Semi-mold bagging - RC Groups
Shop our Airplanes Products Drone Products Sales
Thread Tools
Aug 10, 2011, 09:09 AM
Stealth Plane Works
Anker's Avatar
Build Log

Semi-mold bagging


With conventional bagging techniques there's always a challenge of ensuring that the panel comes out straight, with the correct twist and a straight trailing edge.

This becomes especially challenging when the panel has a dihedral break.

For a long time I have mulled over an approach that will eliminate these problems and produce a higher quality wing, but still be practical for the hobbyist.

The idea is to create a half mold for the top surface and do the top layup in the mold, place the core on the mold, do the bottom surface layup conventionally on a Mylar and then lay the Mylar and panel bottom layup on top. The breather and bag are then laid around the whole shebang so the vacuum will press the layup into the mold. This should produce a perfect top surface (assuming the mold is perfect) and a typical bagged bottom surface. If the mold is straight and solid the wing will come out straight, with the correct twist and a perfect trailing edge, every time.

My main issue has been how to build the mold. I do have an offer to machine an aluminum mold, but even with free machining, the slab is expensive and the mold is really heavy. Cabosil would be inexpensive, but fragile and heavy, so I have decided to create a composite mold with a foam cores attached to a 1/2 inch piece of fiber board.

I decided to start with Supra wing tips and cut a set of top beds with 1/2 inch lead-in on both the leading and trailing edges. I then epoxied the two half panel beds to a fiber board after cutting the board to the shape of the beds. After that I sanded any imperfections at the joint away.

The next step was to lay up a layer of Kevlar and carbon cloth on the mold. I chose this cloth just because I have tons of Kevlar and some heavy carbon that I don't have any other use for. I did the layup on a thin sheet of Mylar and then placed the completed layup on the mold, and put everything in a bag and then into the heat box under vacuum.

My only worry is whether the transition from the wing surface to the lead-in at the leading edge will come out right. The Mylar is thin, so there's a good chance it will. I'll know in a few hours.

As I write this I am waiting for the layup to cure.

I have high expectations for this and will use this thread to advise you of further developments.

If the tip molds come out well I'll create a center panel mold with 5 degrees of dihedral. If that works I'll be immensely pleased.

Anker
Sign up now
to remove ads between posts
Aug 10, 2011, 09:41 AM
Registered User
Very interesting, pictures please!
Aug 10, 2011, 10:52 AM
Stealth Plane Works
Anker's Avatar

Pictures


Here are some pictures before bagging the mold:











I did pull the first one out of the bag and it had some flaws. One flaw was some dry spots that I have filled and will have to sand down. Another is that the Mylar didn't conform quite as well as I had hoped, so for the next one I used extremely thin Mylar. Finally, there are some shallow waves, so before the mold gets used I need to sand it and polish it. But its usable, and I think the next one will come out better.
Aug 10, 2011, 11:08 AM
Registered User
Cool, keep us up on how it goes.
Aug 10, 2011, 11:48 AM
Deniable plausibility
Shedofdread's Avatar
No reason why it shouldn't work - those Aeromod planes work very well. The key will be to make sure there isn't any air / voids between the core and the moulded, upper surface.

Very best of luck and I'll certainly be watching with interest.
Aug 10, 2011, 12:41 PM
Registered User
Ward Hagaman's Avatar
Very cool! Thanks for sharing!
Aug 10, 2011, 01:00 PM
Registered User
Anker where exactly did the mylar not conform on the first trial - near the leading edge ?
Aug 10, 2011, 01:50 PM
Stealth Plane Works
Anker's Avatar

Out of the bag






The second was about the same as the first.

I know why I get the dry spots now. Frekote causes Epoxy to bead and unless you have a very light glass layer on the outside the Epoxy will show these dry spots.

You can see some shiny spots where I dripped Epoxy on the dry spots. When the Epoxy is completely cures, tomorrow, I'll sand the surface smooth. I'll probably sand down to the cloth in a few places. I think its going to be a few says of sanding and filling and finally applying a finish coat.

But it will work, it just isn't as simple as I had hoped.

Anker
Aug 10, 2011, 01:51 PM
Stealth Plane Works
Anker's Avatar
Quote:
Originally Posted by wim5150
Anker where exactly did the mylar not conform on the first trial - near the leading edge ?
Yes, at the leading edge it goes through a 90 degree turn.

Anker
Aug 11, 2011, 03:23 PM
Scott Zastoupil
szastoupil's Avatar
Anker,

This is going to be very interesting. I'm curious about the Foamcutter setup to create an accurate bed. Since the software is written to compensate for to create an accurate core the kerf exists in the bed cutway, but to create an accurate bed the thicknesses due to kerf and skin would need to be pushed into the core.

So in order to create the proper bed size, did you just put the software's calculated kerf values into the skin thickness text boxes? The mold kevlar and carbon thickness would also need to be accounted for in this value.

A flying buddy has been brainstorming about foam molds so I've had some time to think of how I can create something for him using the CNC cutter. Inquiring minds will want to know how the leading edges will work out too.

Scott
Aug 12, 2011, 08:05 AM
Stealth Plane Works
Anker's Avatar

Kerfs


Quote:
Originally Posted by szastoupil
Anker,

This is going to be very interesting. I'm curious about the Foamcutter setup to create an accurate bed. Since the software is written to compensate for to create an accurate core the kerf exists in the bed cutway, but to create an accurate bed the thicknesses due to kerf and skin would need to be pushed into the core.

So in order to create the proper bed size, did you just put the software's calculated kerf values into the skin thickness text boxes? The mold kevlar and carbon thickness would also need to be accounted for in this value.

A flying buddy has been brainstorming about foam molds so I've had some time to think of how I can create something for him using the CNC cutter. Inquiring minds will want to know how the leading edges will work out too.

Scott
Great points.

At this stage I haven't done kerf compensation. I have decided to use a Mylar sheet between the layup and the mold. This will eliminate a bit of the kerf error. If it works well I'll modify my foam cutting program so I can do a negative kerf and get a perfect mold. For now I think its close enough to be a big improvement as is.

I tore the first molds apart and started all over again yesterday. I decided that bagging the molds wasn't optimal and this time I'll brush the cloth on with laminating epoxy, just like it's done when covering model fuses with glass. I'll also eliminate the 90 degree turns over the edges of the mold. I think I can get the cloth to conform better with the mold that way. That will also eliminate the dry spots I was getting before.

Anker
Aug 12, 2011, 12:13 PM
Stealth Plane Works
Anker's Avatar

Much better






This is much better. The cloth doesn't round any sharp corners and it conforms exactly to the leading edge. I used thinned epoxy and brushed it on. When it has set I'll cut the excess off with a razor blade, then clean off the cloth with a light sanding, add another layer of cloth, sand that and finally add a strip on the leading edge lead-in. The last step is not necessary, but I prefer to protect is and make sure it releases if any epoxy leaks on to it when bagging.

Depending on how neat the surface turns out I may finish off with a coat of automotive filling primer and wet sand it to a smooth finish.

Anker
Aug 14, 2011, 10:20 PM
More Pile-it than Pilot
mdennis's Avatar
Anker,
Great project. Trial and error inventing can be frustrating, but it is very rewarding when you finally get it to work.

I am a little confused by your attempt to glass the mold. Are you doing this to reinforce the mold surface or to produce a top skin for the wing?
Aug 15, 2011, 06:43 AM
Stealth Plane Works
Anker's Avatar
Quote:
Originally Posted by mdennis
Anker,
Great project. Trial and error inventing can be frustrating, but it is very rewarding when you finally get it to work.

I am a little confused by your attempt to glass the mold. Are you doing this to reinforce the mold surface or to produce a top skin for the wing?
I am doing it to reinforce the mold surface for durability. For now I plan to use a Mylar sheet between the mold and the layup. Down the line I may eventually decide to do the layup directly in the mold.

I now have two layers in one tip mold and a good foam bed for the second tip. The second time around the inner and outer tip mold surfaces didn't line up properly and after sleeping on it I deceided to rip out the outer tip bed and re-cut a better one. That one lines up perfectly and is now ready for glassing.

Tonight I my start on the center panel mold. This one has 5 degrees of dihedral which I may reduce by a small amount. My program knows how to cut with dihedral, so as long as it doesn't exceed the thickness of my foam it should be easy. (I have said that before and discovered otherwise).

Anker
Aug 19, 2011, 05:37 PM
Stealth Plane Works
Anker's Avatar

Going well


I bought some 2.3 oz fiberglass cloth. It was getting expensive using Kevlar.

I have the second tip done and cut the beds for the center panel with dihedral. I decided to limit the dihedral to what the 2" foam could accommodate, which is 4 degrees for a regular Supra.

One of the F3J juniors wants my scaled-up Supra wing, so I am shifting to building molds for that. The dihedral for the Super Supra is just under 4 degrees, still in the acceptable range.

Anker


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Repairing a minor delam without vac bagging? DeuceTrinal Hand Launch 20 Dec 16, 2013 07:01 AM
Discussion wing joining with dihedral with vacuum bagging Blackrat Composites Fabrication 5 Jul 13, 2011 12:50 AM
Help! Vacuum bagging machine Perth Kaffine Australia 136 Jul 08, 2011 09:02 PM
Discussion Constructing a wing mold for vac bagging Flynn Composites Fabrication 13 Oct 24, 2007 03:52 PM
Help! Wrinkles while bagging into a mold BillO Composites Fabrication 21 Feb 21, 2006 06:20 PM