Views: 0 Author: Site Editor Publish Time: 2020-07-27 Origin: Site
In the hot pressing process of the hot press, the total hot pressing time can be divided into several parts according to the pressure and temperature. The main points to be noted are the closing time and the main hot pressing time. If there is a thickness regulation, the main pressing time is from the contact of the hot plate with the thickness to the opening of the hot plate. Closing time is a function of pressure. Generally speaking, the main pressing time is the most significant and has the greatest impact on the particleboard properties, such as directly affecting the density of the particleboard thickness, the thickness control of the particleboard, the surface quality of the particleboard, the durability of the glue and the pre-curing. In addition to the closing time and main pressurization time affecting these, other times also have economic significance. Any part time reduction can increase productivity and reduce costs. The closing time has an effect on the density of the particleboard thickness. Under most hot pressing conditions, the density forms a gradient, and the surface of the slab is relatively dense and dense. Different pressure and closing time have different effects on the density of particleboard. For example, there are three types of particleboard with the same average density. Except for the closing time of the hot press, the raw materials and manufacturing methods of these three types of particleboard are the same. The closing time of the hot press is short (high initial pressure), the density of the surface layer is high, the density of the core layer is small, and the density difference between the thickness of the same particleboard is large. On the contrary, the hot press has a long closing time (low initial pressure), and the density of each part of the thickness is close to the average density of particleboard. Due to the relationship between density and strength, the former has high flexural strength. Although the plane tensile strength of the latter is increased, its bending strength is much lower than the former.
The closing time of the hot press has an effect on the thickness tolerance of particleboard and medium density fiberboard. The closing time of the hot press is long, especially when hot pressing at high temperature. After the pressure is relieved, the surface layer adhesive is pre-cured, resulting in poor adhesion, so the thickness of the board has a larger rebound rate and the thickness is difficult control. The closing time speed of the hot press is generally controlled at 15-30mm/s. Too fast will also make the internal exhaust of the slab insufficient, resulting in damage to the slab structure.
The no-load fast closing section is an empty stroke, which is mainly to eliminate the space of the upper (lower) hot pressing plate of the slab. This time should not exceed 10s. Too long time firstly affects productivity, and secondly, it is easy to thicken the pre-cured layer on the lower surface of the slab and affect product quality. Of course, too fast may also blow away small materials on the slab surface.
The hot pressing time of the hot press is to make the adhesive in the wood-based panel have a higher curing rate, and at the same time make the wood have a certain degree of plasticity, reach the required thickness of the board, and also avoid the board's process defects, such as bubbling. The hot pressing time of the hot press is mainly determined by the following factors: ① the curing speed of the adhesive; ② the moisture content of the slab; ③ the thickness of the board; ④ the board species; the tree species; the hot pressing temperature, etc.
For example: commonly used urea-formaldehyde resin glue, the hot pressing time of plywood is 0.7 ~ 1.0 min/mm; the hot pressing time of particle board is 14 ~ 40s/ mm; the dry medium density fiberboard is 20 ~ 30s/mm.
In addition, the hot pressing time of the hot press should not be too long, otherwise it will cause the thermal decomposition of the board surface adhesive and the degradation of wood cellulose macromolecules at high temperatures. In addition to the closing time and hot pressing time of the hot press, other time reductions are also economically meaningful. Any part time reduction of the hot press can increase productivity and reduce production costs.