How-to use honeycomb aramid cores

Last months have been quite busy. A pity it hasn’t been on building Schneewittchen… Anyway, we started to finish the mold by sanding up to 3000 grit and polishing the surface. Soon we’ll be able to build the first parts and it is time to find out how to produce the shell of the center section.

A couple of months ago I puchased about 9 m² (100 sq ft) of 5×3.2 mm aerospace grade aramid honeycomb core material. Usually this material  runs as Nomex or Kevlar honeycomb. It’s essentially aramid paper shaped in a honeycomb pattern and soaked in phenolic resin. Manufacturers produce large cubes from which thin sheets are cut off perpendicularly to the honeycomb channels. The usual sheet thickness is a couple of millimeters, where 3, 5 and 8 mm are usual thicknesses in airplanes. The amount I bought should be enough  for the center section including some tests pieces.

If you aim at building ultra light, using this core material is a must. It weights almost nothing (29 kg/m³) and is very strong. However, its handling and application is not easy and needs to be learned beforehand. If done wrong, you either end up with a heavy or fragile piece. As an appetizer, I started with a small flat piece of about 40×20 cm (16×8″):

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A crucial point in using honeycomb cores is to achieve a very good bondage to the skin. Else the skin might peel off causing failure at high shear stresses. This is why usually the skin laminate is let to gel and glued to the core with a freshly wetted thin coupling layer. Vacuum pressing ensures then that the core is tighly pressed into the coupling layer resulting in good bondage. On the one hand, telegraphing might occur when the skin is still too soft, which leads to a weaker sandwich. While on the other, fully cured skins, do not chemically bond resulting in  weaker joints. You want to avoid both. It’s all about the right timing and technique.

Roughly 80% of the sandwich weight is contributed by the skin (typical layup for 5 mm core: 2×105 g/m² and 1×160 g/m² glass fabrics). There is no way to further reduce the weight of the core. Thus, optimization of weight is only possible by reduction of the skin weight. One approach is to use other materials, such as carbon. However, the available choices of fabric weights are limited and oversizing can happen resulting even in an increase of weight. Also, light carbon fabrics increase disproportionally in price: 93 g/m² is almost three times the price of 160 g/m² carbon. Another strategy, is to reduce the amount of resin in the skin to a minimum, i.e. to increase the fiber volume ratio as much as possible.

I chose to test the second strategy and used vacuum infusion for the outer skin and vacuum pressing for the inner skin. For both skins I used the low viscosity HP-E300RI resin designed for vacuum infusion. One problem was to catch the right time when the laminate is geled enough but not too hard (roughly 3 times the „pot life time“, i.e. about 11 hours). Instead of using a thin coupling layer to bond the core, I decided to use an industrial epoxy-adhesive: 3M Scotch-Weld 9323. Pricy (roughly 160 € / Litre) and adds some weight (200 g/m²), but I’ve heard it produces a much better joint that the coupling layer.

So, I started with vacuum infusing the outer skin. The laminate is composed of two quasi-isotropic layers of 105 g/m² aerospace grade glass fabrics and one layer of 100 g/m² flax fabric:

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So why flax? Well because it is a renewable natural product, it has ideal damping properties and is quite strong. It adds also thickness without increasing that much weight. Take, for example, this impressive wind-turbine blade breaking test:

 

The laminate was vacuum infused using DD|compound’s MTI hose and was let to gel for roughly 8 hours (longer would have been better):

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Then a thin layer of Scotch-Weld 9323 was gently spread on the geled laminate. The honeycomb core was vacuum pressed and cured overnight (see first picture on top ). The edges of the core were sanded with 80 grit to obtain a nice wedge. The inner skin—composed of 80 g/m² glass and 100 g/m² flax fabrics—was hand laminated and geled for 11 hours in a strong vacuum to press the excess resin. The laminate was still soft but not too sticky. Again a thin layer of Scotch-Weld 9323 was used to bond the core material and the sandwich was cured in a moderate vacuum overnight.

I’m quite happy about the result (1300 g/m²):

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