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[Neural]

I know a tiny bit about casting, but one thing I can never figure out...

When they make a mold off of a wax piece, how do they know how to cut the mold so that it can be re-used?  Or are they all just one-time use molds?

[hummingbirdstones]

They cut the mold in half.  The object that is used to make the mold can be any material.  Then when they want to make a wax copy for casting, they put the two parts of the mold together and use injection wax to fill the mold.  The use the wax image to invest for metal casting.

There's lots of videos on YouTube, but here's one:

https://www.youtube.com/watch?v=Am7kNd3evLE

[gemfeller]

I know a tiny bit about casting, but one thing I can never figure out...

When they make a mold off of a wax piece, how do they know how to cut the mold so that it can be re-used?  Or are they all just one-time use molds?

Are you asking about molds from master wax models?  If the models are made in the exact size of the casting desired, the shrinkage factor of the mold material is very important.  Master waxes are usually duplicated with RTV (Room Temperature Vulcanizing) material that's generally transparent.  It's pricey though.  Such molds can be cut just like high-shrinkage rubber molds but if you're inexperienced at the task have a first aid kid nearby -- scalpel slippage can be very dangerous!  Watch his video -- scary.  Be sure you know where all your fingers and other vulnerable body parts are at all times!

I have some ideas about making 2-piece RTV molds that don't need to be cut, using mold locks like these Acorn Nuts but I haven't tried it yet:
 https://contenti.com/jewelry-casting-supplies/mold-making-supplies/mold-locks-n-plates/acorn-nuts

Anyhow I did some searching and found this comprehensive website that deals with making many different kinds of jewelry molds including RTV.  The whole thing is really worth a read and archiving if you plan to do any of this work.  His film canister molds and mold-holder suggestions are simply a great idea and save a bunch on RTV compound.  Hope this answers some of your questions.
 http://www.flexiblejewelrymolds.com/blog/

[olgguy]

I am new to the Forum and just found your post. If you are sending your wax to a commercial caster they will make the mold out of a two part liquid mix. When it cures they will cut parallel to the side of the ring. This type of mold is reusable for years. If the wax is broken in the process they will just shoot a new wax from the mold and cast that one.  If you send them a ring in metal they will mold it in rubber if you want.
               olgguy.

[Talia]

I know a tiny bit about casting, but one thing I can never figure out...

When they make a mold off of a wax piece, how do they know how to cut the mold so that it can be re-used?  Or are they all just one-time use molds?

I think what he's actually asking is how does someone who is cutting a mold know where to make their cuts?

The answer is, experience. The more jewelry you make, the better you will understand where you would want your parting lines to fall if you mold it. In general, you want the mold to separate (known as the parting line) at points on the piece of jewelry where it will do the least harm, and leave you with the least clean up to do. So if you have a section of high detail and you don't want flashing there (extra little bits of wax that ooze out through the cuts of the mold) on your detailed area, you might veer the parting line off to the side of that detail into an area that is smooth, and easy to clean up afterwards.

Sometimes this makes for a mold that has wavy-looking cuts, or might have three or four or more sections that all come apart, but that's all part of ending up with an injected wax that needs the least cleanup possible before you cast it.

[Neural]

I know a tiny bit about casting, but one thing I can never figure out...

When they make a mold off of a wax piece, how do they know how to cut the mold so that it can be re-used?  Or are they all just one-time use molds?

I think what he's actually asking is how does someone who is cutting a mold know where to make their cuts?

The answer is, experience. The more jewelry you make, the better you will understand where you would want your parting lines to fall if you mold it. In general, you want the mold to separate (known as the parting line) at points on the piece of jewelry where it will do the least harm, and leave you with the least clean up to do. So if you have a section of high detail and you don't want flashing there (extra little bits of wax that ooze out through the cuts of the mold) on your detailed area, you might veer the parting line off to the side of that detail into an area that is smooth, and easy to clean up afterwards.

Sometimes this makes for a mold that has wavy-looking cuts, or might have three or four or more sections that all come apart, but that's all part of ending up with an injected wax that needs the least cleanup possible before you cast it.

That's more what I meant, yes.  Thank you :)