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Ash Small

Thu Oct 27 2011, 09:04AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I picked up a brand new, unused, large high vacuum rotary feedthrough from Egay for Â£50 a while back.

For your application, which doesn't require 'high vacuum' you could knock up a rotary feedthrough using viton seals, especially as you have access to machining equipment. (you won't need ferro-fluidic seals)

You could then just tip the resin in.

jpsmith123

Thu Oct 27 2011, 11:20AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Tipping it in sounds good in theory, but in practice I've found it's difficult to do it without making a mess.

In my case I have about 0.5" of clearance between the two cylinders, and I'm pouring a viscous mixture - something like heavy syrup. Even when I use a measuring cup with a well-defined pouring spout and I rest the pouring spout on the outer cylinder, I still have to be very careful or it goes all over.

Patrick

Thu Oct 27 2011, 02:21PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I wonder if you should be filling it in progressive stages.

first fill it a third full at STP.
draw down to 50 microns.

fill second third full at STP.
draw down to 50 microns.

fill last third at STP.
draw down and leave for setup/cure.

Ash Small

Thu Oct 27 2011, 04:07PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

jpsmith123 wrote ...

Tipping it in sounds good in theory, but in practice I've found it's difficult to do it without making a mess.

In my case I have about 0.5" of clearance between the two cylinders, and I'm pouring a viscous mixture - something like heavy syrup. Even when I use a measuring cup with a well-defined pouring spout and I rest the pouring spout on the outer cylinder, I still have to be very careful or it goes all over.

Sounds like you need something like a conical flask, something with a 'neck'.

jpsmith123

Thu Oct 27 2011, 05:09PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Basically I do it in a few stages right now.

As soon as I mix the epoxy, I put the epoxy (still in the mixing cup) into the chamber and pump it down for 15 minutes.

Then I pour it into the coil assembly, filling it enough to just cover the last coil segment - which leaves about 1/4" inch room on the end for foaming/bubbling. (And this is not much room, so I have to pump it down slowly and carefully).

Then after about a half hour in the chamber, I remove it and top it off with more epoxy, and then I let it cure.

When the epoxy cures it leaves a concave meniscus (maybe it shrinks a little when it cures?), so I mix up a little more epoxy and top it off again.

The fundamental problem is that the multiple windings take up a lot of room and there's not much room between the last winding and the end of the coil. If there was at least a 1/2" of space, it would be a lot easier.

Anyway, I don't think doing it in more stages, would help very much, mainly because of the finite "pot life" of the epoxy. (In this case 1 hour). Don't forget that as soon as it's mixed, the reaction starts and the viscosity starts to slowly increase. If you filled it and pumped it down in multiple stages, by the second or third addition/pump down cycle, it'd already be thicker and harder to de-air accordingly. Doing it in stages like you suggest would probably help if you were using an epoxy with a longer pot life, however.

Ash Small

Thu Oct 27 2011, 07:56PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

If you pump it down, then pour it under vacuum, it avoids all of the problems mentioned above.

jpsmith123

Thu Oct 27 2011, 10:58PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

If you design and build it properly, then yes, I think it would solve the problems (hopefully without creating any new problems).

I seriously considered trying to do it, but I concluded that to do it right it would be a fairly involved project, and I didn't feel like bothering with it just to encapsulate a few coils.

Patrick

Fri Oct 28 2011, 12:57AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i like my oil filled project, looks cool in a clear case and fewer problems, or at least the same as your vaccuum drawn epoxy.

Ash Small

Sat Oct 29 2011, 10:07AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

jpsmith123 wrote ...

If you design and build it properly, then yes, I think it would solve the problems (hopefully without creating any new problems).

I seriously considered trying to do it, but I concluded that to do it right it would be a fairly involved project, and I didn't feel like bothering with it just to encapsulate a few coils.

I'm planning a similar system for vacuum induction casting, which will also be usable for pouring epoxy, etc.

Ther are alternatives to using rotary feedthroughs, but these consist of using bellows or magnets.

jpsmith123

Sat Oct 29 2011, 01:34PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

If I were to try it, I think I would make two separate vacuum chambers - the one with the epoxy being directly vertically above the one holding the coil assembly.

Then I would pump both chambers down, and then after reaching a predetermined level of vacuum, open a valve or something and let a measured amount of epoxy pour straight down through a tube into the coil assembly.

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