The flowability of high-quality toner is a core indicator of its processing performance and print quality, and airflow milling technology, through precise control of particle surface roughness, has become a key technical path for optimizing flowability. The flowability of toner particles essentially depends on the balance of inter-particle interaction forces, where surface roughness directly affects the particle contact area and frictional resistance. When the particle surface roughness is high, microscopic protrusions significantly increase the mechanical interlocking between particles, leading to decreased flowability; conversely, smooth-surfaced particles, due to reduced contact area and friction, are more likely to form a uniform powder flow. Airflow milling technology uses high-speed airflow to drive particle collisions, achieving ultrafine milling under conditions of no mechanical contact. Simultaneously, by adjusting process parameters, particle surface roughness can be effectively controlled, thereby optimizing toner flowability.
The core mechanism of airflow milling is to utilize high-speed airflow (typically reaching sonic or supersonic speeds) to cause high-frequency collisions and shearing of particles in a turbulent field. Compared to traditional mechanical milling, airflow milling avoids particle contamination caused by equipment wear, and by adjusting the airflow speed, nozzle structure, and stager wheel speed, the milling and shaping process of particles can be precisely controlled. In toner manufacturing, airflow milling offers a unique advantage in simultaneously achieving particle refinement and surface smoothing: the high-speed airflow subjects particles to extremely high impact stress upon collision, rapidly smoothing out microscopic protrusions on the surface. A classifying wheel then uses centrifugal force to separate particles with the required size and surface morphology, while substandard particles are returned to the milling chamber for further processing. This dynamic balance mechanism ensures that the surface roughness of toner particles is optimized to the optimal range.
Controlling particle surface roughness requires comprehensive consideration of airflow velocity and nozzle design. Airflow velocity directly affects the energy density of particle collisions: too low a velocity results in insufficient collision energy and limited improvement in surface roughness; too high a velocity may lead to excessive particle breakage, generating new microscopic cracks or protrusions, thus increasing roughness. Optimizing the nozzle structure is equally crucial. For example, using Laval nozzles can improve the uniformity of airflow velocity, creating a stable turbulent field within the milling chamber and preventing surface damage caused by excessive localized energy. Furthermore, by adjusting the nozzle's spray angle and number, the directionality and frequency of particle collisions can be controlled, further optimizing the surface morphology.
The classifying wheel speed is another core parameter for controlling the surface roughness of toner particles. The classifying wheel separates coarse and fine particles through centrifugal force, while simultaneously screening the particle surface morphology. At higher speeds, only particles with smooth surfaces and excellent aerodynamic performance can pass through the classification zone, while rougher particles are trapped due to greater frictional resistance. At lower speeds, the classification criteria are relaxed, allowing more rough particles to enter the finished product, leading to decreased flowability. Therefore, by precisely matching the classifying wheel speed with airflow parameters, targeted control of toner particle surface roughness can be achieved.
The initial properties of the toner raw materials have a decisive impact on the airflow milling effect. For example, differences in hardness and toughness between resin and pigment lead to different deformation behaviors of particles during collision: hard particles are more likely to form smooth surfaces through brittle fracture, while tough particles may develop microscopic wrinkles due to plastic deformation, increasing surface roughness. Therefore, the optimal combination needs to be determined through pre-experiments during the raw material formulation stage to ensure the stability of the airflow milling process. Furthermore, the particle size distribution of the raw materials also affects the grinding efficiency: raw materials with a more uniform initial particle size can reduce over-grinding, thereby reducing fluctuations in surface roughness.
Optimization of the airflow milling process also needs to be combined with downstream processing requirements. For example, in two-component toners, the surface roughness of the carrier particles and toner particles must be matched to ensure uniform flowability after mixing; in low-temperature fixing toners, smooth-surfaced particles can reduce adhesion to the fixing roller, reducing energy consumption. Therefore, the parameter adjustment of the airflow milling process needs to be coordinated with the overall toner formulation design to achieve comprehensive performance improvement.
The airflow milling process provides an efficient solution for controlling the surface roughness of high-quality toners through particle collision driven by high-speed airflow and precise screening by a classifying wheel. By optimizing the airflow velocity, nozzle structure, and classifying wheel speed, the particle surface can be transformed from microscopic convexity to a smooth plane, significantly reducing the frictional resistance between particles and improving toner flowability. This process not only meets the stringent requirements of modern printing equipment for toner uniformity but also provides technical support for the continuous upgrading of toner performance.
