Plywood Making Technical Skills

Plywood is a three-layer or multi-layer board like material made by rotary cutting wooden sections into veneers or by planing wooden blocks into thin pieces, and then bonding them together with adhesive. It is usually made of odd numbered veneers and bonded together with adjacent layers of veneers with their fiber directions perpendicular to each other. The following is the general production process of plywood:

1、 Raw material preparation

Selection of logs

Choose logs with straight trunks, no pests or diseases, and even annual rings. Common tree species include birch, pine, poplar, eucalyptus, etc.

The diameter of the logs is generally not less than 15 centimeters to ensure sufficient area of veneer is cut by veneer peeling machine.

Cutting and peeling of logs

Cut the logs to a suitable length (such as 2.44 meters) for subsequent processing.

Remove bark through mechanical peeling (such as a drum type peeling machine) to ensure a clean surface of the veneer.

Softening treatment of logs

For hardwood or logs with low moisture content, softening treatment (such as soaking in hot water or steaming) is required to improve the plasticity of the wood and facilitate rotary cutting.

2、 Single board preparation

veneer peeling

veneer  peeling: Fix the logs on the veneer peeling machine, and use rotation and tool feed to cut the logs into continuous veneers (usually 0.5-3mm thick).

Planing: Used for preparing decorative veneers (thin wood), the wooden blocks are sliced into thinner veneers (such as 0.2-1mm) by a planing machine.

Veneer drying

The moisture content of the veneer after rotary cutting is relatively high (about 60%~80%), and it needs to be dried by a dryer (such as a roller type or mesh belt dryer) to a moisture content of 8%~12% to avoid bubbles or deformation during bonding.

Single board sorting

Repair: Fill in defects such as joints, cracks, etc. on the veneer (such as using putty or patches).

Cutting: Cut the veneer to standard dimensions (such as 1220 × 2440mm) and remove any damaged edges.

Grading: Divided into different grades based on the quality of the veneer (such as flatness, number of defects), used for different layers of plywood (surface layer, core layer, bottom layer).

3、 Glue coating and assembly

mixing glue

Common adhesives include urea formaldehyde resin (UF), phenolic resin (PF), melamine formaldehyde resin (MF), etc. Choose the appropriate adhesive type according to the purpose of plywood (such as indoor or outdoor).

Mix the adhesive solution with curing agents and fillers (such as flour and starch) in proportion, adjust the viscosity and curing speed.

gluing

Apply adhesive solution evenly on the surface of the veneer using a glue spreader machine (such as roller glue spreader or curtain coating machine), with a coating amount generally ranging from 100 to 400 grams per square meter (adjusted according to the veneer thickness and adhesive type).

assembly

Layer design: The number of layers of plywood is odd (such as 3, 5, 7), and the fiber direction of adjacent layers of plywood is perpendicular to balance internal stress and improve dimensional stability.

Arrangement principle: Use high-quality veneer (with beautiful texture and few defects) for the surface and bottom layers, and use secondary veneer or scrap veneer for the core layer.

Pre pressing: After assembling the billet, apply low pressure (0.3~0.5 megapascals) through a pre press machine to preliminarily bond the veneer, facilitating subsequent transportation and hot pressing.

4、 Hot press bonding

Hot pressing process parameters

Temperature: Urea formaldehyde resin adhesive is generally 100-120 ℃, and phenolic resin adhesive is 130-150 ℃.

Pressure: adjusted according to wood density and veneer thickness, typically ranging from 0.8 to 1.5 megapascals.

Time: Calculated based on the thickness of the plywood, it takes about 1-3 minutes per millimeter of thickness (such as 18-54 minutes for hot pressing of 18mm thick plywood).

Hot pressing equipment

The commonly used equipment is a multi-layer hot press or a continuous hot press, and the surface of the pressure plate needs to be smooth to ensure that the plywood surface is flat.

Hot pressing post-treatment

After the hot pressing is completed, the plywood needs to be cooled to room temperature (either by air cooling or natural cooling) to avoid deformation caused by internal stress.

5、 Post processing

Trimming

Use a longitudinal and transverse edge cutting machine to cut the plywood into standard specifications (such as 1220 × 2440mm), removing edge burrs and glue stains.

Sanding

Polish the surface of the board with a sanding machine to remove burrs and unevenness, and control the thickness tolerance within ± 0.3 millimeters.

High end plywood requires multiple sanding processes (coarse sand, fine sand) to improve surface smoothness.