Building a mold to pour a part

[Casemaker]

Does anyone have any experience or advice concerning building a mold in order to get a pattern for a metal part. I need to make 4 identical patterns for a left lug and 4 for a right lug (or grouter) that goes on a 1/2 scale steam engine rear wheel. I believe that by creating a mold and then creating 4 parts from the same mold will be the way to go. But what do you make the mold from and where can you get these supplies ? Can you make the mold from clay and then use a plastic type wood filler to pour the part ? What can be used as a release agent in order to get the part out of the mold without damaging the mold ?

[Jim Mackessy]

Hi Casemaker!
A lot depends on how durable your master patterns need to be. With careful handling you can get away with using urethane compounds, but they are somewhat delicate. There are more durable compounds out there, such as Devcon and Flexane, but they can be very pricey. As for release compunds, I use Butcher's Paste Wax, if the pattern is too complex to apply this I have used silicone mold release sprays or even Mazola corn oil spray to achieve a good release. One project I did was a reproduction of a brass name plate, about 3" x 10" x 1/4" thick. I mounted it to a board, and built a frame of 3/4" x 3/4" pine, about 1" bigger on each side of the plate. After a light spray of silicone, I painted a latex resin over it. I don't remember the name of the product, but it came from a hobby shop and is used for making scenery molds and theatrical masks. It is very flexible. After it cured, and before I removed the plate, I pumped the rest of the frame full with Silicone caulk, same as you get at the big box stores, and struck it off level. I removed the
mold after everything had cured, and then sprayed it with the corn oil spray, as that is what was recomended to me instead of silicone. I mixed some Alumilite Super Plastic, ( a two part urethane ), and poured the mold. This stuff got pretty warm as it cured, but it did come out nice. I had some trouble with air bubbles because of the way I mixed it, but on the second try I just rolled the mixture around in the container and poured it. This was not mixed well enough and did not cure entirely, so I doctored up the first piece and used it to send to the foundry. We did get five nice plates off of it. Now,
for the rest of the story.... I found out the latex reacts with the brass pretty good on curing, darkening it up pretty good. About a month after casting the urethane, I noticed the mold was getting gooey and deteriorating. This was due to a chemical incompatibility between the latex on the face of the mold and the RTV Silicone filler on the back. All in all, it was a success, but I think there is a lot to be said for the commercial "system" approach, as all the chemicals are matched to the application. I found the info I used on the internet, I read sites on theatrical makeup and masks, model railroad scenery, and model castles, of all things! I spent less than $100.00 on the whole project, so there's something to be said for the amatuer approach too. Good Luck! - JM

[kerogas]

Those guys at SmoothOn are always blowing something up . The PMC744 is right up your alley . You can use whatever will hold the weight , like a shoe box divied up to save compound . I have made molds on top of the cooler in the tent with a tube of silicone to glue down pieces of a manilla folder I cut up with my knife . The molds had a waffle grid on one side .The trick is remembering to bring the rubber .

You you use rigid mold materials like plaster and clay if the part has plenty of relief . They just arent as durable .
http://www.smoothonsecure.com/store/...p?cPath=289_88

[Jim Tremble]

Originally Posted by Casemaker

Does anyone have any experience or advice concerning building a mold in order to get a pattern for a metal part. ?

Are you trying to make a mold and do the pour yourself or are you going to make a pattern for the foundry to do the pour for you. ?? All they need is the pattern.

Keep in mind, the pattern has to be 10% bigger that the part you will get after the pour. It will shrink 10% to get the right size. This applies to cast iron. I have no info on the shrinkage on cast steel.

Another thought, fill all holes with bondo and you can drill the new casting when you get them back from the foundry.

Here is another thing to think about, half way through your pattern. (center line) you have to have a slight angle on the edge of your pattern. This makes the pattern come out of the mold with no problem.

Hope this helps,

Jim

[Alan Bowen]

I am trying to picture just what you want to pour,,,
What is wrong with making a nice durable pattern from wood?
I guess that would be my method being a 30+ year carpenter/builder.
I am collecting materials to start casting myself.
I have seen the router that will follow anything and make a duplicate from it.
Kinda like you take this pointer and run it around a part and over there the router is making a new part.

I think it was a Sears item.
I am in the planning stages right now.
It shouldn't be too hard to make one that would enlarge your pattern 10%.
Then if you needed a different amount of enlargment you could loosen a screw or two and set it to 8% or whatever.

That is of course assuming you have one and need to cast another.
Or if you had a busted one that you could "fix" with putty long enough to copy it for a pattern.

[Peter]

I think rather than emphysis on the shrinkage problem --the thing to keep in mind is 75% of the time, shrinkahe can be ignored. Almost all the work I have done was to use and origional part as a pattern. Where a surface was to be machined I would build it up.

tractor lugs are a perfect project where an origional can be smooth down, holes filled and used as a pattern. Ship it down to cattail. Holes can be filled with WAX. yes wax. that works fine. no problem. In some rare cases I have used epoxy filler but almost always i used wax to fill or build up. one problem is a lot of origional casting are rough and need to be sanded and that can slightly alter the look of the origional. if you have a lot of lugs then one can be sacrificed as a pattern and made smooth as glass with epoxy.

the genius of using an origional beside the simplicity is since the origional was cast, it by definition meets the requiremtns for casting.

The big problem to making complex parts is core boxes. Thats the pattern for the hollow inside or parts. In this case its not an issue.

If you are thinking of casting your own and using expoxy for tractor lugs. BIG mistake. Have it done in iron. its not very expensive. the parts&kits you buy are marked up due to of all back/forth shipping and middleman profits. Direct from Cat tail is very very reasonable.

[Motormowers]

Ive been toying with the idea of sawing a bad Tillotson carb in half and having a mold made to cast some up. I wonder if it could be done?

[Scott]

Go to youtube and search "mold making" or "sand casting". I found several good videos there. Some of them are 9 minutes long and are very good.

[J.B. Castagnos]

Iron shrinks about 1 percent, aluminum about 2 percent. For iron figure about 1/8" per ft, aluminum 1/4" per ft. 10 percent would be 1 1/8" per ft.

[Jim Tremble]

I stand corrected. 1% sounds like what I was thinking. 1/8" per foot.

Thanks,

Jim