06 October 2021
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Sixth sense - honeycomb as a structural material

Honeycombs, as everyone knows, consist of rows of hexagonal cells made of wax where bees store honey and grow their young. By mimicking the insects’ collective wisdom (and construction methods), manufacturers are able to make panels that have strength-to-weight and stiffness-to-weight ratios far higher than a monolithic block out of the same base material. The introduction of new materials and production methods suggests that hexagons are here to stay, reports Will Dalrymple

Although honeycomb is sold bare, most of its structural applications are within lightweight sandwich panels, consisting of a honeycomb core in between sheets of material including glass-reinforced plastic or metal skin. The honeycomb transfers force between the two sheets, providing rigidity and compressive strength with relatively little material.

Not only does that economise on resources in production, it also plays a key role in making transport more energy-efficient. Goodfellow technical specialist Lidia Wolanicka says: “Honeycombs allow for significant weight savings, which is critical for applications like aerospace and architecture, while offering higher strength-to-weight ratios than solid composite laminates. This translates to lower overall material cost, as well as lower weight of an aircraft or a vehicle, for example, which is critical and translates to huge fuel savings.”

The secret of honeycomb is its structure, she explains: “The high compression strength of honeycomb is due to the hexagonal configuration, where walls support each other. Hexagons are one of the strongest shapes in nature. Honeycomb sandwich panels can be as much as 40 times stronger than laminates in certain scenarios. Also, load carry and load transference are much greater in honeycomb than in laminate.”

HOW THEY ARE MADE

There are two principal types of honeycomb, thermoplastic and metal, and each is manufactured in a different way. Broadly speaking, thermoplastic honeycomb is made by extruding thermoplastics, although there are variations on this. They honeycomb usually offered by Goodfellow is made by extruding the lattice in a single step. EconCore uses a two-step process, extruding a film which it rotationally thermo-forms into a half-hex shape. This is then folded up to make the final honeycomb, and all in a continuous process. The honeycomb then undergoes controlled heating and cooling to fuse parts together.

Goodfellow offers aluminium and aluminium alloy honeycomb from foil sheets, sliced, glued, stacked and placed under heat and pressure (pictured, top right). Once cured, they are cut to thickness and expanded by physically pulling top and bottom sandwich panels in opposite directions. Another brand of aluminium honeycomb panel, larcore (pictured below right), is made by Spanish firm Alucoil, and distributed in the UK by Panel Systems (0114 249 5626).

Once the honeycomb is made, it needs to be attached to the skins. As each individual honeycomb strut has a thin cross-section, there is little surface area to create the strong bond required for structural integrity. When the core or exterior material are the same – for example polypropylene – EconCore uses thermal bonding to join the two. When using GRP panels, the skins are sometimes laid on to the honeycomb wet and placed in a mould. The adhesive used to form the panel then integrates it with the honeycomb.

In cases of dissimilar materials, Wolanicka at Goodfellow suggests that good-quality adhesive needs to be used to securely bond panels and honeycomb. In particular, she adds that the main difficulty posed by thermoplastic materials for honeycomb is achieving a good interfacial bond between the honeycomb and skin. She says that usually two-component polyurethane adhesive thermoplastic film, epoxy film or two-component epoxy adhesives are used (EconCore also uses liquid glues, thermoset resins and thermoplastic heat-activated adhesive films).

Also, when different materials are used for the core and panels – for example, Goodfellow offers aluminium honeycomb with glass fibre and carbon fibre, as well as an aluminium skin – Wolanicka also warns of the risk posed by differing coefficients of thermal expansion; in an extreme case, the panel could fail. However, although chief operations officer Tomasz Czarnecki at EconCore agrees it is something to take into account, he argues that the cellular thermoplastic structure can compensate for expansion and shrinkage.

Both Goodfellow and EconCore’s bare polypropylene honeycomb core can be supplied with a film to improve bonding and prevent ingress of adhesive during the converting process.

Paneltex, which converts EconCore’s ThermHex polypropylene honeycomb, makes completed sandwich panels in two Yorkshire factories using polyurethane adhesives and vacuum and platen presses.

DESIGN PARAMETERS

Unlike bees, manufacturers are able to alter the design and materials used to best suit the purpose. Provided sufficient volumes, they also offer customised solutions to add to the standard offerings.

Goodfellow can modify the cell shapes. While a regular hexagon is standard, that form can be elongated into a shape that more closely resembles a rectangle. Another option is a reinforced hexagon, which contains a bar through the middle for extra compressive strength.

EconCore’s proprietary process produces only one shape, but different moulds offer different cell sizes (3-10mm), and different wall thicknesses. Those two factors are both adjusted to maintain panel density; smaller cell sizes require thinner walls. It produces honeycomb in eight different standard sizes. Czarnecki observes: “There is a general tendency to go to smaller and smaller cell sizes, as for many applications, surface quality is important, and smaller cells achieve a smoother surface.”

Wolanicka adds that increasing core density will also improve shear performance. Goodfellow’s honeycombs are offered in cell sizes of 3mm-6mm (polyaramid), 8mm-10mm (polypropylene) and 3.2mm-19mm (aluminium). For aluminium cores, she points out that density also plays a non-mechanical role: smaller cell sizes that increase the panel density will improve thermal conductivity.

In terms of performance, the other important dimension is the thickness of the panel. That largely determines its rigidity, comments Czarnecki; in fact, rigidity rises exponentially with an increase in thickness. EconCore’s panels can measure from 3-30mm thick (depending on the mould). However, the thickness of the panels also plays a role in weight and rigidity, so sometimes a thick honeycomb layer is compensated by using thinner skins (typical polypropylene sheets are 150-500μm thick). Goodfellow produces different thicknesses of honeycomb sandwich by slicing the extruded ‘cake’ to size. Panel Systems offers ThermHex polypropylene in two densities: 60 and 80kg/m³. Thicknesses range from 3.5-28mm, and cell sizes from 3mm-9.6mm.

JOINING

Honeycomb cores offer such advantageous strength/weight ratios because they are mostly air by volume; that means, depending on the material, that they are more vulnerable to crushing than monolithic forms. “You want to avoid high concentrations of local loads,” Czarnecki states, which typically means avoiding standard mechanical fixtures in favour of special fixtures designed for sandwich panels.

In addition, manufacturers have found other work-arounds. Trailer makers have developed specific connection systems for panels; car manufacturers lay the panels on supports so they do not need joining; others use friction-stir or ultrasonic welding to join metal panels. In other cases where thermoplastic panels are compression-moulded into a 3D part (pictured), overmouldings might include fixation features, adds Czarnecki.

Where panels are used for making furniture, loads need to be transferred from one skin to another, so fasteners might be mounted on the skins’ surfaces.

Working with aluminium is somewhat simpler. Spanish supplier Alucoil, which produces the larcore aluminium alloy honeycomb sandwich panel sold in the UK by PanelTex and others, says that the material can be bent, screwed or riveted, drilled or perforated, glued or cut with a routing tool. Both exterior surfaces are aluminium alloy 5754. It comes in eight thicknesses from 5.5-40mm, with a cell diameter of 9.52mm, and density of 54kg/m² (except the thickest panel, which uses a thinner foil and has a density of 40kg/m²). Although the core is 3005 series alloy (EN 573-3), the manufacturer cautions that the sandwich panels must be protected from the ingress of moisture. Another ‘ultracore’ version has a different (unspecified) formulation intended for areas with high humidity.

Alucoil points out that larcore sandwich panels can be laminated with a wide variety of materials for floors and ceilings, including stainless steel, CPL (coil pressure laminate, for application of wood veneers), high-pressure laminate, stone, VC film (an optical material) or rubber flooring.

MATERIALS

The actual performance of any given honeycomb design depends most of all on what it is made from. As standard, Goodfellow offers cores of polypropylene, said to provide good chemical resistance, and polyaramid (Nomex). In a second processing step, the latter is infused with a phenolic resin to improve mechanical properties and make it fire-retardant for aerospace applications. Phenolic resins are also used in some of its aluminium honeycomb.

In an environmental improvement to the process uses tannin-based resins (from tree bark), although phenolic resins generally pose little hazard, Wolanicka says. They are used to partially replace phenol in phenol resin synthesis, and the resulting resin displays a reduced curing time and lower formaldehyde emission.

In addition to polypropylene material, EconCore also offers PET, which performs in a similar way but comes from recycled consumer and industrial waste (plastic bottles) to improve its eco-credentials. Also, it offers a polycarbonate with higher temperature stability – up to about 140°C, compared to about 100°C of polypropylene, and a polyamide version with temperature resistance up to 180°C and which also offers good fire smoke toxicity rating for aerospace and railway uses.

Goodfellow’s aluminium honeycomb cores are available in pure aluminium or AL300 alloy (Al98/Mn 1/Fe 0.5/Si 0.5) for corrosion resistance. They offer the best performance when used not in a panel but in an energy-absorbing structure such as a vehicle bumper. Alucoil offers honeycomb with only a single exterior sheet for better formability.

Also, as the cells of honeycomb are formed in such a way that their walls are perpendicular to the sandwich, they can be used for non-structural purposes, such as in fuel filters, in optical equipment and for heating and air conditioning.

Asked about the potential of honeycomb, Wolanicka says that it is limitless: “Wherever you need to reduce weight of an element or a part, you can use honeycomb. You can choose a number of different ones, depending on the required properties.”


BOX: Basalt honeycomb for rail

Basaltex (www.basaltex.com) has developed and tested a new composite material solution comprising fibres of basalt combined with EconCore’s patented honeycomb technology.

This new material has greatly improved fire resistance and is highly rigid, as well as offering the usual honeycomb properties of light-weighting, according to Basaltex.

The composite material consists of basalt fibres, a bio-resin and rPET honeycomb.

Filaments of only stone fibres are extruded at a temperature of 1450°C, similar to glass, but with a number of advantages, not least the absence of boric acid in the process, EconCore says.

Railway applications require materials with enhanced fire resistance, and testing has shown that the EconCore-Basaltex solution fits these needs well, at the same time offering drastic weight reduction compared to traditional monolithic GRPs, used rather widely in train interiors.

Such sandwich panels could be deployed in applications including cladding, partitions, tables and flooring.

Author
William Dalrymple

Related Companies

Goodfellow Cambridge Ltd
Panel Systems Ltd

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