Plywood is made from a minimum of three veneer layers which are then cross-glued together. Cross-glued veneer wood layers give plywood very good strength and dimensional properties. Plywood is commonly made from Birch, Spruce, Poplar and Pine veneers or in combinations thereof. Spruce plywood is light and stable and Birch plywood offers high strength and excellent structural properties.
For demanding applications, plywood can be coated using plastic or phenolic films or can be lacquered/painted and edges sealed to be resistant to moisture. Phenolic coated plywoods have excellent water proofing properties. Plywoods are most commonly glued together by using moisture resistant Phenolic Formaldehyde (PF) glues used in exterior applications and Urea Formaldehyde (UF) glues for interior applications. PF glues are dark brown in colour and are visually seen as a dark line between the veneer layers, this is regarded as the strongest glue bond available in plywood manufacturing today.
Plywood faces mainly determine the different types of grades of the panel and can be graded according to the number of knots, splits in the face veneers or the general appearance of the face veneers. A good reference is the “Handbook of Finnish plywoods” manual that is downloadable off of our site.
Appearance of Birch Plywood
UPM’s standard birch plywood classification is by the grades of the face veneers in accordance with SFS 2413 standard.
|Recommended for surfaces to be lacquered. A few pin knots with a diameter of up to 6 mm are permitted as is slight variation in colour. In standard sizes up to a panel size 1250 x 2500 mm the outer ply shall be of one veneer. Does not contain plugs.|
|Recommended for surfaces to be painted with non-transparent paint. A good, uniform grade. Some sound knots with a diameter of up to 20 mm and a limited number of other knots with a diameter of up to 10 mm are permitted. Discolouration is permitted.|
|A grade most used in structures. Minor surfaces defects are repaired with synthetic filler. A few sound knots with a diameter of up to 25 mm and other knots with a diameter of up to 6 mm are permitted.|
|Suitable for constructional purposes where appearance is not important. Open splits, knot holes and other defects permitted.|
Appearance of Spruce Plywood
Standard Finnish spruce plywood classification is by the grades of its face veneers in accordance with Standard EN 635-3.
|Best spruce grade, repaired with plugs, inserts and filler. Unrepaired knots with a diameter of up to 5 mm are permitted.|
|A grade which may have open splits and knot holes.|