Principle and Operation of Packaging Tear Strength Tester

Rationale

Packaging tear strength tester is a special equipment used to quantitatively evaluate the ability of film, paper, non-woven fabrics, composite flexible packaging materials, etc. to resist tear propagation. Its core measurement principle is based on the Elmendorf tearing method, which has been adopted by many domestic and international standards. The instrument calculates the tear strength of a material by measuring the work required to tear a sample with an initial incision to a certain length.

The working process can be briefly described as follows: the specimen is clamped between the fixed gripper and the movable pendulum gripper. At the beginning of the test, the pendulum falls under the action of gravity, and its potential energy is converted into kinetic energy, which drives the movement of the movable gripper, which tears the specimen from the initial incision. The pendulum loses some of its energy during the tearing of the specimen, and the remaining energy causes it to continue to swing to a certain height. By measuring the potential energy difference between the pendulum before and after tearing, the energy consumed by the torn specimen can be calculated. This energy value is converted internally by the instrument and is finally displayed in the form of force (milliN, mN) or tear index, etc. Its basic calculation formula can be expressed as:

\[ F = \frac{E}{d} \]

where \(F\) represents the average tearing force, \(E\) represents the energy consumed during the tearing process, and \(d\) represents the tearing distance of the specimen (usually a fixed value, such as 43mm).

Structure and function

A typical tear strength tester consists of the following key components:

Pendulum system: This is the core power and measurement unit of the instrument. Includes a pendulum with specific potential energy, release mechanism, and angle or position sensors. The mass and initial lift angle of the pendulum determine its initial potential energy to accommodate material testing in different strength ranges.

Clamping device: It usually includes a fixed gripper and a movable gripper mounted on the pendulum. They must ensure that the specimen is securely clamped at the moment of testing to prevent slippage, and that the edges of the two grippers are precisely aligned to ensure that the tear proceeds in the intended direction.

Cutting device: Used to prepare standardized initial incisions on specimens. Typically an integrated or external blade ensures consistent cut length and depth, which is a prerequisite for repeatable results.

Measurement and display systems: Modern instruments often use electronic sensors (such as encoders) to accurately capture the angle change before and after the pendulum tear, and directly convert the angle difference into the tear force value through the built-in microprocessor, and display the results on the digital display. Some models have data statistics and output functions.

Calibration device: Includes pointer or electronic calibration functions to verify instrument friction, as well as standard weights or verification tools for calibrating pendulum shaft friction and air resistance.

Operation process

Standardized operations are the foundation for obtaining accurate, comparable data. The operation process usually follows the following steps:

1. Preparation

Prepare specimens according to relevant test standards (such as GB/T 455, ISO 1974, TAPPI T414, etc.). The specimen size, shape and incision size must strictly comply with the standard regulations. Place the instrument on a stable, level tabletop and adjust the foot to keep the instrument horizontal.

2. Instrument calibration and inspection

Use the calibration tool provided with the instrument to check the friction and air resistance loss of the pendulum. It is common practice to release the empty pendulum and observe its free swing number or the offset of the pointer to ensure that it meets the sensitivity required by the standard. If necessary, the force value should be calibrated according to the instructions.

3. Parameter settings

According to the expected tear strength of the specimen, the appropriate pendulum capacity (i.e., potential energy range) is selected, and the principle is to make the test result fall between 20% and 80% of the full scale of the selected pendulum to ensure the best measurement accuracy. Set parameters such as specimen thickness and number of tests on the digital instrument.

4. Specimen installation

 Use the incision device to make a standard incision on the specimen. Place the specimen flat in the gripper, making sure the cuts are aligned with the centerline of the gripper and that the specimen is aligned with the gripper edge. Tighten the gripper evenly and firmly to prevent twisting or loosening of the specimen.

5. Perform tests

Gently release the pendulum to allow it to fall and tear the specimen. After the pendulum swings back and comes to rest, record the tear force value indicated on the display or on the dial. For digital instruments, data is usually recorded automatically.

6. Result processing

After completing the test of a set of specimens (usually no less than 5 valid specimens in the longitudinal and transverse directions), the instrument or operator calculates the mean, standard deviation, or coefficient of variation for each set of results. The report should indicate the test standard, pendulum capacity, specimen size, average tearing force and unit.

Influencing factors

To ensure the reliability and reproducibility of the test data, the following factors need to be taken into account:

Specimen status: The humidity of the material, temperature history, and the temperature and humidity of the test environment will have a significant impact on the results, and the state needs to be adjusted according to the standard before testing.

Operational consistency: The quality of the cut, the tightness of the clamping, and the smoothness of the pendulum release must all be consistent.

Instrument status: The friction of the pendulum bearing, the neutrality of the gripper, and the sharpness of the blade need to be checked and maintained regularly. The instrument should be regularly verified or calibrated by a qualified institution in accordance with the verification procedures.

Routine maintenance mainly includes: keeping the instrument clean, especially the clamping area; Regularly apply a small amount of special lubricating oil to moving parts such as pendulum shafts; Avoid impact or vibration of the instrument; When not in use for a long time, the pendulum should be kept in a released state to protect the bearing.

Application and standard reference

Packaging tear strength testers are widely used in the following fields: paper industry (to evaluate the tear resistance of paper and cardboard); plastic film and flexible packaging industry (evaluating the durability of shopping bags, laminated films, plastic wraps, etc.); textile and nonwovens industry (evaluation of mechanical properties of nonwovens and geotextiles); Research on relevant materials of quality inspection institutions and scientific research units.

Here are some common reference standards:

References

1. National Paper Industry Standardization Technical Committee. Determination of tear degree of paper and cardboard: GB/T 455-2002[S]. Beijing: China Standard Press, 2003.

2. International Organization for Standardization. Paper — Determination of tearing resistance (Elmendorf method): ISO 1974:2012[S]. Geneva: ISO, 2012.

3. TAPPI Press. Internal tearing resistance of paper (Elmendorf-type method): TAPPI T414 om-12[S]. Atlanta: TAPPI, 2012.

4. National Technical Committee for Standardization of Plastic Products. Determination of tear resistance of plastic films and flakes Part 2: Elemendorf method: GB/T 16578.2-2009[S]. Beijing: China Standard Press, 2009.