Core Materials – Part 2 – Nomex Honeycomb Core

Honeycomb cores offer the highest performance in core selection today. The hexagonal core shapes (honeycomb) and the Aramid fiber / Phenolic resin makeup give the core structure its strength. From commercial and military aircraft, auto racing, transportation, and construction industry; this core material is the top of the line.

As with any thing though, there are always pros and cons. For this material they are:

  1. Advantages: For the Lightest, Most Rigid Components providing proven high performance
  2. Disadvantages: Cost, Experience needed for processing & machining in complex structures

Simple Panels

For flat panels and simple contours on a core exposed sandwich panel, Nomex honeycomb cores are very easy to work with. A razor knife will cut through the material with relative ease if a sharp blade is used.

Note: Nomex Honeycomb cores are only waterproof if encased within a composite matrix. Otherwise, the core will be subject to water intrusion.

However, with more complex structures, there may be additional processing steps of using honeycomb core that makes it challenge. Is it worth the time and effort to achieve the highest part quality in a prepreg or traditional sandwich structure layup? Absolutely. Choosing this type of core totally depends on the end need of the composite structure.

Honeycomb core may be placed into a structure at the time of cure (Co-cure) or bonded secondarily onto cured skins using an adhesive. Co-curing of the cores may be completed with prepreg material with film adhesive covering the core material and placed into an ovens / autoclaves for cure at elevated cure temperatures. Film adhesives aid in the bonding of the core to the
laminate plys and provide a chemical bond between the substrates. Core pieces may need to be chamfered to successfully encase & co-cure in a laminate. Traditional layup methods are generally used when secondarily bonding the cores between 2 cured composite skins. Both applications have advantages depending on the processability and end use needs.

See above for an example of a chamfered edge vs a straight edge that yields a simple sandwich panel. A chamfered edge provides a means in which the plys will not need to go straight down, causing wrinkles, porosity, and other quality issues. A chamfer will provide optimum laminate composite quality in a co-cured sandwich laminate.

Cutting / Chamfering raw Nomex Honeycomb

Cutting Nomex core can be a delicate process that requires patience and experience to perfect. Especially if there is not a 5-axis CNC machine just lying around. As option, use a small amount of honeycomb material to experiment, learn how it reacts to certain cutting tools, applications, and even the orientation of the core while it is cut. If one does enough research,
there are many ways Nomex cores can be cut, though some of them are easier than others.

The first step in cutting Nomex Honeycomb core is to somehow stabilize the structure along the areas in which will be cut.

  1. Masking & Packing Tapes placed over the areas that will be chamfered or cut. Once cut is performed, remove the tape. This generally does not help when chamfering the core, only cutting through it.
  2. Potting compounds can also be added to fill the void areas of the honeycomb in which the cut or chamfer will be performed. (This will make the core weigh a little more)
  3. Film Adhesive added and cured to the top of the core in areas in which it will be cut. This will aid significantly to stabilize the core material on both sides and can be used to chamfer.
  4. Pouring an expandable foam into the areas that will be cut or chamfered will add stability to the structure to allow for better workability. This option will also maintain stability throughout the chamfering process the same as it would on the surface.

If cutting straight thru the core a sharp OLFA type razor blade will suffice. Some engineering / workability will need to be setup to achieve a chamfer. A simple workstation such as a Dremel 220 can be applied to a rotary tool with a cut off wheel to achieve the cut at the angles needed. Usually 45º is a good starting point for a chamfered edge but there is nothing to say the core cannot be cut a steeper or shallower angle. It is advised to use the maximum number of RPMs that a given rotary tool can provide and to cut the core slowly.