www.TK560.com

[Grandpa]

I have seen so many posts on people using this and some "Experts" even recommending it. It definitely should not be used, it is not rated for the heat the wire generates, it will crumble over time and expose your wires to shorting out creating a possible fire hazard!!

I know it is cheap, but do you want to use your fire insurance because you were too cheap to use the correct material?

Fiberboard should be used, it is rated for the heats the wire generate and it used for kilns and can be found under kiln supplies. Another product that can be used is Vermiculite Board.

If you are going to build a machine at least try to make it safe.

[felixx]

I am at the point where i will be making an oven and have been told and read by others that hardieback is the way to go.

Do you have any pics of yours that you can share?
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[jdougn]

Respectfully Grandpa, from where did you obtain the information that hardibacker is not safe nor suitable for vac forming ovens? You list no sources or references for your statement. Please let us know what your personal experience is with vac forming ovens and the materials you suggest rather than just posting unsupported remarks.

The hardibacker on my main oven has been in use going on 3 years. It is a single sheet measuring 29" x 52" and I form thicker ABS requiring long heat cycles. Although there are now some cracks in the hardibacker they are minor and don't even appear on the opposite side. There have been no other problems with the oven . Additionally, the new modular system being used has eliminated cracks in my newest (25x49") oven since the four modular panels are only 12 x 25. Hardibacker have obviously been used in diy hobby vac formers for multiple years with no one reporting serious reoccuring problems from properly built ovens.

However, if a product that can be shown to be more effective or safer is available at a reasonable cost I would be very interested. Perhaps you could support your original post by providing specifications on the products your suggesting, some pricing information, and links to the product source that is available to the typical diy'er?

Respectfully, DougN
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DougN - Just in case you're wondering, I got my "rating" legitimately... by posting aimless drivel, useless advice, and pointless questions.

[felixx]

Hey doug, I have a bunch of 1/4 inch board and wanted to know if that was thick enough for an oven.

as you recall im making a 2 x 3 foot oven

thanks buddy!
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[Grandpa]

jdougn, I have over 30 years experience as a aerospace engineer. Fourteen of those were spent at NASA with six years devoted to High Temperature Refractories.

Now getting to the real problem: Cement board is only rated to 572 degree Fahrenheit, above that it starts to shrink and develop cracks. At 1162 degree Fahrenheit calcium carbonate decomposes and starts to crystallize. I could go on about the other things that will fail, this should be enough information to at least get you thinking about using other materials. Backer boards were developed for a moisture barrier not as a refractory material as some here are using it.

The correct material to use is Ceramic Fiberboard, it was designed to be used for high temperature ovens. It is rated to over 3200 degree Fahrenheit. You can find 2'x4' x 1/2" sections for about $50.

The only one on this board that has addressed this problem is the designer of the Hobby-Vac and Proto-Form machines, all of his heater kits uses Ceramic Fiberboard.

[jegner]

Thurston James recommends calcium silicate millboard for the oven liner. I could find this only at McMaster Carr, and it was $300 for a 4x8 foot sheet. A well respected chemical engineer took a look at the Hardibacker 500 MSDS sheet and it's 60% calcium silicate, and the bulk of the remainder was silica, and a trace of bonding fibers.

I've been running my machine for 5 years, and although the HB500 has cracs, it's work surprisingly well. It's fire rated for noncombustible construction use and is flame resistant.

Cement board is a whole other animal and is NOT recommended. That stuff has a coarse fiberglass matt with cement applied, and appears to have other undesirable properties.

If there is another affordable solution that's easily available I would love to learn about that.

[Grandpa]

[jegner]

There seems to be a lot of questions regarding the James Hardy Hardibacker materials.

I don't want to sound like I am promoting one product over another, so I will leave it up to the builder as to what material to use for their heaters. Here is a link to the Materials Safety Data Sheet [MSDS] for the HardiBacker 500.



http://www.mccraylumber.com/pdf/JamesHardiemsds.pdf

You can make your own decisions.

For the Calcium Silicate mill board, most of the vendors I contacted have to be emailed for specific quantity needs to get a quote. This is not a DIY material, more in the line of industrial construction use material.

McMaster Carr has this material available by the sheet http://www.mcmaster.com/#calcium-silicate-sheets/=421rhw look at item 93505K42 for a rigid high-temp board, or 9353K51 for a slightly cheaper option.

What ever material you decide to use, just be careful and use some common sense.

I personally have had good luck with the 'hardware store' solution, and so have a number of other builders here.

[kayaker43]

Grandpa is stating facts accurately. Hardi backer is the wrong choice from a technical standpoint. It does sort of appear to work for a while, but I could never sell it in my oven kits or recommend it in my plans due to liability reasons.

If I needed a machine for my own personal use and for only one or two uses, I would consider it. Anything more than that and I would get the right material. My oven designs offer much more aggressive heating and the hardibacker always failed my testing. I suspect the lower heat output of the Thurston James design is less of a problem?

Will your homeowners insurance pay off if your house burned down because you used a material at 5 or so times more than its rated temperature? How many have sold or loaned their machine to others? Those people may not understand the risks you are willing to take?

I admire the pioneering attitude and inventiveness of this group, and their willingness to share ideas. Having said that, there are some pretty scary posts here regarding oven designs and that's why I have to keep a repectful distance from this forum.
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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

[speedofsound]

This was a very good read!

I'm sold, going to convert my oven away from hardibacker. Luckily, I can swap out all the panels in an hour, and there are a few others things I'd like to tweak while it's apart.

-Alan

[felixx]

are we talking about the same thing here?

http://en.wikipedia.org/wiki/Fiberboard

made from wood???

why would you want to use that as your oven?
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[crashmann]

Hah-hah, that reminds me of Monty Python and the Holy Grail:

"How do you tell if she is made of wood?"

"Build a bridge out of her!"

Grandpa is not recommending regular wood fiberboard, that would certainly smoke and probably catch fire with the nichrome wire heating against it. He is recommending ceramic fiberboard. A Googling for that returned a bunch of suppliers for building kilns, like the kind used for baking clay sculpture. However, those sites listed the price for a 2'x4' sheet at over $300!

A few months ago I decided the Grand Canyon crack in my oven had grown large enough, and it was time to replace the Hardibacker base after two years of service:



When I removed the sides of the oven, the center sagged tremendously, and I realized the dangerous potential for a huge electrical fire that I had been playing with:



I had no supports under the oven base, and the weight of the side walls were actually helping to support the oven, and prevent it from collapsing in the middle. I suspect that all of the holes I drilled for the wires holding the ceramic donuts weakened the Hardibacker, since the fractures connected the dots.



So I very carefully moved the nichrome coil to a new base. Do you know what it's like to handle a Slinky all stretched out? Now imagine it's 500" long! Don't get it tangled up, you'll never get it straightened out I secured it with cotter pins going through holes drilled into the Hardibacker board. Hah-hah, I've got much fewer holes than last time, so no chance of fractures developing here!







Finally, after all of these years having the wobbly table, I add support braces to the legs. (Are you happy now Jim? )



To prevent any trouble in the future, I also install two wood support beams under the Hardibacker base. In the unlikely event that cracks appear, I won't have to worry about the whole thing collapsing



I carefully measure the resistance of the nichrome coils to ensure there is an equal load on all five segments, then I install the screws for the terminals. Now I connect the wiring underneath for the hot and neutral electrical lines.



On the top, I secure the nichrome to the screws using lockwashers and nuts. No ceramic spacers needed because the Hardibacker does not conduct electricity!





I also replace a couple of the side panels that have cracked, and make a new lid since the old one had corners broken off from meeting the floor a little too hard...



The moment of truth - I plug in the oven and flip on the power switch. I've got an infra-red thermometer with a laser to check the temperature.

It smells like it's getting warm.

It feels warm.

No arcing or sparking.

It must be working! Whoo-hoo!

I let it run for 45 minutes to "burn in"

The following weekend, I pull parts for my sandtrooper backpack kits for 5 hours. The oven works great! One side is at least 20 degrees hotter than the other, but I use that to my advantage when pulling the radio faceplates which have a lot of detail



The next morning, at least 12 hours after my vac forming session, and after plenty of time for everything to cool down, I'm working in the garage trimming the new parts I've just made. When suddenly, I hear a loud "pop" and "sproing" from the oven coils. What the hell?!? Sure enough, the brand spankin' new oven base sprang a crack! It was right above one of the wood supports.



You consarnit piece of shit!

Fortunately, it is supported underneath, and has been running fine for the past 5 months. A second fracture has also developed running lengthwise. Even my new lid started flexing funny last weekend thanks to a new crack. Argh!

So, perhaps Grandpa is onto something here, after all, he is a rocket scientist Maybe Hardibacker is not the ideal material to use for your oven liner. It killed 6 jigsaw blades when I was cutting it to shape (and be sure to wear a respirator because you do not want to breath that dust!)



I don't believe it is water resistant. When preheating the oven, I'll set a few molds on the lid, and when I lift them up, there's lots of water collected on the side facing the Hardibacker - like a soda bottle on a humid summer day!

Perhaps I did not provide enough room for the Hardibacker to expand inside the metal frame - maybe that contributed to the fractures.

Perhaps the wood support retained heat, changing the rate of cooling for that section of the Hardibacker compared to the rest of the base.

Redesigning the coil layout by using 5 separate Hardibacker / nichrome panels and a little expansion room between them would probably be a better solution (and eliminate the 500" Slinky!)

So Grandpa, any "economical" sources for ceramic fiberboard? What do you use to cut that stuff?

Charlie

[jegner]

Thanks to everyone who has contributed to this very informative discussion. Good information. I agree the Hardibacker 500 is not the ideal solution and a true calcium silicate mill board is the better material to use in your home brewed machine. The HD500, over time WILL crack and deteriorate.

I think this grand experiment with the HB500 has been an interesting one, and has functioned, but is not ideal. Time to seek out an alternative.

Suggested materials, sources and prices?

[crashmann]

Going back and reading Grandpa'a first post, he also recommended vermiculite. A quick Googling found Bullseye Glass offers a 24"x36"x1" piece for $35, plus a $60 packaging fee, plus shipping for 19 pounds and an oversize item.

http://www.bullseyeglass.com/products/tools-supplies/kilnwork/bullseye-vermiculite-board-24-x-36-x-1.html

That one sheet is almost big enough for my 28"x36" oven base.

Missed it by that much!



Be very careful to not breathe in the fibers when working with vermiculite. It is not supposed to be manufactured with asbestos anymore, but you still need to wear a respirator when tooling it

http://en.wikipedia.org/wiki/Vermiculite



Anybody find other sources, or sizes?

Charlie

[jegner]

That vermiculite board has to be bisque fired before use. 1500 degrees! Ouch! I don't think my oven can get that hot.

But, I'm still looking... Good idea though!