For cornice panels, curved panels, and counter fronts-important factors in architectural installations and fixture manufacture-special constructions have been developed to meet required stresses.

Special laminated constructions have also been devised for the fabrication of artificial limbs, bobbins, die-boards, and gunstocks— all having the same fundamental characteristics of extra strength coupled with lightness of weight and the elimination of material waste in fabrication.

In the sign-painting field, hardwood plywood has attracted considerable attention. It provides a split-proof surface with fine painting properties. The saving in weight greatly simplifies sign hanging.

A limited amount of plywood has been introduced into the sporting-goods field. Tennis rackets, lacrosse and hockey sticks, toboggans, skis, and surfboards are being made of this material, and a constant chain of experiments foreshadows much wider use.

Practically all games and toys which require flat surfaces, or which are produced by jig-sawing, are being manufactured of hardwood plywood.

In trunk and luggage manufacture this exceedingly thin and lightweight material has proved ideal for side walls and for compartment divisions. Much of the lightest and most durable airplane luggage relies for its strength primarily on the hardwood plywood used in its construction.

Advantage has been taken of the ability of wood to stand molding in the production of woodenware in the form of trays and plates of hardwood plywood. These are practically indestructible and also possess unusual eye appeal.

While many varied industries have already found hardwood plywood indispensable, the field for its specialized applications is broadening steadily. The unlimited range of variations in panel construction gives hardwood plywood a use flexibility of wide appeal to the industrial designer.

WHERE AND HOW TO BUY PLYWOOD

Hardwood plywood can be readily obtained in face, wood, figure, thickness, size, and ply suitable for any purpose. Large warehouse and manufacturers' stocks are immediately available. Many local retail lumber or building-material dealers carry limited stocks or will quickly obtain your requirements for you.

The kind of hardwood plywood to specify will be determined by the use to which it is to be put. Panel faces obtainable range from highly figured (such as curly, mottled, broken stripe, swirls, burls, and crotches) to straight stripe, plain stripe, and comparatively plain or flat grain. Additional beauty is often obtained from the many woods by working them into designs of various combinations of grain and color.

Having determined the figure of the face desired, the color wanted can be obtained by selecting the proper kind of wood; although color can be obtained in the finishing, and not infrequently is varied when desired.

Three-ply and five-ply are most commonly specified. As three-ply construction is stronger than a board of equal thickness, and much

more strongly resists distortion from any cause, so is five-ply stronger than three-ply. The greater number of plies for a given thickness, the greater the strength. Your local building-supply dealer will be glad to advise you. Data are also available in the tables included herein under "Engineering Data."

If the conditions under which you plan to use the plywood are more than normally damp, waterproof glue should be specified, which will permit the plywood to be soaked in water without deterioration of the glue.

Sizes most generally stocked range from 36 inches wide by 48 to 96 inches long. Panels wider or narrower, longer or shorter, are obtainable, but there is a price advantage in buying the sizes most commonly used.

EXPORT TRADE

The volume of hardwood plywood exports is very small despite the acknowledged superiority of the American-made product. Definite data are not easily available, since official export statistics, as published in Foreign Commerce and Navigation of the United States, do not segregate hardwood plywood. Exports for 1937, however, are estimated at about 1,800,000 square feet valued at $120,000; Canada, United Kingdom, Mexico, and Latin American countries were the principal destinations. Europe has derived most of its plywood from the Scandinavian countries, Latvia, Poland, and Russia.

Foreign plywood technique is quite different from the American. The foreign product is usually manufactured in squares about 60 by 60 inches, whereas American plywood is usually oblong. Few European factories produce completely finished plywoods faced with finer woods. This is usually done by the fabricator-the furniture and cabinet maker-often by hand. Birch and alder are the predominant European woods used, although finer imported woods may sometimes be used for the face veneers. The structural and quality requirements abroad are not so exacting as those demanded by the American trade, although the per capita consumption is many times that in the United States.

ORGANIZATION OF THE INDUSTRY

Because of the close relationship between the manufacturer of veneer and plywood it is desirable to review briefly the trade associations covering these two branches of the industry. Although a number of hardwood plywood manufacturers produce a large part of their own fancy face-veneer requirements, it is important to note that in general the production of fancy veneers is a separate and distinct industry. The hardwood plywood industry and furniture manufacturers are the principal consumers of face veneers.

The trade associations representing these industries have been instrumental in creating greater harmony and a better mutual understanding of their basic problems. The associations have offered a ready means for the exchange of information leading to improvement in quality of manufacture. They have rendered great assistance to consumers in the selection of veneer and plywood best and most economically suited for each purpose.

PLYWOOD MANUFACTURERS INSTITUTE

The Plywood Manufacturers Institute (located at 205 West Wacker Drive, Chicago, Ill.) was formed in 1938. The principal objective of the organization is to promote the general development and improvement of the industry. More specifically, the association is interested in maintaining and increasing the use of plywood in both domestic and foreign markets; in encouraging the development of such standardization of the products as will be beneficial to the industry and consumers; in encouraging the preparation and adoption of a code of business ethics and the elimination of unfair trade practices; in collecting and disseminating such trade statistics as may be legally proper and beneficial; and in stimulating such engineering research as may benefit the producers and consumers of plywood.

THE VENEER ASSOCIATION

The interests of manufacturers and consumers of face and commercial veneers used in the construction of plywood are fostered through The Veneer Association (616 South Michigan Avenue, Chicago, Ill.). This association, founded in 1933, represents practically the entire hardwood veneer industry. The Veneer Association is national in scope and is devoted to the interests of wood veneer, regardless of species.

Trade promotion for the benefit of the industry as a whole is the primary object of the association. It was organized to promote, foster, and advance the interests of its members as manufacturers and distributors of veneers; to increase the use of their products; to improve the conditions under which the industry functions; to develop fair and just competitive methods; to collect and disseminate data necessary to the attainment of its objectives.

SOUTHERN PLYWOOD AND VENEER, INC.

The Southern Plywood and Veneer, Inc., consisting of manufacturers of hardwood plywood located in the Southern States from Virginia to Texas, was established in 1939. Although at the present time the organization (with offices in the Johnston Building, Charlotte, N. C.) is concerned principally with problems arising from labor legislation, it is expected that it will eventually expand its activities along those lines of, and in conjunction with, the aforementioned associations.

ENGINEERING DATA

TENSION TESTS

Table 1, following, shows the tensile strength parallel to the grain of the faces of three-ply wood of various common veneer species and the approximate strength of single-ply wood. The strength figures, given in pounds per square inch, correspond to the moisture contents listed. The test specimens used to obtain these values were 3 inches wide by 12 inches long, the central portion being trimmed down to a width of approximately 1 inch. They were held by ordinary flat grips and were tested in direct tension to rupture. The tensile strength is the average stress over the section at failure.

SHRINKAGE

The shrinkage of plywood varies with the species, the ratio of ply thickness, the number of plies, and the combination of species. In order to obtain a general average, the Forest Products Laboratory ran several hundred shrinkage tests on varied combinations of species and thicknesses in which three-ply panels were brought from a soaked to an oven-dry condition. The species included in the tests were mahogany, birch, yellow poplar, basswood, red gum, chestnut, tupelo gum, elm, sugar (hard) maple, black walnut, Spanish cedar, and spruce. The average shrinkage from the soaked to oven-dry condition was about 0.45 of 1 percent parallel to the face grain and 0.67 of 1 percent perpendicular to the face grain, with ranges of 0.2 to 1 percent and 0.3 to 1.2 percent, respectively. It is not unlikely that certain combinations of some other species may give wider ranges than these.

BENDING AND SPLITTING TESTS

The results of the bending tests are given in table 3. The strength values correspond to the moisture contents given. As a rule, bending tests were made on specimens measuring 5 by 12 inches, although some of the thinner specimens were cut to a length of 6 inches. The pieces were loaded as a column, with the greater dimension vertical. In half of the tests the grain of the faces was parallel to the direction of application of the load, and in half it was perpendicular to it. The ends of the test specimens were rounded in approximately a semicircle. Deflections were measured at the center of the panel. The product of the load and the corresponding deflection was recorded as the bending moment. Like the modulus of rupture in bending, the column-bending modulus is not a true stress existing in the fibers at the instant of failure. It is merely a measure of the magnitude of the external bending moment that a piece of plywood can withstand before it fails.

A comparison of the relative resistance to splitting of various three-ply panels will also be found in table 3. For splitting tests square pieces 314 by 34 inches were used. Upon the center of the test piece a conical spear was first dropped from a height of one-half inch. The spear was 8 inches long and 2 inches in diameter at the upper end and (with the shaft) weighed 11.22 pounds. Carrying the test piece upon its point it was then dropped from increasing heights until failure due to splitting occurred. The successive increment change in height of drop was one-half inch.

SUMMARY TABLES

The following tables were prepared by the Forest Products Laboratory. The results are based on tests of over 30 species. In general, 8 thicknesses of plywood, ranging from 10 to 1/2 inch, were tested. Most of the tests were on panels composed of three plies of equal thickness, with all plies of the same species and with the grain of successive plies at right angles. In addition, tests were made on plywood of various numbers of plies, having different ratios between core and total panel thickness, having the plies glued at angles other than 90 degrees with each other, and on plywood in which the core and the faces were not of the same species.

NOTE.-Sample computation.-To obtain the tensile strength of three-ply wood consisting of two 120-inch birch faces and a 16-inch basswood core: Parallel to face grain=2X20X19,820-1,982 pounds per inch of width; perpendicular to face grain=1X16X10,320-645 pounds per inch of width. This computation neglects the tensile strength of the ply or plies perpendicular to the grain, which is comparatively small, and the results are therefore slightly in error.

1 Specific gravity based on oven-dry weight and volume at test.

2 Based on total cross-sectional area.

3 Based on assumption that center-ply carries no load. Data based on tests of three-ply panels with all plies in any one panel same thickness and species.

4 Probably black cherry.

Probably Eastern cottonwood.

6 Coast region.

"Probably white fir.

8 Probably black gum.

9 Probably evergreen magnolia.

10 Probably Khaya species.

11 Probably tanguile.

12 Probably silver maple.

13 Sugar or black.