Core Materials – Part 4 – Foam Cores & Balkore Balsa Core

To finish off our series on Core Materials we dive into Foam Cores
& Balkore Balsa Core. As always should you have any
questions on any of these core materials covered in this series feel free to
contact us as we’re always happy to help!

Foam Cores

Foam cores share a wide range of applications and uses in addition
to being applicable with polyester, Vinylester, and epoxy resins. Foam offers
one of the most light-weight solutions in core materials. One of the biggest
factors in selecting a foam core is the open vs closed cell structure within
the foam. For sandwich composite structures, a closed cell foam should be used
to so that the core does not absorb resin during cure under vacuum. A resin
filled foam core is neither light nor strong. Closed cell foams take in
virtually zero resin in the cure process.

Foam cores, such as PVC foams offer a much lower density solution
than the Soric or Coremat lines. However, they lack in terms of strength. Foam
cores will still add good compressive strength within a structure but not as
high as other options.

All foam cores can easily be cut with hand tools such as a razor
blade and formed with a block sander to create the desired chamfer angle in the
core material as needed. For thinner laminates, the fabric layers may be
matched to the foam core’s thickness to mitigate any possible bagging issues.

As easy as closed cell foams are to machine or cut in placement in
a part, there are challenges around pin holes, dry patches, bridging, and voids
within a co-infused laminate using foam cores. Epoxy resins will stick to cores
with less of an issue. However, relying only on the vacuum pressure to pull
Polyester or Vinylester resins across the foam core can lead to such issues as
the closed cell foam does not take resin well. To solve issues such as pin
holes, dry areas, and other quality issues, the foam core will need to be
perforated, having holes drilled into it for the resin to fill adequately. To
perforate core, drill holes in the foam core with a ~.098” drill bit for every
¾” of foam and score foam between the holes as illustrated below.

There are multiple ways of scoring foam. One way is by use of a
router with a small bit or by using a blade / hot knife to go between the
drilled holes. Scoring cores will aid in increasing resin travel and mitigate
dry patch issues. Scoring core also aids in the foam core’s ability to flex and
conform to more complicated contours.

Foam core may also be secondarily bonded to 2 cured laminate skins
using an adhesive layer between the skins and the foam core. This would make a
lot of sense in doing if fabricating simple or flat parts but is not limited to
that.

When processing or working with foam core before cure, be mindful
that it may crush easily. Be sure to never place too much pressure on the
foam’s surface or it will likely scrap the core piece. Do not place foam cores
in areas that will be commonly walked over or common to bumping into. These
issues can be mitigated by transporting and storing (ALL) core materials
between cardboard sheets, protecting the foam or other core material
inside.

If foam core machining or if complicated shapes are in issues with
utilizing core materials, one option is pourable expanding foam cores. When
poured into a given mold or shape, the foam cores expand and fill to give a
desired shape to lay up over. Expandable foam cores come in an array of
densities and resin type applications from polyurethanes to epoxies. These
cores exhibit the same mechanical properties as comparable sheets but cut costs
and time behind working or machining core pieces.

Balkore Balsa Core

Balsa wood core material utilizes the same hexagonal shape in it
as honeycomb cores. However, it usually is not seen by the naked eye. Balsa
wood’s honeycomb configuration comes from its molecular structure. This
molecular structure compliments its high strength to weight ratio as a great
choice in a sandwich structure. Balsa cores are processed much like that of
foam cores, though offered in thicker options. They may be co-bonded / infused
into the part or secondarily bonded. The same principles with scoring to
conform to complex shapes are an option when utilizing this core
material.

Balkore also offers the
Balsa core grid scored for conforming to complicated surfaces and “scrimmed”.
The scrim aids in its bonding capability while allowing for additional
flexibility on the opposite side. Balsa wood may be cut or machined with
chamfered edges just as foam, honeycomb may be. Balsa is not as easily
chamfered or cut as foam but still much easier than Nomex honeycomb. Balsa is a
great choice for any layup technique and is compatible with all conventional
resin types.