I have seen printed horns coated with the spray-on conductive paint (e.g. MG Chemicals 843AR) whose gain is only a dB below a metal one, but it was a one-off and I never saw any follow-up. That said, it might be enough for the initial strike plate needed.

As far as smoothness and surface sealing, you should look up what people are doing to ABS with acetone vapor. I've tried it a bit and can attest that, by paying careful attention to temperature and duration, you can get quite nice finishes.

Thanks for the suggestions! Spraypainting might be bit tricky inside the waveguide, but we'll certainly try the acetone vapor!

t

I know that plating plastic is possible, and some companies do it well. But the devil is in the details. Not just in the plating but in holding tolerances on the plastic part. I am considering adding an entry on this on our career killer page... good luck!

Steve

So, quick update on this:

After almost a year of trial and error, we finally figured out that a few rounds of Tollens' reagent (https://en.wikipedia.org/wiki/Tollens%27_reagent) works like a charm... much more even finish than trying to electroplate copper, and IL (@ K-band) only marginally below metrology grade waveguide.

Paper will be submitted to IEEE CPMT soon; will keep everyone posted.

That will be a useful paper to many people, thanks!

Steve

So, after the usual trials and tribulations, the paper is now *finally* out, where we describe the process.

Downloadable version here:

https://www.researchgate.net/publication/368909379_SLA_Printed_K-Band_Waveguide_Components_using_Tollens_Reaction_Silver_Plating

What we found is that slowing down the reaction by cooling it gives you a much better finish, and using multiple plating rounds really helps.

It's still a frail metallization approach, however, and easily rubs off with a few not-so-vigorous thumb swipes; fine for lab prototyping, but you'll need plating or some other coating to use it industrially.

I have had good experience using a plater familiar with the Enthone process. The process roughens the surface so the plating can stick better than smooth surface. I have investigated all sorts of methods; vacuum deposition, conductive paint, plasma torch, conductive additive in plastic pellets, etc.

Here is my combline diplexer design (in plastic). Plating kind of sucks inside the resonators but this was the first one and design improvements improved the process. https://www.epner.com/capabilities/plating-on-plastics/

Thanks for the advice!

These SLA resin printed parts are... tricky to plate. As you rightly mentioned, smoothness hampers adhesion for e.g. a palladium colloid needed for electroless plating (or the seed layer for electroplating), so the guys that plate injection molded chrome plated auto parts use chromic acid, hydrochloric acid, potassium permanganate, or any number of other strong oxidizers. Sandblasting will work as well, but not inside complex waveguide assemblies.

Unlike (from what we could gather) most injection molded parts, these SLA parts are porous... not as much as FDM parts, but enough to leave enough residual material that risks contaminating subsequent plating baths.



The other problem is, of course, that students (specifically EE students) and strong oxidizers don't mix well either.

What we really want is a way to get copper on the parts (including inside the waveguide parts) that is cheap, safe, does not require special equipment, and won't peel if you look at it funny. The Tollens' process is cheap(ish) and safe(ish), but the process recipe is still a bit cumbersome and the resilience leaves much to be desired.

So, quick update on this:

We figured out two things the past week which improved the surface adhesion:
- Adding a surfactant: we just mixed in a bit of dishwashing liquid, and
- Adding an oxidizer: we added a bit of household bleach (3% sodium hypochlorite)

Much smoother silver finish, with much better adhesion.

t