Overmolding Propeller Products

Farmer Scott

Not sure if this is necessarily the right place to post this, so I may post it in the Sandbox as well, but here goes...

Anybody overmolded a Propeller based product, what casting material did you use, and what were the results?

I'd like to take some things I'm working on, and mold them in a 'block' of material, such as liquid urethane plastic or epoxy.· Additionally, I'd like to do the same for some sensors (such as DS18B20, DS1620, etc).

I'm currently considering something like Smooth Cast 310 from Smooth-On Liquid Plastics & Rubber (www.smooth-on.com), but am concerned about dramtically altering the electrical properties as well as affecting the accuracy of temperature sensors.·

This is for an agricultural application, so the sensors will certainly see quite a bit of abuse, I can't think of a much better way to protect them.· It will also greatly reduce the complexity of producing sensors.

Any thoughts or insight would be greatly appreciated...

Thanks,

Scott

mctrivia

For my propmod I encase the propeller in epoxy. the only down side is if you discover a flaw in your soldering job after the fact you can't fix it.

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Farmer Scott

Did you use any particular epoxy...or just "the stuff the hardware store had in stock?"

Cluso99

There is also another product referred to as conformal coating which is painted or sprayed on like a varnish. This is often used in the tropics. Perhaps you could google this.

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Timothy D. Swieter

I haven't epoxied Propellers, but other electronics. You need to be careful which epoxies you use. Heat is given off in the curing process and the heat can get intense in some epoxies. I don't know what you have at the hardware store, that stuff probably doesn't cure too fast and therefore the heat is less. You might also need to look at the conductivity of the potting material.

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Timothy D. Swieter, E.I.
www.brilldea.com - Prop Blade, LED Painter, RGB LEDs, 3.0" LCD Composite video display, eProto for SunSPOT
www.tdswieter.com

mctrivia

hardware store epoxy is no good. you want to get an electrical epoxy but they call it "potting compound" instead of epoxy for some reason even though the box says epoxy. MG makes 3 different types that I know about.

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Beau Schwabe

Farmer Scott,

You want to watch out for any "potting compound" or "epoxy" and make sure that it does not contain acetic acid .... A quick test, if it smells like vinegar, it's the wrong stuff.

To re-iterate what Timothy said, heat can be intense. If you have a way to control the catalyst (promoter) the better. A slower reaction will also reduce the amount of "yellowing" from the epoxy and eliminate air-bubbles if you want to be able to see what's inside. Timothy's point on conductivity is also important to consider, another is simply the dielectric property of the epoxy. In most applications this won't be an issue, but other designs might be sensitive to any dielectric changes that can alter the parasitic capacitance significantly.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Tubular

I also looked at low pressure urethane (Optimel) for an agricultural application some months ago.

The material was about US$30 a litre, but the mould die was fairly expensive.

The unresolved concern I had with it was how it would stand up to UV sunlight over time

tubular

SRLM

Whatever your final solution, you could always paint it with some UV resistant paint.

sylvie369

If you're working with significant amounts of epoxy for your first time, be aware that it is easy to develop an allergy to the stuff if you handle it barehanded or breath the fumes too much. I know people who have lost their ability to be around even hardened epoxy, and I have enough sensitivity that I need to keep my exposure down. Wear gloves, and work in a well-ventilated place.

CannibalRobotics

You should contact the manufacturer and find out how the epoxy/silicone reacts with lead, tin, silver and copper. I had a problem many years ago with TrackSTAR boards plugged into TRS-1000 computers (PC clones). The manufacturer had used a silicone bead over the back of a header. This was to protect the ribbon cable from sharp edges. It had an oxidation reaction to the lead-tin mixture in the solder and the oxide formed was conductive.
After about a month in the field these machines all started erasing all disks as the write line shorted to 5vdc. It took us months to figure out what was going on.
Jim-

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When your problem seems insurmountable, read the forum for a while.
Solve someone else's problems and yours won't seem so bad.

Phil Pilgrim (PhiPi)

Many silicones release acetic acid (the acid in vinegar) as they cure. BAD for electronics! If you use silicone, make sure it's acid-free.

-Phil

Chuck Rice

I think this is the type of stuff you are looking for. This is my local electronics store, but I bet you can find them a lot of places.

mctrivia

here is a product on digikey: http://dkc1.digikey.com/de/de/ph/MGChemical/EpoxyTC.html

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

science_geek

i dont know if it works with electronics, but alumilite is an amazing resin that works well when casting plastics, dont know about acid or not, i have used it and love it, it comes in 2 parts that you mix, its fairly runny, but can be easily contained with clay.

mctrivia

the name makes me think it will probably cause problems. makes me think there may be aluminum in it

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Tracy Allen

I use 3M potting epoxy DP270. It is meant for electronic encapsulation and has a low viscosity (to fill voids) and long curing time with low exotherm. It is not thermally conductive though like the MG chemicals product that mctrivia mentioned above. I don't use it on any serious heat producing components. One thing the 3M DP series has going for it is the dispensing gun, dual cartridges and mixing nozzles. No air becomes entrained in the mix and the mix is perfect.

Another material I've experimented with for coating in medium layers is E6000 from Eclectic Products. They suggest that it is suitable for electronic applications, but I have not seen much solid data to back up that claim. It does have desirable properties, one of which is to stick to almost anything, including low surface energy plastics like polyethylene.

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Tracy Allen
www.emesystems.com

Tracy Allen

I'd like to amend my comments about the E6000 and other "GOOPs" from Eclectic Products. I did a little more digging. All of those are silicone rubber based, but the E6000 industrial series uses percloroethylene as its solvent, while the GOOP series are are commonly found in many varieties (household, plumbers, marine, shoe, RV, Craft etc) use toluene as the primary solvent. Other differences are small and have to do with viscosity, final hardness, and UV resistance. Both solvents are quite toxic and should be used in a well ventilated area, and percloroethylene is a possible carcinogen. Neither is good for the shared atmosphere. On the technical side, those solvents might tend to attack plastics if left for any amount of time, also, the presence of a halogenated compound (per_cloro_ethylene) is troubling if any solvent remains after cure.

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Tracy Allen
www.emesystems.com