[SScnc]

I'm planning to make some letters out of casting resin for a project, these letters will each be about 4" high, 3" wide and 1.5" thick. I'm trying to find a material to machine the mold cavities for these letters into. I'd like to use something inexpensive (CHEAP), light, easy and fast to machine. I'm thinking some type of foam, like the 2" thick blue foam but some that will not melt when the resin comes into contact with it and will release the cured resin.

I'm also concerned that this type of foam may have a slightly rough finish in the cavity after machining, it doesn't need to be glass smooth but I don't want it to be "feathered".

Any advice on what material may work good for this ?

Thanks,

Steve

[Kipper]

I'd make an MDF Male plug and then apply "Vinamold" over it (after applying "releasal" otherwise it sticks like **** to a blanket!...then press polyester filler (Car bodyfiller) into the vinamold.....If the finish was sh..."poor" I'd just sand the poly till smooth.

VM is good for 100 pulls+ and best of all is re-usable...Can take a mirror finish impression too. Releasal is a very fine Al suspended in Isopropanol with wax for mould release in sandcasting (I have a thousand years supply of the Foseco equiv)

[RomanLini]

Cut the molds out of wax. You can run a little hot water over it to remove any tool marks if needed and give a glassy finish.

Then cast the resin in the wax mold (make sure you keep it cool with fans etc during the cure) then separating from the wax should be easy enough.

I'm not sure about *cheap* as 4" x 3" x1.5" letters will be about $5 each in resin costs, maybe more.

[SScnc]

Kipper, I'm not familiar with Vinamold, sounds good, I'll check it out. Sounds like it will work good for many projects however, I really want to just do a negative mold in one step if I can find the right material to use.

Roman, that's the idea, I like that but the wax is so expensive. I'll see if I can find something that will work similar. I'm wondering if I were to use the blue rigid foam and coat/seal it with some type of spray paint that the resin will release from (?) Any thoughts about that ?

Also Roman, I just read your excellent thread HERE regarding the home switches and will be making those with the LED's for my new machine. Great job and thanks !

Steve

[HPbyGD]

Vinamold is a great product ! I use it to take casting of ports in two stroke cylinders. They make a couple of different grades and I use the red stuff.

Gary

[Kipper]

Vinamold is a great product ! I use it to take casting of ports in two stroke cylinders. They make a couple of different grades and I use the red stuff.

Gary

Red is the softest...great for replicating every little blemish White is the hardest and used primarily for concrete....I use a microwave for melting (got banned from using the household one as it stinks it up a mite) Strangely the magnetrons are different in all ovens...so it's best to get to know the oven....or it burns

I believe due to the spelling "mold" it's an American product? So hopefully the OP can grab some locally and give it a go....I don't have a bad word for it.

Make sure the pattern is dry...steam is crapola as it makes bubbles!

[RomanLini]

...
Roman, that's the idea, I like that but the wax is so expensive. I'll see if I can find something that will work similar. I'm wondering if I were to use the blue rigid foam and coat/seal it with some type of spray paint that the resin will release from (?) Any thoughts about that ?
...

I don't see the wax as very expensive. It's non critical machining so you can use supermarket white stick candles and buy a few boxes for $10. Then melt them in an old saucepan and of course after molding you can simply re-use the wax on other projects so it becomes an investment in a tool, not a disposables cost.

I'd like to see if you can get a good result from a coated foam female mold. Most foams will melt with the solvents in spray cans. If you could make a paint-on water based (maybe alsohol based?) mold release it might work ok.

The problem will probably be surface finish, if you are making letters you probably want them to look nice and any foam etc stuck to them will need filing/grinding off and then you are up for a lot of finishing around the complex shapes of letters.

I think to get decent surface finish you need a mold release with a decent gloss, excellent release and probably something you can melt residue off the leters or wash residue off the letters with a simple solvent like water or alcohol. Wax is still looking good to me.