Views:2 Author:Site Editor Publish Time: 2019-08-14 Origin:Site
Despite huge advances in computer technology and automation, plywood panels are still essentially compiled by hand. This is because of the large variation seen in the form and colour of veneers; they are not flat, not particularly flexible and are often split. These factors make it difficult for an automated system to cope with veneer. Humans can recognize veneer defects and can modify the layup accordingly. So human resources are largely required by plywood making industry.
The most common method of applying adhesive is with a double side glue spreading machine which applies a known amount of adhesive to both faces. Adhesive is applied to both faces of alternate veneers and not every veneer. For example, in a 3-ply panel adhesive is only applied to the middle veneer and for a 5-ply only to veneers 2 and 4. Other adhesive application systems for plywood making are used including: curtain coating, spraying and extrusion.
The grammage of adhesive applied varies with grade of panel, type of adhesive, wood species and, in particular, the surface smoothness of the veneer. Less adhesive is necessary for smooth faced veneers. A typical grammage will range from 140 to 240 g/m² per face (or glueline). It is sometimes quoted as a double face measure (and so double the figures given in the previous sentence) because the roller spreaders apply adhesive to both faces at once.
When laying-up plywood, the veneers placed on the outer faces should be the orientated so that their tight faces are exposed. If this is not done, then there is a risk of excessive checking on the surface with time as the veneer moves with changing atmospheric conditions. Some mills have semi-automatic lay-up lines where surface veneers are handled by machine.