The thixotropy of PE screen printing ink is a key factor in controlling print thickness. Essentially, it refers to the ink's viscosity decreasing under shear and recovering after standing. This reversible rheological behavior directly influences the ink's transfer and film-forming processes, ultimately determining the uniformity and stability of the final print thickness.
In screen printing, ink is squeezed through the mesh by a squeegee. During this process, thixotropy reduces the ink's viscosity under shear, enhancing its fluidity, allowing it to penetrate the mesh smoothly and adhere evenly to the PE substrate surface. Insufficient thixotropy (excessive viscosity) can result in reduced ink penetration and a thinner print thickness. Conversely, excessive thixotropy can cause the viscosity to recover too slowly after shear, leading to ink accumulation in the mesh, resulting in uneven thickness or blurred edges. Therefore, achieving the right balance between ink penetration and printing accuracy is crucial.
The surface properties of the PE substrate significantly influence the ink's thixotropy. Due to the low surface energy of PE, the ink requires excellent leveling to ensure adhesion. Inks with excessive thixotropy may fail to level properly after standing due to rapid viscosity recovery, resulting in an orange peel or grainy effect on the printed surface. Insufficient thixotropy, on the other hand, may cause excessive ink flow, impairing the clarity of the pattern edges. By adjusting the ratio of thixotropic agent to leveling agent, the ink's spreading behavior on the PE substrate can be optimized, thereby controlling the uniformity of the printed thickness.
The thixotropy of the ink is also closely related to printing speed. At high printing speeds, the shearing time of the squeegee on the ink is shortened, requiring the ink to have a faster thixotropic response—rapid viscosity reduction to ensure sufficient ink penetration. If the thixotropic recovery rate is too slow, the ink may not fully recover, resulting in uneven transfer during subsequent printing, causing thickness fluctuations. Therefore, optimizing the type and dosage of thixotropic agent for different printing speeds is an important means of controlling print thickness stability.
The influence of ambient temperature on the thixotropy of PE screen printing inks should also not be ignored. Low temperatures increase ink viscosity and slow thixotropic recovery, potentially leading to thinner print thickness and slower drying. High temperatures can overactivate the ink's thixotropy, causing the viscosity to drop too quickly, leading to excessive ink bleeding or pattern spreading. Controlling the print shop temperature or adding a temperature-sensitive thixotropic agent can effectively adjust the ink's rheological properties under different conditions, thereby maintaining consistent print thickness.
From an ink formulation perspective, thixotropy works synergistically with pigment dispersion and the choice of resin system. The shape and size distribution of pigment particles influence the ink's thixotropic structure. For example, irregular particles tend to form a stronger thixotropic network, requiring surface treatment to reduce interparticle interactions. Furthermore, the resin's molecular weight and degree of branching can affect the ink's thixotropic recovery rate. Low molecular weight resins generally provide faster viscosity recovery, helping to control ink leveling after printing.
In practical applications, the thixotropy of PE screen printing inks requires repeated trial and error to optimize. For example, when printing high-precision patterns, it's important to choose inks with moderate thixotropy and fast shear recovery to minimize thickness deviations caused by ink buildup. For large-area solid-color printing, however, thixotropy can be appropriately reduced to increase ink fluidity and improve coverage efficiency. By combining the characteristics of the PE substrate, printing process parameters, and environmental conditions, precise control of thixotropy can become a core technology for achieving high-quality control of print thickness.
