The composite fabrication thread

[TDM]

I think this thread is long overdue. We can discuss here anything related to composite fabrication. We can discuss materials how they work proper way to use them and where to get them from. We can discuss fabrication techniques types of molding construction. If you have something that needs clarifying ask questions. I am sure that there are a lot of questions out there.
Lets stay in the modeling realm and not adventure in the aerospace autoclave stuff that no one has the money to invest in. Lets stay in the stuff we can actually do and make, and we can go very far.

[TDM]

1. Regular molded parts where you lay the material in a mold get it wet with epoxy let dry and use the part. This the thing we use when we make a cowl a shell or something like that. This is perhaps the first molded part most people do.
2. The hollow molded parts. Just as it suggests there are two parts molded separately that are joined together afterwards.
3. The solid core parts where there is a core that is laminated with some kind of outer skin.

[TDM]

Lets try to do this as a question and answer kind of thing. 

[TDM]

First answer. You mix epoxy with microbaloons some add a foaming agent to the epoxy (AKA splurge), then you put this stuff in a syringe or a triangular bag like the one used for icing the cake, and just like you ice the cake you put an bead of this splurge on the edge and close the mold. Wait a day then remove the part. This is a technique that is used on most if not all hollow molded wings. Splurge is a gap filler and can have all kinds of compositions depending on where and what function it has.
Splurge is epoxy mixed with fillers: microbaloons, cabosil, chopped fibers (glass or carbon balsa dust). The epoxy can be further modified by adding foaming agent, this is done after you mix the epoxy and helps it expand a little.
I use splurge for general closing purposes or to create hard points in the part. You can use it in sharp corners where the fiber doesn't want to lay flat. Each purpose uses a different cocktail of additives. You can go from light to heavy from hard to soft depending on the mixture.

[big ron]

TDM If you were planning on building a composite model and wanted to have a set of molds built what would be the best option? I kept up with your build of revolutions and am interested in this construction but don't have access to the machinery to create such molds.

[TDM]

Ron be more specific. Best option for you? You can always make the mold the old fashioned way balsa with glass to make a plug then make molds from that. Machining make things a lot more precise and for me it eliminated the plug and mold and left me with polishing the mold.

[big ron]

If I were looking to have a set of molds machined is there a source that you know of that could take a drawing of what I want and turn it into accurate molds like the ones you used? And if so what do you think would be the cost?

[Gerald Arana]

First answer. You mix epoxy with microbaloons some add a foaming agent to the epoxy (AKA splurge), then you put this stuff in a syringe or a triangular bag like the one used for icing the cake, and just like you ice the cake you put an bead of this splurge on the edge and close the mold. Wait a day then remove the part. This is a technique that is used on most if not all hollow molded wings. Splurge is a gap filler and can have all kinds of compositions depending on where and what function it has.
Splurge is epoxy mixed with fillers: microbaloons, cabosil, chopped fibers (glass or carbon balsa dust). The epoxy can be further modified by adding foaming agent, this is done after you mix the epoxy and helps it expand a little.
I use splurge for general closing purposes or to create hard points in the part. You can use it in sharp corners where the fiber doesn't want to lay flat. Each purpose uses a different cocktail of additives. You can go from light to heavy from hard to soft depending on the mixture.

TDM, you mentioned "chopped" fiber. Personally I prefer "milled" fiber. Chopped fiber is course and milled fiber is fine. I generally mix it to about the consistency of Vaseline and there are lots of uses for it.

Cheers, Jerry

PS: Looking forward to learning something new.

[TDM]

If I were looking to have a set of molds machined is there a source that you know of that could take a drawing of what I want and turn it into accurate molds like the ones you used? And if so what do you think would be the cost?

Usually expensive. You are better off buying your own machine and do that. 4-5000$ investment plus CAM and tools. A lot less expensive is to make your own plugs.

Gerald yes that is another type of filler. There are many types of fillers out there all depends on what the end goal is.

[Dave Hull]

MM,
If you bag your parts, you are going to need to use a few more items inside the bag. One of these is a drain. It gives a path for the excess resin to be pulled away from your part. We need to find you a diagram showing the setup.

Dave

[TDM]

I just realized I'm too dumb to know what to ask lol.

How many layers of 1.6 glass cloth would be needed to make a wing shell?

Motorman

Motorman you need to be more specific and give more details on what kind of wing construction (hollow mold vs solid core) is this the wing or the stab, and also what layout are you planing to have.

[TDM]

Ok gentleman here is a Utube channel where you can watch most of them to get a good start on understanding types of constructions and some layout ideas. This should add some 10000% to your current knowledge in making composite parts. At very least we can start to speak the same language. These videos are full of techniques used in composite model parts in particular for wings and tails.

https://www.youtube.com/channel/UCWaPcbCQVYwQQ-zrQ5jpv7A
This one in particular  will show a lot of stuff you canuse to make a cheap wing without breaking the bank. I find it great for first try and all applies to our wing construction too. Note he uses 60psi foam from home depot cut in 1mm thickness (for our purposes 1.5 or 2mm is better) You can skip the Disser but layout the glass at 45 degrees bias.

[Brent Williams]

For the time challenged, here's a 2 minute time lapse video of a similar method, minus the Disser arrangement.



Here is a longer, 20 min version with commentary by the same builder.

[pmackenzie]

You need a sandwich for each skin, with foam or balsa core.
Glass alone will either be too weak/flexible if it is light enough or too heavy if it is strong/stiff enough.

[Target]

Motorman-

As Pat said above, for a "hollow molded wing" for each skin, you need an outer shell layer, an inner core material, and an inner shell layer. The core spreads the two pieces apart, and this is what makes the assembly rigid.
So, you might use 1/2oz glass outermost, then 5.9oz carbon fabric inside of that, then a layer of 1/16" light balsa, then 1.6oz glass on the inside.
All of this would be vacuum bagged to the mold, which is pre-waxed, and usually sprayed with PVA, a release layer.
All this would be cured, then when cured, it would be trimmed.
Then this shell and the mating one would be joined, with a spar usually in the middle. The two molds are made to index and mate to each other. "Splooge" is the glue that bonds the top and bottom shells to one another, as well as the spar (which needs to be EVER SO SLIGHTLY UNDERSIZED, like .005-.0010", preferably). After it is all cured, the molds are taken apart and the molded piece extracted.

^^^^^^^^^^^^^ This is completely different from making a vacuum bagged foam wing!^^^^^^^^^^^^^

So, for a bagged wing, think of it as using fabric and epoxy in place of balsa as a skin. Normally on a bagged wing, 100% of the foam core stays there between the composite skins. On a sailplane, with a 7-9% thick wing section, it is not a lot of weight, and it adds to the structure.
On a stunt plane, especially since you would want and need to use the 15-40psi foam, this WILL add to the weight with a larger penalty.

It is my theory that someone could CORE the foam AFTER each wing half is vacuum bagged, and remove some excess weight. I think that with a simple spar running the full span of the wing, or maybe 1/2 span, you would have enough strength to fly stunt with, and hopefully it should be light enough.
Remember that you would NOT have to add a finish to the outside most likely, except at the leading edge. So you are going to save a lot of time there and weight on finishing, (I think).
The snag points are for hinging flaps, and a few other details like landing gear and the bellcrank. The last two are probably easily solved already by doing what Bob Hunt has done for foam wings, with maybe some slight mods for the materials being thinner.
A wing with no flaps and fuse mounted landing gear makes the idea of a vacuum bagged wing very attractive to me. I would just need to make some foam cutting templates for stunt wings. And tails.
I hope this helps with the confusion. Hollow molded and "stressed skin, solid core, vacuum bagged wings are two completely different animals, although they both use epoxy and fiber fabrics, and a vacuum bag.

The hollow molded construction has many advantages- The outside surface retains the exact same coordinates no mater how light or heavy you build the inside. Every wing you build form molds SHOULD fly the same. It is likely also lighter. But the process can cost much more, and be more complex.

A vacuum bagged foam core wing starts with a hot wire or CNC cut foam core, and since layers are added to that, the more material you put on it, the dimensions will change slightly.
I don't view this as a big deal, as ANY BALSA SKINNED FOAM CORE WING has the same issues since you sand it, etc., there will be variences anyway. Since the "skin" of a composite vacuum bagged wing is thinner than the balsa skinned wing, I can only assume that the accuracy should be improved....


Kind regards,
Target

[Target]

MM,
If you bag your parts, you are going to need to use a few more items inside the bag. One of these is a drain. It gives a path for the excess resin to be pulled away from your part. We need to find you a diagram showing the setup.

Dave

Dave-
I have never used a drain for vacuum bagging any parts. This is where the resin control comes into play. More resin than is needed is flooding the fiber layup, then excess is removed. Then it is put under vacuum. Inside the vacuum bag there is the foam core, the skin layup on a Mylar sheet carrier, then a poly sheet with a breather cloth. All this is slid into the vacuum bag, which is then sealed and vacuum applied. Typically the part is then put back into the foam shucks and weighted slightly, to maintain no twist in the parts.
Excess resin, which should be minimal, will often bleed into the outer breather cloth (breather cloth is necessary to get the vacuum to the far end of the bag, away from where the vacuum line is).

There IS a method, you may be thinking of, that is called infusion bagging or something like that. The layup is done dry, and very very low viscosity resin introduced at the far end away from the vacuum source, and the epoxy is "drawn through" the dry fabric. To me, this is way overly complicated, and unnecessary for our needs. The static wetting out of the fabric on Mylars, then vacuumed down works great as it is.

[TDM]

Motorman Target is right you need a core material in the layout.

If you want a hollow molded wing on the cheap do this for a layout. 1.6 glass outer layer at 45 bias 1.5-2mm (1/16-5/64) foam sheet (high load 60) and a layer of 1.6 glass on the inside. For spar use 100g/M3 carbon unidirectional from Easy Composites they sell it or under 20$ a square meter. Put a vertical wall about the same position where you put a D-box sheer webs 6mm foam is enough and add some 2-3 ribs inside mainly to support the rear skin. You night want to add some patches to reinforce wherever you need extra strength.
Another foam option is the home depot foam which is about 40PSI strangth and if you use that you need to cut it thicker at about 2-3mm (3/32-1/8in).

Actually anyone that wants to adventure in this direction I recommend they use exactly this layout (for making wings) for the first tries. If you have a fuselage with tubular shape use this same thing again.

Tips recomendations
Stay away from bleeder cloth drain stuff that removes excess epoxy. If you look carefully at the video I recomened when Tom is weting the inner layer of fiberglass the one that was attached with 3M77 to the blue foam sheet look at what he does with that roller after he adds resin to it, he rolls it on the surface of the table before he puts it on the glass, he is using a dry roller, also he is using indirect light to gauge  when it is wet. Every detail is important.
Stay away from trimming excess material to parting line after bagging, it will damage the mold and you do not need surprises like that. Instead cut the core material just short of the parting line and use splurge to fill the remaining gap when you close the mold.

Another recommendation is to use thinner core material on smaller parts. you can scale it down some.

http://www.easycomposites.co.uk/#!/
http://www.easycomposites.co.uk/#!/fabric-and-reinforcement/carbon-fibre-reinforcement/unidirectional-carbon-fibre/carbon-fibre-unidirectional-100g.html this stuff is great.

perfect machine to make cores and flat sheets of foam.

[TDM]

There IS a method, you may be thinking of, that is called infusion bagging or something like that. The layup is done dry, and very very low viscosity resin introduced at the far end away from the vacuum source, and the epoxy is "drawn through" the dry fabric. To me, this is way overly complicated, and unnecessary for our needs. The static wetting out of the fabric on Mylars, then vacuumed down works great as it is.

For our purposes this method would add a lot of weight because it is not only filling the fiber with resin it is also filing the pores in the core material with resin which add perhaps even doubles the amount of resin in a part.
Remember fiber gives strength resin holds it in shape and goes for the ride. And the strength is in the direction in which the fibers are laid. 

[Target]

I only brought it up because Dave mentioned using a drain.

[Curare]

Vacuum infusion is used in boat construction pretty regularlly these days, but in order to do that they have some form of epoxy flow medium to allow the epoxy to travel though the layup, Lantor Soric is good one, but it's completely unfit for modelling applications.

I know that some of the guys who were doing DLG moulds here were doing corian moulds with drainage channels around the periphery of the part (in that case the wing) , these collect any excess epoxy, and make for chambers to better allow the vaccum bag to work.

Is that what you're talking about Dave?

[TDM]

The drain channel in DLG wing molds is not really used to drain. It is used more for splurge exit during closing process and to have a witness mark to where to trim the wing TE.
The drain in the DLG fuselage mold is used to drain the excess resin the bladder pushes out.

For our purposes we don't really benefit from a drain channel. But on the other hand it doesn't hurt either. It is easier to add it at the beginning than try to put it in later.

[Double Deuce]

Can anyone suggest an adhesive to use when trying to assemble primarily, carbon tow, before moving to the mold and applying epoxy. I need an adhesive that does not cause issues with the epoxy (815)

Read some data years ago for using spray contact cement to cloth and sticking the cloth to the item to be covered. Looked doable except when I discussed it with a buddy who did repairs to full size boats. He said it came loose every time. My thoughts on this was the glue was contaminating the resin.

Items being made are Speed propellers and at the revs turned, I do not want a failure. What I am trying to do is to prevent slippage of the tow and the ensuing balance problems. Machined aluminum mold being used does not leave much excess material for balancing without getting into weakening the blade.

Uni-directional ribbon is stuck together with something that obviously does not contaminate resin. I may be stepping on propitiatory issues here but does anyone know what it is?

Help appreciated. As things have turned out, machining the mold was the easy part.

Double Deuce

[Target]

Can you use uni instead? That would be easier I would think.
Usually when tow is used, it is strung around two nails on a board to keep the fibers straight. That might give you some ideas.
If not, what about having the tow inside a woven cf sock?

[phil c]

There is a simple technique for making cowls, wheel pants, and other smaller parts.  It could even be adapted to a fuselage.

Virginia and Ray Randall won the carrier NATS in 1959 and 1960 flying a Grumman Tigercat model.  Ray developed a very simple method of molding the fiberglass cowls.  The article American Aircraft Modeler can still be found online.

Make an internal model of the cowl.  It can be turned from wood, built up with plaster, or otherwise sculpted with something hard.    The only thing necessary is that the rear dimensions be larger than the front and there are no undercuts.

Mount the mold from the rear on a sturdy stick of wood.  Clamp it in a vice with the front pointing up.  Take a round balloon and blow it up.  Position it over the part and push down while slowly releasing the air.  You need to push the partly inflated balloon past the part before letting out all the air so the balloon tucks under the part and holds itself in place.

Build up several layers of 2-3oz glass cloth, saturated with epoxy on the mold.  The glass has to go around onto the front face.  Mop up and excess epoxy.

Blow up another  balloon with a piece of string hanging down inside.  Again force it down  over the glassed mold.  Push the balloon well past the part and slowly let the air out.

I've made several dozen parts this way.  It was even possible to mold cheek cowels in place  by taping them in place on the basic part.  Once the part is done and the outer ballon removed the main part can be removed and the cheek cowels will come out with the inner balloon.

Obviously the balloons aren't usually reusable, but they're cheap party balloons.