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Old Jul 25, 2009, 02:27 PM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
4,540 Posts
Quote:
Originally Posted by Kevin Cox
Very nice Dan!

Do you any pictures on how you did the mould for the nose cone? I really need to know how you guys do this .
Hi Kevin,

As I said before, the process of making molds isn't difficult, but there are some pitfalls to avoid. The worse-case scenario is having your plug stick inside the mold and you have to destroy the plug to get it out and maybe even destroy the mold trying to get the plug out.

I destroyed the plugs for the F-4 parts seen in the photos. I think the problem was that the wax wasn't compatible with the PVA and the mold grabbed onto the paint. Luckily, I was able to save the molds. When this happens I've found that Popsicle sticks which have chisel-shaped tips work best for breaking/scraping the paint loose from the mold without scratching the mold.

On the other hand, the inlet molds for the Su-27 worked flawlessly.

For the Su-27 parts, I used Partall Paste #2 and Partall Film #10, which are compatible with each other. I used Dolphin Wax for the F-4 parts and it's clear they weren't compatible with each other because the parts stuck in the mold. Don't get me wrong. Dolphin Wax works great for molding parts. I use it all the time to mold parts without PVA. But, it didn't seem to be compatible with the PVA or the paint I used.

I used to try to put a flawless finish on my plugs and polish them until they were mirror-like. But once you put the PVA on, the finish of the mold with only be as smooth as the PVA barrier. So, the Ferrari finish and polishing seemed to me, to be a waste of time. If you apply the PVA carefully enough, it will have a mirror-like sheen of its own, even over primer.

The inlet plug seen in the photo below was painted with cheap white primer, then I applied 4 coats of Partall #2. In the past, I've done more coats of wax, up to 10-15 coats. But even when I've done that I've still had plugs stick in the molds. It seems to work best by making sure to get a good even coat of PVA.

Here's the pics I took these of the starboard inlet duct mold set for the Su-27. It's pretty much the same for a nose cone except that with a nose cone mold, you only need one end plate.

1) Here's the stages you want go through: plug and parting box, master mold and final part.

2) Here's the plug and parting box and end plates. The box is made from 1/8" masonite and glued together with Zap-a-Gap. I bought a 4' x 8' sheet at Lowe's that is painted on one side, which cuts down on the amount of wax it takes to seal the paint. I had them cut the sheet into 12" x 48" strips so that it would be more manageable in my tiny shop. I use a couple of sheet metal screws to hold the plates in place.

3) As you can see, I take as little time as I can making the box and supports. I make the templates for the supports from balsa, then when the part is the right height I cut it from masonite and glue it into place.

4) The part is in the box with the end plates in place. If you look closely at the end plates you can see the triangle piece I glued into place to keep the top part of the end plate from pulling away from the plug.

5) From here the clay is applied to the inside corners. I bought this clay at the local Aaron Bros. art supply store and paid $6.50 cents. I got snookered. Search around. You can get it lots cheaper elsewhere.

6) I used to hand roll my clay filler. Then, I saw this extruder. It has many different shape bits, but this one which squirts out a 1/16" diameter string of clay that works great to fill the gaps. I put the excess back into the syringe. It's easier to extrude when it's warm.

At this point gently polish the waxed mold and parting box and end plates to remove any excess clay from the surface.

7) I like to build a dam of balsa around the outside of the molds to keep the epoxy from running and to make the molds easier to get a speader between the halves. First, I run tape along the edge of the mold. Then, I apply an even coat of PVA and let it dry. I try to keep the PVA off the tape. Lastly, I glue 1/4" balsa squares to the tape to form the dam.

8 & 9) Foam brushes work really well for applying PVA. I treated the foam brush more like a marker than a paintbrush. You want to be careful not to apply too much PVA that it runs.

10) I've also used this Preval sprayer to apply PVA with good results. I found this one in my neighborhood Ace hardware store for about $10-15 dollars for the unit and about $5 for the refill.

11) From this point, it's a matter of laying in the fiberglass and resin to build up the mold surface. I like to mix the resin, then add some cabosil to harden the cured epoxy and then add pigment to form a gel coat. I'll put a layer on and let it gel, then apply the cloth and resin. I lay on 4 layers of 5-6 oz. cloth.

12) Once the resin has cured I gently remove the end plates, then separate the mold and plug from the parting box. I don't usually remove the plug from the mold. I clamp some waxed plates to the flange, then apply clay between the end plates and plug. Then, it's tape, PVA and balsa dams.

13) I use a round dremel bit to grind a small dimple in the surface of the flange to form the female side of an alignment pin before molding the second half.

Don't pay attention to the holes drilled in the flanges. These were done after both halves of the mold were done.

Dan
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Last edited by DanSavage; Jul 25, 2009 at 02:43 PM.
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Old Jul 25, 2009, 10:43 PM
2014 EDF JET JAM We be Jamming
Kevin Cox's Avatar
St. Louis Intl, Missouri, United States
Joined Jan 1997
6,718 Posts
Hi Dan,

Thanks so much for posting this! I helps me out a lot. I may have a few questions later.

That F-4 is going to look great. I know it is a bit early but what paint scheme do you have in mind
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Old Jul 25, 2009, 11:09 PM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
4,540 Posts
You're welcome.

Thanks.

What else? Just as I've never built an F-4, I've never had a model with the Blue Angels paint job.

When my wife mentioned to her mother that I was building an F-4, her mother asked if I was going to paint it like the Blue Angels.

Dan
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Old Jul 25, 2009, 11:53 PM
DELTAS RULE
corsair nut's Avatar
tehachapi, CA
Joined Jan 2006
20,831 Posts
i think the blue angels F-4's look hot. go for it!!
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Old Jul 28, 2009, 12:40 AM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
4,540 Posts
Thanks!

I installed the inlet ducts this weekend. The inner wall is 1/8" balsa sheet and the outer wall is made from 100# smooth Bristol paper.

I built the aft duct first, then built the front section. Once the base layer of paper was in place, I went back and glued a doubler and tripler to the outside to stiffen the paper and keep it from sucking in under power.

I am going to extend the aft doubler so it wraps all the way to the balsa inner wall. Right now, mine doesn't extend all the way and it's flexing a little more than I'd like to see.

Once the duct is stiff enough, I'll move on with the fuselage construction. I'm planning to bolt the fan into the model and run it up to test the duct before I sheet the fuselage.
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Old Jul 28, 2009, 01:43 AM
smug in granny panties
monkamarm2000's Avatar
NorCal Silicon Valley
Joined Aug 2002
11,778 Posts
great stuff Dan! and yeah I've been using sprayed PVA for a while now cause it can be treated as a smooth last paint job if layed on right.


Barry
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Old Jul 28, 2009, 11:52 AM
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Daren's Avatar
Los Alamos, NM, USofA
Joined Nov 2000
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Very nice, Dan. The pre-sheeting test is a good idea too. Much easier to see if anything is flexing and fix it while you have access to it.

Daren
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Old Jul 29, 2009, 12:11 AM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
4,540 Posts
Thanks, Barry. I found that the finish on the molds gets smoother and glossier with each generation.

Quote:
Originally Posted by Daren
Very nice, Dan. The pre-sheeting test is a good idea too. Much easier to see if anything is flexing and fix it while you have access to it
Thanks. That's what I was thinking.

Having the fuselage off the building board with all four sides open made the inlet duct pretty painless. A lot easier than the F-106, even.
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Old Aug 04, 2009, 10:33 PM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
4,540 Posts
Having completed the inlet ducts, it was time to move on to smaller jobs for a change.

First up, construct the spar pockets. I used this design on my F-106 and I am very happy with how well they work.

This design is a variation on what we're doing with the Su-27 spars. The big difference between the two is that the Su-27 spars and the bulkheads to which they are glued, are exposed and so they can be clamped while the glue dries.

With this model and the F-106, the bulkheads and spars are buried under the fuselage sheeting. In reality, the spar pockets are really just hidden clamps that keep the spars in contact with the bulkheads while the glue dries.

Next is the battery tray mounts. I had one very hard flop in my F-106 when the motor died on take-off. It did the infamous delta-tailstand and pancaked hard on the dirt as the nose came down.

One of the weaknesses that crash highlighted was that 1/8" birch ply isn't sturdy enough for as many batteries as I was carrying. This model uses 1/4" light ply, which weighs about the same as the 1/8" birch, but has more material, so it should be able to better withstand hard landings without breaking in two.

I used 1/2" triangle stock on both sides of the bulkheads for better support. Because this is the prototype and I'm still not exactly sure where the batteries will need to go to balance the model, I made this tray extend a lot further back than will probably be necessary on the production model.

Nose gear servo and nose gear retract hardware goes in the forward bay and the receivers and switch harness will go in the aft bay ahead of the battery tray.

I still need to make up the linkage and trim the servo throws and then finally, the nose gear itself.

After that, I'll make the canopy mounting plate/hatch and that should pretty much complete the nose area. Then it's time to move back to the tail area and start work on the elevator and rudder servos and their linkages.

Dan
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Old Aug 04, 2009, 11:42 PM
deltas are cool
AIR SALLY's Avatar
Tehachapi ,CA.
Joined Apr 2006
20,002 Posts
Dan great build ....now can i ask a stupid Ques.....why did you make a mold of the ducts with the smooth side out? were you just making some hard tooling so you could lay up ducts smooth side in ?.
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Old Aug 05, 2009, 10:06 AM
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Daren's Avatar
Los Alamos, NM, USofA
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Howdy!

Looks great, Dan. Pretty soon it will be sheeting and sanding time. The tail stuff should go pretty quickly.

Daren
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Old Aug 06, 2009, 01:45 AM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
4,540 Posts
Quote:
Originally Posted by AIR SALLY
Dan great build ....now can i ask a stupid Ques.....why did you make a mold of the ducts with the smooth side out? were you just making some hard tooling so you could lay up ducts smooth side in ?.
Thanks.

Not a stupid question at all. Yes. Those are the molds that make the inlet ducts.

Dan
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Old Aug 06, 2009, 01:50 AM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
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Quote:
Originally Posted by Daren
Howdy!

Looks great, Dan. Pretty soon it will be sheeting and sanding time. The tail stuff should go pretty quickly.

Daren
Thanks, Daren. Yes, and the sheeting won't take too long.
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Old Aug 25, 2009, 01:00 AM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
4,540 Posts
It's been a couple of weeks since my last update. All of the basic construction is done.

I've buttoned up the fuselage. The next step is to sand everything flush, then glue on the nose cone and tail cone doublers, then round the corners.
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Old Aug 25, 2009, 08:05 AM
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Daren's Avatar
Los Alamos, NM, USofA
Joined Nov 2000
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Very nice, Dan. Now comes the fun part of shaping, final assembly and finishing. Then of course, the really fun part: FLYING!

Daren
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