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Casting Silicone (Red High Temperature)

12/04/2015 12:24 PM

I am working on some novel designs for thermal batteries. Current research involves two electrically bonded pieces, one of aluminum, one of copper.

I need to form an impervious seal at the joint between the metals, seal the edges of both, seal the bottom of one piece, and the top of the other piece. This allows me to stack these up like a stack of dominoes with different values (of potential) on each end.

There has to be a gap of about 0.025" on each exposed metal face, so that when two faces are placed adjacent, there will be 0.050" of the matrix (natural fibers and some crystals) between them, leaving the other faces of the metal covered in the silicone.

Thus, once stacked, I have a group of cells connected in series to form a battery of selected potential. Parallel connections of these engineer the current, and output resistance to the target values.

The question is about how to form the molded silicone around the metal parts and achieve open faces and covered faces. Sounds like I need to use a casting wax to cover the faces that will be exposed later, and somehow still keep the covered faces level in the mold (not tipping down to the bottom). I supposed I could pre-case some silicone the right thickness (0.025"), then while it is past the open time, simply cut these into convenient little round standoff stubs, and then set these in the poured silicone (at the correct places), and place the metal "coupon" domino down on top. Wax rectangle would cover the faces of metal to be opened after taking the silicone with the embedded metal out of the mold. I can see that there might be a small problem getting the upper part of the pour to level at the correct height, unless I put some on top of that to "squeeze the pour out", such as a piece of wood with small vent holes.

Comments on how you think this will work? Ideas to improve on this.

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#1

Re: casting silicone (red high temperature)

12/04/2015 12:48 PM

I am attempting to provide a visual clue to what this looks like:
I hope this helps anyone trying to figure out what I am talking about.

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#2

Re: casting silicone (red high temperature)

12/04/2015 12:51 PM

Thanks.

I assume that you will use the proper primers for adhesion.

I'd pour some silicone on a plate and stand the cells off that plate by .025.

After cure, trim the excess.

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#3
In reply to #2

Re: casting silicone (red high temperature)

12/04/2015 1:06 PM

I think I agree with that completely. There has to be a stand-off, or it will never fly. The stand-off does need to be on both parts to produce the 0.025" thickness all around. I see that you are telling me to simply cut away the silicone from the metal faces that will be exposed in the cell stack. How brilliant and simple! That way, there is no need to get all crazy with wax pads and such. Getting the wax back off the metal left me with trepidations. A smartly preparing cutting guide should not be much of a problem.

It is no problem at all to make the little riser pads, just pour them and cut them out later, then cast over them, and heck, even use them for pour depth gauges. This is not rocket science, now is it?

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#4
In reply to #2

Re: casting silicone (red high temperature)

12/04/2015 1:07 PM

About the primers, that is going to be something of an issue, since I want the silicone to adhere (which it will not without a lot of help) in some areas (edges, joints), and let go when cut away from the faces to be exposed to matrix.

Do you of any Smooth-On primer that will work for cohesion to copper (cleaned) and aluminum (cleaned)?

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#5
In reply to #4

Re: casting silicone (red high temperature)

12/04/2015 1:39 PM

Luckily RTV 630 (if that's what you're using) and all of the addition cured silicones won't stick to much, except themselves. So, getting it off the unprimed surfaces will be easy.

Whatever brand of silicone you are using should tell you what primer to use. They are water thin and easy to apply. Cu or Al will both prime well.

I've been out of that business for 30 years.

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#7
In reply to #5

Re: casting silicone (red high temperature)

12/04/2015 3:15 PM

Thank you! First pour will reveal all. I am using Mold Max® 60.

It makes working at high temperatures possible.

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#9
In reply to #7

Re: casting silicone (red high temperature)

12/04/2015 3:29 PM

I mis-spoke anyway. RTV 60 is the FeO2 filled silicone.

RTV 630 is what the Apollo Astronauts boots were made from.

Told ya it'd been a long time.

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#6

Re: casting silicone (red high temperature)

12/04/2015 1:41 PM

If I may, please let me recommend yo a company in the silicone business called Nusil.

I have contacted these guys with different problems and they always had the right product for my application ... or a good advise to make my job easier.

Good luck in your project!

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#8
In reply to #6

Re: casting silicone (red high temperature)

12/04/2015 3:15 PM

Thank you for the tip, I looked into it, and this is what i found:

http://nusil.com/product/fs-3511_liquid-injection-molding-fluorosilicone-elastomer

The only drawback I see is the 150 C cure temperature requirement. This product is recommended for injection molding, but I suspect it can be poured just as well.

If the Mold Max 60 does not hold up to the matrix, I will already have an answer in hand, since this product is 100% fluorosilicone, and is chemically inert, for nearly anything.

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#12
In reply to #8

Re: casting silicone (red high temperature)

12/05/2015 9:57 PM

Every time I contacted this company, they called me back to know exactly what my project was and offered different alternatives (many not displayed on their site), so I do not hesitate in recommending their services. Depending on the project, their prices may be a drawback, but well... this is a very personal issue.

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#10

Re: Casting Silicone (Red High Temperature)

12/04/2015 4:20 PM

We use RTV silicones for potting high volt electronics. As Lyn says the compound flows like water, the primer is similar. Where we don't want silicone we use syringes of the stuff dentists use to take casts of your teeth. It sticks well to surfaces but peels of cleanly. Don't know the name off hand but obviously comes from dental suppliers.

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#14
In reply to #10

Re: Casting Silicone (Red High Temperature)

12/07/2015 8:24 AM

I suppose the trick is to prime the surfaces that need to bond to the silicone, and maybe talc the areas where it needs to release (or buy some actual mold release, and spray that on these areas. It might be just a tag tricky masking off for spraying mold release after priming the other areas, I do not know yet.

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#15
In reply to #14

Re: Casting Silicone (Red High Temperature)

12/07/2015 9:04 AM

We use mould release but paint it on rather than spray. It's tricky to keep it to a small area, it tends to run everywhere.

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#16
In reply to #15

Re: Casting Silicone (Red High Temperature)

12/07/2015 10:05 AM

Probably, the first attempt, I will just immerse the assembly in the silicone to a controlled depth, allow cure, then attempt to cut away the sections that need to be exposed for further assembly of the stacks. If I encounter problems there, then I will start over, and figure out how to get the silicone to release only from those areas, and not the parts that need to be hermetic seal. Something of a challenge, as I see it.

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#17
In reply to #16

Re: Casting Silicone (Red High Temperature)

12/07/2015 12:38 PM

No challenge, really. Silicone does not inherently adhere to anything much, except more silicone and silicone primer.

Primer is water thin and can be applied with an artist brush to those areas where adhesion is desired.

Try a sample first. We used to made silicone molds of many (some unimaginable) things on the lab.

To keep the silicone from sticking to itself we used TFE spray silicone. Miller-Stephenson MS 122. We wiped the mold release off the metal (usually) master because it left a foggy finish on the silicone. The parting line of the mold didn't matter.

Time to try a sample.

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#18
In reply to #17

Re: Casting Silicone (Red High Temperature)

12/07/2015 1:26 PM

I have some materials already on order, if they will just hurry up and get here.

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#19
In reply to #18

Re: Casting Silicone (Red High Temperature)

12/10/2015 3:03 AM

CR4 cannot help with that.

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#11

Re: Casting Silicone (Red High Temperature)

12/05/2015 6:19 AM

We used to use "DYMAX" for flowable silicone materials. We used them for PWB encapsulation. They were either UV or heat cured. Stuck like you know what to a woolen blanket and were really good on copper tracks.

Most of their materials came standard with a UV fluorescent pigment so that you could detect whether the contact areas were clean or contaminated.

Be especially careful with RTV materials. Most have citrate activation (acidic byproducts) that compromised life expectancy of copper components.

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#13
In reply to #11

Re: Casting Silicone (Red High Temperature)

12/07/2015 8:21 AM

That is interesting about the citrate activation. It is possible to include some citric acid the the matrix (crystals) I use, and this reacts with the alkaline materials present to form various citrate salts.

I like the fluorescent pigment idea, hopefully, though this will not be needed, as I plant to talc areas where the silicone will later be removed, then jet clean the metal.

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#20
In reply to #13

Re: Casting Silicone (Red High Temperature)

12/11/2015 12:11 PM

some adhesive / masking tape will be of help to cover areas you don´t silicone to get bonded to.

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#21
In reply to #20

Re: Casting Silicone (Red High Temperature)

12/14/2015 11:16 AM

Thanks. I have the first materials I ordered, the red high temperature silicone from Smooth-On, and some Insta-mold from another supplier.

I really do not think I need to Insta-mold, as I plan to simply make a wooden frame mold (a really small one), and coat the inside of it with a specialty wax I have available (an additive in lost wax casting, that is a hard wax of high melt viscosity, that is added to paraffin to produce a yellow casting wax of minimal shrinkage).

I have brazed, soldered together 7 of 13 pieces of "domino" copper-aluminum. Alumiweld on the aluminum, resin-core solder on the copper, after small holes (two) drilled in each piece, then filed out to make a key hole shaped opening in the edge of the metal pieces. Then the parts are laid edgewise (on a piece of travertine tile) and heated and scratched with a nail until the metal and braze material is wetted (contact angle problem), and the parts join. The problem is getting all the Alumiweld filed off areas just adjacent to the joint, so this does not affect results (phonon cell) later.

Interestingly enough, a microphotograph of the joint shows copper joined to aluminum, with a slightly bluish hue metal between (the result of combination of Alumiweld and typical solder).

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