower it down inside the oven box. What caulk will seal that can withstand the elements/heat?

For the oven wiring, can someone point out how to physically wire it for 220volts? Schematic?

Also, what is the best way to mount the ceramic spacers to the metal plate?
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drcrash
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PostPosted: Mon Oct 02, 2006 9:51 am    Post subject: Reply with quote

Whoops, I responded in the other thread... and forgot you'd said you have a steel box.

Considering that you have a stainless steel box, I'd think it'd be reflective enough that you don't have to worry about the distance from the coils to the plastic. Most of the IR is going to bounce around until it hits plastic.

The exception to this is is around the edges, where you want uneven heat to counteract the effects of convective cooling (from the bottom) around the edges. The outer row or two of coils needs to be fairly close to the plastic to get that effect.

That brings up another issue. In the Thurston James design, the outer row of coils is further out than the edge of the usable part of the plastic.

That means that the frames themselves shadow part of the plastic around the edge, blocking the IR from the outer coil from hitting the plastic just inside the frame.

That may limit the effectiveness of the outer coil for heating the very edge of the plastic, especially if your frames are made out of large stock.

Instead of putting the edge coil outside the footprint of the plastic, it may be better to put it just inside the frame's inside boundary, but close to the plastic, and the other coils further away.
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Mattax
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PostPosted: Mon Oct 02, 2006 11:31 am    Post subject: Reply with quote

drcrash wrote:


Instead of putting the edge coil outside the footprint of the plastic, it may be better to put it just inside the frame's inside boundary, but close to the plastic, and the other coils further away.


That means making a post to mount the ceramic spacers so as to bring that section of heating element down closer to the plastic.
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drcrash
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PostPosted: Fri Oct 06, 2006 11:01 am    Post subject: Reply with quote

Quote:
That means making a post to mount the ceramic spacers so as to bring that section of heating element down closer to the plastic.


Yes---or, depending on the exact size of your oven, mounting them on the wall instead of the floor/ceiling. If your last row of coil goes just inside the wall, which is common, you can mount it on either the floor/ceiling or the wall. Mounting it to the wall lets you put it at whatever height you want without an extra spacer.

You can also mount several ceramic doohickeys on a strip of something, and mount that strip on a few riser posts. (You could likely find some kind of of metal strut with convenient holes already drilled in it at regular intervals; like maybe one of those extruded metal verticals that holds up movable shelf brackets.)
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Mattax
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PostPosted: Fri Oct 06, 2006 3:00 pm    Post subject: Reply with quote

I can make a plate with all the necessary holes for the ceramic spacers and then set that up into the oven and bolt it down with 4 holes through the Stainless steel top which seems to me will reflect the heat back down better than making a metal grid which will absorb the heat.

I was reading in one of your other posts that the spacing between coils in the spiral is 2 inches, the distance from the coils to the plastic needs to be 2x this. Therefore, 4 inches. My oven box is 12 inches tall, so with the above plate, I would need to drop it down eight inches minus the ceramic standoffs distance. Would this be correct?
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drcrash
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PostPosted: Fri Oct 06, 2006 3:41 pm    Post subject: Reply with quote

Quote:
I can make a plate with all the necessary holes for the ceramic spacers and then set that up into the oven and bolt it down with 4 holes through the Stainless steel top which seems to me will reflect the heat back down better than making a metal grid which will absorb the heat.


I think either will work fine. A metal grid will mostly reflect the IR, just like a metal wall will. (It will absorb a little bit of it.) The effect of the grid would be to just be to scatter some of the IR. (The small fraction that hits the grid wires, minus the small fraction that it absorbs.)

That's kind of like making a wall a little more matte-finished and less shiny. It doesn't much affect what fraction of the IR (or light) is reflected, just the direction.

(A matte finish is just a finish that's irregular and bumpy on a very fine scale, so that it scatters light in all directions, rather than reflecting it coherently.)

If you have even "illumination" to start with, and a matte-finish reflective box, it will illuminate just as evenly as if it were shiny. The light will take different paths, but still come out even.

(And if you have uneven illumination to start with, reflecting it off a matte finish reflector will make it <i>more</i> even---the hot spots will get scattered and spread, and tend to fill in the cold spots.)

Quote:
I was reading in one of your other posts that the spacing between coils in the spiral is 2 inches, the distance from the coils to the plastic needs to be 2x this. Therefore, 4 inches. My oven box is 12 inches tall, so with the above plate, I would need to drop it down eight inches minus the ceramic standoffs distance. Would this be correct?


I'm not sure what you're asking. The distance from the coils to the plastic should be at least three or four inches, but it can be more and that won't hurt the evenness---it can only help, because the bands of IR illumination spread and overlap even more. (It also won't hurt the efficiency significantly, assuming the walls are reflective.) I'd go with five or six inches if I had the room, but beyond 4 it probably doesn't make a significant difference.

The only way more distance can hurt is when you want the illumination uneven---more distance will partly undo that by letting your band of extra IR illumination spread out more before it hits the plastic.

Also, if possible, I'd put the plastic up inside the oven a few inches if you have the room.

So for example, if your plate or grid or whatever puts your coils 2 inches below the ceiling, and you have 10 inches left, you might raise the plastic four inches into the oven, and you'd still have 6 inches between the coils and the plastic.
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Mattax
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PostPosted: Fri Oct 06, 2006 3:58 pm    Post subject: Reply with quote

drcrash wrote:


Also, if possible, I'd put the plastic up inside the oven a few inches if you have the room.

So for example, if your plate or grid or whatever puts your coils 2 inches below the ceiling, and you have 10 inches left, you might raise the plastic four inches into the oven, and you'd still have 6 inches between the coils and the plastic.


The plastic cannot go into the oven. It will stop at the bottom of the oven and the sides of the plastic holding frame will go one inch up around the outside edge of the oven.
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PostPosted: Fri Oct 06, 2006 4:42 pm    Post subject: Reply with quote

Quote:
The plastic cannot go into the oven. It will stop at the bottom of the oven and the sides of the plastic holding frame will go one inch up around the outside edge of the oven.


OK. You might want to put a skirt around the bottom, to keep breezes and natural convection from cooling the bottom of the plastic while you're warming it from the top. (Especially at the edges and corners.)

The hot plastic will warm a layer of air next to it, and if it doesn't get blown away and has no place to escape to, it'll tend to stay there. Without something kind of windbreak, the warmed air will to try to go upward and escape around the sides of the oven---especially since the warm oven sides will generate an upward current next to them, which will tend to pull more air from below the oven.

(Just the thickness of the frame below the plastic actually help significantly, with no additional skirt; I don't really know.)
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Mattax
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PostPosted: Fri Oct 06, 2006 9:56 pm    Post subject: Reply with quote

Since I cannot show you a picture here is what it should look like:
Quote:


-- l
l l l OVEN
--
---------------------- Plastic sheet
--
l l <- Bottom of plastic holding frame
--


I hope this makes sense.
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PostPosted: Sat Oct 07, 2006 4:33 am    Post subject: Reply with quote

I think maybe your ASCII art picture got mangled by the html processing software, which may be stripping out "extra" white space.

If you edit it in a text editor with a fixed-with font (so that what you see is what you get), cut and paste it into your posting, and put a Code tag (not a Quote tag) around it, it should come out fine.
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Mattax
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PostPosted: Mon Mar 24, 2008 3:15 pm    Post subject: Reply with quote

Been a while but here is a nice PAINT cross section version of the pic above.



So you can see there will be a small gap all away around the bottom edge of the oven to the plastic. However, the plastic holding frame whihc is 1 inch square will go up almost one inch surrounding the edge of the oven box and the oven mount will also act as a seal on the top.

The bottom plastic holding frame will be the "skirt" going down one inch all around the edges. The open area is just over 24 by 24 inches.
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PostPosted: Mon Mar 24, 2008 3:24 pm    Post subject: Reply with quote

Also, in goinv with what you said above. I should place the outer ring of colis within 2 inches of the edge of the oven and keep the other coils at the 6 inch height.

This image is showing the upper height of the coils at 4 inches but that can easily be moved upward to 6 inches.




Now, if the coils were mounted at 6 inches, what happens with the 6 inches of space above the coils? Should I block it off with another stainless steel plate? Or can I just use the wire grid method to mount the standoffs to?
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PostPosted: Tue Mar 25, 2008 9:59 am    Post subject: Reply with quote

I notice your frames are not exposed to the heat of the oven. I think most if not all the the ovens shown on this site have the frames exposed to the direct heat of the oven. Cold frames may make it harder to heat the sides and corners, especially on the thicker plastics. Anyone else have an opinion on this ? ...............Fredo
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PostPosted: Tue Mar 25, 2008 10:52 am    Post subject: Reply with quote

I have to agree. The frames should really be inside the oven to allow the entire surface of the plastic to be exposed directly to the heat source. If the edges of the plastic are not as hot as the rest of the sheet, it will be hard to make an air tight seal against your platen, which is vital for reaching maximum vacuum.

I originally had my frames on the outside similar to this design, only after reading through many of the threads on this site that I decided to change it so the frames went inside the oven. Now, every single part of the sheet is warmed up with no clod spots what so ever.
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Mattax
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PostPosted: Wed Mar 26, 2008 1:12 am    Post subject: Reply with quote

That sucks. But thank you for bringing it to my attention, however late. Sad

I will have to redo the entire oven box as it is stainless steel and add about 1 1/8 inch to all four sides.

And only the top frame will be able to go into the oven box anyway as the bottom frame is attached to the supports that go to the guide tracks.

That said, if I did have the outer ring of heating coil down low to the plastic, would that not heat it up more to prevent cold spots?

Another idea is to heat up the frame itself instead of sticking it inside the oven box. This would take out the cold spots problem. A heating strip placed directly on all four sides of the frame would pre-heat the frame and take out the potential cold spots. I envision a simple heat strip with a coiled wire hanging to the back side of the upper frame back to the main frame of the vac table. The coiled wire is SO CORD that is meant to stretch and can easily handle the 3 feet of travel the frame is meant to go.

Since the bottom frame will never get heated by the oven anyway, additonal heating strips can be placed on the bottom frame and remove any potential cold spots from it as well.

I would place a second coiled SO CORD as a ground wire to the frame in case of accidental shorts from the strip heaters.

Wonder how long it would take a strip heater to heat up a one inch square by 27 inch long piece of aluminum.

Thoughts on this? It would definitely save me money on redoing the stainless steel oven box and the mounts needed to secure it to the top of the frame. I am talking aprox $200 to $400 ($400 if the fabrication shop said they would have to make the oven from scratch).
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