[AR] Re: Electroforming Experiment
- From: Peter Fairbrother <peter@xxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Sat, 13 Jul 2019 16:30:40 +0100
On 13/07/2019 04:07, Ed LeBouthillier wrote:
On Fri, 2019-07-12 at 15:01 -1000, David Summers wrote:
That looks really good! How complex is the design? Are you leaving
the chamber walls that thin?
This was just a first experiment. I wanted to establish the rate of
deposit. My goal will be 0.015" wall thickness, 0.015" cooling channel
thickness and 0.015" closeout thickness, for a total thickness of
0.045" inches. At my current amperage level, that'll take a 60 hours
per layer (per 0.015"), or about 240 hours total. There is some benefit
to plating slower; the quality of the deposition is better. If one
increases the amperage, then spikes and trees will form. I had no
spikes or trees so I feel that I shouldn't try to speed things along.
Agitation is one way to control spikes and trees. But the best way is to
periodically reverse the current. What happens is that spikes do form,
then they are preferentially removed in the reverse phase.
I use 1.6 seconds on, 0.4 seconds reverse, no gaps, same current both
directions, but ymmv. I am only getting a 60% overall rate - effectively
1.2 seconds plating in 2 seconds - but as I can use a much higher
current, the overall speed of forming is faster.
Some commercial people use ratios as low as 30% or so. Sometimes they
use gaps between phases, or higher currents in the reverse phase. but
the chemistry and math here is a) complex, it depends on the bath
chemistry, temperature, agitation, flow, rotation etc and b) a lot of
it is proprietary, so trial and error is probably the best guide.
Periodic reversal is particularly important for combustion chambers,
where organic brighteners and levellers are a no-no - they destroy the
high-temperature properties of the copper deposit.
About the only useful additives for high temperature deposits are very
small quantities of pentoses or triisopropanolamide - and chloride, if
you can call it an additive, though often you are trying to lower
chloride levels.
What bath chemistry are you using? "Acid copper" covers a multitude of
sins, from high-speed baths at > 300 g/l copper sulphate to high throw
baths at less than 60 g/l.
Temperature? Agitation? Flow? Rotation? Filtering? Chloride?
chambers for the SSME used:
5s forward 2s reverse
20C
5A/dm^2
187 g/l or 221 g/l Cu2SO4.5H2O
74 g/l H2SO4
chloride ion 81ppm by weight
xylose 0.3 g/l
Peter Fairbrother
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