Magnet Plate Casting - What Are We Doing Wrong?

[JasonPAtkins]

We cast our first magnet plate this past week.  The resin dried outside here in MI, so it took about 18 hours to harden (using half the hardener as suggested by the book.)  We ran into a few issues that I'm hoping have simple answers.  So we come to the collective old sage seeking advice.

The resin shrank, as we knew it would, but more than expected.  We poured on a level surface and filled the mold, when we put the top plate on resin was being squeezed out all around the perimeter of the circle.  When it dried, as you can see in the pictures, the edges are barely covering the magnets.  We ended up with one big pocket that doesn't have resin, though we're going to try to patch that when we pour the next one.

So, did we do something wrong in the pour, or is this just the way they look before you clean them up?

The next thing is, I don't understand the directions for the middle 6.5" disc, because they call for it to be 1/2" thick, which is the thickness of the magnets.  That makes sense except when you remember that the fiberglass mat on top of the magnets adds thickness, which it did, making the resin run on top of the 6.5" disc, which I had to chip out.  Should that disc really be 5/8" thick to allow some room for the fiberglass mat?

That leads to a bigger question about tolerances.  The book says there should be 3/16" between the face of the magnets and the stator.  However, the resin+mat on top of the face of the magnets is like a heavy 1/16" thick, so is the 3/16" including that, or is it 3/16" from the surface of the resin?  Or are we supposed to sand the fiberglass down almost right to the surface of the magnets?  It seems like if we start sanding on them, the mat is going to be exposed and then just get all frayed on the surface.

We have three more plates to pour, but would like to know if there's something else we should know before doing those.  Should we open the mold after a few hours to add a little more resin to fill the space that's been vacated by shrinking resin?  Should the 6.5" disc really be a little thicker?  What kind of gap are you really talking from the face of the magnet to the wire in the stator?

Thanks in advance for all of the help!  I'd had a set of model blades spinning in the backyard not connected to anything for the last six months and I can't wait to get them connected to something!  I want to just forge ahead, but I also want a well balanced assembly that's going to adequately protect the magnets from the elements.  Thanks again!

(The resin is bondo resin and we used the cloth rather than the shredded stuff.)

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Fixed your gross violation of image width restrictions. I only do that once, please do not post big wide photos without proper width tags. Next time the post goes away.
TW.

[JasonPAtkins]

Added to clarify, the second two pics show the edges, and I'm worried that if we sand it down so that the edge is even, we're going to get really close to the magnets, which seems like a bad idea both from a strength and magnet protection standpoint.  Or is a coat of paint enough to protect even if we sand the fiberglass to almost touching the magnets?

[Rover]

OK, I've never poured around magnets, so I have no idea about that (hopefully, others will answer about that) . It is the void that caught my attention, I do a lot of mold pours, some with polyester resin (bondo), and they can be tricky.. couple of things caught my attention

1. You said the resin dried out?
   
2. 18 hrs to harden?/cure? (what is your temp?)
   
   

It looks like the resin was either very thick , or the resin stuck to the top while gelling..(still thick resin due to temperature?)

3. Any additives to the resin? polyester resin cure times vary widely when adding additives.
   
4. Was the resin chunky (not unusual, but should not be used), seen this numerous times with box store aged resins , including bondo
   
   

 

[Rover]

Adding..It really looks like you casted too cold, and the mold was not level or had a leak, especially since it looks like the void extends under the two magnets adjacent , and the layer of fiberglass is white in that area and sucked in.. way to much to account for shrinkage alone.

I won't pour in less than 65 F and not more than 80F, polyester or epoxy. Temp has a dramatic effect... either way...

with the extended cure time, the epoxy had the time and the fluidity to seek out a leak or to flow over the downside of the mold.

 

[wdyasq]

First, you used 'Bondo brand' - indicates polyester resin ... the fact of 'half hardener' sort of confirms it.

I wish I could think of something good to say about polyester resin.

IMO, epoxy resin should used for magnet casting. Vinyl-Ester resin should be used for coil casting.

Polyester resin MIGHT make a good suppository for those who recommend it.

Ron

[Rover]

"Polyester resin MIGHT make a good suppository for those who recommend it."

Ouch.... that might hurt, the following day of course

I use epoxy resin (MAS, West, and others)  70% the time, polyester resin does have it's place though...I would also use epoxy in this case.

[Blutoy]

Clean it up and use it.

[JasonPAtkins]

1. I didn't mean it dried out, just that it cured.
   
2. We left it outside at about 40f, which judging by the other responses was our problem.
   
3. No additives.
   
4. Not chunky at all, it was thick but smooth.
   

[JasonPAtkins]

It sounds like that is our problem.  We cast it outside (it was about 40F) because we were worried about fumes.  I figured after reading a few posts about stuff curing in 30 minutes that since it was our first time, the slower the better and would give us plenty of time to work with it.  It seems in retrospect that this is just plain too cold, huh?  We did the second one today inside the shop at about 65F with ventilation fans running, so tomorrow we'll see how they turned out.  Hopefully it will work a little better.  Live and learn, right?  

Thanks for all of the replies, everyone!

[JasonPAtkins]

We're planning on doing exactly that.  I figure it might not be the prettiest, but as it's the first of many, that's not the worst thing that could happen!  We might just have to sand it down flush with the steel, which wouldn't "grab" the disc as well, but should work just fine.

Thanks again for the advice everyone!

[Rover]

couple of tricks i find usefull for pouring in colder weather..

1. keep the mold indor until the last minute
   
2. keep the resin , somewhat warm (not hot ) in a tap water bath
   
3. If you have an electric blanket, heating pad etc, laying around, use it, cover the moled with cheap plastic drop cloth, place the heating blanket on top, and another blanket. (you don't need to heat the room, just the mold and the epoxy). Leave a thermometer under the top blanket to keep an eye on temp and runaway heat for when the epoxy starts to cook off ( usually within two hours)
   

[ibeweagle]

scrap that idea of casting magnet get new design with 16 magnets and metal magnet holder if interested e-mail ibeweagle@hotmail.com

[Dave B]

Bondo is not an adhesive. Casting the rotors is not necessary. Properly glue the magnets with an "adhesive" usually epoxy not super glue and fillet around them. No fuss, no muss, no big expense, no cracking and trapping water and no extra weight issues either. Why is banding used if casting the rotors ? Just something to think about, KISS.  Dave B.

[Flux]

The casting doesn't need to be pretty and what you have will work.

I don't see any form of band round the magnets and that thin bit of polyester resin will give little support above what the glue gives you ( assuming that you have glued the magnets on. If you are in a dry climate the protection from weather should be adequate but I am not convinced that the protection from polyester is much anyway. I have noticed rust penetration between the steel and resin on some of mine.

If you really need weather protection then something that sticks such as epoxy is far better. If you must use polyester for support then you need a band round the magnets or larger discs with a significant resin casting round the outside and you need to think about keying the whole resin block to the discs, it really doesn't stick to anything but the attraction of the magnets keep them on so if you key the resin block so it cant get off centre the whole thing will stay put.

Unless you live in a harsh maritime climate you may be better off without potting the things, just stick them on with something decent and perhaps a light coat of epoxy or something weatherproof. Where I live I feel some form of total covering is necessary but I have not been impressed with polyester.

Flux

[bob golding]

it will work ok,but if you can get vinyl ester that will be a lot better,and is not that much more expensive. cheaper than epoxy. i just cast 2 magnet rotors in cold conditions and they worked fine. took about 24 hours to go off properly but seem fine. i used 50 % fillite filler in mine. a few minor bubbles but nothing serious. i used polyester for my last stator casting and i can tell you  vinyl ester is a lot easier to use. i haven't cast my new stator yet, but going on the magnet rotors i dont see too many problems.