I have a project that may need to use dental acrylics to pot a mini
PCB within a plastic housing to form a solid object the size of a
stick of chewing gum.
Injection plastic, the normal industrial process, to pot the PCB had
proved a nightmare as the injection temp softens the housing and the
injection pressures of 20K psi wreaks some of the internal parts.
Other problems include internal flow turbulance (lots of internal
housing support ribs result in bubbles) and the plastic cools
prematurely before it gets to fill all the crevices, etc.
Low pressure cold injection dental acrylics may be my salvation.
The dental acrylic cures in boiling water for 40 minutes. I'd rather
cure it in a dry oven. I can fab my own draft free oven. The heat
element will probably be as simple as a bank of IR heat lamps.
I need a temperature controller. Is there an ordinary commercial
thermostat that will shut off the power between 80 to 100 deg C range?
It need not be precision since I can always modify the inputs to get
the ideal steady conditions.
As I write this I am testing if my kitchen oven can be used. Its set
at 250 deg F and I have a standalone digital thermometer that is
reading just the desired temperature range 70 to 90 deg C as it cycles
between on and off. I have placed inside it the plastic materials I
am using to see what happens. I may get lucky after all.
Thermostats of many types are commonly available. Try
http://www.grainger.com/ . It's a bad idea to do what you are proposing in a
"food" oven. But, a cheap toaster oven may work without much or any
modification. There are many ways of potting electronics that do not require
much higher than ambient temp. 10 min epoxy from the hardware store is used
by many hobbyists.
I'll experiment with the kitchen oven before building a production
one. The materials are non toxic and ordorless. At the cure temp of
boiling water there are no outgassed fumes.
It is not straightforward potting although that word gives the best
mental image of the problem. I need to use food grade resins as the
item stays in the mouth for exrended periods.
The original plan was industrial injection molding. Since that
doesn't look feasible and my production run is only 200 to 300 I will
have to do it at home. I can't even hire someone as it requires skill
and care in assembly and set up before the final potting. The potting
material has to bond the two halves of the housing, to include two sub
surface plastic parts, to produce a cosmetically attractive product.
Think of a two color molded plastic toothbrush handle with a PCB
embedded inside. The handle has a small area that is flexible enough
to move slightly when given a bite. Every part of the product design
worked except the final potting. It is a complex high tech problem
involving multiple materials.
I managed to get in touch with the dental tech I am seeking advice
Not advisable to use a dry heat oven.
By staying with the boiling water cure the temp control will never
rise above 100 deg C anyway.
In a boiling water cure the two part resin expands (in the process the
pressure collapses the raw mixture bubbles) to produce a clear
polymer. This particular dental resin is used to make a prosthesis
that helps people who gnash or grind their teeth when asleep.
My experiment by just boiling the raw mixture resulted in a bubble
filled lump about twice the volume. Its consistency is useable,
similar to a moldable sports teeth guard.
My next problem now is the mold to encase the object while it cures in
water. Will have to think that through.
You might want to run your ideas past a prototype maker with a background
in biomedical engineering. A local engineering school or university medical
center would be a good place to look for this sort of advice. If this device
goes inside the body then safety issues are really important. Knowledgeable
advice may save you a lot of time
A double boiler may fill your bill since it will not get above 100 C and
yet will keep your components out of the water.
<< I have a project that may need to use dental acrylics to pot a mini PCB
within a plastic housing to form a solid object the size of a stick of chewing
Any potting compound that you use will shrink when it cures, worse if it is
heat cured. The least shrinkage will be a filled epoxy, room temperature cured.
There are probably dozens of decent commercial compounds available; do a Google
search to start. If your device doesn't have to be monolithic, consider
elastomeric compounds, silicones or polyurethanes. Even quicker, cheaper and
shock resistant are closed cell urethane foams. HTH
On 06 Feb 2004 01:27:13 GMT, email@example.com (Joe Bobst) wrote:
Thanks for all the replies. To undertake the project I had engaged an
industrial design engineer and we came up with the current design. We
had tried all sorts of combinations, based on Sanaprene, a food grade
plastic, without success. The plastics experts at the Alberta
Research Council and another at the Technical College, NAIT) pretty
much have the same solution we had tried so far and that is to keep
trying different resin combinations and vary the injection and
temperature parameters. They are very interested in how things will
I had thought of dental acrylics to think outside the box so to say.
To move things along. Of course neither the injection plastics people
nor the dental people have an idea how to implement this application
nor do they want to spend lab time on it since I am not paying them to
do so. So I am doing this out of curiosity as much as of necessity.
Fortunately this project opened for me many doors to meet all sorts of
interesting people and experts who volunteered gratis their time and
It has to be monolithic, for mechanical strength and to hermetically
seal the electronics as the device will be cleaned regularly with soap
and water. Silicone does not bond to the injected plastic body, it
doesn't cure evenly or fast enough and it is mechanically weak.
Urethane, I'll try it but it will probably open another can of worms
with regard to health, taste and other properties.
There aren't very many polymer materials that are acceptable for in vivo
use. As much as I like urethanes they are not good for in vivo use.
A technique that I have used for implantable electronics in the past is
to satisfy the "hemeticity" requirement by first coating the device with
parylene, a very effective conformal moisture barrier which is
acceptable for in vitro use. I've used it for devices implanted in the
chest and eye with good success.
I don't know how much mechanical strength you need but another of the
few good polymers for use with human subjects is the family of
siloxanes. High molecular weight siloxanes exhibit some strength and
will cure at low temperatures. They are also acceptable for in vitro
use. Satisfying your need mechanical strength with the potting material
may not be as effective as addressing it separately.
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