Friday, 9 November 2012

A Waiting Game

Buried beneath the white gloop are some masters for the ballast wagon kit - I don't know when I will see them again....

There are lots of tricks, tips and shortcuts on the learning curve that is resin casting, but for me the darkest art is making the silicone rubber moulds.

It's not so much the end result - the mould - that has me scratching my head, it's more the preparation of the rubber compound, and specifically how long it takes to set.

You see, it's never the same twice.

The stuff I'm using requires you to mix in a catalyst with a ratio of 5% of the rubber mix by weight.

That's the tricky bit - especially when I'm only needing to mix a small amount at a time.

The 3 moulding boxes in the picture above needed about 15g - which required 0.75g of catalyst.

I'm sure the boffins at Pfizer or GSK would have no bother measuring out such small amounts accurately but it's not so easy on my bargain basement set of electronic scales.

It's supposed to take 24 hrs to go off. And in fact the first mix I ever made did precisely that.

However subsequently I've had batches which set in 6 hours flat (Opps! Too much catalyst!) and others that were still soft after 3 days, which had me questioning whether I had forgotten to add in the catalyst.

These latest moulds show every indication of being a 3 day-er.

The good news is I've not noticed any appreciable difference in the quality of the mould regardless or how fast or slow it has set, but it's just a little frustrating.

1 comment:

  1. My wife (former Pfizer & GSK boffin) suggests a syringe if it's liquid. It's density is likely to be similar to water so just use ml instead of g (and maybe add a bit since it still seems ok if you use too much). If powder she suggests making a simple balance and using and appropriate amount of water to measure it. She also suggests that other factors such as temperature could make it take longer to cure.