Epoxy molding, encapsulation, molds and stuff like that!

[DTJ]

I'm working on an ongoing ultrasonic measuring device that's used under water, anything up to 10m or more marine, fresh, brackish or sewer.

To protect the electronics its encapsulated in PU epoxy. We precision locate the PCB and piezo elements in a CNC machined aluminium mold and then just pour the epoxy in.
We then wait 24 hours and then push the units out of the mold using a release pin located in the bottom of the mold

The top of the mold is open so the 'base' of the finished product ends up being what was the exposed opening of the mold.
The mold is coated with a PVA release agent prior to the potting process.


       Pour epoxy in here

                   |
                   |
                  \/

|                                                                     |
|                                                                     |
|     ========= PCB =============      |   <-- mold
|                  | PIEZO ELEMENT|                      |
\__________________________________/



All this works ok but it would be nice to perhaps fully enclose the potted part so we get a nice finish on all the external faces.

It's also a bit messy and labor intensive. Typically we manufacture 10 units at a time.


Is there any resources that cover this sort of process? I was kind of hoping there were documents describing how others do this sort of thing or perhaps others that had done it.

We're going through a model change and I guess I'm wondering if we can be doing this a better way.

[DTJ]

I'm also interested in ways people might have to precision position components inside the mold as the ultrasonic tx/rx disks need to be set up at a precise distance from the potting compound surface.

[thm_w]

For the height offset:
- pins from the mold rising up to touch the PCB in an appropriate place
- PCB tabs that stick out the side and engage with the mold

In both cases I think this would require additional sealing step after removed from the mold though..

Maybe a plastic cylindrical housing that the PCB sits in, extending down to the mold to set the appropriate height.
We used Al housing but that is not cheap.

Google ultrasonic waterproof sensor, most are either plastic housing, aluminum housing, etc. Some have a step in them, possibly where a PCB sits?


https://www.taiwantrade.com/product/40khz-18-mm-waterproof-ultrasonic-sensor-transducer-1782294.html
https://www.makerfabs.com/water-proof-ultrasonic-ranger-jsn-sr04t.html

[DTJ]

For the height offset:
- pins from the mold rising up to touch the PCB in an appropriate place
- PCB tabs that stick out the side and engage with the mold

In both cases I think this would require additional sealing step after removed from the mold though..

We get around this at the moment by supporting it from above the mold. As you say it means having to seal it after demolding which as an added process is a pain.

An aluminium external housing would be nice, we're fully potting to minimise costs.

[Benta]

Please educate me:
what's "PU epoxy"?
I know PU and I know Epoxy, but I've never seen that combination.

Thank You.

[Flexbex]

I don't really understand the question. But if I get it right you want a nice finish on the outside. For this you could look up what the surfboard shapers are doing with fibreglass and PE or Epoxy. You'll get a nicer finish with Polyester Resin. Maybe you could even put fibreglass in the mold and then just do one coat with PE glosscoat.

[Gregg]

To get a nice shiny surface, the actual mold could be vacuum molded such that the mold becomes part of the product. I’m thinking of the type of mold maker that heats a sheet of plastic and then vacuum forms it over an object.  The plastic used would have to be compatible with the epoxy.  It could be marketed as an extra layer of protection.  The real upside is the low cost compared to nice polished molds and sticking ejector pins.

[DTJ]

Please educate me:
what's "PU epoxy"?
I know PU and I know Epoxy, but I've never seen that combination.

Thank You.

\

You are correct. I stuffed up - we have another product that we pot with PU and I just plain old mucked up.

[DTJ]

Maybe you could even put fibreglass in the mold and then just do one coat with PE glosscoat.

That could be an option. Thanks.

[DTJ]

To get a nice shiny surface, the actual mold could be vacuum molded such that the mold becomes part of the product. I’m thinking of the type of mold maker that heats a sheet of plastic and then vacuum forms it over an object.  The plastic used would have to be compatible with the epoxy.  It could be marketed as an extra layer of protection.  The real upside is the low cost compared to nice polished molds and sticking ejector pins.

That would create a nice looking part but the ultrasonics would probably complicate things as they interface directly through the 'outside' of the mold if that makes sense. The thickness of the epoxy over the sensor is only ~1mm.

[T3sl4co1l]

Is this vacuum degassed BTW?  (Would imagine so, voids must kill the ultrasonics?)

Obviously, you need a two-piece (or more) mold to enclose it; or if it's still 1-piece, it's got to be fairly deep and slightly tapered (draft), and leaves a smaller free surface.  Molds can be rigid, or silicone rubber -- allowing some undercut and quite complex shapes to be made.  Filling sprue should be at the top, with a generous funnel to allow for expansion when vacuuming, or at least pouring when filling.  Then cut off the excess and you're good.

Tim

[Flexbex]

Silicone won't hold against PU expansion. The mould must be strong. They even take concrete sometimes. To hold the pressure

[T3sl4co1l]

Err -- FYI, the polyurethanes used for potting are solid, non-foaming.   OP also corrected that to just epoxy.

Tim

[Flexbex]

Oh there is non foaming PU?
I was referring to this one
https://youtu.be/K1eqfklRENA

[Flexbex]

Btw if you experience air bubbles in your epoxy you could put epoxy in vaccum before pouring. It will get most of the air bubbles out. Also let it cure under pressure will help. Vaccum will get air out. Pressure will compress air and make air bubbles small