Relationship between slab moisture content and temperature of hot press

Hot-press pressure, hot-press time and hot-press temperature are the three elements of the hot-pressing process. In fact, the status of the artificial board slab during the hot-pressing process (water content, glue type, artificial board thickness, board type, etc.) It will also affect the hot pressing effect.

The moisture content of the slab is an important factor affecting the performance of the artificial board. Today we take a look at the moisture content of the slab.

The water content of the slab before hot press comes from three aspects. First of all, after veneer drying, it is absolutely dry according to the requirements of the process, and still maintains a certain amount of moisture in the material. The moisture content of dry shavings and fibers is generally 2%-5%, and the moisture content of veneer is generally 8%-10%. If powder adhesive is used, a higher water content should be used. In this way, it is beneficial to keep the resin glue layer when mixing glue.

Secondly, the liquid adhesive added when mixing the glue is the second source of moisture in the slab. The solid content of urea-formaldehyde resin glue used for particle board is generally about 60% to 65%, while that of plywood is 55% to 60%. The solid content of phenolic resin glue varies greatly. The common solid content of phenolic resin glue is 40%, that is, 60% is moisture. In many cases, too much moisture will increase the final moisture content of the slab. Recently, phenolic glue with a solid content of about 53% has been developed. In fact, these solid contents are not entirely phenolic resin glues, and some of them are fillers, which are used to reduce the moisture in the resin.

The third source of moisture in the slab comes from the polycondensation reaction when the resin glue is cured. The resin glue undergoes polycondensation to produce a portion of water. According to reports, urea-formaldehyde resin glue increases solid resin by 6%, which can increase the slab moisture content by 0.9%. In any gluing operation, the excess water content can be kept in the glue between the two pieces of wood. The average moisture content of the slab is usually controlled at 6% ~ 15%. Excessively high water content should prolong the hot pressing time: while the water content is too low, problems may occur when the press is closed, and the plate thickness cannot meet the requirements.

During hot pressing, a temperature gradient is formed in the slab, and its change is a function of the hot pressing time. When the hot press plate is closed, the surface of the slab immediately contacts the hot press plate, and the surface temperature of the slab rises quickly. At the beginning of hot pressing, the temperature in the slab is room temperature. With the extension of the hot pressing time, the steam quickly moves to the center and the temperature of the core layer rises. The temperature gradient between the surface layer and the core layer of the slab gradually narrows, prompting the resin to cure quickly. The moisture content gradient and temperature gradient exist not only in the thickness of the slab, but also in the plane, that is, there is also a temperature gradient between the core layer and the edge of the slab. The slab is pressed in a hot press for a long time, and the temperature gradient difference can be gradually reduced. Once the core reaches 100℃, the urea-formaldehyde resin glue can be cured within 30s, and the phenolic resin glue takes longer. The core layer temperature at the corners of the slab is difficult to reach this level unless the slab is placed in a hot press for a long time.

Different raw materials also have an effect on heat and moisture transfer. The relationship between heat and moisture transfer in the slab of two different shavings. When using granular wood shavings, if the water content increases, the time for the slab core layer temperature to start rising is slightly shortened, and the temperature of the core layer is kept constant, the water becomes steam, and the period of time when it evaporates from the slab has a great influence. Once the temperature of the core layer reaches the boiling point, all the heat will be transferred to this part, which is used to evaporate the remaining water. If there is a large amount of water, it will take a long time to evaporate these waters. When there is no moisture in the slab, the temperature curve is the same as the general heating curve.

All particleboards made of flake shavings have difficulty moving water. When the water content of this kind of slab is high, after the core layer reaches the boiling point, water vapor is not easy to come out, the steam increases, and the pressure also increases, so that the boiling point of water is raised. The steam pressure continues to increase until the steam pressure is sufficient to vaporize the moisture, and the moisture is reduced after evaporation. As a result of the reduction in steam pressure, the temperature drops to nearly 100°C, and the temperature rises again until the end of the constant temperature period.

Particleboard density also directly affects the time-temperature curve. The high-density particleboard is difficult to move due to strong pressure. The temperature rises slowly at the beginning, but continues to rise throughout the hot pressing process. After the core layer reaches the boiling point, if the moisture content of the slab increases, the steam pressure is higher and the temperature rises faster. As a result of the temperature rise, the moisture begins to evaporate and continues to evaporate, and the temperature-holding phase tends to be 100°C or more. For slabs with low density (steam is easier to escape), the temperature of the core layer rises rapidly and can quickly approach the boiling point of water. Because the slab is loose, it is helpful for the steam to pass from the surface layer to the core layer in the initial stage of hot pressing. Moisture begins to evaporate and continues to evaporate, keeping the temperature at the constant temperature stage not rising, and the steam pressure is not very high, which is unfavorable for the flow of steam to the edge of the board. The heating time during hot pressing is longer than normal. These can explain the effect of moisture content and temperature in the slab during hot pressing.

The temperature change inside the plywood is basically close to the particle board. However, since the hot pressing temperature is lower than the hot pressing temperature of the particle board, its initial rising temperature is different. This shows that the hot pressing temperature affects the initial rise temperature of the board.

The above is the relationship between the moisture content of the slab and the hot pressing temperature during hot pressing. I hope it will help you. If you have any questions about plywood machines, you can contact us, we will provide a complete set of plywood production solutions.