Apart from mending broken glasses and suchlike, you can make quite big things using epoxy resin and a mould made from bits of old plastic pipe:
The epoxy turns up nicely on a lathe as long as you don't add silica or anything that blunts metal tools.
That's mighty impressive. Which product did you use? I've done similar things with polyester, but nothing as precise as that.
Impressive piece of work!
It's a very liquid epoxy resin with a hardener added in a 5:1 ratio. The amounts have to be precise or it won't set. I coloured it with blue and yellow pigment because I didn't have green.
The epoxy doesn't stick to the plastic pipe that goes under a sink. That's actually where the pipe I used came from!
I tried a fast hardener but it got very hot and started smoking so I had to dowse it in the sink. The next mould leaked like a sieve all over the bench and I had to keep scraping the epoxy up and putting it back in the top. The third one was successful. I patched the others up and here they all are:
If you don't add some glass filler it doesn't last long if its subject to vibration IME. I am talking about stuff with a design life of 20+ years though.
Aargh now you tell me! It is subject to extreme vibration. And every now and then a man in a 25 ton vehicle hits the insulator with a big steel pole!
Phenolic microballoons (from West System) are good as a filler if you need to machine it afterwards.
My father used to use epoxy for encasing electrical assemblies, some of these very large. One of the key factors is working out what setting time you need so the body of the epoxy can conduct the heat away at a rate which is fast enough to not over-heat. Epoxy isn't a good heat conductor anyway, and with the reaction being exothermic, and with increasing temperature making it run faster, it's very easy to have to run away, and wreck whatever you're encapsulating and have the epoxy crack itself. I believe some of the larger ones were designed to take weeks to set, in order to stay cool and stable during the process.
I was wondering.
Well, from your other post, I would guess that it is part of the overhead electrical supply system for a tram.
Chris
Yes it holds the wire up!
The epoxy seems to be very strong. The bolt inside has a flange so that even if all the insulation broke off it the bolt cannot pull through the support that is holding it up. In that case the circuit breaker for the power supply would trip. The old insulation is made from asbestos and maybe mica and some horrible compound that breaks easily when I hit it with a hammer. The epoxy does not break when I hit it with a hammer!
Then it possibly isn't epoxy,.
Epoxy is not 'hardened' by a catalytic hardener,. It sets by direct chemical reaction between two precisley mixed constituents. The mixture ratio is crucial.
That's very polyester like behaviour.
Beware of the propensity of resins like these to carbonise and track.
Structurally its a good material, but I have some reservations about electrical properties.
The largest castings I made set in a few hours, although they might take a day to get really hard. I made thousands of these 40 years ago using epoxy resin in silcone rubber moulds. I added a huge amount of silica flour to the mix. That made it much cheaper and it was a better heat conductor and slowed the reaction down. Red oxide was added to the hardener to show that it was mixed well. It also looked more attractive than grey! Here's a 33kV audio transformer and a couple of current transformers. The Buddha is polyester resin.
That's what I just said. It is epoxy.
I made 33kV insulators using epoxy resin 40 years ago and they are still fine. I tested insulation to 80kV and epoxy was almost the best insulator there is.
33 kV!.. Whatever was that for electrostatic speakers or an AM transmitter?...
So why don't you consider '5:1' to be precisely mixed? There are at least two major manufacturers of epoxy resin systems that specify a
5:1 ratio, West and SPS, there are also resin systems with 3:2, 5:2 and 1:1 ratios.Epoxy can also exotherm, when this happens, other than a lack of styrene smell what you observe during such an occurrence is identical, and the precautions taken to avoid exotherm are identical regardless of whether you use polyester or epoxy resins. You could take any one of dozens of resins of varying chemistry and by intentionally introducing process errors during mixing or application and they would all without exception exotherm with an ambient temperature of 15 deg C, yet you could also mix and apply those same resins in a different manner and they wouldn't exotherm when the ambient temperature was raised to 25 deg C.
Also a 'fast hardener', when used as specified by the manufacturer is no more prone to exotherm than a 'slow hardener'. All it requires is attention to ambient temperature, mix volume, mix vessel geometry, application vessel geometry, layup thickness, and curing temperature.
Last job I did with epoxy it began thermal runaway, due to the size of the thing. It does that if you put too much of it in one lump. Solution was to submerge in cold water once it began to set. Fillers reduce the tendency simply because there's then less epoxy in a given volume of mix.
NT
country underneath 33kV powerlines using the same pylons. Every now and then the 33kV wires would fall down on to the telephone line and the telephone girls used to complain about their telephones exploding. There were circuit breakers and fuses but those took too long to operate. The transformer was required to withstand 33kV between primary and secondary for up to a minute, as well as passing 30,000 Hz plus 17 Hz ringing frquency. This it did very well. I've always though there must be a use for such a transformer these days. It could pass up to 100,000 Hz without too much loss. It was rated at 100 watts. The production version had a toroidal core of grain-oriented silicon steel. I am the only person alive who knows why it works at such a high frequency, and the special insulation that was successful after many prototypes failed. The picture is of prototype 1, of 12.
Right!.. This surely wasn't in the UK?. I know they sling wires on low voltage 230/415 lines but can't say I've ever seen them on anything higher volts...
Matty F submitted this idea :
..and that would be something to do with the MOD or Civil Defence?
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