Forming characteristics and uses of high density polyethylene board

Forming characteristics and uses of high density polyethylene board

1. Material preparation
The high-density polyethylene board production step is the material preparation. The main equipments are chippers, belt conveyors, sieving machines, bucket elevators, storage bins, etc. The raw materials for the production of high-density boards are mainly wood fibers, including small diameter wood, branch wood, fuel wood, and processing surplus materials. The ideal raw materials are 60% -70% of coniferous wood, 30% -40% of hardwood, and bark content not exceeding 5%.
2. Preparation of fibers
The preparation of fibers is mainly the separation of fibers, which is the core of the production process of MDF. Fiber separation equipment includes sawdust silo (or preheating silo), small silo, thermal grinding, melting wax application device, sizing and measuring device.
The fiber separation steps are as follows:
A: Hot grinding. Qualified sawdust enters the sawdust bin, and the sawdust is softened by cooking and transported to the grinding chamber body. Wood chips with high moisture content are compressed by a hot mill to squeeze out water.
B: Waxing. Paraffin is heated and melted by a steam tube. The molten paraffin is pumped into the sawdust and then into the grinding chamber. After paraffin is separated into fibers, it is evenly distributed on the fiber surface.
C: Sizing. Urea-formaldehyde resin is injected into a double metering tank through a filter and a rubber transfer pump, and then into the discharge pipe of the pump to adjust the fiber size. The fibers in the discharge tube are in a high-speed flow state, and the glue solution is atomized and sprayed uniformly on the fiber surface. The curing agent and formaldehyde trapping agent can be added to the rubber tank for mixing.
three. dry
The main equipment of the high-density polyethylene board production process during the drying process includes a drying host, a drying pipe and a cyclone separator, a fiber conveying device, and a dry fiber silo. The hot mill discharge tube sucks up the wet fibers of the drying tube and makes full contact with the hot air. The fibers are transported in air by the air in suspension. The fiber runs in the air duct for 4-5 seconds, and the moisture of the fiber is quickly evaporated to reach the required moisture content (8% -12%).
4. Forming
Paving is an important part of the production process of high-density polyethylene boards. This process mainly includes planking, pre-compression, edge smoothing and cross section. Paving process requirements: uniform and stable board density, consistent thickness, continuous and stable control of board weight per unit area, and a certain degree of compactness.
5. Hot pressing
At present, the hot-pressing process for manufacturing high-density polyethylene boards in China is an intermittent multilayer hot-pressing process. Various process factors have a significant impact on the performance of MDF.
A: Hot pressing temperature. The selection of the hot pressing temperature mainly depends on the type and performance of the plate, the type of adhesive and the production efficiency of the press. The choice of temperature mainly depends on the comprehensive factors such as raw materials, tree species, fiber moisture content, adhesive properties, board thickness, heating time, pressure and equipment conditions.
B: Thermal pressure. During the hot pressing process, the hot pressing pressure changes. When pressure is applied to meet the thickness requirements, the pressure gradually increases. That is, stress should be reduced. When the adhesive is cured, various cohesive forces are formed between the fibers. The evaporation of water is mainly completed in the low pressure section, and the pressure in the low pressure section is generally 0.6-1.3Mpa.
C: Hot pressing time. The determination of the hot pressing time is mainly related to the properties of the adhesive, curing time, fiber quality, moisture content of the board, thickness, hot pressing temperature and pressure. Hot pressing time is usually expressed as the time required for a 1 mm plate thickness.
d: moisture content of the board. During the hot pressing process, the role of moisture in the high-density polyethylene board is to improve the plasticity and thermal conductivity of the fiber. Therefore, the proper moisture content can ensure the quality of the cardboard, which is generally controlled at about 10%. If it is too high, the density gradient between the surface and the core layer will increase, and the bonding force between the core layer fibers will be poor. When the pressure is reduced and the steam is discharged, the water vapor is difficult to remove, causing foaming and delamination in the board. If too low, the surface


Tag: High density polyethylene board


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