Help with designing a mold?

[Chris Moliere]

I've recently come across an incredibly rare item used for the MST3K bots - an original Money Lover Barrel. I'm planning on making casts, and selling them to fill the current void of available parts. However, I'm having difficulty figuring out how to design the mold. It has no visible seam line around the vertical edges, as far as I can tell, and there's no seam crossing through the black rings. Has anybody ever molded from one of these before, and if so, how should I make the mold? If not, what would be a good way of going about it in order to prevent the seam from showing? This mold is being designed for use in a rotocasting machine.

 

[udog]

Take in account the seam could have been sanded down and "hidden" after demolding.
I think you could make a brush up silicone mold with fiberglass mother mold/cases
If you are using it for rotocasting it could be a three part mold. The third piece would be the bottom part. You might want to create a plug there that you can take off for pouring material and easily close to rotocast.

 

[JPH]

Use a thin styrene sheet to make a divider. Make registration dents in it, then you can brush on mold for details. Make a hard, outer shell then pour in the rest of you mold material.

 

[Zinger]

Use a "glove mold" made with a brush on silicone, like Smooth-On Rebound 25. This video shows how to make the mold. The second part shows how to build the rigid shell (the "mother mold") that supports the floppy mold during casting.

A few lessons I've learned. Don't skimp on your registration keys. Look how big Milo's are and the taper angle. You need those keys to lock firmly into the mother mold. Otherwise, you get waves, wrinkles and other defects in the casting. Have a nice, wide flange with plenty of keys, like in the video. That makes the mold easier to handle. Finally, when you do your clay up, have the clay inside the base of the mold and take your time to get the junction clean. That makes it easier to trim the part when it comes out of the rotocaster and reduces clean up.

 

[Zinger]

I based my rotocaster on this design. Scaled it up to fit the mold for my bat. DIY Rotocaster

 

[Zinger]

Use a thin styrene sheet to make a divider. Make registration dents in it, then you can brush on mold for details. Make a hard, outer shell then pour in the rest of you mold material.

If you're going make a split mold like that, you don't want a straight seam. It can slip inside the mother mold and cause defects in the casting. Either make a block mold and cut a zig-zag seam into it. That will act as its own keys. Or put registration keys along the seam so they lock into the mother mold.

I didn't see the keys in the styrene at first. That could work.

For something this shape, I'd skip the seam altogether. The base is so much wider than the rest of it, a seamless glove mold will pull right off.

 

[JPH]

If you're going make a split mold like that, you don't want a straight seam. It can slip inside the mother mold and cause defects in the casting. Either make a block mold and cut a zig-zag seam into it. That will act as its own keys. Or put registration keys along the seam so they lock into the mother mold.

For something this shape, I'd skip the seam altogether. The base is so much wider than the rest of it, a seamless glove mold will pull right off.

Chris is molding a round object. If he uses one styrene sheet on the back side of it, he will have minimal seam. Use the styrene sheet 180 degrees from the beak.

Cutting a zigzag guarantees defects and seams. Seriously, zigzags up and down multiple curves, where the mold is supposed to join(!?!?!?!) that is begging for defects.

A 1mm straight sheet with dents in it for registration will vastly minimize seams, even if he doesn't use the styrene during the casting process. Even if he doesn't use the lumpy styrene sheet during casting, 1 mm to the diameter is a few inches will be negligible.

IF Chris wants to mold in sections, a one-piece mold for the bottom is a great idea, but I am under the impression he wants to do it all as one.

 

[Chris Moliere]

Looking at the replies I realize I forgot to mention that only the barrel (the green object) is being cast, all the other items are being made separately. Although if I do need to make single piece casts I will definitely keep this in mind.

 

[Zinger]

Then a glove mold is the way to go. It will peel right off that shape.

 

[Zinger]

Chris is molding a round object. If he uses one styrene sheet on the back side of it, he will have minimal seam. Use the styrene sheet 180 degrees from the beak.

Cutting a zigzag guarantees defects and seams. Seriously, zigzags up and down multiple curves, where the mold is supposed to join(!?!?!?!) that is begging for defects.

A 1mm straight sheet with dents in it for registration will vastly minimize seams, even if he doesn't use the styrene during the casting process. Even if he doesn't use the lumpy styrene sheet during casting, 1 mm to the diameter is a few inches will be negligible.

IF Chris wants to mold in sections, a one-piece mold for the bottom is a great idea, but I am under the impression he wants to do it all as one.

Self keying zig zags are the standard way of making a cut mold for objects of any shape. They interlock preventing the mold from slipping past itself.