Teflon as spark plug insulator

[lee9966]

I have seen comments from respected members about using PTFE (Dupont brand = Teflon) as an insulator core for a diy spark plug insulator. Some valid comments dealt with the difficulty of machining this material, and I agree. But... Working on a video about my Tiny IC redesigned as a twin I decided to run it hard to see what would fail; after all its been sitting on a display shelf for over 10 years.

I was surprised by two things. The fun one is with some fiddling with timing and carb tuning it hit over 10k RPM. After about a minute, mostly at idle, it failed, and it turned out that the Corian insulator had burnt a bit and cracked apart.

So, since I don't mind hard to machine materials for just a few parts, are there any other reasons not to use PTFE rod?

PTFE specs say:
"While PTFE is stable and nontoxic at lower temperatures, it begins to deteriorate at temperatures of about 260 °C (500 °F), it decomposes above 350 °C (662 °F)"

versus Corian - referencing the manufacturer's website where they (very cautiously say):
"Heat resistance: the material is heat resistant up to 100 °C (212 °F), but can be damaged by excess heat. DuPont recommends the use of trivets when the material is installed in kitchens"

Both have health concerns when overheated.

Even the melting/softening/molding temps show the same 2x higher temps for PTFE.

 

[stevehuckss396]

Take a look at peek. It's a plastic with a much higher temp rating than corian.

 

[Asm109]

when Teflon decomposes, the byproducts are extremely toxic.

"Overheating of PTFE generates fumes of highly toxic PFIB and poses a serious health hazard to the human respiratory tract. PFIB is approximately ten times as toxic as phosgene [2]. Inhalation of this gas can cause pulmonary edema, which can lead to death."
Perfluoroisobutene
https://fluoridealert.org/wp-content/pesticides/teflon.decomposition.prod.htm

 

[mrehmus]

And it deforms under pressure. Sort of like hard grease.
But:
Who says modelers can't contribute to scientific endeavor?
Bay Area Engine Modeler Club member (www.baemclub.com)

Paul Knapp’s spark plugs are in space! Dawn Aerospace of New Zealand has been using Paul's spark plugs to ignite thrusters for satellite propulsion systems. His are the only spark plugs that have been up to the task of performing flawlessly under wild extremes of temperature.
Dawn Aerospace of New Zealand has been flinging his little masterpieces into orbit for some time and is delighted with its satellite mobility performance.
The thrusters are beautifully crafted as a single structure using Inconel 718. They are supplied as single, four, or eight thrusters to be easily configured to various satellites.
The fascinating details and photos can be found at: dawnaerosystems.com
But far better is to go to YouTube and type in: Dawn Aerosystems for a variety of excellent videos of the thrusters in action. There is even a musical production with B20 thrusters playing a waltz!

 

[solarenergyadventures]

Why would anyone use Teflon in a spark plug when this stuff is readily available? McMaster-Carr
If that is too spendy, there is also this stuff. McMaster-Carr

 

[stevehuckss396]

Because the cheaper of the two is almost 180 dollars a foot. A foot of material will make 8 parkplugs maybe 9. That's 20 bucks for just the insulator. At that price just get a rimfire.

Most engine builders don't run 10,000 rpm for extended periods of time so corian works great and lasts forever. Last time i purchased corian bout 1.5 year ago, one square foot of corian was about 20 bucks and can make about 400 insulators. That's 1/4 inch. #8's is 1000. 1/4 inch that's 5 cents each.

I have been driving to work and back in a ford focus since 2010. Never a problem except a new battery. I could have done it in a 2 million dollar Bugatti but why waste the money when the 13,000 dollar focus can do the same thing. Makes no sense.

 

[Gabe J DiMarino]

Since we are talking about spark plugs can a welding tungsten be used as an electrode.

 

[GailInNM]

Another insulator material that I have been using for the last year or two is pultruded fiberglass epoxy rod. It is in common use as the shaft for bicycle safety flags so is easy to find. If you or a friend has an axel mount safety flag you can even pull it out of the stamped mount and saw off enough for half a dozen plugs then put it back in the mount and nobody will even know the difference. It has better qualities than Corian and is easier to machine. There are a few more details at:

https://www.homemodelenginemachinis...nstructlon-revisited.30665/page-3#post-360405
Gail

 

[solarenergyadventures]

Because the cheaper of the two is almost 180 dollars a foot. A foot of material will make 8 parkplugs maybe 9. That's 20 bucks for just the insulator. At that price just get a rimfire.

Most engine builders don't run 10,000 rpm for extended periods of time so corian works great and lasts forever. Last time i purchased corian bout 1.5 year ago, one square foot of corian was about 20 bucks and can make about 400 insulators. That's 1/4 inch. #8's is 1000. 1/4 inch that's 5 cents each.

I have been driving to work and back in a ford focus since 2010. Never a problem except a new battery. I could have done it in a 2 million dollar Bugatti but why waste the money when the 13,000 dollar focus can do the same thing. Makes no sense.

You could have just said you thought it was too expensive Steve.

 

[Steamchick]

Since we are talking about spark plugs can a welding tungsten be used as an electrode.

YES. - Or old electrodes recovered from old spark plugs, especially the Platinum tipped plugs.
K2

 

[Steamchick]

Lee,
A guy at my local model engineering club uses fuse bodies from domestic 13A fuses. Some ceramics explode, but some are softer and simply machine as required. They are the right size for his electrodes, and plug bodies... glued in place with epoxy resin. But at one show there was a small bang and one of his engines stopped immediately - as the ceramic had overheated the epoxy, which failed, so the plug body popped out! His plugs with machined shoulders don't do that, but he had made a plug with a ceramic that wouldn't machine and was parallel.
K2

 

[giel]

why not use alumina ceramic tube?? that's made for it costs Penny's and can be baked with a microwave kiln..

epoxy s and plastic don't belong into an engines business end ( burning chamber) as it easy heats up to 300⁰c +

 

[Charles Lamont]

I recommend a read of this:

https://modelengineeringwebsite.com/Miniature_spark_plugs.html

 

[stevehuckss396]

You could have just said you thought it was too expensive Steve.

I agree. My apologies.

 

[DKGrimm]

My experience with Corian is that it all depends on the design and application of the plug. I've had a Corian insulator on my low tension igniter running for hundreds of hours, with no sign of deterioration. On the SAME ENGINE, I made long-reach spark plugs with a long, tapered insulator around the center conductor (like a 'hot' spark plug). I could smell burning plastic as it started up, and within 60 second it had burned away all the insulator and shorted out. I think the difference is simply that the long-reach plug had a relatively large area of Corian exposed directly to the combustion flame front, whereas most of the insulator in the ignitor is sort of 'hidden' and heat-sunk in the body of the ignitor.

 

[TSutrina]

Working on an engine design. Need a spark plug. Really not happy with plastic as the insulator. The temperature limits shown by others is why. The outline is for a purchase plastic insulated spark plug. Put ceramic in the hot zone. The steel electrode is a poor heat conductor and so is the thin anulus air passages. Likely they are flame arresters. The cool end of the ceramic is held with epoxy. You could fill the air gap with silicone rubber.

 

[stevehuckss396]

Is this engine going to do work or run at high rpm for extended periods of time? If not corian is more than fine. I have had corian plugs in my peewee for 12 years and you can ask anyone I run mine alot when at the shows.

That dimensioned drawing looks familiar.

 

[TSutrina]

Working on an engine design. Need a spark plug. Really not happy with plastic as the insulator. The temperature limits shown by others is why. The outline is for a purchase plastic insulated spark plug. Put ceramic in the hot zone. The steel electrode is a poor heat conductor and so is the thin anulus air passages. Likely they are flame arresters. The cool end of the ceramic is held with epoxy. You could fill the air gap with silicone rubber.

I would either pot the electrode to the ceramic insulator top to bottom or just the top leaving air. Reason is that the silicone rubber will flex and can stand likely as high temperature as corian. Then pot the spark plug with epoxy. The reason is that the threaded body is heat sink to the cylinder and head. The epoxy will conduct the heat from the ceramic through it to the steel. So the temperature rise of the ceramic will not be that high. The electrode is cooled only by the wire connection and being out of steel is a poor conductor likely the fin efficiency is below 70% The tip of the electrode is going to be hot. That is why I prefer to not pot the full length in rubber. The ceramic can take compression but not much tension. So the steel housing will expand more then the ceramic. The highest by far expansion rate is the epoxy. The epoxy is weak compare to the steel or the ceramic so it's diameter will be determined by the steel on the and apply pressure to the ceramic, compression stress in the ceramic.

 

[xander janssen]

For my new engine, I need some very special spark plug design as it has to mimic a Diesel injector and there is only room for a 3 mm (1/8 inch) diameter.

For this I bought some Macor which I found at a German supplier which also sells to privat pers o ns after payment in advance. They have Macor rods ranging from 1.75 mm up to 50 mm diameter

https://www.mc-i.de/macor/aktuelle-preisliste/
Not cheap, but for me Teflon is out of the question given the risk for negative health effects.

 

[TSutrina]

Gave some thinking of making a ceramic spark plug insulator. Skip firing and sealing. Dry out the green clay it will shrink. steel expands greater then the clay, but the clay before sintering may slump on the OD of the clay. The electrode will push the clay out so the sintered ID of the insulator will be bigger. In use the temperature will be lower then sintering so there will be a gap on the ID of the insulator to the electrode. On the OD of the insulator the steel cold if slumping occurred will compress the insulator and when hot that pressure will reduce and could turn to a gap. Ceramic is strong in compression. Thus the steel of the spark plug is unlikely to break the ceramic.
How to make the spark plug. If you can the use of a pottery kiln that is the easiest. Search for a place that teaches pottery even a school and get your plugs put into a firing and glazing firing. Else you can look up how to pit, sawdust, barrel fire clay. These methods are thousands of years old and you can learn how they sealed the porosity. A pit built like a rocket stove with a fan flowing in are is much hotter for the final step up. Note this is how they smelt steel for hundreds of years. So you get to glazing temperatures. There is also a very fast DIY glazing approach on the web. The problem is that the faster you heat up the ceramic or clay the more likely that it will crack. And a spark plug is not the ideal shape for thermal shock.