Effect of pressure on transfer rate
In the offset printing process, printing pressure is one of the key variables that affects the efficiency of ink transfer from the plate to the substrate. The print suitability meter simulates different pressure levels through an adjustable pressurization mechanism, which can quantify and analyze the functional relationship between pressure change and ink transfer rate. Ink transfer rate is defined as the ratio of the ink mass transferred to the surface of the substrate to the initial ink mass on the plate, usually expressed as a percentage.
According to relevant standards (such as GB/T 18723-2002 for printing suitability measurement), the experiment often uses a fixed ink supply, the same printing speed and specified environmental conditions (temperature 23±2°C, relative humidity 50±5%), and only changes the printing pressure variable. The pressure range is generally set between 50 N/cm² and 400 N/cm², covering the pressure range commonly used for offset printing. Three parallel samples were taken for each experiment to calculate the average value to reduce the accidental error.
Experimental data usually show the following trend: when the pressure is low (e.g., 50-100 N/cm²), the ink does not fully fill the microscopic depression on the surface of the substrate, and the transfer rate is low, about 30-40%. With the increase of pressure to 150-250 N/cm², the transfer rate increases rapidly, up to 60-70%, and the increase of pressure at this stage makes the ink layer and the substrate achieve closer contact. When the pressure continues to increase above 300 N/cm², the increase in transfer rate slows down and peaks around 350 N/cm² (about 75-80%). Subsequently, excessive pressure (e.g. over 400 N/cm²) can lead to ink extrusion deformation or plate wear, resulting in a slight decrease in transfer rate.
To establish a quantitative description, a simplified model can be used: the relationship between transfer rate R and pressure P can be approximated by a logarithmic function. The formula is expressed as follows:
R = a · ln( P ) + b
Among them, a and b are the coefficients obtained by the least squares fitting, and the specific values depend on the ink viscosity, the surface energy of the substrate and the characteristics of the instrument. For example, when using general-purpose offset printing inks on standard offset paper, the typical fit values a is about 22.3 and b is about -62.1, and the formula applies to the pressure range of 100-350 N/cm², and the fitting determination coefficient R² is usually higher than 0.95.
The following table lists some of the typical data points for experiments:
| Printing Pressure (N/cm²) | Ink Transfer Rate (%) |
| 50 | 35 |
| 100 | 45 |
| 150 | 58 |
| 200 | 67 |
| 250 | 72 |
| 300 | 76 |
| 350 | 79 |
| 400 | 78 |
It can be seen from the table that when the pressure is in the range of 200-300 N/cm², the transfer rate increases significantly with the pressure. After exceeding 350 N/cm², the transfer rate began to stabilize or even decline. This indicates that there is an optimal pressure range where ink transfer efficiency is highest and fluctuates the least. Beyond this range, the ink may be over-squeezed, causing the dots to enlarge, or causing slippage between the plate and the substrate, affecting transfer uniformity.
In practical application, the following points should be paid attention to when using the printing suitability meter for simulation: first, the printing plate and imprint drum should be cleaned before and after each experiment to avoid residual ink interfering with subsequent data; secondly, it is recommended to use the same batch of paper for the substrate to reduce the difference in surface structure; In addition, the ink should be fully stirred and sampled, and the viscosity change should be controlled within ±5%. These operational details directly affect the repeatability of the pressure-transfer rate curve.
In summary, the print suitability meter can effectively simulate the offset printing process under different pressure conditions, and the relationship between pressure and transfer rate shows a typical relationship of first rising, then flattening and then decreasing slightly. By controlling the pressure between 200-300 N/cm², a relatively stable high transfer rate (about 67-76%) can be obtained, which provides a data reference for the actual printing process parameter setting.
Cited Sources:
1. General technical requirements for printability measurement, relevant industry standards.
2. Analysis of influencing factors of offset printing ink transfer rate, printing technology research literature.
3. Laboratory simulation of the relationship between printing pressure and ink transfer, instrument application technical report.