The peeled veneer has a high moisture content (M/C) ranging from 30% to 110%. The M/C is determined by wood species, heartwood/sapwood ratio, heat treatment, etc.
In order to continue with the gluing and hot press process of plywood making, the veneers must go through a drying process. Veneer drying can be done by natural circumstances or veneer dryer.
Problems with Wet Veneers
The over amount moisture in the veneer can affect the spread of glue on the veneer surface. Wrong material can cause blister, glue penetration, glue peeling, etc.
High moisture veneer is not suitable for hot press process of plywood manufacturing. It will prolong the hot press time and discount the quality of wood panels.
Moisture Movement in Veneers
Water exist naturally in the wood fiber. It moves along with the direction of wood fibers.
However, the veneer is a long size thin layer, the moisture goes out of the veneer mainly by lateral direction in the veneer drying process.
In veneer drying, the direction of moisture movement is mainly the transverse conduction perpendicular to the fiber arrangement direction. So the biggest resistance is the resistance of the pits, followed by the resistance of the cell cavity and cell wall.
The greater resistance that affects the evaporation of water, and the critical layer where the surface of the veneer contacts the air. The widely used air jet dryer, based on the existence of the critical layer, uses a high-speed hot air stream perpendicular to the surface of the veneer to break through the critical layer to improve thermal efficiency and accelerate the evaporation of moisture.
Veneer Drying Process (With Veneer Dryer)
Veneer drying is a forced evaporation of moisture by heating in a modern veneer dryer or drying machine. In China plywood industry, this process is done with a dryer or natural drying.
1. Heating period
When the veneer first comes into contact with hot air, the surface temperature is low, and the vapor pressure of free water is lower than that of hot air. The moisture in the veneer cannot evaporate, and the heat in the hot air is mainly used for surface heating.
At this time, the surface temperature of the veneer is lower than the dew point, and the water vapor in the hot air condenses instantaneously on the surface of the veneer.
This time is very short during the veneer drying process.
2. Constant drying period
When the temperature of the veneer rises to the dew point, the surface moisture begins to evaporate, and the temperature continues to rise. The moisture on the surface of the veneer evaporates into the air through the critical layer, causing the capillary pressure difference between the surface and the inside, forcing the free water inside to move to the surface.
Due to the low resistance of free water movement and the severe drying conditions, the evaporation of water is large. The rate of evaporation of water during this period is roughly equal, so it becomes a constant-rate drying period.
The water evaporated on the surface is mainly supplemented by the capillary conduction near the surface. With the extension of the internal water conduction distance, the resistance becomes larger and larger, and the time for the evaporation rate to be consistent with the diffusion rate of the water inside the wood is very short. Therefore, the constant drying time is also very short.
During this period, free water was mainly evaporated, but due to severe drying conditions, almost at the same time as free water was evaporated, absorbed water also began to evaporate. This is manifested in the phenomenon that the veneer begins to shrink when the veneer begins to dry and the moisture content is far above the fiber saturation point.
3. Deceleration drying period
This period can be divided into two stages: the first stage and the second stage.
In the first stage, the temperature of the veneer generally remains at the wet-bulb temperature, and the speed of the internal water movement begins to be lower than the surface evaporation rate. Part of the surface is dried below the fiber saturation point. As the drying progresses, the internal moisture diffusion resistance increases and the drying speed of the veneer gradually decreases. In this stage, the free water has basically been evaporated, and most of the absorbed water has also been evaporated. This is the main stage of veneer drying.
Only part of the veneer in the second stage absorbs water, because the microcapillary shrinks and the gap becomes smaller, which makes it more difficult for water to move. Basically, the movement in the vapor state is the mainstay. At this stage, the heat supplied by the hot air removes evaporation In addition to moisture, the temperature of the veneer gradually rises until it approaches the dry bulb temperature
Among the above three periods, the first period of the first period, the second period and the first period of the third period, although the temperature of the heated air is very high, the temperature of the veneer is not much higher than the wet bulb temperature, so the high temperature causes less loss to the wood. Therefore, the temperature at the inlet end of the current new dryer is very high to improve the efficiency of the dryer. In the second stage of deceleration drying, because the veneer is close to the dry bulb temperature, the temperature at the end of the dryer should not be too high at this stage to avoid high temperature damage to the veneer.
Factors On Drying Speed
Veneer Factors
- Wood Species
- Initial moisture content
- Veneer thickness
In order to ensure the uniformity of the final moisture content of the veneer, it should be classified according to the tree species, the initial moisture content and the thickness of the veneer before drying, and then the respective drying process should be used for drying.
Drying Conditions Factors
- Temperature of hot air
- Relative humidity of hot air
- Air speed
Quality Control
| Defects | Reasons | Improvements |
|---|---|---|
| Edge cracking | Damage during handling Too High Temperature Over Done Drying Dryer problem Wet veneer over time stocked | Adjust the right temprature Spray water to edges before drying Carefully handle veneers |
| Discolouration | Too high temperature Dryer stopped during the drying | Set proper temprature Avoid dryer stop working |
Good veneer drying practice is essential for making good quality plywood.
