The definition and detailed classification system of plywood
Plywood, as an important engineering wood material, is a type of board made by bonding and pressing three or more layers of thin wood veneers together with adhesives in accordance with the principle that the grain directions of adjacent layers are perpendicular to each other. This unique structure endows plywood with excellent physical and mechanical properties as well as dimensional stability.
According to different classification criteria, plywood can be divided into various types:
Classified by use:
Ordinary building plywood: mainly used for concrete formwork, temporary buildings, etc
Plywood for furniture manufacturing: It requires high surface quality and good environmental protection performance
Packaging plywood: Emphasizing strength and durability
Plywood for vehicles and ships: Special moisture-proof and anti-corrosion treatment is required
Classified by water resistance:
Class I plywood (weather-resistant plywood) : Made with phenolic resin glue, it can be used outdoors for a long time
Class II plywood (water-resistant plywood) : It uses urea-formaldehyde resin glue and is suitable for humid environments
Class III plywood (moisture-resistant plywood) : Suitable for general indoor environments
Category four plywood (moisture-resistant plywood) : For use only in dry environments
Classified by surface treatment:
Sanding plywood: The surface has undergone sanding treatment, making it smooth and flat
Veneered plywood: The surface is covered with decorative paper, wood veneer or plastic film
Coated plywood: The surface is treated with paint or other coatings
The complete process flow and technical key points of plywood production
1. Log selection and processing
The production of high-quality plywood begins with strict selection of raw materials. Common tree species include poplar, eucalyptus, birch, pine, etc. The diameter of the logs is usually required to be over 20cm and the length to be 2 to 4 meters. Logs need to undergo steaming treatment (soaking in hot water at 60-80℃ for 12-24 hours) to enhance their machinability.
2. Rotary veneer cutting process
Rotary cutting is a key process in the production of plywood. Modern rotary cutters can cut logs into continuous veneer strips with a thickness of 0.1 to 4.5mm. When rotary cutting, it is necessary to control the cutting Angle (usually 89°-92°) and the gap between the knife door (approximately 80% of the thickness of the veneer) well. High-quality veneer should have uniform thickness and a smooth surface without tears.
3. Veneer drying technology
Veneer drying usually employs a drum dryer, with the temperature controlled between 120 and 180 degrees Celsius. The moisture content of the veneer after drying should be controlled within the range of 8% to 12%. Excessive moisture content can lead to poor gluing, while too low a moisture content can easily cause the veneer to become brittle.
4. Gluing and assembly process
Common adhesives include urea-formaldehyde resin adhesives, phenolic resin adhesives and melamine-modified adhesives, etc. The amount of adhesive applied varies depending on the type of adhesive, usually ranging from 120 to 200g/m² (on both sides). When assembling the veneer, the textures of adjacent veneers should be vertically crossed, and the number of layers is generally odd to ensure a symmetrical structure.
5. Key technologies of hot pressing forming
Hot pressing is the core process in the forming of plywood. The typical hot pressing parameters are: temperature 110-140℃, pressure 1.0-1.5MPa, and time 0.6-1.2 minutes /mm depending on the thickness. Precise control of hot pressing parameters is crucial to product quality.
6. Post-processing and machining
The hot-pressed boards need to be cured for more than 24 hours, and then undergo edge cutting, sanding and other treatments. High-end products still need to undergo deep processing such as defect repair and surface coating.
A complete index system for the quality evaluation of plywood
Physical performance indicators:
Moisture content: The national standard requires 8%-14%
Density: Usually between 0.5 and 0.8g/cm³
Thermal conductivity: Approximately 0.12-0.15W/(m·K)
Mechanical performance indicators:
Bonding strength: ≥0.70MPa for Class I plates, ≥0.70MPa for Class II plates
Static bending strength: ≥30MPa (along the grain), ≥20MPa (across the grain)
Elastic modulus: ≥3500MPa (along the grain), ≥2500MPa (across the grain)
Environmental protection performance indicators:
Formaldehyde emission (in accordance with GB/T 17657-2013) :
E0 grade ≤0.5mg/L
E1 grade ≤1.5mg/L
E2 grade ≤5.0mg/L
VOC emission: Complies with the requirements of GB 18580-2017
Appearance quality requirements:
Surface flatness: ≤0.5mm/m
Edge straightness: ≤1mm/m
Surface defects: No serious defects such as decay and dead joints are allowed within the limit
