The three elements of the hot press-the role of temperature

In the production process of artificial board, the slab is sent to a hot press for heating and pressurization, and maintained for a certain period of time, so that the adhesive in the slab solidifies into the processing process of the board. This process is basically the final process that determines the inherent performance of the wood-based panel product. The correct hot pressing process and reasonable equipment selection ensure the quantity and quality of products. To understand the hot press, we need to understand the basic principles of some hot presses.

Seeing you on the surface, the three elements of hot pressing are hot pressing pressure, hot pressing time and hot pressing temperature. In fact, the process of hot pressing has a great relationship with the condition of the slab, such as moisture content, rubber type, Thickness and board type. During hot pressing, the three conditions of the slab and the hot pressing process should be combined.

We first understand the role of the temperature of the hot press.

The heat is transferred to the backing plate through the surface of the hot-pressed board (when using the backing plate), the backing plate is then transferred to the board and the surface layer, and then gradually transferred to the center of the slab to promote the curing of the adhesive. Generally, the hot pressing temperature refers to the temperature of the hot pressing plate, and is regarded as a constant heat source.

The selection of hot pressing temperature should mainly consider factors such as the type of adhesive patch, board type, equipment capability tree type, slab moisture content and board thickness.

Generally, the temperature required for the phenolic resin glue is greater than 139°C; the urea-formaldehyde resin glue theoretically cures at 20 to 30 seconds at 100°C. However, the temperature varies greatly depending on the type of board. If the particle board is produced with urea-formaldehyde resin glue, the old hot pressing temperature is about 140°C. At present, the hot pressing temperature of many factories reaches 190～200℃; the hot pressing temperature of plywood is 105～120°C, and some poplar plywood factories only use 95℃. This mainly depends on the degree of easy discharge of moisture in the board, the speed of temperature transfer, the plasticity of the surface of the board, and the bonding performance.

When the plywood reaches the specified bonding strength, it is not appropriate to choose a higher temperature. The higher the temperature, the higher the water vapor pressure in the board, and the layered veneer is prone to process defects such as bubbling due to hindering the migration of water, and the temperature inside the board is difficult to release during the stacking process, resulting in the heat of the adhesive layer. Decomposition; the slab moisture content of plywood is also slightly higher than particle board and medium density fiberboard, high temperature is easy to cause deformation of the board; from the point of output rate, the production of plywood is expected to have less plastic deformation; for trees with large acid buffer capacity , Like poplar,

When producing urea-formaldehyde resin plywood, the hot pressing temperature is high, and the phenomenon of bulging and delamination is particularly serious; it is not appropriate to use a higher temperature for plywood with a large thickness and number of layers. Considering the above reasons, even if the phenolic resin glue is used to produce plywood, the hot pressing temperature should not exceed 150°C.

For new particleboard or fiberboard, hot presses generally use higher temperatures. For single-layer hot presses, hot press time is particularly important. The heat of the hot press can also be supplemented by the heat released from the polycondensation reaction where the adhesive is cured. According to reports, the heat released by polycondensation in the reaction is up to 20% of the total heat, usually 10% to 15%. In the early days of particle board development, it was pressed at low temperature for a long time. Later, the new type of adhesive required high temperature heating, the temperature reached 150-200 ℃, shortening the hot pressing time. The higher the temperature, the shorter the time required for hot pressing. The limit of the heating temperature is appropriate not to pyrolyze the wood. In the hot press, in addition to curing the adhesive, the high temperature can also increase the plasticity of the wood, which is convenient for compacting the slab. Studies have shown that increasing the temperature and increasing the water content of the surface shavings will produce plastic deformation under pressure. If the hot press does not heat or the heating is insufficient, higher pressure is required to achieve proper contact of the shavings.

Heat also affects the polycondensation process of the adhesive. While heating the glued material, the adhesive is subjected to heat, the internal frictional force begins to decrease, and the viscosity drops to a minimum at 60 ~ 80 ℃, thus improving the contact between the glue and the raw material. This flow time is very short, as the temperature in the slab increases, the internal friction of the glue increases rapidly after a few seconds, and finally the adhesive is fully cured. When the internal friction starts to decrease, the surface tension of the adhesive also decreases, making the surface adjacent to the material to be wetted easily, and the adhesive liquid is also easily transferred from this surface to another surface. Heating the slab causes water and glue to move under vaporization, diffusion and capillary action, and melts and diffuses on the surface of all glued materials.

The thickness of the slab also has an effect on the core temperature. The time for the core layer to reach 100°C has a great relationship with the thickness of the slab. The greater the thickness of the particle board, the longer it takes for the core layer to reach 100°C. For example, the thickness of the flower plate was increased from 5mm to 50mm, that is, the thickness increased by 10 times, and the heating time increased from 90s to 43min, the time increased by 27 times. In fact, for thin particleboard (such as less than 8mm), the core layer is kept at a constant temperature for almost no time, especially when the moisture content is low. On the other hand, when the actual wood-based panel is hot-pressed (the temperature is higher than 150°C), the temperature of the center layer of the thick slab (thickness greater than 12mm) rises to a critical value, and the temperature rarely changes within a period of time, which can be approximated as Constant 103-105℃.

So what happens to the temperature inside the board after heating?

After the slab is hot pressed, that is, starting from the opening of the press, the temperature of each layer inside the slab changes differently at different hot pressing temperatures. Take the temperature change inside the ordinary particleboard after hot pressing as an example. The internal temperature change of the slab is not only determined by the slab's own factors, such as the density of the slab, the thickness of the slab, and the moisture content of the slab; but also due to changes in external conditions, such as ambient temperature, air flow speed, etc. Knowing the law, it is convenient to make full use of the residual heat in the board and solve the actual production problems. For example, shorten the hot pressing time, use the appropriate time for the post-curing of the adhesive; control or reduce the free formaldehyde in the board; arrange the cooling board time and process reasonably.

The above is the function of the temperature of the hot press. If you have any questions about the machine, please contact us at any time. Our company can provide you with a complete set of plywood production solutions according to customer needs.