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About Balsa

Balsa trees (ochroma lagopus) grow naturally in the humid rain forests of Central and South America and New Guinea. However, the small country of Ecuador, on the western coast of South America, is the primary source of model grade balsa in the world. Balsa needs a warm climate with plenty of rainfall and good drainage. For that reason, the best stands of balsa usually appear on the high ground between tropical rivers.

There was no such thing as forests of balsa trees, until now, growers are now producing plantations. Commonly the trees grow singly or in very small, widely scattered groups in the jungle. As balsa wood has become very popular in the modeling industry, growers had to find a way of producing a greater abundance of balsa, therefore plantations. For hundreds of years, balsa was actually considered a weed tree. They reproduce by growing hundreds of long seed pods, which eventually open up and, with the help of the wind, scatter thousands of new seeds over a large area of the jungle. There they lay and accumulate until one day there is an opening in the jungle canopy large enough for the sun's rays to strike the jungle floor and start the seeds growing.

Balsa trees grow very fast. In 6 to 10 years, the tree is ready for cutting, having reached a height up to 90 feet tall and a diameter up to 45 inches. If left to continue growing, the new wood being grown on the outside layers becomes very hard and the tree begins to rot in the centre. Not harvested, a balsa tree may grow to a diameter of 6 feet or more, but very little usable timber can be obtained from a tree of this size.

The start of the balsa business was during World War I, when the allies were in need of a plentiful substitute for cork, used primarily in floatation devices. The only draw back to using balsa was, and still is (in some cases), the back breaking work that is necessary to get it out of the jungle. Because of the way the individual balsa trees are scattered throughout the jungle, it has never been possible to use mass production logging procedures and equipment, until now. Before, the plantations, the best way to log balsa trees is to go back to the methods of Paul Bunyan - chop them down with an axe, haul them to the nearest river by ox team, tie them together into rafts, and then float the raft of balsa logs down the river to the saw mill. At the saw mill, the balsa is first rough cut into large logs or blocks, and then carefully kiln dried, and finally packed into bales for shipment. As a result of the balsa tree's fast growth cycle, both the quality and lightness of the timber obtained from a balsa tree can vary enormously depending upon the tree's age at the time of cutting.

The secret to balsa wood's lightness can only be seen with a microscope. The cells are big and very thinned walled, so that the ratio of solid matter to open space is as small as possible. To give a balsa tree the strength it needs to stand in the jungle, nature pumps each cell in the wood full of water until they become rigid - like a car tire full of air. Green balsa wood must therefore be carefully kiln dried to remove most of the water before it can be sold.

Most people are surprised to learn that compared to other woods, balsa is only about the third or fourth lightest wood in the world. It is not until balsa is reached in that line that there is any sign of real strength combined with lightness. In fact, balsa wood is often considered the strongest wood for its weight in the world. Pound for pound it is stronger in some respects than pine, hickory, or even oak.

There area two properties of commercial Balsa, Density and Cuts. Understanding these two properties will help you to select and utilize this material’s light weight while maintaining strength for your need.

The hobby industry demands the highest quality balsa but accounts for only 10% of the market. The largest user is the composite marine industry. In the 1940's the U.S. Navy's PBY flying boat is an example. Over 3 million water craft have been cored with various forms of balsa. Other industries using balsa are transportation, aerospace, surf boards, and motion picture props. The supply is virtually unlimited as the trees seed themselves long before the time they are harvested. Even though the hobby industry demands the highest quality, the quality within the industry varies greatly. Perfect, flawless wood, with exact density desired, accounts for 1⁄2 of the finished hobby product. The remaining half (which is considered scrap) is plagued with defects and saw kerf loss. Some of these defects affect the integrity of the wood while others are superficial.

THE DEFECTS AND HOW THEY AFFECT QUALITY

WEIGHT/DENSITY - Balsa, being a product of nature, is intrinsically inconsistent. The weight range is wide, ranging from 3 to 30 pounds per cubic foot, with the average about ten pounds. (Balsa less than 4.5 lb. is difficult to manufacture) Another factor taken into account is density distribution. A block or sheet of balsa that is light on one end may be heavy at the other end, or in the middle. There are no industry standards for density or weight groups.

GRAIN PATTERN/Irregular Grain Pattern - Balsa is usually described as A, B, or C grain. Contrary to the opinion, which says A grain is Excellent quality, C grain is bad, and B grain is average, or somewhere in-between this is NOT TRUE. A, B, and C, grain, actually designate the grain pattern and the way in which the block was cut from the log. If the log was cut so the annular rings run across the thickness, this is termed "A" grain . The cut is made tangent to the side of the log. If the annular rings run throughout the thickness, this is "C" grain. "B" grain is a cross between "A" and "C". "B" grain accounts for most of the grain found in balsa blocks. Since we are dealing with a product of nature, "A" grain may exhibit swirls, or the grain may not run parallel to the long side.

DISCOLORATION is a grayish color in the grain ranging from a few spots to complete coverage. It's caused naturally by minerals in the water absorbed by the tree. Although there is no structural weakness, the balsa just doesn't look good. Very light balsa exhibits discoloration. Discoloration can also be caused by a tree that was lying on its side on the wet forest floor for a period of time causing rotting. In this case the rotted portion is very soft and has no structural integrity. It is unusable.

WORM HOLES - Obviously caused by nasty little creatures that like to use balsa as a home. Worm holes vary in size from tiny pin holes to holes 1/4". Sometimes the holes are not visible on the surface of the block and they only manifest themselves after the block is cut into sheets. The manufacturer has several options to deal with worm holes. The portion of the sheet containing the hole can be cut out making a smaller sheet, the wood can be sold as a factory reject, or if need be, just ignored. If there is only one small pin hole in a large sheet, the problem is sometimes ignored as the sanding process tends to fill the hole with balsa dust and the pin hole is invisible.

KNOT HOLES can vary in size from 1/16" diameter on up. A knot hole of any size is cause for reject. Fortunately, the sheet of wood can be recycled by cutting down the width or length of the wood eliminating the defect. After the balsa is imported, the distributor, or any of the other Balsa converters, has little to say about the quality, except to specify for the highest quality available at the time. Like fine wine, seasonal variations in the weather effect the quality of balsa. If there was a long drought, one would expect a few years afterwards when the trees are harvested, the industry would be plagued with heavy wood. On the other hand, if the rainy season was exceptionally wet, we would be inundated with very light wood. When a trailer load of wood is received from the exporter , properties range widely. Therefore, grading and sorting the wood is the most important step in the manufacturing process, which accounts for most of the labor cost. The hobby industry uses various misleading adjectives to describe the quality of balsa. These terms or phrases are totally subjective in their meaning. Terms such as, Contest Grade, AAA Grade, Super Light, Virgin Quality, Light to Medium Weight, and Hand Selected, are only marketing oriented. There are no industry standard specific numbers placed on these terms.

CONSISTENCY - Just because a block of balsa is light and the grain looks fairly straight, that doesn't mean every piece of balsa within the block is useable. One end of the block may be light, but the other end may be heavy. The same is true with the grain and other natural occurring irregularities. The larger the block, or the larger the final sheet or strip, the more the effect of inconsistency.

WIND CHECKS - When the wind blows a little too hard in the rain forest, the balsa tree may bend just enough to crack. This crack is usually healed with time but unfortunately will show up during the manufacturing process.

DENTS: are usually caused by improper handling, not by nature. Strapping used to hold the bundles during shipping, are a major cause for this defect. Other dents are caused by tools and rough handling (Customs) or during the process of cutting down the tree and during the manufacturing process. Balsa being so soft doesn't take much abuse!

SPLITS - A split usually occurs at the end of an "A" grain sheet, perhaps by the manufacturing process or naturally. If the sheet is not handled, the split may not even be visible. It sometimes is found when the ultimate user it

Balsa is purchased in 20 and 40 foot containers from several plantations in Ecuador. We can specify balsa quality, width and length. Thickness varies from an inch or two to as much as 6" to 8". Specifications of grain, density, flatness, and color are usually random and up to the supplier.

After a month or two from ordering, the container of Balsa arrives at the port in Long Beach where it is sent to customs. At that point, customs decides whether or not to inspect the container, as shipments from Central and South America are suspicious of concealing drugs. If customs inspects the shipment, they can do serious damage to the Balsa, like drilling out core samples and pounding the Balsa blocks to listen for hollow sounds. This procedure causes extensive dents. Inspections could take up to six weeks, which we pay for.

After the Balsa is delivered to the distributor, the raw blocks go through a series of grading and manufacturing processes. The initial grading is one of the most important steps requiring an experienced operator with considerable knowledge in the characteristics of Balsa.

Each block of Balsa is separated and graded for density, quality, defects, and grain pattern. At that point one side of the block is carefully selected for certain characteristics, and then run through the joiner to provide a true surface for the next operation. After joining, the block of wood is rough sawn into sheets varying in thickness from 1/16" to 1", on large industrial type band saws which run continuously. They generate so much saw dust, they require their own dust collection system.

The sheets of rough-cut balsa are then put through an impressive looking machine, called a double- sided, wide belt, belt sander. This machine sands the rough band-sawn cut sheets into the final thickness, as required. Tolerances are held to within a few thousandths over the length and width of the sheets, which can be up to 12" wide and up to six feet in length. For this reason, the belt sander undergoes constant maintenance. After sanding, sheets are trimmed to length and undergo another critical inspection, for density, grain, and quality. The quality of our product is not what the customer gets, it's what the customer does not get! At this intermediate grading, Balsa is separated into premium, commercial, and utility grades.

Selected sheets are cut into sticks of various sizes using specially designed slitting saws. Warped sticks are rejected. Sticks may be further processed into special shapes such as, triangles, ailerons, trailing and leading edges. This operation uses shapers and specially designed cutter blades.

Now, why don't kit manufacturers use the best density and grain structure for the intended purpose. The answer is an easy one; economics and availability.

Again, balsa being a product of nature, grain structure is not consistent. a sheet of balsa may contain mixed grain, that is, some of each, A, B. and C. The larger the sheet, the likelihood of mixed grain increases. Therefore, when a customer specifies 'A" grain, the likelihood of that sheet being 100% "A" is remote unless the customer is willing to pay for the sorting and selection process.

Picture this scenario. A thermal glider kit manufacturer needs 1000 - 1/16" x 4" x 48" sheets of 4 - 6 pound per cubic.. foot, clear, 'A' grain balsa to used on the wing leading edge. This would make a beautiful wing but to provide wood to those specifications would probably require sorting through 10,000 blocks. After the block is cut into sheets each sheet would again require weighing and inspection for clear "A" grain. And, still, the order may not be filled. Why? Consider this. 5% of the balsa will be 4 - 6 pounds. Of that, 25% would be true "A" grain. Also, light wood has a tendency to contain spots and stains therefore only 50% of the remaining balsa would be usable. Other reject criteria (table 1) would reduce the yield to practically zero. In other words, perhaps 200 - 300 sheets of balsa may be found in an entire boxcar load to meet that specification. And at what cost, obviously so high that the manufacturer could not be competitive.

Another problem with grain is consistency. It's rare indeed when a sheet of balsa contains one type of grain structure. The larger the sheet the more likely it will contain mixed or cross grains. So a sheet of balsa can contain A, B, and C, grain and combinations of each.

Another factor that effects quality is the uniformity of grain, that is, are the grain lines more or less equally spaced or do they have a swirled pattern. Grain uniformity effects warpage. Sticks cut from non uniform grain patterns will warp or twist. This warp is due to internal stress in the balsa. If used in a structure, the resulting structure will eventually warp.

At the distributor balsa is selected for its intended use. Long straight grain wood of medium to hard density is ideal for the smaller sticks. The larger the stick used, the more of a medium weight. These properties will insure that the stick will not warp. Sticks found in retail outlets are almost invariably bent. This bending is due in part to the internal stresses of the wood and humidity. But most likely, because the balsa manufacturer did not take the effort to select balsa of the proper characteristics.

Balsa is purchased from the mill in blocks of wood ranging in size from 2" x 2" x 24" to 4" x 6" x 60". The larger the block, the more the cost per board foot. Example, the 2" x" 2 " block may cost $1.00 per board foot, whereas the 4" x 6" may cost $2.00 per board foot. When specifying balsa for purchase, two of the three dimensions may be specified. The third dimension is random. In most cases, the width, either 3" or 4", and the length, either 36 or 48" is specified and the width is random. This width varies from 2" to 6" or more, however wide blocks are becoming scarcer.

The 3" or 4" dimension (used for sheets of 3" & 4" wide) is controlled by the mill. If incorrect the block must be reworked and used for a smaller size. Stick sizes and sheet thickness are controlled by the manufacturing process and pose no problem as it is under the manufacturers control.

IN SUMMARY

Do these defects described in the article really effect the aerodynamic or the structural integrity of models. Do we really need perfect balsa?

If you are a modeler, the answer lies in the ultimate use you have for the wood.

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