Ideas for casting this part?

[Rogviler]

I'm trying to figure out an easy way to cast this...

http://img35.imageshack.us/img35/420/ringwn.jpg

http://img37.imageshack.us/img37/1817/ring2kk.jpg

Of course the first inclination is to create an open-face mold just as it sits, but the problem is that the bottom and its relation to the two sides is what matters and needs to be perfectly flat, which doesn't always happen with that type of mold (and I don't trust sanding it). This isn't a cosmetic part, but will serve to align a short cylinder with a flat surface plus a conical part over that. So pretty doesn't matter, nor does the top (hence the roughness).

Anyway, if I mold it the other way so a rough top wouldn't matter, I could foresee problems getting it back out without tearing the mold.

I only need a few of these, too. Four for sure, plus a couple for just in case. So, the thought of making an elaborate two-part mold isn't appealing...

I'm hoping there's just a tip or trick I don't know about.

-Rog

[Johnnycrash]

Of course the first inclination is to create an open-face mold...

And that would be the best way to do it. And, to get the flat surface you need, use a "squish plate". Basically, you slightly over fill the open face mold, and then put something flat on top of it. I use the flat surface of old molds. It's silicone, so won't stick. You can also use a piece of plastic, you will just need to spray it with release agent.

[Ziz]

Look at the way I make some of my parts -

http://www.modular-models.com/Bee/interface%20800.JPG
http://www.modular-models.com/Bee/singl ... %20800.JPG
http://www.modular-models.com/Bee/passe ... %20800.JPG

Make a thin ring of plastic on the larger surface of the ring, the part it's resting on in your picture. Then add a wider ring on top of that. You're essentially creating a ring with a T-shaped cross section. Use white glue to glue the whole thing to a piece of spare sheet. Build up your mold box, pour your rubber, de-mold, make your part.

The T-shape is effectively your sprue. Trim it off and sand the surface back down to smooth.

[Rogviler]

Make a thin ring of plastic on the larger surface of the ring, the part it's resting on in your picture. Then add a wider ring on top of that. You're essentially creating a ring with a T-shaped cross section.

How's that for unmolding? I think it would work if the ring was thin enough, because the whole bottom surface doesn't have to be perfectly flat, just most of it. But then it would be harder to get out...

I like the "sort of two-part" method with a squish pate. If I can get a stiff enough top piece that would probably work for this.

It's just tough because it is structural. If it were cosmetic, it would be easy to sand and putty things to "look" right. I guess that's my anxiety over it.

I've test fit this part and it's so perfect it hurts, so the copies have to be exactly the same. It feels like I'm talking myself into a two-part...

-Rog

[Johnnycrash]

It feels like I'm talking myself into a two-part...

You are WAY over-thinking this. Just glue it down to a flat piece of plastic, make your mold box, pour RTV. Wait till cured, demold master. Mix and pout in resin, and slap something flat on top that won't stick. Wait till cured. Demold perfect part. That's it, that's all.

A stiff top piece is anything flat that can be greased up, or a piece of old mold that's flat. Hell, you could even pour a new piece for the squish plate, just make sure it has at least one side as smooth as possible. Like a piece of plastic.

It's not that hard. You just need to STOP over thinking it.


As far as being able to demold it... What RTV are you using?? If it's not flexible enough to pull that part out (in it's inverted form - small part at the top - where you pour in the resin), then the rubber you are using in WAY to fricken' hard. You need to start using a more flexible RTV. And if you are so worried about the mold tearing taking out such a simple part, again, you are using the RTV. I have done more complex parts than that with incredible undercuts, no problem. Pick the right rubber for the job.

[Rogviler]

It's not that hard. You just need to STOP over thinking it.

Usually I'm hearing that from a woman...

All right, I'll give it a go with the simple method. I think my RTV is getting past its due date anyway, so I wouldn't mind using it all up.

-Rog

[Rogviler]

When I dug out my casting RTV I realized I didn't have nearly enough, so I ended up doing an old (and cheap) trick using GE Silicone I (caulk). It's too thick for delicate parts, but it came out great with this simple shape:

http://img689.imageshack.us/img689/9053/lemony.jpg

The test casting with plaster turned out great, as you can see just a little flash that could be removed easily.

If this method doesn't work I'll try making the mold upside-down. Even the GE-I is flexible enough I think. I realized I was thinking of it like the inside would be undercut as well, which would certainly make it hard to remove, but only the outside would be, so it shouldn't be a problem.

-Rog

[Ziz]

Make a thin ring of plastic on the larger surface of the ring, the part it's resting on in your picture. Then add a wider ring on top of that. You're essentially creating a ring with a T-shaped cross section.

How's that for unmolding? I think it would work if the ring was thin enough, because the whole bottom surface doesn't have to be perfectly flat, just most of it. But then it would be harder to get out...

I like the "sort of two-part" method with a squish pate. If I can get a stiff enough top piece that would probably work for this.

I tried the "squish plate" method and while it does work, it's not fool-proof. You have to make sure your mold is sitting perfectly level or the plate will just slide out of place by the displacement of overpoured resin. Putting a weight on it is also iffy because now you're not leaving any way for air bubbles to escape from the top of the resin as it starts to cure.

The T sprue solves both problems at once - it doesn't matter if the sprue section is perfectly smooth OR level at the mold edge because you're going to trim it off anyway. Getting the part out of the mold isn't an issue because the mold is rubber, remember? It flexes. For larger shapes I'll cut the mold part way down the height so that separate it a little more. Then for casting, I build clamps out of Legos to hold the mold together.

[Rogviler]

Yes, I know rubber flexes, it's mostly the center that would be a problem I think. The outside has room to move, but the center is "captive"... Maybe it wouldn't be too bad, but if I can do it where it's not a problem at all, the why not?

Okay, so after messing with this first mold I think I'm going to try the "sprue" method, but instead of a T I'll use an L, keeping the inside smooth. That way the center of the cast part will lift straight out, and the mold should be able to flex easily over the undercut part on the outside. That seems the best of both worlds.

Thanks for the ideas!

-Rog

[Ziz]

Your part only looks to be about 1/4" thick - that means your mold will only need to be about 1/2" thick or so - plenty of flexibility in there to pop the part out of the mold.

[Rogviler]

It's probably a little over 1/2" tall and a little under 1/2" wide at the base. So a good chunk of material making up the "donut hole". I'm building the sprue right now, and figured I could overhang the center a bit to give a little more room to work. So basically an L that sticks out over the inner lip slightly. I've got much more confidence in this one...

-Rog