"Help! My vulcaniser keeps destroying my masters!" Topic

No Im still fairly new to this, but Ive made probably two dozen moulds without problem at this point – but my vulcaniser doesnt have any sort of indicator for pressure, and the temp dials are just the kind you would get on an oven, theres no digital read out – so both could be wrong and I wouldnt know.

Whats happened is the last two moulds Ive made the masters were melted: completely in the case of some 6mm's and 'pockmarked' in the case of some 28mm's. This has only happened the last two times.

Looking at the image of what's happened to the surface of the masters does anyone recognise it and can tell me what's gone wrong straight off the bat? Otherwise I see no option but trial and error to find out what I did wrong, or just crossing my fingers next time.

link

Thank you so much for any help you can give me!

If it helps, I preheat the can and the mould slightly, and I vulc at 175'c-180'c (according the dials on my machine).

what are your masters made of?
it could be one or both the plattens are getting too hot, can you use a thermometer to check how hot they are getting?
are you using natural rubber moulds?
shaun

Hi RF, if I recall the temp for the vulcanising process to do its job without destroying your masters is about 140'c at longer period of vulcanising, this is for the medical rubber that is used here ( pale blue or green in colour) perhaps this helps.
Cheers

J Cokers Organic Rubber J100s (The ones with the peelable layers) – what temp do others vulcanise theirs at?

The Masters are GW Neales C90 alloy – pewter which has a tiny amount of lead in.

Im going to use a thermometer now to check what temp they are getting – I could understand if there was slight fluctuations due to the warm weather in the UK right now, but that should be +20'c at most, not the extra hundred or so to melt the white metal!

You are vulcanizing at the right temp (@350 F) if the Coker's J100 is a black rubber. I don't think it's your pressure either which should impress and cook fine anywhere from 10-30 tons psi.
I'd say it's the C90 alloy. In the USA the 90 usually means the tin percentage content. If you know there is only a small – 1 % lead – then the remainder is probably antimony and/or cadmium. That would make it an excellent casting alloy but it is not a good vulcanizing alloy and it would account for the severe pitting of your masters. The softer antimony or cadmium is not holding up under the pressure and heat and going pasty or melting in the cavity.
Toby Barrett
Thoroughbred Figures

Awwwww cr*p – so I should use less pressure/lower temp or give up totally on minis with lead content? (And buy myself a different alloy for masters in future. I nearly always use 3D printed masters so don't keep a separate stock of alloy for masters.)

I get a melting point of 228'c for the c90 – does the pressure decrease the melting point?

Thanks MasterCaster – this is proving most informative.

As for the temp, checked it now with a proper electronic thermometer and there's only about 5-10'c fluctuation from what the dials are marked up as. I figure I can knock it down a notch while the weather is so hot.

Moldmaking and Casting really is a "dark art"! :-)

yes you can say that again

I have no idea why it keeps rewriting my posts with random stuff… has there been odd behaviour elsewhere on the boards? :S

It sometimes happens when we have a lot of traffic, particularly if someone is editing a message.

Use Neales L grade and Coker moulds for my production moulds with no problem, platterns set at 140 from memory pressure set at a few PSI ( never bother to remember the number as just know the right point on dial lol). More than likely the metal, vaguely remember having problems with pewter masters way back.
If in doubt about the Masters use the silicone low temp from Coker tad pricier but cheaper than ed Masters.
L

Well it appears the problem has been found – the temp dials on my vulcaniser are just basic knobs with markings around them, no digital readout – over time they had come loose inside, so the markings no longer bore any relation to the temp, I had to open up the machine and install some new knobs, and test it a few times with a seperate digital thermometer.

When I thought it was at 170-180'c, it was actually at over 280'c+, easily enough to completely melt the masters and turn the insides of the can into a pressurised mess of bubbling rubber and molten splotches of pewter.

Oh & LeonAdler – you mention using the low temp silicone from Coker, Ive only just started using this to master things from 3D prints and hopefully you have superior experience with it – Do you bother to preheat the stuff at all?
I find that it starts to set at even the slightest warming (50-60'c), and that its so soft anyway there doesnt seem to be a point. On the other hand though, it seems to have virtually zero flow properties, meaning anything but the smallest pieces have to be cut in *exactly* or the detail doesnt form… any tips you have from using it?

Rf,
Basically just used it as a standard 'squash' mould just making a few indentations occasionally when required ( dentist tools or small slot head screwdriver works well………….oh I do like to be technical lol).
I'd reckon my Masters are a lot tougher than yours as the 3D prints Ive seen have been of lightweight plastic type stuff so I get less of those sorts of problems. Never pre heated them its so soft and pliable as is didnt seem any point. Arent 3D print models usually silicone pour moulded rather than vulcanised?
L