How it works:
The Sheffield Smoothness Tester is a precision instrument based on the air leak method for quantifying the surface smoothness of paper, cardboard, and other sheet materials. Its core principle is to measure the time it takes for a certain volume of air to pass through the gap between the sample being measured and the standard measuring head under specific pressure conditions. The smoother the sample surface and the tighter the fit to the measuring head, the greater the resistance to air leakage, resulting in longer leakage time. This time value directly characterizes the smoothness of the material's surface in seconds. The instrument usually consists of parts such as the measuring head, pressure control system, flow sensor, and timing unit, ensuring that the testing process complies with relevant international standards, such as ISO 8791-2 and TAPPI T 538.
Function
The smoothness of the paper surface is one of the key factors affecting the quality of printing. In the printing process, smoothness directly affects the transfer of ink, the clarity of dot reduction, and the gloss and uniformity of printed graphics. Objective quantification with Sheffield smoothness analyzers provides critical data support for paper producers and printing companies. Through accurate measurements, it is possible to predict the printability of paper on high-speed presses, evaluate the improvement of surface structure by different coating processes or calendering, and help optimize printing parameters to reduce ink consumption and avoid defects such as printing spots, thereby controlling production costs while ensuring print quality.
Testing process
The standard testing process begins with the preparation of the sample. Samples are pretreated under standard temperature and humidity conditions (typically 23±1°C, 50±2% RH) and cut to size. During testing, the sample is placed on a measuring table so that the measuring head is in contact with the sample surface at standard pressure. The instrument automatically applies constant air pressure and starts timing until a predetermined volume of air passes through the contact surface. The test is usually repeated at several different locations on the sample to obtain an average value that represents the overall surface. The resulting data can be read directly, the smoothness value (T) is calculated based on the air leakage time (t), and the basic relationship can be expressed as:
T ∝ t
That is, the longer the leakage time, the higher the smoothness value. When analyzing the data, it is necessary to combine statistical parameters such as coefficient of variation to evaluate the uniformity of the paper surface. The test results can be systematically recorded in the table below.
| Test items | Typical Data Range and Description |
| Smoothness value | Tens to thousands of seconds, depending on the type of paper |
| Test pressure | According to the standard, it is usually a specific kilopascal value |
| Single point test time | The instrument automatically measures and records |
| Multi-point test average | Used to characterize overall surface properties |
| Data coefficient of variation | Evaluate surface uniformity, the lower the value, the more uniform |
Influencing factors
To ensure the accuracy and comparability of measurement results, a number of technical details need to be paid attention to during operation. The ambient temperature and humidity must be strictly controlled, as the moisture content of the paper affects its softness and compressibility, which in turn changes the contact between the measuring head and the surface. Regular calibration of the instrument is essential, and the accuracy of its barometric system and timing unit is verified using a standard reference plate. The characteristics of the sample itself, such as thickness, stiffness, compressive elasticity, and surface porosity, can affect the test results. In addition, the cleaning and maintenance of the measuring head should not be neglected, as any contaminants or wear may introduce errors. Understanding these factors helps operators interpret data correctly and effectively correlate lab test results with actual print performance.
Industry Applications
Sheffield smoothness testing is widely used in the paper, packaging and printing industries. In the papermaking process, it is the core means to monitor the surface quality of coated paper, art paper, high-grade writing paper and other products. In the packaging sector, it is used to evaluate the printability of corrugated cardboard face paper or label materials. This test method is closely tied to multiple industry standards, providing a common technical language for quality protocols in the supply chain. Its test data can not only be used for internal quality control, but also as the basis for product specification confirmation and acceptance between suppliers and customers, promoting the standardized management of product quality.
Summary
Sheffield smoothness analyzers play an important role in paper printing surface analysis by providing objective, quantitative surface smoothness data. Its measurement principle based on the air leak method is scientific and reliable, and the standardized test process ensures reproducible results. Correctly understanding the working principle of the instrument, standardizing the execution of tests, and considering various influencing factors can obtain data that truly reflects the characteristics of the paper surface. This data has clear practical value for guiding paper production, predicting print performance, optimizing process parameters, and ensuring the quality of the final print.
References
ISO 8791-2: Paper and board — Determination of roughness/smoothness (air leak methods) — Part 2: Sheffield method.
TAPPI T 538 om-19: Smoothness of paper and paperboard (Sheffield method).
Pulp and Paper Testing Methods, relevant chapters on surface properties.