Aug 17th 2021
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 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.