Degrease
Aluminium Honeycomb Core is manufactured through a series of steps and processes. The first process is receiving in the raw aluminium foil from the mill as a large continuous roll. The material comes untreated and is contaminated with oils and greases, which left as is would cause problems when trying to bond to it. To overcome this problem, the raw material is loaded and unrolled carefully through a degreasing chemical, removing any grease and leaving the aluminium clean and contaminate free.
Oxide Removal
The next stage is to remove the Aluminium Oxide layer on the surface which is formed when Aluminium reacts with the oxygen in the air. This layer of Oxide is brittle, and when bonded to, fails at a much lower load than if the aluminium is bonded directly to. To remove the Oxide, the aluminium foil goes through a deoxidiser, usually an acid which etches and removes a very small amount of material from the surface, and in process removes the oxide.
Corrosion Protection
Following the deoxidisation process, leaving the aluminium exposed, the material needs to be protected before it has time to re-oxidise with the oxygen in the surrounding air. The roll is therefore passed through a protective treatment bath, providing a corrosive stable surface (also known as a conversion coating) ready for adhering to. Once treated the material is wound already for applying the adhesive node strips.
Adhesive Application
The treated aluminium roll is placed into the adhesive strip application machine and is passed through a series of tensioning rollers, prior to reaching the adhesive application roller. This roller has high precision raised strips equally spaced circumferentially that picks up liquid adhesive and deposits it onto the sheet aluminium – the spacing of the strips ultimately determines the cell size or density of the core, where the closer the strips are place the smaller the core cells will be. The aluminium with adhesive strips is then passed through a drying process, where it is carefully dried at a specific temperature ensuring not to start adhesive curing process, similar to an adhesive film. This drying process allows the aluminium to be wound without it sticking together, ready for the cutting and stacking process.
Cutting and Stacking
The next process is to cut and stack the adhesive stripped aluminium roll into sheets with the length of the sheet making up the width of the core slice. The sheets have to be precisely cut so that its adhesive strips are staggered in relation to the previous sheet. It is this staggering that creates the honeycomb pattern when expanded at a later date.
HOBE Bonding
Once stacked the adhesive strips are required to be cured and bonded together to create the HOBE (HOneycomb Before Expansion). This is performed in a heated hydraulic press where the stack is heated and held at temperature under pressure to allow the adhesive strips to flow bond to the adjacent sheet. Once cured, the stack is now classified as a HOBE, whereby expanding it would create a honeycomb lattice, albeit a very large and deep lattice since it has yet to be cut into slices at the required thicknesses.
HOBE Slicing
The HOBE is cut into what is known as a honeycomb slice, where they are cut to bespoke customer required thickness and are still in the pre-expansion state. The cutting is performed using a large deep throat bandsaw specially adapted with feed-controlled sliding beds and is clocked to ensure a parallel cut. The sliding beds are usually slanted to present the HOBE to the bandsaw blade progressively, resulting in a cleaner cut. The purpose of using a bandsaw over other cutting methods, is used to ensure the blade / cutting teeth have enough time to cool prior to cutting again, resulting in less risk of disbanding due to temperature build up.
Quality Control
Due to the special processes used to manufacture Honeycomb core, it is vital to from a quality control perspective that each HOBE is verified and checked for conformity. Therefore, each HOBE is checked by extracting the center slice and destructively testing it against qualification standards to it fails above the pass criteria.
Slice Expansion
Once in slice form, the slice can be expanded into its honeycomb lattice state. The expansion process is performed by pulling the slice sides evenly until the core is fully expanded, allowing for spring back by over expanding it. The cells are then quality checked to ensure they are correctly formed and are not distorted. It is then ready to be used in our lightweight panels and structures.
Honeycomb Preperation
As explained earlier, adhesive bonding is all in the preparation and cleanliness of the adherends or material required to be bonded. For this reason, the processes required to create, cut, expand, machine and bond Aluminium Honeycomb must be performed in such a way as to not contaminate it. Contamination can drastically reduce the mechanical properties and could lead to premature failure. Core slices are therefore carefully manufactured and delivered clean and contamination free, ready for bonding, any handling of the core should be kept to a minimum and carried out with lint free cotton gloves to prevent skin oils being transferred.
Machined Honeycomb
It is not unusual that honeycomb is machined, chamfered or worked post its sliced and expanded states for design purposes. When worked, it is vital to maintain cleanliness, to ensure it is not contamination, therefore all machine surfaces and tooling should be cleaned and degreased before use.
Cleaning Expanded Honeycomb
If the core becomes lightly contaminated, it is possible to degrease and clean it via a vapour degreasing process, flushing it with solvents or wiped with a clean lint free cloth, dampened with a solvent. Before use in product, the core should be thoroughly dried. Disclaimer – It should be noted Bonded Components does not take any responsibility for damage to the core, or its nodes or deterioration of any mechanical properties caused by cleaning, and the content on this page is for information purposes only.
Perforated / vented core
Most honeycomb structures, create a closed non-permeable cell once the skins are adhered to the core, preventing any fluid or gases from migrating between the cells. If water or any other fluids does however, manage to penetrate the adhesive bond line, fluid can build up in a cell. This fluid then becomes in contact with surrounding cell bond lines, and can continue to breakdown the bond line. Although rare, it is very difficult to detect the defect when in its premature state, and left for extended periods can lead to premature failure. This has causing certain industries, such as Aerospace, to ensure honeycomb panels when used with fluid requiring a dual walled structure, to have a method of detecting fluid or leaks into the core interspace.
This can be achieved by using perforated or drilled honeycomb, allowing any fluid or gases to migrate between cells to a collection point, where it can be inspected to establish whether the duel walled structure has been compromised.
Perforated cores can also be used to prevent over pressurisation to the structure caused by differential pressures.