The hot press is the most important procedure in plywood production. This process is irrevocable.
If problems occur during the hot process, such as BLOWS (hollow spots inside plywood), it would cause big losses for the manufacturers.
Delamination Signs
- Bubbles or lumps on the plywood surface: These are sure signs of ply delamination.
- Blows: Blows are the hot spots inside the plywood. They are caused by the excessive moisture in the veneers. The blows can’t be found unless you cut the plywood.
Three Main Reasons For Delamination
The most important reasons are the veneer’s humidity content and the hot press workflow.
1. Veneer Humid Content
High-humidity veneers mainly cause bubbles and blows. The veneer’s humidity content should be controlled at less than 8%-12%.
If the humidity content is higher than 15%, there is a big chance that this plywood will have bubbles at the surface or blow inside the panel.
2. Hot Press Plate Temperature
The higher the temperature of the pressing plates is, the easier the plywood has bubbles and BLOWS.
But to ensure the bonding strength and glue curing, the pressing plate temperature can not be too low. For example, pressing the melamine glue board temperature can not be less than 105 degrees.
Therefore, temperature control is crucial in the hot press procedure. It should be set between 105 degrees and 120 degrees for producing plywood core board.
Since the hot press machine can’t have the same temperature at every corner of the plywood, workers should check its temperature frequently.
3. Pressure Reducing
The veneer and glue contain lots of humidity, which is not easy to get out during the hot press.
When the plywood temperature increases and high-pressure steam is generated inside the board. In the process of pressure reduction, this steam will get out of the board quickly. To make sure the steam doesn’t create bubbles and blow, workers shall reduce pressure slowly and carefully.
How to Find the Delaminations
The most common way in the Chinese plywood industry is to hit each piece of plywood and listen to the sound. If the plywood sounds hollow, then it should be picked out as a rejection.
Conclusion
The veneer quality and hot press temperature shall be controlled carefully so that less problems will occur for the plywood.
