Saturday, May 2, 2009

This blog is dedicated to the advancement of a new type of honeycomb core which we have named "Hexaflex". As you can guess from the name, this core is based on a hexagonal lattice framework, and exhibits distinct advantages over the more traditional kinds of industrial honeycomb.  

Up until relatively recently, we had found it a challenge to be able to manufacture Hexaflex.  With the more recent developments in 3D Printing, what was probably not even possible before, has now been produced as an attainable, and multi-functional, construction element, that shows us a viable, alternate method of manufacturing into the next millennia. 

Scan to the bottom of this blog for recent updates.

Friday, May 1, 2009


The rising demand for new materials with higher strength to weight ratios has created a dramatic growth in sandwich composite technology. Sandwich construction employs a lightweight core that has a flexural strength and flexural modulus far exceeding that of the skin laminates alone. The most common type of core material is honeycomb, which is used primarily in the aerospace industry.
The main disadvantage of honeycomb type cores used in the Aircraft and Aerospace industries is that of delamination which can cause a catastrophic failure of the vehicle. This is caused primarily by the failure of the epoxy adhesives to maintain a bond between the facing skins and the core because of the very small bonding area that honeycomb cores edges offer.
This is further exacerbated by the fact that honeycomb type cores create pockets of trapped air within the closed cells of the core when the skins are attached. The air pressure experienced at high altitudes is much lower than the trapped air within the cells with the result that the skin is pushed away from the inner core by the air pressure. Ingress of water into already partially delaminated cells at high altitude freeze into ice particles which expand and force the skin to separate from the core. Eventually after many cyclic operations the skin will delaminate. Additionally, lightning strikes can cause entrapped moisture within the core to immediately turn to steam with catastrophic results to the integral strength of the panels.
The Aerospace Industry remains the greatest consumer of honeycomb materials, whether for civil aircraft, military jets, helicopters, aero-engines or the newer space satellite and launchers.
The Director of NASA’s Marshall Space Flight Center once stated that “our (composite) technology has not yet advanced to the point that we can successfully develop a new reusable launch vehicle that substantially improves safety, reliability and affordability.” He was referring to the failure of the composite fuel tank panels of the NASA X-33 Reusable Space Vehicle due in part to honeycomb delamination.


Regular Honeycomb is a series of cells, nested together similar in appearance to the cross-sectional slice of a beehive. In its expanded form, honeycomb is 90-99 percent open space. Honeycomb is lightweight and has good impact resistance. It offers the best strength to weight ratio of all the core materials. (The blue lines indicate bonding area and the red lines are the glue joints between the ribbons)


In this 3D view of Hexaflex, one can very quickly grasp the fact, purely by observation, that the hexagons have a much larger surface area available for bonding to the face sheets in comparison to the surface area of the edges available in conventional honeycomb. The blue color denotes bonding area.
Hexaflex core design has two different surface architectures. the side which is shown above, which we think of as the outside surface, and the other, inside surface, below, which as you can see, has only half as much bonding area as the outside, but none-the-less, still has a large bonding area in comparison to conventional honeycomb.

Each cell in a honeycomb sandwich is an airtight vessel. When heated, the air in each expands, increasing the pressure. If the pressure gets too high, the film adhesive bond may fail, initiating a delamination.
Hexaflex overcomes any possibility of this occurring because it is an open fast-venting core design that prevents any pressure differentials from building up within its geometry.
Some adhesives give off gases or solvent vapors during cure, which can interact with resin systems in some non-metallic cores, or with the node adhesive in some metallic honeycombs. The entire bonding process must be checked to ensure that no reduction in mechanical bonding properties has occurred.
One could quickly purge the assembled Hexaflex sandwich panel with gases or liquids by virtue of the ventways that inherently run through the core design. (see red arrows)
These ventways could also be utilized for service runs for electrical pneumatic or hydraulic lines.

Compression and shear forces can be tailored to suit the application by placing foam metal hexagonal cross-sectioned inserts into the blind hexagonal cells on the one face of the core material.
Hexaflex conforms naturally to compound curvature with its cells normal to the face surface, without the need for curving, rolling or heat forming operations. It does not suffer from cell wall damage, columnar failure, node separation or distortion of the hexagonal cells when subjected to compound curvature.
Honeycomb cores are heavier due to the fact that they consist of multiple ribbons of core material glued together (see red lines below) requiring one of the six cell walls of every cell in a regular honeycomb core to be double thickness.
Hexaflex core has no glue, is a single thickness throughout and is formed from a single sheet. In the event of excessive forces the core will demonstrate superior structural integrity.

Hexaflex core material can be edge-lapped upon itself allowing hexaflex panels to be joined together with superior seam strength. Hexaflex core material can be stacked upon itself to allow efficient storage.

Nested partially deployed configuration


These next 2 illustrations show the steps taken to form Hexaflex from it's initial flat sheet to the final folded configuration. It is easy to imagine that Hexaflex does not have to be formed out of metal.  The intended use will to a large degree influence the choice of materials used in it's fabrication. 


aerospace, automotive, defense, test facilities, industrial machines, marine, nuclear, rail
Thermal Protection System, satellite launch vehicles, space shuttle, adaptive structures
crash test barriers, door and roof panels, fairings, heat exchange panels, double-skinned exhaust manifolds, flexible fuel tank, adaptive structures
flaps, ailerons, struts, radomes, doors, rudders, flooring, cowlings, adaptive structures
wall panel, false ceilings, roof panels, flexible tubular structures, Interstitial Space
hulls, decks, bulkheads, stringers, bunks, covers, hatches
double-skinned oil tanks, oil pipelines, flexible body armor, soil stabilization mat, derigibles, athletic shoes, solar energy panels, sound attenuation panels
doors, floors, energy absorbers/bumpers, ceilings partitions
Snowboards, Wake Boards, surfboards, motorcycles
Nanotechnology, capsids, intercellular matrix, medical stents, surgical mesh


To read more about Hexaflex and the journey which brought us to where we are today, go to the enclosed link where you will find other related structures and additional details and photos of the Hexaflex chronology.





As stated in previous posts, we envisage this core to be made out of whatever material is suited to the end use.  It is therefore apparent that there can be different ways of manufacture depending on that use.  For example, a metal sheet would be cut out into the required pattern, and then after creasing and scoring, would be folded into the required core configuration.  Similarly, paper or cardboard would be diecut and folded.  Another method would be to chop-spray a material over a prepared form. Another method which occurred to us more recently, is that of vacuum forming, and as the photos indicate, we set about making a rig that would enable us to prove the concept.  The picture at left shows a single formed panel, approximately 12" in diameter and 1" core depth. and pictures 2 and 3 show how the panels edge-lap together to form required shapes.

Utilizing the properties of various materials including Kevlar or Aramid fiber, or even recyclables, it would be possible to create cores with a variety of different characteristics such as added strength and heat resistance.  


Due to the unfortunate circumstance of inventor Bob Burdon being diagnosed with an Inguinal Hernia, we realized that perhaps Hexaflex could be used to create a hernia mesh.  

The use of mesh has become essential in the repair of all hernias. To move forward into a new era of hernia mesh prosthetics, a panel of nine experts in hernia repair and experimental mesh evaluation agreed that new technologies and novel approaches must be investigated and designed. We propose a new concept in the design & manufacturing of a prosthetic latticework for inguinal, ventral or incisional hernia repair.

The 'smooth' side, having a small pore size, is placed adjacent to the bowel and resists tissue attachment.

The unique geometry of the lattice allows it to stretch in more than one direction and then return to its original shape. Existing hernia meshes are made of relatively stiff and inelastic material.  We strongly believes that these characteristics may be a contributing factor for hernia recurrence and patient discomfort.

The proposed lattice easily assumes the conformity of the abdominal wall musculature anatomy and thus improves the long term comfort and well-being of the patient.

The 'rough' side, with a more open pore size, is next to the tissues that surround the bowel where tissue incorporation is an advantage. Lattice cell size of 4mm (5/32nds) and thickness of 2mm (5/64ths).  Lattice width of 150mm (6”).

The method of manufacture of this surgical lattice uses 3D printing technology. 
The basic materials are:
  • ePTFE. (expanded polytetrafluoroethylene ) The use of modified ePTFE surface in hernia repair enables early tissue attachment, reduces adhesions, and could reduce the incidence of recurrences. This would be the first layer that is printed (smooth side down)
  • Polypropylene. This material has been used for the past 20 years because of its stability, strength, inertness and handling qualities. Polypropylene is overprinted on the PTFE layer and provides the basic structure of the lattice.
  • Collagen. A final layer of collagen is printed to encourage speedy host tissue incorporation into the latticework.

Potential attributes of lattice.
  • May result in the permanent repair of the abdominal wall, to reinforce and replace tissue for long-term stabilization of the abdominal wall.
  • Ingrowth characteristics that mimic normal tissue healing. May stimulate adequate fibroblastic activity for optimum incorporation into the tissues. May prevent adhesions. The ePTFE  protects the edge of the lattice minimizing tissue attachment to the material.
  • Strong.  May provide sufficient biomechanical strength to meet physiological requirements in order to permanently protect the fascial defect.
  • Pliable. It has elasticity in more than one dimension, allowing it to stretch in more than one direction and then return to its original shape. Easily assumes the conformity of the abdominal wall musculature anatomy
  • Handling characteristics should be amenable to laparoscopic instruments.
  • The lattice may have an adequate adhesive quality that requires minimal or no additional fixation, even for large defects.
  • Non-allergenic.
  • Inert.
  • Non-biodegradable.
  • Non-carcinogenic.
  • Cuts easily without fraying.

It is worth noting that due to the relatively recent upsurge in 3D printing technology, what we envisage for the mesh has only just become possible.  The ability to create on demand, one-of-a-kind meshes for each patient is new and gives the medical profession opportunities they never had before.  It will be interesting to see where this fledgling technology goes.

In addition to mesh creation, we also envisage a 3D printed system of tubular structures for blood vessels, veins, and even muscles and valves. We are at the beginning of a new age in terms of what 3D printing can bring to the (operating) table!  

So we are at an interesting phase. We are talking to mesh manufacturers and medical companies that use hernia mesh, gauging their interest and believing strongly that this hitherto unrealized possibility, could well provide the stimulus to take the whole Hexaflex concept to the next level.

September 2014

We were asked by a potential client to come up with a working method to produce a metallic honeycomb, and to those ends have added more details to a manufacturing concept we first outlined in our patent.  

By using 2 sets of dies (A) - top and bottom, shown here in red and pink, and placing them between top and bottom clamp plates (B), when the clamps are drawn together, the dies, which are constrained by a sheet of polyester which we have called "living hinge," (C) come together to crease and fold the metal sheet.  D, E and F show the range of motion where D is the start before any compression, E is half complete and F is a closed version of conceptual Hexaflex honeycomb.

If this proposal works, it will be the first time that Hexaflex honeycomb has been manufactured in any sort of quantity. The project will call for about 50- 75 sheets using a hexagon size of 12" and a core depth of around the same.  We will keep you posted on the project as we move forward.

January 2015

During the course of our research into the possible uses of Hexaflex, we have come to the realization that the core is lacking in compression strength when compared to standard honeycomb. This shortcoming can be solved by the ability of this design to be injected/infused with syntactic foam into its galley ways, completely filling all voids between the face sheets.Compression forces can be tailored to optimize the structure by strategically infusing syntactic foam of varying crush strengths. The high crush strength and low density of syntactic foam makes it an ideal core material for hybrid sandwich composites. This latest modification to the property mix of Hexaflex, indicates a strong likelihood that delamination can be avoided in most incidences, a condition that the aerospace industry will no doubt welcome.

Shear experiments have showed that the bond between a syntactic foam core alone and the composite face sheets could be a weak link in a standard honeycomb sandwich design.  However, by combining the attributes of Hexaflex and syntactic foam together, it is possible to create an ultra lightweight hybrid core, wherein Hexaflex provides the tensile properties and the syntactic foam provides the compression properties.

September 2015

We have known for some time that one of the best ways to manufacture Hexaflex, using a variety of different materials, is to use 3D printing. After some months of file preparation and modification, we submitted our designs to Shapeways, obtaining some 150 3D printed tiles, a mixture of hexagons and squares, and assembled them into a sheet of Hexaflex. 

One of the first things we noticed was the comparative ease by which they could be assembled into such objects as spheres, domes, buckeyballs, nanotubes, torus/donuts, terraced planes, rhombic dodecahedrons and parabolic dish structures, to name but a few.

This unorthodox building kit could be an educational toy incorporating multiple layers of learning. These layers, which promote both critical thinking and discovery, include:

1. Exploring the world of polyhedra.
2. Discovering the intricacies of Nanotechnology.
3. Creating robotic structures.
4. Integral construction possibilities with Lego

January 2016

Incorporating a earlier invention called the Nodlet truss into the hexaflex pattern, we were surprised to discover an entirely new form of curvable lattice, not unlike a mechanical skin...

Using components engineered in 3D printed nylon, inventor Bob Burdon discusses a newly-discovered foldable lattice created using Hexaflex and a earlier invention called the Nodlet.  We are currently embarking on a program of reaching out to major aerospace and automotive companies who are looking for adaptive structures that change shape according to desired performance criteria.

April, 2019

Elon Musk made the decision to use stainless steel instead of carbon fiber/epoxy materials for his Starship.

Stainless steel properties:
·     Very ductile, very tough. No fracture issues
·     At cryogenic temperatures, the strength is boosted by 50 percent.
·     “it’s obviously cheap, it’s obviously fast.”
·     It can safely withstand a temperature of 1600 degrees Fahrenheit.

Elon Musk also proposed a transpirational heat shield:

“You just need, essentially, [a stainless-steel sandwich]. You flow either fuel or water in between the sandwich layer, and then you have [very tiny] perforations on the outside and you essentially bleed water [or fuel] through them … to cool the windward side of the rocket.” – SpaceX CEO Elon Musk.

Open view of a proposed panel section

A cutaway view showing lower nozzles and interior Hexaflex framework

Interior view of Hexaflex without side walls

This could be the another Aerospace application for Hexaflex.
Hexaflex can be fabricated from a continuous sheet of stainless steel that has been die cut with a repeating geometric design, creased and folded concertina style in upon itself to make a double sided core material that is flexible, able to vent, exhibits good bend and shear strength and has a large surface bonding area rendering it suitable in the construction of lightweight sandwich panels.

A transpirational heat shield must be able:
·      to efficiently join inner and outer shell surfaces with stringers
·      to provide a clear interstitial space, allowing fluids to flow through its structure
·      to be formed into compound curvatures
·      to serve double duty as structure.

The ability of a Hexaflex panel to serve double duty as a structural component opens up the possibility of making the entire rocket skin from Hexaflex panels. They can be stacked in multi-layers or plies in order to create alternative storage for gasses liquids or rocket fuel

The future is exciting.  With 3D printing fast becoming a legitimate manufacturing platform, and machines being developed that can print multiple materials concurrently - a concept completely in sync with the Hexaflex system, it remains to be seen where Hexaflex will fit in, and which applications will be among the first to find a viable and sustainable market.