Manufacturing production process Hot pressing process of wet    hardboard

Manufacturing production process Hot pressing process of wet    hardboard

 

Hot pressing is the process of making  hardboard with moisture content close to zero and thickness of 3-4 mm from wet slab with relative moisture content of 65%-70% and thickness of 20-25 mm by heating and pressurizing. The combination of heat and pressure during hot pressing

 

Hot pressing is the process of making  hardboard with moisture content close to zero and thickness of 3-4 mm from wet slab with relative moisture content of 65%-70% and thickness of 20-25 mm by heating and pressurizing.

 

In the hot pressing process, under the combined action of heat and pressure, the thickness of slab decreases, density increases, moisture evaporates, water repellent reallocates, adhesive solidifies, and a series of physical and chemical reactions occur among wood components, so that the fibers are firmly bonded to form products with certain mechanical strength and water resistance.

 

  1. MECHANISM OF WET-PROCESS FIBER BOARD BINDING TO PLATE

 

The fibers interweave with each other and combine under the action of mechanical force.

 

Lignin melts at high temperature to produce adhesive force.

 

The free hydroxyl groups in wood components form chemical bonds.

 

The curing effect of resin adhesives binds the fibers together.

 

Some of the free hydroxyl groups or hydroxymethyl groups in the resin glue did not participate in the polycondensation reaction, and formed chemical binding force with the free hydroxyl groups in the wood components.

 

  1. Hot-pressing process of wet-process rigid hardboard

 

The hot pressing process of wet    hardboard usually takes the form of three stages of pressure, as shown in Fig. 6-7.

 

Pressure-time curve during hot pressing

 

Figure 6-7 Pressure-time curve during hot pressing

 

(1) Water squeezing section

 

The water content of the slab from the forming machine is very high, and there is a large amount of free water and bound water. After entering the hot press, free water in the slab is almost eliminated in liquid form by mechanical force in the water extrusion section. At this time, the relative moisture content in the slab is about 35%. By squeezing water, the slab structure is initially compact, and plays a role in reducing evaporation and saving heat energy consumption.

 

This stage requires high pressure, usually at 5-7 MPa. It would be better to have a shorter period of time. However, the maximum boost speed of hot-pressing plate should be equal to the drainage speed in the slab. Otherwise, if the pressure is raised too fast and the water can not be discharged, the slab will be broken by water pressure. In order to facilitate drainage, a metal mattress is set on the lower surface of slab, so that the water in the slab is mainly discharged from the mattress. But the boost speed should not be too slow. Otherwise, it not only prolongs the hot-pressing cycle and increases the consumption of heat energy, but also easily causes sticky plate phenomenon, resulting in waste products. The reason is that under the dual action of high temperature and water, lignin melts, hemicellulose hydrolyzes and caramelizes, forming sticky plate phenomenon. Therefore, the water squeezing section should stay for a short time after reaching the maximum pressure required by the process as soon as possible, and then immediately step down into the drying section. It takes 1.0-1.5 minutes to produce 3-4 mm wet-process rigid  hardboard.

 

(2) Drying section

 

By drying, the bound water in the cell wall and the water film attached to the surface of the fibers are discharged in the form of gases with heat energy.

 

Conventional hot press uses the principle of heat conduction to heat the slab, which belongs to porous material and has low heat transfer efficiency without contact pressure. But too much pressure is not enough. Otherwise, the void fraction of slab decreases and the resistance of water vapor movement increases. At the same time, the temperature of water vaporization in the core layer of slab rises with the increase of pressure. Excessive force prolongs the time of water evaporation. In order to ensure both efficient heat transfer and difficult water vapor discharge, the unit pressure in the drying section should be controlled at 0.5-1.0 MPa. At this stage, the temperature is generally controlled at about 130 C, and the relative moisture content of slab can be reduced to about 4%.

 

(3) Plastic section

 

A series of physical and chemical reactions should be produced in the plasticizing section of slab, which can make the slab structure finalize and reach the required density of the product; the residual water helps to form hydrogen bonding force; the pyrolysis reaction of the chemical components in slab and the re-melting and flow of the crust material on the surface of fibers to form bonding force; and the waterproofing agent. (Paraffin) Melting, spreading and redistributing on the surface of the fibers, and fixing on the fibers. In order to achieve the above objectives, both heat and pressure are needed: the process parameters used in the plasticizing section are generally: pressure 5-7 MPa, temperature about 200 C.

 

When producing    hardboard with thickness of 3-4 mm, the whole hot pressing period is 8-12 minutes with the above process parameters.

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