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The goal of printing ink production is to disperse pigment particles in the binder evenly, adding appropriate additives based on the specific ink type and performance requirements to form a stable suspension. The printing ink manufacturing process includes preparation, batching, mixing, milling, inspection, and barreling. The main equipment required for ink production includes resin refining equipment, mixers, kneaders, three-roll mills, ball mills, and sand mills.
Preparation refers to the processing and preparation of the main raw materials needed for ink production by ink factories, primarily including the preparation of pigments, binders, and fillers. Raw materials purchased by ink factories, such as pigments, often do not meet ink production requirements and require further processing until they are suitable for ink production.
The most common preparation phase involves binder refining and pigment production. Binder refining is a crucial step in the preparation phase. It involves selecting a few suitable raw materials from hundreds of binder sources based on a formula to create a binder with specific properties. Binder refining includes vegetable oil refining, resin-based binders, and solvent-based binders. Vegetable oil refining involves refining the drying vegetable oil to remove impurities and pigments, and then heating and refining it to produce a varnish of a desired viscosity. Resin-based and solvent-based binders are prepared by dissolving a solid or high-viscosity resin in drying vegetable oil and ink, or in an organic solvent, to form a sol-like substance.
The production of pigments mainly involves the synthesis of certain specific organic pigments in order to stabilize their color.
Batching is the process of placing the pigments, binders, and auxiliary agents specified in the recipe into a dedicated container. Different types of printing inks often utilize completely different raw materials and production processes, so the production of a specific ink requires batching according to that specific ink’s recipe. Batching is a key factor in determining the ink’s various properties, directly impacting its quality and printability. This crucial step in ink manufacturing ensures that the ingredients of each ink are strictly adhered to the specified recipe.
Mixing uses mechanical force to mix pigment powder with the binder, encouraging the binder to enter the pigment’s interstices and pores from a specific direction, wetting the pigment while removing air and moisture from the pigment’s surroundings, achieving a coarse dispersion. Mixing is accomplished in a blender, typically a planetary blender, butterfly paddle blender, high-speed impeller blender, or biaxial blender. Regardless of the blender type, the initial stirring speed should be low to prevent unwetted pigment particles from flying or escaping. After a certain period of stirring, high-speed stirring is applied to increase the shear force on the ink and break up the pigment clumps. When the ink becomes a paste, the machine can be stopped, and the ink barrel can be removed for rolling and grinding.
Ink factories also use kneaders for mixing ink. They are particularly suitable for mixing pigment filter cakes with water and the binder. Compared to blenders, kneaders are not only more efficient but also provide better dispersion. Because the kneading machine is composed of a trough and two rotatable stirring blades, when the kneading machine is running, the squeezing effect of the stirring blades repeatedly presses the ink onto the trough saddle, grinding, kneading, and mixing it until it is uniform.
A kneader typically consists of a W-shaped horizontal trough and two counter-rotating impellers in a Z-shape (or other special shapes) (S-shaped impellers are the most common).
Powerful Shearing, Squeezing, and Folding Actions:
The two impellers operate at different speeds, generating tremendous shear forces that quickly break down material agglomerates.
The narrow gap between the impellers and the trough wall creates a strong squeezing and grinding action on the material.
The movement of the impellers continuously divides, combines, and folds the material, much like kneading dough, achieving uniform mixing in all directions.
Suitable for Extreme Conditions:
A kneader can process pastes with viscosities as high as one million centipoise (cP), which are beyond the capabilities of high-speed dispersers.
It is particularly suitable for initially mixing large quantities of dry powdered pigments and fillers with viscous resins/binders to form a uniform paste.
Grinding, commonly known as rolling the ink, is a key process that mechanically transforms pigment into primary particles and disperses them in the binder, creating a stable suspension of the ink. However, grinding can only break up pigment aggregates and allow them to be wetted by the binder; it cannot crush the pigment to particles smaller than its primary size. Furthermore, while grinding can fully disperse the pigment, it can cause agglomerations to reappear upon storage, affecting the fineness and fluidity of the ink. Therefore, achieving a uniform and stable dispersion of the pigment in the binder requires more than simply increasing grinding. Improved wetting properties of the pigment and the binder are essential. The addition of surfactants can improve these properties.
Different types of ink mills are used to grind different properties, but all rely on shear, compression, and friction to disperse the particles. Typically, a three-roll mill is used to produce thicker, paste-like inks, while ball mills and sand mills are used to produce thinner or volatile inks.
A three-roll mill consists of three rollers of equal diameter. These rollers rotate at different speeds, with a speed ratio of 1:3:9. This speed difference creates a shearing effect that crushes the particles. The pressure between the three rollers can be adjusted, and the pressure affects the fineness of the ink after grinding.
Both ball mills and sand mills place a certain amount of balls or sand beads in a closed container. The relative motion creates impact and friction, pulverizing the liquid ink and achieving dispersion. Examples of this type of dispersing equipment include horizontal ball mills, vertical ball mills, horizontal sand mills, and vertical sand mills. The most common type is the horizontal ball mill, which features a simple structure and easy operation, but is relatively inefficient. Sand mills offer excellent dispersion, minimize color contamination, and enable continuous production with high efficiency.
A horizontal ball mill consists of a steel cylinder, a set of small balls, a reduction gear, and an electric motor. When the motor rotates, the reduction gear drives the cylinder. This rotation creates a rushing, pouring, or centrifugal motion within the cylinder. The repetitive pouring or downward rolling of the balls creates impact and friction with the ink, dispersing the toner.
A sand mill consists of a round tank, an agitator and hard sand. When grinding in a sand mill, ink is pumped into the bottom of the tank using a pressure pump. The high-speed rotation of the agitator drives the sand beads, grinding the ink from bottom to top. After filtering through a screen at the top, the ink exits the tank. Because small sand beads replace small steel balls, their contact area with the ink is increased, accelerating ink dispersion, shortening grinding time, and improving production efficiency.
In summary, the manufacture of printing inks is a precise and complex process. It goes far beyond simple mixing; it involves a systematic engineering process that closely integrates chemical formulations with mechanical processing technologies. From the meticulous preparation of raw materials and rigorous scientific proportioning to efficient mixing and stirring and final fine grinding, each step plays a decisive role in the ink’s ultimate properties—such as color intensity, stability, rheology, and printability.
The ultimate goal of this process is to achieve uniform and stable dispersion of pigments in the binder at the micron or even nanometer level, ensuring smooth ink transfer on the printing press and producing vibrant, long-lasting prints on substrates. It is precisely through precise control of these key steps and the efficient use of specialized equipment (such as kneaders, three-roll mills, and sand mills) that the modern printing industry can produce high-quality inks that meet the needs of various printing methods and end uses, continuously driving developments in fields such as packaging, publishing, and commercial communications.
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