Mill visits to manufacturers and users of edge-glued panels were conducted in order to characterize the quality of edge-glued joints in appearance products. During these visits, panels with gluelines of good and poor quality were collected for further analysis in Forintek’s laboratory. Microscopic measurements served to determine that the maximum acceptable width of glue joints in edge-glued panels is 0.05 mm. The main causes of troublesome gluelines resulting from ripping operations are splintering at the juncture of the edge and flat surface, excessive edge roughness, and uneven straightness of the saw kerf, although the right angle of the saw is also a critical parameter. The percentage of mill-rejected panels as a result of these problems ranges from 0.5% to 3%.
A series of edge roughness measurements taken from a sample of strips from participating mills set the stage for the development of representative roughness values for the edges of strips used in the industrial production of edge-glued panels. Edge roughness measurements taken from strips ripped in the laboratory showed the impact of various factors on edge roughness values: saw blades, feed speed and chip load. Measurements taken from edges ripped with worn saw blades indicated that edge roughness cannot be used to determine saw blade wear values because average roughness values obtained with such blades were found to be similar to those of strips ripped with well-sharpened saws.
Following the laboratory assembly of panels using strips exhibiting a wide range of edge roughness, measurements revealed that edge roughness contributes to increased glueline width, a greater proportion of gluelines wider than 0.05 mm (too apparent) and a reduction in glueline shear strength. Glueing parameters (type of glue, clamp pressure, ambient temperature, etc.) were constant throughout the production of laboratory panels.
Finally, the results of this study suggest that edge roughness values of 9 µm for Ra and 80 µm for Rt allow large-volume manufacturing of panels with good quality gluelines and that an increase in edge roughness will result in more apparent gluelines.