Plastic layers between foam ribs and carbon for removing foam for extra lightness

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MotoFairing

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Jul 9, 2020
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I just came up with an idea for removing the foam ribs in a composite mold for increased lightness.

Step
1) Layup 1. Carbon fiber(s) layer (vacuum bag).
2) Plastic layer.
3) Foam rib, clay, whatever needed to get the desired shape.
4) Plastic layer.
5) Layup 2. Carbon fiber(s) layer (vacuum bag).
6) Open plastic section, remove rib material.
7) Glue back together for completed part.

This method could also be used for creating two oddly shaped parts that closely fit together.

I'll be using this method to reinforce a body molded seat and for attaching components to that seat.

I'm sure I'm not the first to come up with this idea.

Does anyone have more information on this method or similar?
 

wsimpso1

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I hate to sound like some sort of grammar police, but we have an entire collection of words used in composites fabrication and airplane construction already established and in common use. Using the established nomenclature is not esoteric and meant to exclude - using established jargon allows us to talk to one another in fairly compact form and avoid confusion. So, let's get some terms squared away:
  • Mold usually means a tool that does not go in the part - molds can be heavy, they do not fly, and reducing weight of them rarely has much value;
  • Rib usually is structural to the parts, and tends to be perpendicular to main molded part(s) - It sounds like you are talking about a temporary core between laminated parts;
  • Plastic? The whole thing is plastic. What kind of plastic are you referring to? It sounds like you are talking about a film that the resin will neither attack nor bond to, allowing the various laminations to be separated later.
Let's see if I understand what I think you are proposing - you want to laminate one piece on the mold, put in a release ply, then an expendable core, then another release ply, then mold a second piece over all that, then separate them all and bond the two laminated pieces together. Is that what you had in mind?

If yes, then I will point out that usually the layer of adhesive to hold it all together will weigh more than if the assembly had been laminated in one, two , or even three vacuum bag sessions with an appropriate core left in the part. In most of our parts, the lamina on each side of the part are too thin and flexible to stand alone - they need the back-up of the core. There are places where the laminate against the Outer Mold Line (OML) and the liner are both sturdy by themselves and are bonded together. They may be used in heavier structures, like in large boats, but they are rare in airplanes, race cars, and racing sailboats.

Billski
 

MotoFairing

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I hate to sound like some sort of grammar police, but we have an entire collection of words used in composites fabrication and airplane construction already established and in common use. Using the established nomenclature is not esoteric and meant to exclude - using established jargon allows us to talk to one another in fairly compact form and avoid confusion. So, let's get some terms squared away:
  • Mold usually means a tool that does not go in the part - molds can be heavy, they do not fly, and reducing weight of them rarely has much value;
  • Rib usually is structural to the parts, and tends to be perpendicular to main molded part(s) - It sounds like you are talking about a temporary core between laminated parts;
  • Plastic? The whole thing is plastic. What kind of plastic are you referring to? It sounds like you are talking about a film that the resin will neither attack nor bond to, allowing the various laminations to be separated later.
Let's see if I understand what I think you are proposing - you want to laminate one piece on the mold, put in a release ply, then an expendable core, then another release ply, then mold a second piece over all that, then separate them all and bond the two laminated pieces together. Is that what you had in mind?

If yes, then I will point out that usually the layer of adhesive to hold it all together will weigh more than if the assembly had been laminated in one, two , or even three vacuum bag sessions with an appropriate core left in the part. In most of our parts, the lamina on each side of the part are too thin and flexible to stand alone - they need the back-up of the core. There are places where the laminate against the Outer Mold Line (OML) and the liner are both sturdy by themselves and are bonded together. They may be used in heavier structures, like in large boats, but they are rare in airplanes, race cars, and racing sailboats.

Billski
Oh that's very interesting, yes you have interpreted my post according to my intentions.

Darn I thought I was onto something haha.

Thank you for your reply.
 

wsimpso1

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I am jaded. 37 years of doing engineering for a living... My apologies for smugness. Having been on patent committees in two major companies and holding patents myself, I can tell you that "new" is kind of hard to come by. For instance, we have had competing outfits designing powered airplanes for nearly 120 years, and composite materials have been applied to aviation for 80 years, with thousands of years of working with wood (Nature's Composite) before that. During that time literally millions of smart folks from all around the world have done a lot of thinking (and patenting) and building on these topics. While something "new" is certainly possible, I would bet that if it can be thought up by anyone new to the field, the odds are really good that somebody else has already thought it through too.

Then, once we have a scheme for building, and a part that will do the job, I like to apply Billski's first rule of airplane design: WEIGHT IS THE ENEMY. Compare your method to all of the other ways we might do what is needed, rank them by weight, and work from the light end of the scale.

Sometimes a cored skin is lightest, but we have a paradox where a hollow wing (cored skins, ribs, spars) weighs more than massive core wing (spar, skin, and a hot wire cut solid core) until the chord reaches somewhere around 2 meters. In composites, adhesives and hard points tend to be heavy, so we tend to use as little glue and as few bolted attachments as we can. Conversely, metal skinned airplanes are literally full of holes that are then plugged with rivets to join parts to make the structures.

Billski
 

Burgher

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I am jaded. 37 years of doing engineering for a living... My apologies for smugness. Having been on patent committees in two major companies and holding patents myself, I can tell you that "new" is kind of hard to come by. For instance, we have had competing outfits designing powered airplanes for nearly 120 years, and composite materials have been applied to aviation for 80 years, with thousands of years of working with wood (Nature's Composite) before that. During that time literally millions of smart folks from all around the world have done a lot of thinking (and patenting) and building on these topics. While something "new" is certainly possible, I would bet that if it can be thought up by anyone new to the field, the odds are really good that somebody else has already thought it through too.

Then, once we have a scheme for building, and a part that will do the job, I like to apply Billski's first rule of airplane design: WEIGHT IS THE ENEMY. Compare your method to all of the other ways we might do what is needed, rank them by weight, and work from the light end of the scale.

Sometimes a cored skin is lightest, but we have a paradox where a hollow wing (cored skins, ribs, spars) weighs more than massive core wing (spar, skin, and a hot wire cut solid core) until the chord reaches somewhere around 2 meters. In composites, adhesives and hard points tend to be heavy, so we tend to use as little glue and as few bolted attachments as we can. Conversely, metal skinned airplanes are literally full of holes that are then plugged with rivets to join parts to make the structures.

Billski
Could'nt agree moe!!! when you factor in typing errors😂 and misunderstandings it make me reluctant to post. Just an A&P who worked the big tin along w/GA aircraft but the endless posts disagreeing over minutia is "probably" due to those very new to aviation without a clue and I don't mean that in a bad way. My typing sucks along with gramar and spelling.....😛 . That said I do read a bit of posts and have learned some pretty weird stuff along the way!! Yeah I dig rivets too!!
 

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Burgher

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First class view from MoJo whilst powered by the Military standard 84 incher. When she flys again she'll have the 1/2 VW in her.............
 

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Lendo

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Something told to me by Steve Rahm (Vision Designer), is that the Core of any laminate supports the Skin and helps the skin in sharing loads and stresses between the skins. This would be true of a Solid wing Foam Core, or Cored skins. Also the substrate helps support the skin or the skin relies on the substrate for support to maintain it's strength. That is to say if the skin is damaged, it is unlikely to maintain it's strength.
Often light skins may be strong enough but lack 'impact resistance', it's a real catch 22 as you can't have one without the other, sometimes you can have very light foam core but need extra thickness skins. I don't know where the trade-off is, but maybe different in different applications and priorities.
George
 

MotoFairing

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Upon further research and the replies here I see I won't be able to use empty space in an attempt to reinforce the part.
 

MotoFairing

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Is there a formula for balancing the carbon fiber skin with the core material thickness for optimal stiffness to weight ratio?

Or is it still an iterative process?
 

Lendo

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Short answer- NO! Not that I know of anyway!
However because Carbon is very stiff and Core thickness increases Stiffness, you could reduce the core thickness to make take advantage of the Carbon stiffness. For instance A Glass core would be .5" to keep the shape in manufacturing, but strength wise in could be as low as .25". That's if you could keep it's shape through post-cure.
I have seen photos of a European build with very thin cores when using Carbon, however I don't know if I could personally go below .25" without the engineering knowledge and or experience they obviously had. Maybe in the Wings you could go as low as 3 to 4MM, but just don't know.
George
 

Riggerrob

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Dear Motofairing,
Your concept reminds me of my earlier ramblings on a foam and fiberglass seaplane hull concept.
Basically, I wanted to use Rutan's mold-less construction methods with vacuum-infused resin. The problem is that vacuum would collapse a hollow hull, so I thought about hot-wiring a bunch of cuts in the huge - hull shaped - block. Stack all the smaller blocks together and use them to support the outer shell while it is glassed and infused.
After outer resin has cured, remove the inner, sacrificial foam blocks.
I still wanted to retain some foam to provide emergency floatation.
 

wsimpso1

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Is there a formula for balancing the carbon fiber skin with the core material thickness for optimal stiffness to weight ratio?

Or is it still an iterative process?
Maintaining face sheet materials and thickness, bending (and torsional) stiffness and strength keep going up as you make the core thicker.

If you start with a certain bending stiffness and/or bending strength (or torsional stiffness and/or strength), and it takes a certain thickness of face sheet material with no core, adding and then increasing core reduces the needed face sheet thickness to make strength and/or thickness.

In most design areas, we have minimum gage for materials to stand handling, building, etc. Sometimes min gage is set by buckling stiffness, sometimes strength, but almost always, it is a matter of making the material thicker than is strictly needed for carrying primary loads. It might fly fine, but if you crumple the skin pushing it into the hangar, it is still a bad deal. So, it is with composite face sheets.

Elsewhere we have talked about minimum gages on composite facesheets backed up by various foams:
  • 21 oz/yd^2 of glass-epoxy laminate on the outside of 2 lb/ft^3 styrene and polyurethane foams are widely used;
  • 12 oz/yd^2 graphite-epoxy is common over 4.5-6 lb/ft^3 foams;
  • 6 oz/yd^2 graphite-epoxy is common over PVC foams in the sailplane world.
These and others seem to be practical minimums for face sheet thickness. YMMV. Stiffness and sturdiness of the underlying foam and stiffness of the facesheet both play big roles on how thin you can make face sheets and not face a fragile structure. If you keep making the core thicker and the face sheets thinner, eventually, your airplane becomes too fragile, and you back up a step or two.

Billski
 
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