1. Spindle speed
The dispersion efficiency of the sand mill is directly proportional to the number of rotations of the dispersion disc. Under certain conditions, increasing the number of rotations of the dispersing disc can improve the dispersion efficiency of the sand mill, but it also increases the power consumption and equipment wear rate, so it needs to be weighed and considered. The domestic sand mill is generally 800~1000r/min.
2. Knot average size
In order to ensure the formation of a satisfactory grinding area and improve the dispersion efficiency, the structural dimensions of the sand mill are usually as follows:
a. Cylinder diameter (D) and dispersion disc diameter (d)
D/d =1.1~1.5
Dd=40~100mm
b. Spacing of dispersing discs
H=0.2～0.4d
c. Dispersion disc diameter (D) and cylinder length (L)
L/D=4.0~5.0
3. Grinding media
a. Variety
The types of grinding media include quartz sand, glass beads, corundum porcelain beads and artificial agate. At present, only glass beads are commonly used in CHINA, which limits the improvement of the dispersion efficiency of sand mills.
b. Particle size
Generally, the finer the required fineness of the product, the smaller the diameter of the grinding media used, because the smaller the diameter of the media, the greater the probability of collision between the spheres and the shearing effect, and the higher the grinding and dispersion efficiency. However, if the diameter of the sphere is too small, the shearing force will decrease correspondingly, the wear will increase, and the dispersion efficiency will decrease instead. The diameter of the bead is generally 1~3mm.
Because it is difficult to obtain a good dispersion fineness with coarse-grained grinding media, and the fine-grained grinding media is used in the coarse dispersion stage, and its service life is low due to its small particle size. Therefore, it is recommended to fill the barrels of sand mills connected in series with grinding media of different diameters. The front sand mill is filled with medium with larger particle diameters, which can make use of its strong shearing force to make coarse pigments finer; the back sand mill The media with smaller particle size is filled inside to improve the grinding precision. Thus, dispersion efficiency and product quality are improved.
c. Density
Grinding media with different densities are suitable for different material viscosities. Generally speaking, glass beads can achieve good results in low viscosity materials, while low viscosity materials will cause excessive wear if used with high density media. It is a way to improve the efficiency of grinding and dispersing by using a slurry pump with a strong feeding capacity, and using high-density corundum porcelain beads to disperse pigments with a high density of thick friction slurry.
4. The loading amount of grinding media
Generally, the larger the filling amount of the medium, the higher the grinding and dispersing efficiency. However, due to the limitation of the density of the tribute, the consistency of the paint paste and the power of the main shaft of the sand mill, the filling volume should not be too large, otherwise it will be difficult to start. Generally, the sand loading capacity of the vertical sand mill is 60% of the simplified capacity, and the sand loading capacity of the horizontal sand mill is 85%.
5. Operating temperature
Although sand mills are equipped with cooling jackets, the operating mixing degree must be higher than room temperature. Practice has proved that temperature has a great influence on pigment dispersion. Since the structural strength of the grinding mill slurry decreases with the increase of temperature, it can be operated at a temperature of 40~50°C to improve the effectiveness. On the contrary, for those materials that are unstable at high temperature, they should be dispersed in a closed sand mill and the cooling effect of the sand mill should be improved.
6. Pre-mixing degree
The quality of the pre-mixing of the paint paste directly affects the normal operation of the sand mill and the dispersion efficiency. Therefore, attention should be paid to the sufficient wetting of the tumbling pre-mixing stage, so as to ensure the dispersion efficiency of the sand mill.
7. Composition of grinding slurry
In the case of the above factors being the same, the ratio of pigment-resin-solvent in the grinding mill base has a decisive influence on the grinding and dispersion efficiency of the sand mill. It is recommended to use the "sand mill grinding base composition" for production.
