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Soldering Hollow Forms

I’m about to build my own press for use with impression dies…all new to me. I’ve always used solid shapes like balls ,wire, shot plate designs, etc. Are there special soldering “techniques” for ensuring all contact points of the hollow design type are soldered down well?

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I hope I’ve understood the question correctly.

Before I retired I used to solder quite a lot of hollow shapes in sterling and occasionally in fine silver, so that the interior was completely isolated from the exterior. Such as you would expect from a hollow Xmas bauble for example. Five-pointed stars soldered as hollow cushion shapes were popular.

You’re quite correct, the periphery of both surfaces have to mate with virtually no visible gap. That’s pretty easy to do if both mating surfaces get about the same amount of rubbing against abrasive paper glued to a flat glass sheet. Both surfaces, because usually as you abrade the contact surface to make it flat, it recedes into the form. You want both surfaces to still mate perfectly, so they have to both recede by the same amount (hopefully, if your pressing was accurate, they won’t need much abrasion).

The most important thing I learnt was that you must not use any water, either in the flux or for subsequent quenching / cooling. Once steam gets inside a hollow form which is subsequently sealed, air pressure will collapse it once it cools. That is because even a small amount of water produces a lot of vapour which will displace all the air. Then when it condenses to water when cooler, it essentially makes a perfect vacuum inside (the vapour pressure of liquid water is very low). And of course it is then going to be exposed to the 15lbs/sq.inch atmospheric pressure which usually squeezes it flat (or badly distorted).

So use dry powder fluxes only, and after soldering, allow it to air cool (forced air is often good). Apply more dry flux if necessary and reheat to melt and finish the soldering.

This relies on the fact that the interior pressure of the very hot air is not a lot different to the same air when cooled to room temperature. It will drop of course, but not sufficiently to put it at risk of being collapsed by atmospheric pressure.

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In addition to JelfO’s suggestions, You can also leave a small hole in the piece somewhere where it won’t be seen to allow the pressure to equalize.

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Thanks Paul, I would never have thought about the moisture issue. With regard to “dry powder flux”, would that be borax or boric acid? I didn’t find anything known as that when I searched. I assume that the best way to solder these pieces is to spread the dry flux under the contact area(s) and lay chip solder around them? Much appreciated!
Bruce

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If I remember rightly (it’s four years since I retired) I tended to use boric acid and Johnson Matthey EasyFlo flux powder. The latter makes an excellent additive to either borax or boric acid, it flows at a lower temperature than either and gives excellent flow to the molten solder, but is quite expensive and the fumes are definitely not good for you.

My favourite method of applying boric acid was by dissolving it in methanol (which is also quite toxic) and painting it over the seam. Ethanol won’t work unfortunately. Some people use meths (ethanol denatured by the addition of methanol, at least in the UK) but I find that a waste of time. Methanol is incredibly cheap but you do need to read up on the H&S aspects.

A thin layer of boric acid dissolved in methanol painted on the seams, clamp them together, solder chips on the outside (glued there with the same solution if you find them difficult to place), use a red hot solder pick to drag EasyFlo over the gaps and, bingo! all the solder chips (I normally only used hard) should melt perfectly and be dragged by capillary action into the seam.

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Super helpful Paul…thanks for the detail!

Bruce

It was already suggested to drill a small vent hole ( 1/64") making the assembly process much safer avoiding explosions and collapses !! and then plug the vent hole with solder a process used in the electronic industry for sealing capacitors. Also the hole can be concealed if the form needs and attachment to be soldered after.

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That is exactly what I do and was taught so!

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