Views:7 Author:Site Editor Publish Time: 2020-04-20 Origin:Site
After the production of the plywood veneer production line is completed, we need to reprocess and pass the veneer through a series of production processes such as dryers, hot presses, and cold presses to finally produce plywood. When we use veneer dryer to dry the veneer, we will require the moisture content of the veneer. What determines the moisture content of the veneer?
The drying time of the veneer, the shrinkage rate of the veneer and the quality of the glue all affect the moisture content of the veneer. If only the drying efficiency of the veneer dryer and the output rate of the veneer are considered, a higher final moisture content is better. If we consider the bonding of the adhesive and the veneer, if the moisture content of the veneer is high, it will affect the strength of the bonding, and some problems such as degumming and deformation will occur. If the moisture content of the veneer is too low, it will also affect the bonding strength. Therefore, the final moisture content of veneer drying should be determined according to factors such as tree species, gum species and what kind of plywood is produced.
Different tree species require different veneer final moisture content. In the case of pine, etc., because the veneer itself contains a large amount of resin, the air permeability during hot pressing is poor, and the final moisture content is high, which will cause problems such as bulging. However, it will be different if it is ash, etc., because ash will have good air permeability and water permeability, and the slightly higher water content during hot pressing has little effect on the bonding strength.
The glue used is different, and the final moisture content of the veneer will also be different. For synthetic resin glue, the final moisture content of the veneer can be lower, if using protein glue can be slightly higher.
Various plywood factories in China require phenolic resin and urea-formaldehyde resin plywood to have a moisture content of 6% -12% after drying. The final moisture content of urea-formaldehyde resin plywood after drying is 5% -10%. Blood glue and soy bean plywood require a final moisture content of 8% -14% after drying.
Whether the moisture content of the veneer reaches the standard after drying is an important criterion to measure the quality of drying. So how to measure the final moisture content of the veneer?
There are two measurement methods, one is the weight measurement method (dry method), and the other is the continuous measurement method. The weight measurement method is to determine the moisture content of the veneer by weighing. The method is simple, accurate and reliable, and the single measurement period is long and lags behind. The continuous measurement method is to use the instrument for continuous measurement, which can achieve the integration of measurement and control, but requires that we must frequently calibrate the instrument to prevent errors.