How Do You Test the Smoothness of Paper? - Bekk Method Principle and Procedure - Shandong Annimet

2026-07-21 16:06:55

How Do You Test the Smoothness of Paper? Bekk Method Principle and Procedure Explained

Paper smoothness is a core physical indicator for evaluating the surface evenness of paper, directly affecting ink transfer uniformity, dot reproduction, and print clarity. The most widely used testing method is the air leak method, with the Bekk smoothness tester being the most common instrument in the Chinese paper industry. This article explains the definition of paper smoothness, the working principle of the Bekk method, the testing procedure, influencing factors, and relevant standards.

Introduction

In the paper and printing industries, paper smoothness is a critical quality indicator. For writing paper, sufficient smoothness ensures fluent writing; for printing paper, smoothness directly determines the contact tightness between paper and printing plate and the ink absorption performance, thereby affecting the clarity and vividness of printed images and text. Paper with high smoothness achieves good contact with the printing plate or blanket during the impression process, enabling uniform ink transfer and producing prints with clear images, complete text, and rich tones. So, how exactly is paper smoothness measured? This article provides a detailed explanation.

1. What Is Paper Smoothness?

Smoothness is a physical indicator that evaluates the degree of surface unevenness of paper or paperboard. It reflects the flatness, uniformity, and smoothness of the paper surface, determined primarily by fiber structure, processing technology, fillers, and surface treatment.

The level of smoothness essentially reflects the size and number of surface voids on the paper. The more even the paper surface and the more uniform the fiber distribution, the higher the smoothness; conversely, when the paper surface is uneven with wrinkles, embossing, hard spots, folds, or other defects, smoothness decreases significantly.

2. Main Testing Method: The Air Leak Method

There are many methods for measuring smoothness, including the air leak method, optical contact method, stylus method, water trace method, printing test method, capacitance method, friction plate method, and liquid extrusion method. Each method has different types of instruments, but the most widely used is the air leak method.

The basic principle of the air leak method is: under specified contact conditions and a certain pressure differential, measure the time required for a certain volume of air to pass through the gap between the paper surface and another surface, or measure the airflow rate, flow volume, and air pressure changes. The higher the paper surface smoothness, the slower the air passes through, and the longer the time required; conversely, the lower the smoothness, the faster the air passes through, and the shorter the time required.

Nearly 10 types of instruments have been designed and manufactured worldwide based on the air leak principle, including the Bekk type, Williams type, Gurley H-P-S type, Bendtsen type, Sheffield type, and PPS type. Among these, the Bekk smoothness tester is the most commonly used instrument in the Chinese paper industry.

3. Working Principle of the Bekk Smoothness Test

The Bekk smoothness tester is a specialized testing instrument designed based on the internationally recognized Bekk smoothness principle, complying with ISO 5627 and GB/T 456 and other international and national standards.

Core Testing Principle

The core principle of Bekk smoothness testing is: under a certain vacuum degree, measure the time required for a certain volume of air to pass through the gap between the specimen surface and the glass surface under a certain pressure, expressed in seconds (s). Specifically, the paper or paperboard is placed on a precision glass plate, a specific pressure is applied to create a partial vacuum, thereby drawing in air and passing it through the contact surface, and the time required for the vacuum degree to change within a specified range is measured.

After the instrument generates a vacuum internally, it measures the time required for air to pass through the sample surface, causing the internal vacuum to drop from 50.66 kPa to 48.00 kPa. For very smooth samples, the instrument is equipped with a 1/10 vacuum chamber (small volume) for testing, which can significantly save testing time.

Test Procedure

The standard test procedure for Bekk smoothness testing is as follows:

1. Sampling and Preparation: Sample according to GB/T 450, cutting specimens of at least 60mm × 60mm evenly from the large paper sheet, 15mm from the edge along the cross direction, ensuring sufficient specimens for 10 tests on each side, and ensuring the specimens are free from creases, visible cracks, or other paper defects. Specimens must be conditioned at 23±1°C and 50±2% relative humidity for at least 24 hours.

2. Mounting the Specimen: Place the specimen on the precision glass anvil surface, ensuring close contact between the specimen surface and the glass surface.

3. Applying Pressure: Apply a constant pressure of 100±2 kPa to the specimen. The effective test area is 10±0.05 cm².

4. Starting the Test: The instrument generates a vacuum internally and measures the time required for a certain volume of air (typically 10 mL; 50 mL may be used for low-smoothness paper) to pass through the gap between the specimen surface and the glass surface.

5. Reading Results: The instrument automatically records the smoothness value in seconds (s). The longer the time, the higher the paper surface smoothness.

Interpretation of Test Results

Smoothness test results are expressed in "Bekk seconds." Different paper types have significantly different smoothness requirements:

4. Factors Affecting Smoothness Test Results

Smoothness test values are affected by multiple factors, including:

Environmental Factors: Changes in atmospheric pressure, temperature, and humidity all affect smoothness test values. Therefore, standard testing requires strict control of temperature and humidity conditions.

Rubber Sheet Thickness and Hardness: Softer rubber sheets produce higher smoothness test values, and vice versa. GB/T 456 specifies that rubber sheet thickness should be (4±0.2)mm and hardness (40±5) IRHD.

Vacuum Chamber Volume: Volume has a linear relationship with smoothness test values. Older instruments may experience volume reduction due to back-suction of vacuum pump oil and grease, leading to lower test results; new instruments using oil-free vacuum pumps and high-seal solenoid valves have essentially eliminated this issue.

Pre-pressure Time: Longer pre-pressure time produces higher smoothness values. The standard specifies a pre-pressure time of 1 minute.

Pressure on Specimen: Smoothness values increase with increasing pressure. Increased pressure enlarges the contact area between the specimen and the glass anvil and compresses the paper itself.

5. Practical Value of Smoothness Testing

Paper smoothness testing has extensive practical value in the paper and printing industries:

Print Quality Assurance: Smoothness is the primary condition for obtaining high-quality prints. Insufficient smoothness leads to uneven ink transfer, weak dot reproduction, and mottled or faded print colors.

Process Optimization Guidance: Through smoothness testing, paper manufacturers can evaluate the effects of beating, calendering, coating, and other processes on paper surface properties, thereby optimizing production processes.

Product Grading Basis: Smoothness is an important basis for paper product grading, with different paper uses requiring different smoothness levels.

Batch Consistency Monitoring: Through regular smoothness testing, manufacturers can monitor production process stability and ensure consistent quality between batches.

Conclusion

Paper smoothness is a core indicator for evaluating the surface evenness of paper, directly affecting print quality and writing performance. The Bekk smoothness test, based on the air leak principle, provides a simple and reliable way to quantify paper surface smoothness and is an essential tool for quality control in the paper, printing, and packaging industries.

Shandong Annimet Instrument Co., Ltd. has nearly two decades of experience in laboratory testing instruments, providing fully automatic smoothness testers compliant with GB/T 456, ISO 5627, and TAPPI T 479 standards, suitable for precise smoothness testing of various papers and paperboards. For more product information or assistance with smoothness testing solutions, please call 400-9933-057 or visit https://www.annimet.com