This article explains the calibrated plywood manufacturing process in a typical plywood factory in China.
- Log Cutting
- Veneer Peeling & Cutting
- Veneer Drying
- Glue Spreading
- Veneer Laying Up
- Cold Press && Surface Repair
- Hot Press (1st Hot Press)
- Calibration & Sanding
- Face Veneer / Film Lamination (2nd Hot Press)
- Quality Control
1. Log Cutting
Logs are cut into small length suitable for veneer slicing.
2. Veneer Peeling & Cutting
The veneers are peeled from shortened logs. The veneers are graded after the peeling. Two types of veneers are used in the plywood production for cost reasons.
- Thick and low grade veneers for plywood core
- Thin and higher grade veneers for plywood face
The plywood core has no requirement in apprearance so lower grades are acceptable for core composition. As long as its strength is good enough.
Scarf joint veneers are half sanded on the tip to obtain a scarf joint effect.
A certain amount of timbers are wasted in this process due to the peeler limitations and wood defects.
3. Veneer Drying
Newly peeled veneers are fresh and have a high humid content rate from 35% to 45%. In order to absorb the glue properly, they have to be dried properly in a dryer or hot press machine. Veneer shrinks about 4%-10% after a drying process. The veneer drying process will improve plywood characteristics making it more sturdy.
What if the veneers are not dry? The glue won’t be absorbed into the grains and adjacent veneers won’t bond together. Bubbles would emerge in the hot press stage.
4. Glue Spreading
A glue spreader will spread the veneers with glue on both sides. Only half the veneers should be glued to make sure a dry veneer is layed up with a glued veneer. Glue types depend on the plywood quality requirement.
1. Glue Viscosity
2. Moisture Content of Veneer
3. Glue Spreader
5. Veneer Laying Up
Glued veneers are layed up based on the structure type. To be clear, structural plywood and non-structural plywood have a totally different structure.
Unlike plywood mills in Europe or USA, Chinese plywood manufacturers use mostly labour in this process because each layer of the plywood core are not whole piece. Each layer contains either 3 pieces or 4 pieces veneer sheet.
Before any kind of pressure on stacked veneers, the veneers should stay certain amount of time. This time is to make sure veneers can absorb the glue evenly.
The grains of adjacent veneers are perpendicular to each other. This is to balance the strength of veneers to make sure the finished panel is balanced.
1. Loose and tight faces of veneer sheets should not be messed up.
2. Symmetry and Balance
3. Veneer layers should be odd numbers
6. Cold Press & Surface Repair
Cold press is a pre-press to save energy consumption. It can save time for hot pressing thus save production cost.
Surface repair after cold press can improve the hot press results. Cracks on top veneers are fixed with putty or veneer slice. All the veneer joints are filled with putty to make sure the surface are good enough before sanding.
1. Cold press time should be longer if the formaldehyde content is lower in the glue.
2. Flour can increase glue initial viscosity so to reduce cold press time.
3. Veneer moisture content should be 8-12%.
4. Cold press time should be adjusted according to the temperature and moisture content of the working condition.
7. Hot Press (1st Hot Press)
This is the key process for plywood manufacturing. The glue is cured under pressure and heat. Veneers are bonded together to make a solid board. Plywood without this process is called one-time press plywood, which is lower quality than two times pressed plywood.
- Load Plywood and Increase Pressure
- Keep Pressure (105-130°C for urea glue, 130-150°C for phenolic glue)
- Decrease Pressure & Unload Plywood
Phenolic glue plywood should be stacked after 1st hot press. It let the moisture go through the whole sheet and ventilate through the edges only. This can stop the phenolic glued plywood from warping.
The panel is cut into a square sheet in this process. A large amount of plywood residue is produced in this process. After cutting the edge off, workers will do the necessary fixing on the plywood edge. The fixing includes manual polishing and hole filling.
Trim the plywood vertically first, then trim horizontally.
9. Calibration & Sanding
The plywood sheet is not even in thickness because of the veneers used in each plywood are not always same. A calibration process can make same batch plywood thickness even.
Both faces of the raw plywood are sanded. After this process, the top veneers are polished to fine finishes.
Sand belt for pine veneer sanding shall be rougher than other wood species.
10. Face Veneer / Film Lamination (2nd Hot Press)
This is the last stage before you can get the finished product. It requires a thin decorative veneer or phenolic coated film. The decorative veneer might be polished for better appearance.
This step is considered the second hot press.
11. Quality Control
Quality control started already from the veneer peeling. After the surface veneer is laminated, there will be a final quality check before packaging. Defects on the surface and edge can be found out through visual examination.
The defective products need to be repaired.
Quality Control Factors:
- Dimension: Thickness, Edge perpendicularity, Verticality, Warpage
- Appearance: Grading.
- Physical properties: Density, Moisture Content, Formaldehyde Emission
- Mechanical Properties: Bonding strength, shear strength, wood breakage rate
All exported plywood is packed on crates with plywood strapped by steel or PET straps. The packaging is sea-worthy and fumigation-free. No solid timber are used in the crates. Film faced plywood edges are coated with waterproof paint before the packaging.
The plywood manufacturing process is a little different outside of China. For example, some birch plywood is made with whole piece veneers rather than joint veneers. See the below video to find out how plywood is made in the United States
There are a lot of new techniques developed for plywood production. A lot of new production lines come with robots which will replace many labors. Until now, most Chinese plywood mills are still using traditional production processes and machinery to do their work.