Composition & manufacture
Veneer plywood
Standard plywood veneer is still produced using a lathe, which peels a log in a similar manner to a blade pencil sharpener. Most decorative veneer is sliced from flitches after the log is cut into quarters. Small strips of veneer may be jointed into full-size sheets by edge gluing, stitching or using perforated paper adhesive tape. Open defects, such as knot holes, may be repaired using plugs or filler to upgrade the panel in accordance with grading rules. The dried, clipped or reconstituted veneers are sorted into grades, usually by visual inspection.
Synthetic resin adhesive is applied to the veneers which are then assembled with the grain of each normally at 90° to the adjacent veneer. (Plywood with special characteristics is produced when this rule of bonding at right angles is not followed.) The resultant assembly is known as a lay-up which are then subjected to pressure and heat in batches, most commonly in a multiopening (multi-daylight) press. This results in a compressed and cured panel which, after cooling, is trimmed to size and, if necessary, sanded.
Core plywood (Blockboard/Laminboard)
The technique of manufacturing blockboard and laminboard developed alongside the plywood industry from the turn of the century. Blockboard uses strips of wood about 25mm wide for its core, whilst laminboard cores are composed of strips of veneer on edge (or occasionally strips cut from plywood). Introduction of block or laminboard manufacturing facilities can help a mill to utilize residues by producing types of panel suited to interior purposes such as joinery, door blanks, furniture and shopfitting. The method of production is similar to that for plywood and the ‘wet’ stages of veneer manufacture are identical.
Main types of resin used for plywood manufacture
The relevant European standard describing Plywood bond performance is EN 314 which sets out three bond performances classes:
EN 314 Class 1 for Dry Interior uses
EN 314 Class 2 for High Humidity environments such as covered exterior uses
EN 314 Class 3 for Exterior uses out of ground contact
Plywoood manufactuers should describe their product using one of the three bond classes above, but there is still common useage of the withdrawn term WBP which in performance terms should be equivalent to EN 314 Class 3. This standard makes no restriction on the type of resin to be used but the most common types are:
Urea-formaldehyde (UF) Panels made with this type of resin normally achieve EN 314 Class 1 Bonds and are therefore only suitable for interior use.
Melamine Urea-formaldehyde (MUF) Panels made with this type of resin normally achieve an EN 314 Class 2 bond as they are more resistant than UF to moisture/weather. However, use of additional Melamine allows some manufacturers to achieve exterior EN 314 Class 3 bonds or even marine plywoods using melamine based adhesive.
Phenol-formaldehyde (PF) Panels made with this type of resin will achieve EN 314 Class 3 bonds and are therefore suitable for use in humid or exterior situations.
Plywood Durability
The relevant European standard for specification is EN 636 which allows manufacturers to declare their Plywood as suitable for use in particular end use circumstances.
EN 636-1 for Dry Interior uses
EN 636-2 for High Humidity environments such as covered exterior uses
EN 636-3 for Exterior uses out of ground contact
Clearly the primary criteria for determining Plywood durability is selection of the appropriate bond performance class hence the close link between the classes in EN 314 and EN 636. However, for exterior end uses the durability of the veneer species should also be taken into account when selecting plywood because of the potential for expose to prolonged high moisture content. In order to achieve long term performance in exterior (EN.636-3) uses all veneers used should ideally have a durability rating of Class 3 (moderately durable) or better in accordance with EN 350-2. See DD ENV 1099 for additional guidance on plywood durability.
Surface Appearance
The classification of Plywood by surface appearance is covered by many different standards around the world but in Europe this is dealt with by EN 635 “Plywood – Classification by surface appearance”, as follows:
• EN 635 Part 1. General.
• EN 635 Part 2. Hardwood Plywood
• EN 635 Part 3. Softwood Plywood
Limits are set for 5 different grades of veneer with the best grade “E” being virtually clear of all defects. Grades I, II , III permit increasing levels of defects with the lowest class IV having almost no limit on type and size of defect. The limits vary between softwood (EN 635-3) and hardwood (EN 635-2), so care has to be taken to select the correct definition of the grades. Manufacturers, declare the face grade followed by the back grade to achieve panel descriptions such as I/II or II/III.
Although the quality of veneer used in the manufacture of plywood will affect its mechanical properties, the veneer classes given in EN 635 are intended only for use in determining the visual appearance of the panel. They are not intended to be used as a basis for defining a structural grade of plywood.
The final colour of the panel will be affected by the species of veneer used, the finish applied and the effects of weathering and ageing. . Part 4 of EN 635 provides additional guidance for finishing/coating of plywood.
Applications
The range of species and bond qualities means that plywood can be engineered to have specific properties making it suitable for a wide range of applications. It is the only wood-based panel having established design values that can be used in structural applications under external conditions in accordance with EN 1955-1-1 Eurocode 5 (EC5) or BS 5268-2.
