How it works:
A three-roller grinder is a kind of equipment that finely grinds and homogenizes materials through shearing, extrusion and dispersion. Its core consists of three rollers arranged in parallel with different rotational speeds, usually made of cold carbide cast iron or ceramic, and the gap between the rollers can be precisely controlled by means of an adjustment device. When working, the material is first fed between the slow roller (rear roller) and the middle roller, and is subjected to strong shear force in the roll gap; Then it is passed between the middle roller and the fast roller (front roller) for secondary grinding and dispersion. Finally, the scraper collects the finished product from the surface of the fast roller. This process can effectively break up agglomerated particles and achieve uniform dispersion of pigments in the binding material, especially suitable for processing high-viscosity systems.
High viscosity ink usually refers to a system with an apparent viscosity of more than 10 Pa·s at 25°C, which is commonly used in screen printing, gravure printing and some special printing fields. These inks contain a high proportion of solid pigments, fillers, and high molecular weight resin binders, resulting in poor fluidity and strong cohesion. In the process of dispersion, the main challenges are as follows: pigment aggregates are difficult to open completely; The heat generated by shear during the grinding process may lead to changes in the heat sensitivity of the resin. Difficulty in material conveying and roll gap filling at high viscosity; It is easy to produce bubbles that affect the quality of the final film layer. Therefore, process control needs to take into account both dispersion efficiency and system stability.
Process parameter control
Successful processing of high-viscosity inks relies on co-optimization of multiple process parameters. The roller clearance is the primary control variable, usually set in two stages: the first stage clearance (between the rear roller and the middle roller) is wide, mainly used for preliminary mixing and feeding; The second stage clearance (between the middle and front rollers) is narrow and provides the main dispersion of shear forces. For high-viscosity materials, the initial clearance needs to be moderately increased to ensure stable feeding, and then gradually narrowed to the target fineness. The roller speed ratio (e.g., 1:3:9 for the rear roller: middle roll: typical for the front roller) determines the shear rate gradient, and a moderate speed ratio should be used for high viscosity systems to avoid excessive temperature rise. The roller temperature is controlled by an internal circulating cooling medium, and it is recommended to maintain the operating temperature 10-15°C below the resin softening point to prevent sudden changes in viscosity. The number of grinding passes is determined based on the initial particle size and target fineness, which can be monitored online by the Hegman fineness meter until a predetermined standard is reached (usually a fineness ≤ 15 μm is required).
Process flow
The complete process consists of the following steps: Pre-mixing, which uses a planetary mixer to preliminarily mix pigments, resin binders and some additives into a homogeneous paste. Secondly, the initial feeding is carried out, the gap between the rear roller and the middle roller is adjusted to 0.5-1.0mm, and the roller is started at low speed and evenly loaded. This is followed by the first stage of grinding, where the material passes through the gap of the first roller and is collected to the pallet at the front end of the middle roller. Then the secondary grinding is carried out, and the gap between the middle and front rollers is adjusted to 0.02-0.05mm for fine dispersion. After that, cycle and test, return the collected material to the feeding end, and take samples to test the fineness after each grinding until it is qualified. Finally, the finished product is collected, and the material is completely scraped off from the front roller with a scraper, and defoaming is carried out if necessary.
The relationship between shear stress τ and viscosity η, shear rate, γ during grinding can be roughly described by the following formula: τ = η × γ. In the clearance area of a three-roller grinder, the shear rate can be estimated as γ = (v₂ - v₁) / h, where v₁ and v₂ are the adjacent roller line speeds and h is the roll gap spacing. For high-viscosity systems, a high shear rate is usually required to achieve the shear stress required for effective dispersion, which is achieved by adjusting the velocity ratio and clearance. In addition, the relationship between dispersed energy input E and the final particle size d can be empirically expressed as d ∝ E^(-k), where k is a constant related to the properties of the material, indicating that sufficient energy input is essential for refining the particles.
FAQs
| Problem phenomenon | Possible causes and adjustment directions |
| The material cannot enter the roll gap | the initial viscosity is too high; uneven pre-mixing; the roll gap is too small; Add wetting agents appropriately or increase the premix temperature |
| The fineness is never up to standard | insufficient shear force; the number of grinding passes is not enough; improper pigment surface treatment; Check the roll clearance to speed ratio, increase the number of passes or adjust the recipe |
| The temperature of the grinding process is too high | insufficient cooling; the speed ratio is too large; high ambient temperature; Strengthen the cooling of the rollers, reduce the speed ratio, and grind in batches |
| The finished product contains air bubbles | material entrained air; improper angle of the scraper; Lack of defoaming process; Optimize the feeding method, adjust the scraper, and increase vacuum defoaming |
| The surface of the roller is worn or contaminated | The material contains hard impurities; incomplete cleaning; Regularly check the roller surface and strengthen the front filtration and rear cleaning |
Notes:
Before operation, it is necessary to confirm that all guards are intact and that the roller clearance adjustment must be carried out in a stopped state. It is strictly forbidden to touch the roller area with hands or tools during operation. High viscosity inks may contain volatile components and should be operated in a well-ventilated environment. After daily work, use special cleaning agents to thoroughly clean the rollers and scrapers to prevent residue from solidifying. Regularly check the parallelism of the roller and the surface accuracy, calibrate the gap scale, and lubricate and maintain the transmission components. Record each process parameter and output quality, and establish a process database for optimization.