"unusual problem" Topic

I've been mould making and casting for a dozen or more years and today encountered a problem I've never seen before which I thought I'd mention here to see if others have?

I was vulcanising a black organic rubber mould for some gaming tokens essentially coin like flat discs (1mm thick 18mm diameter) with various complex raised lettering and symbols on both sides.

I don't know if this is in any way relevant but as occasionally happens if the pressure is just a tad too high the graphite powder I use as a release agent wasn't very effective at the edges of the mould so it was a bit of a fight to first separate the two halves but the master patterns came out cleanly such that I only noticed the problem with the mould when the castings it was producing were defective.

Whilst exaggerating simply to better illustrate the odd nature of the problem some of my flat 2D coin like cast tokens were "inflated" being more spherical but still with clear legible lettering on the now domed surfaces almost as if the rolled discs of green-stuff had inflated when heated under vulcanisation. Others just showed inflated dimples on parts of their surface. However the patterns removed from the mould were exactly the flat shape they should be so if they had inflated as the mould baked then they deflated as it cooled.

I'm familiar with the irregular warts you can get on a casting if you get an air bubble trapped on the surface of the rubber but in this case it was as if the surface detail was then on top of the cast wart rather than lost beneath it.

I'm most inclined to ascribe this problem to air bubbles trapped in the green-stuff but I mixed it by hand as I always do and it was relatively new pack so shouldn't have been "off", then only thing different in it's use was that I first rolled out thin sheets from which I cut out discs, whereas normally I'd build up chunkier shapes over a wire armature when sculpting figures.

Are they "inflated" on both sides?

Taking a guess here, but it could be a problem with your vulcaniser not heating the two plates evenly which might show up more on a very flat object?

Mike

My vuclaniser did fail on me recently so had two new elements only about three months ago, as I didn't want to only replace the failed element resulting in different types top and bottom.

In terms of the inflation then I would say one half of the mould showed this problem more than the other but it wasn't clear cut as the better half still showed some tokens with more "problems" than the problem half which did have some clean tokens.

Sounds like uneven heating or catalyst.
Never been impressed with green stuff myself for this sort of thing and havent used graphite for 25 years lol Messy stuff.

What temps you operating on?
L

Some thing i have notice is that my top and bottom spacers have to be metal – to give even temp. I had use wood which gave me a uneven and unseperating rubber.
a while back i bought a lot of rubber and found that my hoarding caused problems as well.

The only times I've seen anything like that it was air trapped in the epoxy. I've never rolled it though.

Were the discs straight epoxy or did they have a metal core?

I find graphite is good as a separation agent but it's not so good at allowing gas to escape the mold.