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'[OT]:Casting plastics?'
2002\03\27@130029 by Jay Hanson

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I would like to encase a small circuit board in some sort of clear plastic.
Should I melt lexan, plexiglass, or ....?  Is there is good liquid for this?
Fiberglass resin?

What are the rest of you doing?

Jay

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2002\03\27@134153 by Dale Botkin

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Check hobby and craft stores, there is casting resin you can buy by the
pint or gallon.  Mix with a little bit of catalyst, pour it into the mold,
hardens into clear plastic - commonly used for paperweights and such.
There are colors available too, and I also found Web sources.  Search for
casting resin, I think.

Dale
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On Wed, 27 Mar 2002, Jay Hanson wrote:

> I would like to encase a small circuit board in some sort of clear plastic.
> Should I melt lexan, plexiglass, or ....?  Is there is good liquid for this?
> Fiberglass resin?
>
> What are the rest of you doing?

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2002\03\27@134539 by Rick C.

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There is a product used in setting flowers and the like. You mix two portions
and pour it on your project. It will harden to a clear plastic. Check your local
flower shop/gift shop for the product.
Rick

Jay Hanson wrote:

{Quote hidden}

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2002\03\27@155251 by Dave King

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At 01:43 PM 3/27/02 -0500, you wrote:
>There is a product used in setting flowers and the like. You mix two portions
>and pour it on your project. It will harden to a clear plastic. Check your
>local
>flower shop/gift shop for the product.
>Rick
>
>Jay Hanson wrote:
>
> > I would like to encase a small circuit board in some sort of clear plastic.
> > Should I melt lexan, plexiglass, or ....?  Is there is good liquid for
> this?
> > Fiberglass resin?
> >
> > What are the rest of you doing?

The stuff they use for flowers is usually a polyester (activated resin + MEK
as catalyst). Epoxies will handle a bit more heat but generally the tg points
are in the 150-250°F range (TG = transistion to Goo ;-]) I'd think the commercial
 conformal coating resins would be modified for a higher temp.

Dave

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2002\03\27@165331 by Jinx
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> > I would like to encase a small circuit board in some sort of
> > clear plastic.Should I melt lexan, plexiglass, or ....?  Is there
> > is good liquid for this?
> > Fiberglass resin?

If you use fibreglass resin (polyester) you can improve the
look of final casting by pre-coating the board and components.
The reason is that the reaction between the polyester and the
peroxide catalyst is exothermic, which causes the resin to
shrink away from the potted object as it sets rigid. What you'll
see then is silvering as a gap is created between the resin
and the object

The higher the level of catalyst, the more exothermic the
reaction, sometimes to the point of cracking the resin. In the
lab we used to see test curings often melt nylon potting cups
and on one occassion actually catch alight, so don't go mad
with the peroxide in a big casting. Depending on the shape
and volume of the resin, anywhere from 0.25% to 1% is
about normal, but check out the particular resin's promoter
content (often metal salts like cobalt naphthenate) and
manufacturer's recommendations

The trick to getting a good finish is to pre-coat the object,
particularly metal as it doesn't bond well to p/e. As the
volume / surface area is low and heat can escape better
you can use a higher level of catalyst, 2% or more but YMMV.

The pre-coat surface after curing will not be hard, as oxygen
inhibits the cure, (which is why a higher level of catalyst helps)
but it bonds extremely well to the next layer of resin. To get a
good hard flat final surface you can use a glass sheet (with
release wax - furniture polish is OK but no excess !!) or the
plastic used for laminates, I think a polyethylene derivative.
This doesn't bond to polyester at all, but finish isn't as good
as glass. To buff up the casting, use brass or silver polish

You don't need to let the pre-coat set absolutely rigid. To do so
causes a visible line, at some viewing angles, between it and
the final potting layer. It's a little bit of an art to find the time at
which
you pot after the pre-coat to create the appearance of a single
volume of resin with no join lines, but a bit of experimentation
will put you straight.

Perhaps the best advice is not to be in a hurry. Speeding up
the process by over-doing the catalyst or heating the resin is
"more haste, less speed". To get really slow stable cures for
decorative objects we'd put the resin in the fridge for a while.
This lengthens the working time, one benefit of which is that
there's longer for bubbles to leave the casting

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2002\03\27@192458 by Ashley Roll

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Hi Everyone,

Another trick is that with the "casting" resins, you can do it in layers and
as long as you're in a clean environment to stop dust settling on the
layers, you won't see the line between the layers as it will bond together.

Doing it in layers means that there is less resin to cure at a time and the
heat generated is much more easily decapitated so you can use a faster
curing rate by using more catalyst.

This is how most of the embedded objects are done IIRC, the first layer
provides the base, then the object is placed in it and the mould filled.

Cheers,
Ash.

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Digital Nemesis Pty Ltd
http://www.digitalnemesis.com
Mobile: +61 (0)417 705 718




> {Original Message removed}

2002\03\28@030950 by Daniel Webb

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> The stuff they use for flowers is usually a polyester (activated resin + MEK
> as catalyst). Epoxies will handle a bit more heat but generally the tg points
> are in the 150-250°F range (TG = transistion to Goo ;-]) I'd think the
> commercial
>   conformal coating resins would be modified for a higher temp.

You might be able to get it cheaper in bulk if you buy it at the hardware
store (polyester resin for fiberglass).  Also, in the original post the
poster was talking about melting plastics.  The plastics being discussed
by everyone else are thermosets, which means they don't melt.  The Tg is
more a measure of where they degrade, not a melting temperature as with
thermoplastics such as polyethylene.

 Sounds like other people here are more expert at this stuff than me, so
correct me if that's wrong.  I got a B in my polymers class :(

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2002\03\28@050549 by Jinx

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> The plastics being discussed by everyone else are thermosets

If you don't want thermosetting (polyester or epoxy) then
go for acrylic. Do a Google search for "acrylic casting"
and you'll get plenty of info

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2002\03\28@094738 by Spehro Pefhany

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At 07:48 AM 3/27/02 -1000, you wrote:
>I would like to encase a small circuit board in some sort of clear plastic.
>Should I melt lexan, plexiglass, or ....?  Is there is good liquid for this?
>Fiberglass resin?

There are several types of castable plastics- epoxies, urethanes and acrylics.
The first two types are binary system, you mix two substances and it sets
in a while. A vacuum chamber is useful to get the bubbles out, some of them
are very low viscosity (almost like water) so they fill nicely. Others are
thicker. Sometimes you want a thixotropic type so you can just dip the
item.

Melting thermoplastics like polycarbonate (Lexan) or acrylic (Plexiglass)
won't work very well, they are very gooey. They are normally injection
molded or extruded with thousands of PSI of pressure.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2002\03\28@112349 by SHAWN

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We do a lot of potting on surface mount circuits and perhaps these
recommendations may be helpful:

  i) If you are working with surface mount, it a good idea to apply a few
coats of silicon conformal coating to your circuit before potting. This
prevents the potting from flowing underneath chips. Some potting materials
expand on drying, causing enough pressure to literally 'pop' off the chip.
ii) Appication of heat speeds drying but can  be very counterproductive.
Excessive heat may damage components or make the potting so viscous that it
simply flows off the board. A 100 watt light bulb is often enough to speed
the process considerably. When in a rush, we cure pottiing in a 80C oven,
but I like to plan ahead and allow the potting to set overnight at room
temperature.
iii) If you are using an encapsulating material that is not specifically
designed for electronic components be certain you understand its electrical
properties. The encapsulating material may affect circuit performance. Also,
you may find that your circuit doesn't work properly until the potting has
fully set, and this may take days. Myke Predko mentions such a problem in
his PIC book.

The following link is for a potting system from M.G. Chemicals. If I
remember correctly, this product sells for under $15. It is designed
specifically for electronic circuits.

http://www.mgchemicals.com/products/832.html

Shawn

----- Original Message -----
From: "Ashley Roll" <KILLspamashKILLspamspamDIGITALNEMESIS.COM>
To: <RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU>
Sent: Wednesday, March 27, 2002 5:22 PM
Subject: Re: [OT]:Casting plastics?


> Hi Everyone,
>
> Another trick is that with the "casting" resins, you can do it in layers
and
> as long as you're in a clean environment to stop dust settling on the
> layers, you won't see the line between the layers as it will bond
together.
>
> Doing it in layers means that there is less resin to cure at a time and
the
{Quote hidden}

> > {Original Message removed}

2002\03\28@114414 by Micro Eng

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Just be sure you never want to access the circuit again.  One of my clients
found out the hard way on this...and now brings out the ICD pins outside the
cast and then clips them after testing.

If you want to protect for moisture, there is some sprays I think that will
coat it pretty well.

Another possible solution is to actually coat it in parafin (wax) and that
way you can scrape it away.

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http://photos.msn.com/support/worldwide.aspx

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