Substitute material for bakelite - electrical control

[JohnInOttawa 104]

So the strange question of the day.  I bought a scrapped three stage STACO autotransformer.  It was brand new, got damaged during installation.

 

The nature of the damage was that the unit was dropped on the end that had the control knob, broke the knob and I am considering that stage to be scrap.  The other two stages are unaffected and are fully functional variacs.  What I lack is a control knob. 

 

Despite repeated attempts to order said knob over the past number of months, it just is not happening, so I think it is time I just print one.  The issues are, of course, in service durability and electrical safety.  The original knob was bakelite, a great insulator but almost a ceramic in how it broke.  The replacement part does not need to be super strong, but tough and non conductive would be ideal.

 

I have nylon/CF and nylon/glass here that I would like to consider for this project.  nylon/CF is a better color (black) and the manufacturer says it is not conductive.  Are there better choices though? 

 

Thanks!

John

[geert_2 558]

I would have some doubts about carbon-filled materials. The leads of pencils are carbon-filled too, and are *highly conductive*: short-circuit a battery with a pencil lead, and you get a really nice welding arc... Black anti-static mats get their properties from the carbon too.

 

Make sure you measure conductivity on a small test print, and also the breakdown voltage.

 

So the glass-filled looks like a better idea to me.

 

Maybe PET should also do, at least for normal house-hold voltages? But I have no clue about its high-voltage properties.

 

Also, have a look at the working temperature at high loads: some equipment might run at 100°C or more, too high for most 3D-printing materials. In that case, you might consider printing a mould, and casting it in a thermoharder or two-component material. Maybe that is why they used bakelite in the first place?

 

[JohnInOttawa 104]

Those are all good considerations.  I am wondering if a mold is the right solution here as well.  One aspect I had not considered when I posted originally is that the air gaps present in normal FDM printing can defeat any intrinsic insulating qualities in the filament itself and can contribute to shock hazard around higher voltages. 

 

This VARIAC can increase output voltage above the source value and is rated over 500V so to your point, maybe I need to weigh what the original designers considered and go with their solution too.

 

J

[Torgeir 248]

Hi John,

 

I'll think as Geert, -try using mold and epoxy resin. 🙂

Epoxy is a very good insulator, but will suffer if heat is to high (but normally a bit over 100 deg. C).

There's plenty of brands, so if you can find one with increased heat properties it may fill your needs.

You can also mix in the color you'd like.. (But check for type).

 

Torgeir

 

[JohnInOttawa 104]

OK.  This sounds like the wisest plan.

 

I've been looking for an excuse to try mold making.  Looks like I found it 😉

 

Now a bit of homework for me....

 

Thanks again!

John

[geert_2 558]

A few tips (maybe you already know them, maybe not):

 

If you would go the moulding and casting route, be sure to post-process your mould very well: remove all layer lines as much as possible by sanding, coating/painting, or chemical smoothing. Otherwise the cast may be very hard to remove, as each layer line acts as a tiny undercut. Don't ask how I know...  :-)

 

I do the smoothing with dichloromethane now. (See my separate post on: chemical smoothing PLA and PET with dichloromethane, should come up in search.) And I make my moulds in silicone from 3D-printed models, and in 3D-printed hard shells (unless I want to cast silicone).

 

If casting PU or epoxies, (1) use plenty of release coatings in plastic moulds, or it will glue like hell (that is why I use silicone moulds, and even then plenty of release spray). And (2) use low exotherm epoxies, or else a 3D-printed mould will melt, and the cast itself might catch fire or explode, if too much epoxy is used at once, since the heat can not radiate out quickly enough. I have had plastic cups and even metal (lead) cups melt...

 

For 500V, I wouldn't worry too much about arcing or breakdown, if the material is thick enough. I don't remember, but isn't the general rule in air that you need a safety gap of 3mm per 1000V? Or was it 6mm? If you can find that rule, and multiply it a few times, you should be safe.

 

[JohnInOttawa 104]

This is great!  The layer line advice is good.  Fortunately the level of detail is low so I can smooth aggressively.  I am looking at potentially just getting some polysmooth filament and manually treating it with isopropyl.  Haven't used it before, so maybe overthinking things a bit.

 

With a wind chill below -20 these days, the notion of anything generating heat gets me excited, but I will heed this warning 😉

 

John

[Carbon 28]

We use this and it passes our conductivity testing.

 

https://www.kimya.fr/en/product/abs-esd-3d-filament/