Mold Making Problems / Questions

[ramius-ii]

Hi All,
I am using a RTV based silicon mold making materials from Micromark 10:1, 24 hour cure, high tear strenght. The part I am trying to mold has some very small parts with areas .024 across. While the results seem to be, for the most part, accurate, the smaller areas seem to become smaller (.014) as if when I remove the master the mold making material has expanded! Is this normal? Is there a better product that does not seem to change size? When I pour the mold making material into its form it is placed in a pressure pot to remove any bubbles. Could this be the cause? Thanks in advance.

Ed

[pirate]

Re: Mold Making Problems / Questions

Can you post a picture of your finished mold?
You may need to use a harder RTV mold material that will not sag into the void left when the part is removed. If your mold is very thin perpendicular to the void, it could be sagging down into that space. Seeing an image may tell us that.

There is not much chance of air bubbles re-expanding unless, again, it is a very soft RFV.

Pete

[JWLaRue]

Re: Mold Making Problems / Questions

While the results seem to be, for the most part, accurate, the smaller areas seem to become smaller (.014) as if when I remove the master the mold making material has expanded! Is this normal?

Hi Ed,

It's not clear what is the wrong size, when....but if I had to guess, I'd say what you might be seeing is the effect of the shrinkage that is more-or-less normal for RTV. Does that sound like it might be the cause to what you are seeing?

-Jeff

[ramius-ii]

Re: Mold Making Problems / Questions

Hi All,
I will work on taking a picture or two. Basically the master has a square mesh screen with .024" verticle and horizontal "bars". After the RTV material is cured, the spaces for the bars become smaller so a cast part will have .014" bars!

[JWLaRue]

Re: Mold Making Problems / Questions

Definitely sounds like shrinkage of the RTV. Do the instructions for the RTV give you any guidance on expected rate of shrinkage?

-Jeff

[ramius-ii]

Re: Mold Making Problems / Questions

Hi!
I am probably not explaining things correctly. You have a total of 9 square openings in the final product. Each square is seperated from the next by a distance of .024". When you pull the master from the RTV the distance between the squares becomes smaller by .01" as if the RTV is expanding leaving less gap between the squares. The square holes are now larger as the gap between them is now smaller. The gaps in the mold are to define the width of the spaces however thes gaps are now smaller. If the RTV was subject to shrinkage then the gaps would become larger and the square openings smaller. This is the opposite where the gaps to be filled with plastic when casting the final parts are now smaller as if the RTV is expanding.

[ramius-ii]

Re: Mold Making Problems / Questions

Hi All,
So here is a picture so you can see what I have been trying to describe. Notice the mold has much smaller verticle and horizontal lines in it than the master!





Thanks in advance, Ed

[salmon]

Re: Mold Making Problems / Questions

Ramius-II,
It may still be shrinkage. The larger base will pull your finer details, much like squeezing a sponge, the top balloons out. I would definitely talk to the supplier of your molding material (or contact the manufacturer). They may suggest a smaller base or different product to use.
Peace,
Tom

[ramius-ii]

Re: Mold Making Problems / Questions

I understand and the mode is poured basically upside down. The picture is from the bottom of the mold

[Guest]

Re: Mold Making Problems / Questions

Perhaps a harder compound would work better?

How many parts are you looking to mould?

In the past I've used candle wax for moulds where I only need a few parts. They're sacrifical, but it only takes a few minutes to make quite a few moulds as melted wax soon hardens off, and the spent moulds can be remelted and used over and over again.

I spray a little water onto the master before pouring the wax on to help it release. I would also solder or glue on a hoop of some kind to the master to pull it from the mould.

[ramius-ii]

Re: Mold Making Problems / Questions

Thank you Sub Culture,
This red RTV, as opposed to the blue, seems harder than most mold materials I have used before and it is a great idea. I did also tried making a mold without using a pressure pot and a thinner bottm (top when poured) with the same results. If I did not need 200 pieces then I would not go through all this! Typically for such a quantity I will make 25 molded parts and then use those parts to make a larger mold. Having the bars become smaller when a part is molded the bars in the final results would be too think to be useable. Thanks again for the ideas.

Ed

[Guest]

Re: Mold Making Problems / Questions

How about getting the grills photoetched, or cut by waterjet?

[scott t]

Re: Mold Making Problems / Questions

This is not a solution for the RTV problem but an idea how to make a multiple mold.

You may have seen a letter being sealed with wax and a stamp pressed into it.

I was thinking maybe you could make your part into a stamp and press it into a sheet
of wax multiple times to form a molds. Then pour and squegee the molding material
into the depressions. Some one on these forums made deck boards in a similar manner.
I don't think they used wax though.

Another idea is to make a punch out of your part and press it through something like the
sides of soda bottles. The material would be cheap and won't tarnish.

Scott T

[ramius-ii]

Re: Mold Making Problems / Questions

Wow!
Some really great ideas! Thank you all! The catch is that the part is not only 1/4" square, it is also 1/4" high and hollow. Basically a piece of 1/4" square tubing with the "screen" at one end. The screen material is brass and comes with the square holes in it. To make the master was a bit tricky, I had to stack several "screens" on top of each other then use small square pieces of aluminum and solder the whole thing together, then remove the aluminum. This way the solder would not "stick" to anything except the brass screens. This has been a real "challenge" to create! Lol

Best, Ed

[Guest]

Re: Mold Making Problems / Questions

Why do want a part with that sort of draught, seems a bit excessive?

Othere solutions to this problem I can think of-

Spark erosion. You would have to make a custom tool to make the parts this way though.

3D printing. A company like shapeways could print these off for you, and as the parts are small, and the volume low, the cost should be reasonable.