Question about casting a "cone"

[jmartin]

Well, technically, it would be a frustrum, but no one likes a smart @$$, right?

Skip over this part if you like. I sometimes feel that I have to over-explain myself. Also, this isn't actually about construction of a Sci-Fi model, but this is where I post and hang out, so I didn't think anyone would mind.

I'm taking a break from the 1/350 NX-01, and even all of my unbuilt Fine Molds Star Wars kits, to finish a couple of 1/32 Tamiya F-14s. One is being built as a "B" (VF-103) and the other as a "D" (VF-2). One (I haven't decided which) will be gears down, the other in flight. To convert the Tamiya "A" kit to either a B/D, you must swap out the P&W TF30 engine nozzles with the GE F110 nozzles (along with some body work). For one kit, I was able to snag a CAM conversion kit. I'll put this on the one that will sit on its wheels. The CAM nozzles are in the "open" configuration, which (I assume) means that engine is at a higher output, afterburner or not. For the one that I am going to pose in flight, I have to go with the nozzle in the "closed" position, since almost all photos of in-flight tomcats show this. (For the record, I'm going with the wings extended for the flying model, and wings swept back for the sitting one.) Closed nozzles for the F110 are not available at this time (to the best of google's ability), so I scratched one up.

So, my question is: How do I cast this so that I don't have to scratchbuild the other one?

I'm not asking for casting advice; I've read a lot about resin casting (mostly here) and will try some simpler objects before I give this a go. However, I can't wrap my head around how to fill the RTV around the master. The best I can imagine is that after making a rubber mold, I will have to remove the "plug" in the center first, then the master. So how do I line up the "plug" when pouring the resin? For it to look half-way decent, I would have to center it with respect to the rubber on the outside.

If any of you have any thoughts, I would appreciate it.

If any of you would be willing to take my original and make me another copy (or a few in case things go bad), I would be interested in talking to you.

Thanks in advance for any input.

John

[en'til Zog]

Um...O.K. Is this a nozzle like the standard injection molded ones? If so, couldn't you attach the flat base, the part that attached to the model, onto a flat sheet of plastic, and then surround the whole thing with a wall or cylindar making a cup with the nozzle sticking up in the middle - pour in RTV and let set. Then pop off all the plastic, remove the original nozzle, and you're left with an RTV mold with the back part - what goes to the model - open so you can pour in resin?

[jmartin]

Um...O.K. Is this a nozzle like the standard injection molded ones? If so, couldn't you attach the flat base, the part that attached to the model, onto a flat sheet of plastic, and then surrounr the whole thing with a wall or cylindar making a cup with the nozzle sticking up in the middle - pour in RTV and let set. Then pop off all the plastic, remove the original nozzle, and you're left with an RTV mold with the back part - what goes to the model - open so you can pour in resin?

Thanks for the reply.

That's what I'm trying to do, but here's what I can't figure out. The wider part of the nozzle, like you said, is what attaches to the model. In order to remove the original, I'd first have to remove the rubber away from it in that direction (in the forward direction). However, the rubber on the outside would have to go the other direction. I know that the rubber is flexible, but I can't imagine it would let me squeeze the original master through. (The "hole" in the front is about twice the size of the one in the back.)

The "open" nozzles that I bought from CAM are set up how you described, with a resin "base" at the attachment point. However, the diameter of the open part is only slightly less than the other side, so I imagine they were able to pull the rubber in the center out pretty easy.

I think I need to have the rubber separate into two parts, the outer mold and the inner "plug." But I am not sure how to line them up when casting in order to keep the correct "thickness."

John

[Stu Pidasso]

A picture of the master part would help! PM me as well.

[en'til Zog]

Ah. So the inside of the nozzle is wider at the base, near where it connects to the model.

Hm.... You might have to 'cheat' the inside of the cone so the RTV can slip off the nozzle 'backward' - thicker walls of the nozzle at the base where it connects to the model. At least that way you can have burner detail molded into the inside bottom of the nozzle.

What Stu Said....

[Joseph Osborn]

Mold it in Smooth-On's "Smooth-Sil 910." It's Shore 10A and will stretch waaaay farther than you'd ever believe. I've been doing some insane 1-part molds with this stuff lately and I may never go back to Mold Max rubbers.

[Chacal]

...The best I can imagine is that after making a rubber mold, I will have to remove the "plug" in the center first, then the master. So how do I line up the "plug" when pouring the resin? For it to look half-way decent, I would have to center it with respect to the rubber on the outside...

John

A hinged arm to hold the 'center plug' in position.

To make the female mold of the outside of the nozzle you'll plug the insides with plasticine, then build a box around the nozzle to dam the rubber, right?

Now, after it's cured, you flip it over, remove the plasticine, 'grease' the rubber in the center of the 'hole' (so it won't stick to the 'plug' rubber), right?

Now, before you pour the rubber to make the male mold of the inside of he nozzle, build a jig either a hinged arm or a beam with notches to lock onto the frame of the mold box (notches on both the box and the beam, so they only fit one way).

To that beam, you'll screw an eyelet in a position that, after you flip the beam down or 'lock' it in position it will be in the middle of the nozzle cavity.

After you're done, flip it down or 'lock' it up in position and pour the rubber for the second part of the mold.

If all goes well, you'll have the 'plug' holding tightly the eyelet, so when you remove/flip the beam up, it is held firmly in position.


Like this: MOLD (sorry 'bout the quality of the image. Didn't want to fire up the 'ol scanner, so I shot the sketch up with the cellphone camera — sucks!)


When you pour the resin, fill partially the cavity of the female part, then flip down/position the beam so that the male (inside) mold displaces the resin up.

[en'til Zog]

COOL!

Great idea, Chacal!