Principle
Flat mills and sanders are two common pieces of equipment in pigment dispersion processes, and their core goal is to break up pigment aggregates so that they can be evenly distributed in the carrier to achieve the desired fineness and stability. Flat grinders usually refer to laboratory instruments based on the principle of plate grinding, such as manual or automatic grinding devices with scraper fineness meters. It shears and rolls the sample at a set pressure and trajectory through a fixed grinding disc and a rotating grinding disc or scraper. Its dispersion mainly depends on the high shear stress generated in the narrow gap between the two surfaces.
The working principle of sanding machines, especially small sanders for laboratory use, belongs to media grinding. It drives the grinding media (such as zirconium beads and glass beads) to move in the closed cavity through the high-speed rotating stirring shaft, causing impact, shear and friction on the pigment slurry, so as to achieve the crushing and dispersion of particles. Its energy input is more intense, and the dispersion process is dynamic and random.
Fine-grained evaluation method
The evaluation of pigment dispersion fineness is usually measured directly using a scraper fineness meter, and the results are expressed in micrometers (μm). The fineness value reflects the size of the largest particles in the slurry, which is the key index for evaluating the dispersion effect. In addition, dispersion stability, color intensity, gloss, etc. are also important indirect evaluation parameters. Fineness (H) is correlated with particle size distribution, and the degree of dispersion can be approximated by empirical formulas, for example, dispersion uniformity can be correlated with the change in the surface area of particles.
The average particle size of the initial aggregate is assumed to be D0, and the target particle size is D after effective dispersiont, then the increase in specific surface area ratio ΔS can be approximately expressed as: ΔS ∝ (1/Dt - 1/D0)。 This relationship qualitatively shows that the decrease in fineness means a significant increase in the surface area of the pigment, which puts forward higher requirements for dispersion energy.
Parameter comparison
The following table compares the characteristics of the two devices in terms of dispersion pigment fineness from key dimensions, based on general technical principles and common standard operating procedures.
| Compare projects | Flat mill instrument |
| Core dispersion mechanism | High shear stress and rolling |
| Typical application stage | Preliminary formula research, rapid evaluation, fineness detection and sample preparation |
| Sample throughput | Less (usually gram) |
| Process controllability | The pressure, trajectory, and number of times can be set precisely, and the repeatability is high |
| Ultimate fineness potential | It is effective for soft aggregates and is difficult to break through some hard aggregates |
| Energy input mode | Static, directional, and shearing |
| Operation and cleaning | Relatively easy and fast |
| Compare projects | Sander |
| Core dispersion mechanism | Media impact, friction and shear |
| Typical application stage | Process optimization, small batch production, difficult dispersion |
| Sample throughput | More (up to hundreds of milliliters or liters) |
| Process controllability | The speed, time, media type and filling rate are adjustable, but there is media wear |
| Ultimate fineness potential | It can handle hard agglomeration, making it easy to achieve a lower, more uniform fineness |
| Energy input mode | Dynamic, random, and shock-oriented |
| Operation and cleaning | Relatively complex and involves media separation |
For flat mills, the grinding pressure, the number of cycles, the initial viscosity of the slurry and the wettability of the pigment are the main influencing factors. Insufficient pressure or too few reps may lead to incomplete dispersion; High viscosity may affect the effective transmission of shear forces. There is a plateau period for the dispersion effect, and the improvement in fineness is no longer obvious after a certain number of cycles.
For sanders, the choice of grinding medium (material, particle size, density), media filling rate, rotor line speed, and dispersion time all determine the dispersion energy density and efficiency. Smaller media particle sizes provide more contact points, which is beneficial for finer particles, but may also increase the risk of media wear contamination. Optimizing these parameters is key to achieving the target granularity.
Select a suggestion
The flat mill is suitable for rapid comparison of pigment dispersion, preliminary screening of coating ink formulations, and preparation of standard fineness detection samples with scraper fineness meters. Its advantages are that it uses a small amount of samples, has a high degree of standardization of operation, and has good repeatability of results, which can effectively simulate certain shearing processes in production.
Sanders are more suitable for research and development work that requires lower fineness and higher stability, or for process studies of difficult-to-disperse pigments, such as some inorganic pigments. It can simulate the actual production sanding process and provide a direct reference for process parameters for scale-up production. When choosing, it is necessary to comprehensively consider the pigment characteristics, target fineness, sample volume and subsequent process connection needs.
Conclusion
Flat grinders and sanders have their own emphasis on dispersing pigment fineness. As a static shearing device, the flat grinder is valuable in standardized testing and rapid evaluation; With its dynamic media grinding capabilities, sand mills show advantages in the pursuit of ultimate fineness and the handling of difficult-to-disperse systems. In actual R&D and quality control, the two are often used complementarily. Process personnel should reasonably select and use these two types of equipment according to the specific pigment system, fineness goals and research and development stage, and strictly standardize the operating parameters to ensure the accuracy of dispersion effect evaluation and the scalability of the process.