Plywood grades

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Grading rules differ according to the country of origin, and the following list is therefore a general indication only.

A Face and back veneers practically free from all defects. A/B Face veneers practically free from all defects. Reverse veneers with only a few small knots or discolorations. A/BB Face as A but reverse side permitting jointed veneers, large knots, plugs, etc.

B Both sides as reverse of A/B.

B/BB Face as reverse of A/B. Reverse side as reverse of A/BB. BB Both sides as reverse of B/BB. WG Guaranteed well glued only. All broken knots plugged. X Knots, knot-holes, cracks and all other defects permitted. Marine Quality High-quality product usually of fir, makore, mahogany or similar highly durable wood throughout, bonded with phenolic resin glue to WBP specification. (Must withstand 72 hours boiling in water without failure of the glue-lines.)

Additionally the United Africa Company grade their Cresta plywoods under Gold, Silver and Blue labels. Construction is the same throughout, and the grading refers to the face appearance only. The bonding is phenolic resin to WBP specification, and every sheet is impregnated against insect attack (lyctus, furniture beetle, termites, etc.), which does not affect the gluing or polishing properties.

In practice it is not wise to go below BIBB, BB or grade 2 for veneering, while Douglas fir ply is altogether unsuitable owing to the upraised grain, and therefore grading rules for this particular kind of ply have not been included. It can be assumed that, unless specifically stated, all plywoods are dry glued, i.e. the veneers dried before gluing, and that except for the very lowest grades not suitable for furniture both the face and reverse are sanded flat for immediate use.

Plywood dimensions

In all usual sizes the length of the face grains is given first, irrespective of the length of the sheet. Thus a plywood sheet quoted as 48 in (122 cm) by 72 in (183 cm) will be 72 in (183 cm) in actual length and 48 in (122 cm) wide across the sheet and parallel to the direction of the face grain. Such a sheet is cross grained as distinct from long grained, as the face grain of the former runs counter to the length of the sheet, and it is important to remember this when choosing sheet sizes, for subsequent face-veneering which must be at right angles to the face grain of the sheet. Thus two wardrobe ends each 6 ft (183 cm) by 18 in (46 cm) which are to be face-veneered must be cut from a sheet 36 in (92 cm) by 72 in (183 cm), and not from one quoted as 72 in (183 cm) by 36 in (92 cm) in which the face grain runs the length of the sheet. Thicknesses in plywood commonly available with their nearest inch equivalents are as follows:

mm 0.8

1/32

mm 8

5/l6

12.5

3/8 1/2

3

1/8

15

5/8

4

5/32

18

3/4

5

3/16

22

7/8

6

1/4

25

1

Plywoods can normally be obtained in any of the following wood species: agba (tola branca), alder, basswood (Japanese lime), European and Japanese beech, European birch, Chilean pine, Douglas fir (Columbian or Oregon pine), elm, gaboon (okoume), lauan, limba (afara), African mahogany, makore, red meranti, obeche, Parana pine, European pine, poplar, sapele, sen, red and white seraya, red and white sterculia, tamo (Japanese ash), utile and yang. Most suppliers will, however, only stock the most popular, i.e. gaboon, lauan, birch, alder, Douglas fir, and limba, etc. Of these gaboon, birch and limba are excellent for veneering, but gaboon is inclined to woolliness in face-work, and birch is difficult to stain evenly. On the other hand, limba is medium textured, moderately hard and stiff, and its natural light gold-brown colour can be stained in simulation of oak, walnut and mahogany, therefore making it a good all-round choice. In all cases the wood species quoted refer to the face veneers only, and not to the cores, which may be of low-density woods.

Bending properties

Although plywood is normally used as a flat material, its natural resilience enables it to be bent to reasonably small radii of curvature without fracturing. The safe minimum of curvature will depend on the overall thickness, the individual thickness of the veneers, the species of timber employed and, to a lesser extent, the nature of the adhesive, while three-

layer plywoods composed of equal veneers will bend more easily than plywoods composed of thick cores and thin outer veneers. It is always better, therefore, to make up thick curved panels out of several thin plies rather than one thick ply, as the following table shows:

Radius of curvature

Thickness Along grain Across grain

1/2 in (12.7 mm) 8ft Oin (2.438 m) 6 ft Oin (l.828 m)

5/8 in (15.8mm) 10 ft Oin (3.048 m) 8 ft Oin (2.438 m)

The above figures were prepared by the Forest Products Research Laboratory in connection with the use of Douglas fir plywood, but also offer a reliable guide as to the bending movements of most other hardwoods bent dry at ordinary room temperatures. Synthetic resin bonded plywoods can also be soaked or steamed, and will then bend to smaller radii dependent upon the wood species. As will be seen from the table above the bending movement is greater across the grain than along the grain, therefore cross-grained panels should be chosen (see below). Plywood can also be scarf jointed provided the same type of adhesive be used, and the inclination of the joint is not less than 1 in 10 for boards under 1/2 in (12.5 mm) thick and 1 in 8 for over 1/2 in (12.5 mm).

Stability

As wood is a hygroscopic material, shrinking and swelling under varying temperature/ humidity conditions, plywood, which is composed entirely of wood, must also shrink and swell, but the total movement is very small owing to the strength of the glue-lines which lock the fibres in all directions. For instance, the Forest Products Research Laboratory has established in tests on 22 different species of 3/16 in (5 mm) thick three-ply that the mean swelling is only 0.18 per cent along the grain, and 0.27 per cent across the grain when subjected to a humidity range of 30 to 90 per cent, equivalent to raising the moisture content from 7 to 20 per cent. For all practical purposes in furniture the movement can be disregarded, although it will show on the edges of untreated ply as a fractional creep of long-grain veneers. The thicker the ply the less the movement is and the greater the measure of stability, while synthetic resin glues give greater measures of stability over the other glues. An additional advantage is that plywood has no line of cleavage and therefore will not split under adverse atmospheric conditions. This absence of lines of cleavage also affects the impact strength of plywood which is greater than that of solid wood subj ected to a similar loading; and in all practical applications thicknesses over solid timber can be reduced, with a 3/4 in (19 mm) thickness of ply roughly equivalent to a 1 in (25 mm) thickness of solid wood.

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