Advice please on shrinkage during mould making and casting in wax then silver

Hi Everyone,
I am very new to this forum and would appreciate some advice ...I have designed a pendant with Illustrator which I want to make in acrylic (perspex). I eventually want to have it cast in silver but before that, I gather I first need to make a rubber mould and cast in wax before sending off to a foundry to be cast in silver. The question is how much smaller in percentage terms will the finished piece in silver be compared to the original perspex pendant? I really need to know this before I create the pendant as I don't want to make it either too small or too large!! Any help would be much appreciated.
Carl

First Carl, you do not need to make your own rubber molds and waxes, most decent casters will make these for you, for a fee. I do not know your design, but please remember that any shrinkage is in all sides of the metal, so to make it easier to understand if you cast a ring from a master pattern, the outside diameter of the casting will end up slightly smaller, but the inside diameter or the hole in the ring will end up larger, so if I read you right and you are making a pendant with something that fits inside an opening, then the opening will be larger than the opening on the pattern when cast. There is no hard and fast rule re shrinkage as different alloys and casting methods give different results, as a general rule if the item's size is critical I would add 0.5mm to each dimension and then file the casting to fit. I hope this makes sense.
James

the shrinkage in my experience is pretty small to the point of insignificance. The only issue is if you are getting bits that fit together. Like James said, most casters will make the molds and wax patterns for you and it's not terribly expensive. I think mine are about £12 for the mold and then 50p per wax.

One thing to be aware of is the molding process can get quite hot and might possibly damage your perspex pattern, so it may be advisable to go for one of the lower heat molds which are not quite so durable in terms of mass production, although you could always get a high heat mold taken from one of the metal castings.

Thanks that's incredibly helpful...but rather challenging! I didn't realise that the inside of a ring gets larger - I would have imagined (like most beginners) it would get smaller just like the outside! The design has two elements and yes one does fit inside the other...I need to think about it and figure out what will happen when it's cast. Yes it makes sense to get a mould made professionally, it doesn't sound overly costly.
carl

Carl,
If you have time. why not get the outer section cast first and then see for yourself how the casting resizes and then you can perhaps make the inner section slightly larger than needed so that a little bit of filing will make the perfect fit. As for casting companies, some will give you the molds and others will keep them and give you a reference number for repeat castings. As Medusa says there are mixtures for making cold molds if your master pattern is not metal. If you do wish to attempt making your own molds and patterns check out this company for mold making supplies; http://www.tiranti.co.uk/
Good luck,
James

I'd never thought about asking who actually owns the molds I've had made. I assumed that I owned them but keep them with the company because they make the waxes and do the casting for me.