Testing method for properties of medium density board
The main production process of MDF is chipping → hot grinding → drying → paving → hot pressing → sawing → sanding → plate embryo sorting → packaging and delivery. In the test of tracking and measuring the temperature change trend of the plate core during hot pressing, the slab is equally and evenly divided into two layers, separated by yarn mesh (the purpose is to enable the thermocouple contact to accurately contact the position of the plate core, so as to accurately measure the temperature change of the slab core during hot pressing).
We mainly measure the absolute moisture content, density, plane tensile strength, static bending strength, water absorption thickness expansion rate of MDF. First, use an inductive moisture content tester to preliminarily measure the moisture content of the board, and determine the ratio of the difference between the mass of the test piece before and after drying and the mass after drying. The strength of the inner junction platform is the ratio of the maximum tensile force perpendicular to the plate surface to the area of the specimen. After microwave drying, the cross section density distribution of MDF is very uniform, which is obviously different from that of conventional hot pressing plate. This is due to the reduction of the temperature difference between layers of the slab by microwave preheating. After microwave preheating, the minimum density of the board was higher than that of the ordinary hot pressed MDF, and the percentage of low density of the core layer was significantly higher than that of the board without microwave preheating.
Because the microwave preheating enhances the heat transfer effect, the temperature difference is reduced, and the time required for the board core to reach the curing temperature of the adhesive is reduced. The assimilation effect of the adhesive in the MDF core has been improved. If the density difference between the surface layer and the core layer becomes smaller and the profile density distribution curve tends to be flat, then the changes of various strength properties will be opposite, that is, the internal bonding strength will increase, the nail holding force of the material will increase, and the mechanical processing performance will be better, but the static curvature will decrease slightly. Microwave preheating can improve the density of the core layer and the internal bonding strength of the MDF; MDF board surface density is still higher than the core density, which can maintain the hardness of the board surface and good finishing characteristics.
