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New oven design
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ANH trooper
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Joined: 20 Oct 2005
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Location: UK

PostPosted: Fri Aug 05, 2011 2:25 am    Post subject: New oven design Reply with quote

Hi guys,

I am wanting to make a new oven for my vacuum forming machine and I am thinking of making the coiled elements plates rather than the TJ style oven I have now (below)


The problem with this set up is it is heating the edges too much when I use plastics other than ABS. This design was created for a heater under the plastic, not above, so it actually heats the edges too much while the middle sags away and doesn't get an even heat.

Now, there may be a way to use this coiled oven and lay out the coil in another fashion or I could make the plates that Doug (I think that's who created them - it's been a while) made using a hardibacker plate with an oblong shape coil. I would just need to know the length or coiled wire and the resistance needed. I do have Doug's plans but they don't tell you the heater specifics.

Any help and advice much appreciated!

Cheers,

-Paul.
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spektr
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PostPosted: Fri Aug 05, 2011 6:30 am    Post subject: Reply with quote

Before you go to the trouble of making a new oven, why dont you try shading the center of the oven with screens or even steel flashing. I've regulated ovens that way for years.....
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jdougn
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PostPosted: Fri Aug 05, 2011 12:39 pm    Post subject: Reply with quote

Yeah, Spektr's got a good suggestion on using shading. Also, what thickness & type of plastic are you trying to heat?

If you decide to build an oven the modular system that Doug Walsh designed works real well. He recommends using his special insulated oven board that comes with his oven kit. It's probably much better than Hardibacker. Also look at the modular tile system that Spektr posted up a month or so ago. It looks like it would last longer than Hardibacker also.
dn
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ANH trooper
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PostPosted: Fri Aug 05, 2011 5:38 pm    Post subject: Reply with quote

Hmmm....not sure what you mean when you talk about shading?

The photo above was taken when the machine was still in production and the side wall are covered with the same Asbesolux (fireproof) material.

It's great for .080" ABS and has been used constantly for about six years now with no problems. But, I want to use .060" PolyProp now and maybe PVC and both plastics are very different from ABS. I have formed both I the past and PVC is much better than PP, but not without problems. PP is just a nightmare on my machine and it's all down to the uneven heating.

I'll have a dig around to see if I can find the thread spektr started.
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spektr
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PostPosted: Sat Aug 06, 2011 7:45 am    Post subject: Reply with quote

what i mean by shading is to make thin steel flashing parts that go between the oven element and the plastic being heated. if you remove direct heating paths to the center of the sheed by creating a shadow on it, you can solve lots of uneven heat issues.. Also, put a heat controller on it and turn the heat down.......
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ANH trooper
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PostPosted: Sun Aug 07, 2011 2:24 pm    Post subject: Reply with quote

I see what you mean now.

I think shading the two outer coils that are close together might help. It's the edges of the plastic that is getting too hot so I think I will give it a try.

BTW, I found you post about the ceramic tile elements. They look good!
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spektr
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PostPosted: Sun Aug 07, 2011 7:24 pm    Post subject: Reply with quote

normally i find the center of the sheets get too hot, id begin there,
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ANH trooper
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PostPosted: Mon Aug 08, 2011 1:50 am    Post subject: Reply with quote

It's the opposite for me. My oven is above the plastic and the TJ coil layout was originally designed for an oven under the plastic which allows the plastic to sag into it so the centre is not so hot while the edges needed to be hotter to compensate.

My problem when heating PP is the edges just burn through before the centre reaches forming temperature. PP does not heat up like ABS and kind of warps and wrinkles as it heats, and doesn't sag either. I think I will shade the outer edges and see if it has any affect on my results.
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spektr
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PostPosted: Mon Aug 08, 2011 6:38 am    Post subject: Reply with quote

The oven probably needs to be cooled off, the easiest way is to get the plastic to oven distance larger... that cools everything at the same time..... The second easiest way is an infinite heat switch, third easiest way is a solid state controller.....
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ANH trooper
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PostPosted: Mon Aug 08, 2011 6:03 pm    Post subject: Reply with quote

I have a setting on my lifting mechanism to allow me to set the plastic at three different heights, so I think I will give your suggestion about lowering the plastic a try first before I start making shading plates.

I have no idea how to install an infinite switch or even what one is to be honest. I do remember someone was tinkering with their oven and having a duel setting or dual coil lay out, where one ring was hotter than the other. I can't remember who it was though as it was a few years back now, but it might be good idea if I need to get the center hotter than the edges.
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DarthVader1
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PostPosted: Tue Aug 09, 2011 4:22 pm    Post subject: Reply with quote

It might the reason that you're using METAL WALLS instead of using the same concrete sheet (hardibacker 500 or equivalent) used for hold the coils.

Any metal is a HEAT CONDUCTOR, and it will reflect heavily when closest to the coils. Also, as you stated, the design was meant for the oven to be UNDER and not over.

My only recommendation is to RE-WIRE (or redo the oven part... yeah, I know) but instead of having the more close coils on the outer edges, do so but in the MIDDLE. It's costly, but it MIGHT help.
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ANH trooper
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PostPosted: Wed Aug 10, 2011 2:18 am    Post subject: Reply with quote

I already have enough coiled wire to make another oven from my original purchase and making a new oven has crossed my mind. That is why I was asking about the oblong elements that Doug designed. It might not be a case of getting the middle hotter than the edges but an even heat across the sheet?

BTW, the oven walls do have the same insulation material as the oven bed - I just hadn't finished the oven when that photo was taken some years ago Smile
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ANH trooper
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PostPosted: Wed Jan 25, 2012 2:25 pm    Post subject: Reply with quote

Right then, I am ready to attempt to make a new over head oven for my former and could do with some help from you guys with a coil layout that gets hotter in the middle rather than the edges.

My current oven is the same as the TJ oven with the outer coils very close together. I could do with someone who knows about electrics to give me some advice on coil layout and tap in points.

It's been a few years since I made my oven and I will be going back to my original thread to read up on the oven to refresh my memory. I did all the testing and work myself and it has worked flawlessly since day one. But it is only good enough for ABS as it is and I need to use other plastics so I have to give it a go.

Any help and advice is very much appreciated Smile
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crashmann
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PostPosted: Thu Feb 02, 2012 11:39 pm    Post subject: Reply with quote

What type of power are you working with in your workshop?

In the US, I've got 120 volts AC with a 20 amp breaker. Then I had a 220 volt AC outlet with a 30 amp breaker installed in my garage.

Let us know, then we can crunch some numbers Smile

Charlie
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ANH trooper
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PostPosted: Sat Feb 04, 2012 4:24 am    Post subject: Reply with quote

Hmmm...good question!

Electrics are not my thing but here goes.....We use 240V here in the UK and the breaker says 80A, though there are 3 trip switches on the box. The trip switch for the sockets has a 32 on it.

The plastic I am wanting to form is PolyPropylene and I had another go at it the other day after forming some ABS. The PP was a total distaster and I know it is all down to the heating.

The plastic distorts so badly that it touched my coils and burned through on the first attemp so I sat the frames lower under the heater on the second attempt. This was much better but the plastic still was heating unevenly and had hot and cold spots.

I really don't think the wire coils are up to the job for this kind of plastic. I have been looking at ceramic heating elements that should heat the entire oven bed evenly. If the coils can be used then I guess they will have to be placed much closer together and maybe have seperate circuits so the outer edges are not as hot as the middle?

It's the edges that are heating up too much at the moment as my oven has two outer coils closer together and obviously the plastic sags away from the overhead heater which means the outer edges held in the frames get the most heat. This is not good for PP unfortunately Sad
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