A question was asked about why use end grains balsa,
From Composites World publication (article linked below)
"BALSA WOOD CORES
Low-density balsa wood is another popular core choice because of its relatively low cost combined with high compression and shear strength and excellent fatigue performance. Balsa trees mature in about seven years, reaching heights of 27.5m/90 ft. Balsa is, in effect, a natural composite bundles of cellulosic fibers are held together by lignin and resemble a honeycomb under a microscope.
Harvested balsa lumber is first air dried, then kiln-dried to reduce moisture as much as possible. Depending on the environmental conditions, average moisture content of balsa varies from about 10 to 14 percent. The dried lumber is planed, cut to length, then precisely measured and weighed to determine density. After sorting, the lumber is glued together, pressed into large blocks and cut into sheets, with the wood fibers oriented perpendicular to the face of the core sheet. This end-grain orientation demonstrates the highest compression and shear properties, fundamental for good sandwich construction. Sheets are sanded, and then prepared for customer specification. While balsa can be supplied in rigid panels, many fabricators prefer flexible sheet material in which the panel is cut into small squares held together with a fabric scrim backing that allows the core to conform to a curved mold surface.
A common rap against balsa wood cores, particularly in the marine industry, is that the material will rot in the presence of fresh water. But marine specialists say moisture uptake occurs only when spaces between the cut core squares are not completely sealed with laminating resin or bedding compound, or if deck fasteners that penetrate the cored laminate are not properly gasketed and sealed against rainwater intrusion. Resin infusion molding methods evacuate air under vacuum pressure, ensuring that all voids fill with resin. Properly fitted interior insulation material also can help resist water accumulation from condensation, caused by temperature differences between the inside and outside of the hull.
The oldest and largest balsa core manufacturer is Baltek Corp. (Northvale, N.J., U.S.A.). Founded by the Kohn family, the company began importing tropical hardwoods to Europe as early as the 1880s. The first lumber mills and facilities were established prior to World War II and supplied balsa wood to military customers like De Havilland in the U.K., which used end-grain balsa core laminated with plywood veneer for its Mosquito bomber. Today the company has five plants in Ecuador, with over 17,000 acres of balsa plantations. Baltek was recently purchased by Alcan Inc. of Canada, and is now known as Alcan Baltek Corp.
Baltek developed the first scrim-backed flexible core, which it calls ContourKore. The company has, over the past several years, developed a new controlled density end-grain balsa product called SuperLite. Based on careful selection and harvesting of genetically modified trees with very little variation in moisture content (and thus density), core densities are available from 4.5 lb/ft3 up to 16 lb/ft3, in 1-lb increments. "They are the first balsa trees grown for the one purpose of becoming core material," says Alcan Baltek's Dohn. Like other suppliers, the company supplies core kits to simplify laminate construction and reduce waste for customers.
Although it may seem counter-intuitive, balsa wood actually performs very well in fire-critical applications. According to Dohn, a product's available combustion energy is a function of its density; a typical lightweight balsa doesn't offer much fuel, and it burns with a nontoxic white smoke. If the wood does come into contact with flame, a uniform char layer forms that protects unconsumed celluose from the heat source. In contrast, foams and particularly PU foams produce a black smoke that, depending on their chemistry, may contain toxic byproducts. For these reasons, balsa is approved in most transit applications. This is one reason that infused, balsa-cored, fiberglass-skinned panels have replaced phenolic honeycomb-cored laminates in the floors of Bay Area Rapid Transit (BART) trains that operate in the greater San Francisco metropolitan area, because of lower cost and better durability performance, says Dohn."
https://www.compositesworld.com/articles/getting-to-the-core-of-composite-laminates
