How Does a Bonding Tester Work? - Internal Bond Strength Testing - Shandong Annimet

2026-07-17 15:44:17

How Does a bonding tester Work? Understanding the Pendulum Impact Principle

A bonding tester (also known as an internal bond tester) is an essential instrument for evaluating the interlaminar bonding quality of paperboard. Its working principle is based on a pendulum impact mechanism — a pendulum of controlled mass and velocity strikes the specimen, measuring the energy required to delaminate the internal fibers in the Z-direction. This article explains the working principle, sample preparation, impact process, and data interpretation of bonding testers.

Introduction

In paperboard production and quality control, interlaminar bond strength is a critical performance indicator. It directly affects how paperboard performs in subsequent converting processes — printing, die-cutting, folding, and box-making. Insufficient bond strength can lead to surface blistering or delamination during printing, and in severe cases, cracking or layer separation in finished products. The bonding tester (Internal Bond Tester) is the specialized instrument used to quantify this key. But how exactly does this instrument work? What is the testing principle behind it? This article provides a detailed explanation.

 1. What Is Bonding Strength Testing?

Bonding strength testing — also known as internal bond strength testing or the Scott Bond test in the pulp and paper industry — measures the energy required to rapidly delaminate a paperboard specimen along its thickness direction (the Z-direction). This reflects the degree of adhesion between internal fibers or between different layers within the material.

Unlike traditional static tensile tests, bonding strength testing simulates the dynamic loads that paperboard experiences during actual converting processes — high-speed peeling in printing presses, instantaneous impact in die-cutting, and rapid folding in box-making lines. For this reason, bonding testers use a pendulum impact method, applying force to the specimen at speeds far exceeding those of static testing.

2. Core Working Principle: Pendulum Impact

The core working mechanism of a bonding tester is based on the pendulum impact principle. The entire testing process can be broken down into the following key steps:

 Step 1: Sample Preparation

Before testing begins, the paperboard specimen must be prepared to a standard size (typically 25.4mm × 25.4mm). The specimen is sandwiched between two layers of double-coated tape, forming a "double-coated tape — specimen — double-coated tape" structure. This sandwich structure is then pressed between a metal flat anvil and an aluminum block under a set pressure (typically adjustable from 345 to 1379 kPa) and dwell time. This step ensures that the double-coated tape fully adheres to the specimen surface, preparing it for the subsequent test.

Advanced bonding testers are equipped with automatic sample preparation stations that can accurately press and cut up to five specimens simultaneously, significantly improving preparation efficiency and consistency.

 Step 2: Pendulum Impact

Once the specimen is prepared, it is mounted in the tester's fixture. The core component of the instrument — a pendulum with a controlled mass and controlled velocity — is released, delivering a high-speed impact to the specimen.

This impact speed far exceeds that of traditional static testing equipment: the pendulum's impact velocity exceeds that of tensile strength testers and other dead-weight testers by more than 6,000 times. It is this high-speed impact that simulates the dynamic loads paperboard experiences during high-speed converting processes.

 Step 3: Energy Measurement and Data Output

After the pendulum strikes the specimen, the instrument measures the total energy required to delaminate the internal fibers of the specimen in the Z-direction into two plies. This energy value represents the paperboard's bonding strength (internal bond strength).

Test results are expressed as strain energy per unit sample area, with common units including J/m², ft·lb/in², and kg·cm. The instrument automatically records impact angle, energy values, and other data, and can perform statistical analyses such as average and standard deviation.

 3. Testing Standards

Bonding strength testing follows a range of international and national standards to ensure the authority and comparability of test results:

• TAPPI T 569 (Scott-type internal bond strength): This is the most core international standard, specifying the detailed method for determining the internal bond strength of paper and paperboard using the pendulum impact method.

• ISO 16260: An international standard for internal bond strength testing developed by ISO, compatible with the TAPPI T 569 method.

• GB/T 26203-2010: The Chinese national standard "Paper and board — Determination of internal bond strength (Scott type)," applicable to single-ply and multi-ply paper and paperboard.

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 4. Practical Value of Bonding Strength Testing

Bonding strength testing has extensive practical value in paperboard production and quality control:

Printability Assessment: Internal bond strength directly affects paperboard printability — insufficient strength can lead to surface blistering or delamination during printing.

Converting Performance Prediction: Bonding strength data can be used to predict paperboard performance in subsequent converting processes such as die-cutting, folding, and box-making.

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

New Material Development: During paperboard formulation adjustments or new fiber material development, bonding strength testing is an important tool for evaluating changes in material properties.

5. Limitations and Considerations

While bonding strength testing is widely used, it has its scope of application and limitations. Because sample preparation requires pressing double-coated tape onto both sides of the specimen under relatively high pressure, this method is not suitable for low basis weight (below 40 g/m²), porous, soft, or low-density materials, such as tissue paper. Additionally, strength measurements obtained by this method do not correlate with Z-direction tensile strength tests (ZDT) — the two measure different mechanical properties.

Conclusion

The bonding tester operates on the pendulum impact principle, applying force to paperboard specimens at speeds far exceeding those of static testing, precisely measuring the energy required to delaminate internal fibers in the Z-direction, and thereby quantifying interlaminar bond strength. This testing method is widely used in paperboard production quality control, printability assessment, and packaging material development.

Shandong Annimet Instrument Co., Ltd. has nearly two decades of experience in laboratory testing instruments, providing bonding strength (internal bond strength) testing solutions compliant with TAPPI T 569 and GB/T 26203 standards. For more product information or assistance with testing solutions, please call 400-9933-057 or visit https://www.annimet.com

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