80-90 degC temp controller

[Klm]

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.

[David Martel]

Klm,

Thermostats of many types are commonly available. Try

. 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.

Good luck, Dave M

[RB]

Grainger has a few thermostats that will work in this range but a more economical approach might be to use a forced air furnace fan limit control, also available from Grainger.

RB

Klm wrote:

[Klm]

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.

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I managed to get in touch with the dental tech I am seeking advice from.

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.

[RB]

Have you looked at the moldable mouthpieces for football players and scuba divers? There may be something there you can utilize.

RB

Klm wrote:

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[David Martel]

Klm,

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.

Dave M.

[Joe Bobst]

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

Joe

[Klm]

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 turn out.

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 expertise.

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.

[RB]

You may want to consider separately addressing these requirements. 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.

RB

[Klm]

Hi RB,

That's a fantastic lead. Can I contact you offlist? My email address is [e-mail]