In this article we explore the best adhesives currently used to bond honeycomb panels; how they achieve their effectiveness; and a brief overview of honeycomb structures used in the aerospace and rail industries.
Honeycomb panels utilize the geometrical structure of a honeycomb, which is usually hexagonal, in order to achieve unique physical properties that offer many advantages. Depending on their desired applications and specifications, they are manufactured using a variety of different materials, ranging from paper, reinforced plastics, aluminium or thermoplastics (polypropylene).
They are most commonly manufactured using an expansion production technique and by cutting and bending rolls of metal. A honeycomb core, composed of hollow hexagonal cells between thin vertical walls, is layered between two thin face layers, forming a sandwich panel. The distance between the centroid and the face layers leads to a high moment of inertia, meaning that the panel will withstand a very high net force before rotating.
Composite honeycomb panels are used in many scientific and engineering applications, including those of the automotive, aerospace, and sports industries. Most honeycomb panels encountered in aerospace are made from aluminium or Nomex®. They are extensively used in the aerospace and rail industries due to their peculiarities; a few of which have been described below.
Main advantages of honeycomb panels
High out-of-plane compression and shear strengths due to the efficient hexagonal configuration
Superb rigidity at minimal weight due to the sandwich structure
High specific strength, which is useful in applications in slightly curved surfaces
Can be used to reduce or create wind turbulence when used as a mesh
The significance and usage of adhesives in honeycomb panels
To prevent the two face layers outside the honeycomb core from slipping and working in opposition to one another, they must be structurally bonded to the core. When a load is applied to the panel and a bending force is present, it is the role of the adhesive to counteract that force by providing a shearing force, consequently stabilising the face layers and the whole panel. Therefore, it is essential that the adhesive used is of supreme quality and allows the shear loads to be transferred effectively.
We are now going to explore the most suitable structural adhesives for this purpose from the Araldite range of products. 3 different honeycomb sections that these products have been used with can also be seen below:
Adhesives used to bond honeycomb panels
Araldite 420 A/B, a two-component epoxy structural adhesive system, is a commonly used adhesive for bonding honeycomb panels in the rail and aerospace industries due to its appropriate properties. It offers great moisture resistance, excellent lap shear and peel strength, and is extremely resilient and tough. It cures at room temperature and is suitable for bonding a wide variety of substrates, though it is known for its ability to bond honeycombs and fibre reinforced composites.
Araldite 2015-1, a two-component thixotropic toughened paste, provides resilient bonds for honeycomb panels and cures at room temperature. It is a high shear and peel strength adhesive; as well as being gap filling, non-sagging up to 10mm thickness. Additionally, as it is very resistant to weathering, it is suitable for applications in aerospace and rail.
Surface preparation for bonding honeycomb panels
To obtain good strength bonding between honeycomb panels, ensure that the surfaces are, at the very least, free of all traces of dust, oil, grease or dirt by cleaning them with a good degreasing agent. We recommend any good quality IPA wipe. Never use petrol, paint thinners, or low-grade alcohols. For optimum strength bonding, the honeycomb surfaces can be gently mechanically abraded (“pickling”) with an emery cloth, eliminating the oxide layer, then wiped again with a solvent wipe to remove all loose particles, before applying the adhesive.
For further information regarding surface preparations, read: What you need to know on bonding plastics.