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The Significance of COF Test of Flexible Packaging Film and Test Method

Oct 15,2020

Source: Link Testing Instruments Co., Ltd

Coefficient of friction (COF) is one main physical index of plastic film or sheeting, it  describes the ratio of force of friction when the material sliding over itself or other substances under specified conditions.

The flexible packaging film should be of proper coefficient of friction otherwise negative effects may be caused during the operation. To be specific, if the coefficient of friction is too high, it will require a higher pulling force to unwind the roll of the plastic film. As a result, the production efficiency will be compromised and even worse the film may be deformed after being stretched excessively. On the other hand, if the coefficient of friction is too low, the films may slip or fall off from the roll, that may cause suspension of production and unnecessary production cost. Therefore, it is quite important to the manufacturers and users of flexible packaging film to control its coefficient of friction and select suitable packaging materials through COF tests.

There are various factors that may influence the coefficient of friction of flexible packaging film, including additives, lubricants, surface texture and temperature, etc. The manufacturers can adjust the coefficient of friction of films by modifying those factors mentioned above taking into consideration the COF test results.

The coefficient of friction of the same material on distinct surfaces may be different. In order to measure the coefficient of friction of flexible packaging materials precisely, suitable surface should be selected. In the whole manufacturing process (unwrapping, bag making, product filling and heat sealing) with flexible packaging materials, the friction occurs between the inner surface (heat-sealable surface) and external surface(non-heatsealable surface), inner surface and inner surface, external surface and external surface, external surface and machinery, so when measuring coefficient of friction, the friction surfaces should be accordingly inner surface against external surface, inner surface against inner surface, external surface against external surfaces, inner surface against test plane, external surface against test plane. To be specific, if the coefficient of friction between inner surface and inner surface is too low, the packaging materials may slip and cause incorrect sealing edges. If the coefficient of friction is too high, it may be difficult to open the package. The coefficient of friction between the inner and external surfaces may influence the unwrapping of the packaging materials.

Currently the COF of flexible packaging film can be tested according to ASTM D1894: Standard Test Method for Static and Kinetic Coefficients of Plastic Film and Sheeting. The simplified testing procedures are provided in the following for your reference.

Link Testing LTCOF-05A and LTMXS-06 Coefficient of Friction Testers are used as testing instruments for testing the coefficient of friction of flexible packaging materials, paper, textiles, non-woven fabrics, woven bags, rubber, printings, aluminum foils, metals and their products. Those instruments are featured with high accuracy, intelligent design and simplified operations.

Figure 1. LTCOF-05A Coefficient of Friction Tester

Figure 3. LTMXS-06 Coefficient of Friction Tester

The test can be performed according to the following procedures.

  • Cut specimens. Do not touch the test area of the specimens when preparing and mounting the specimens. The specimen surface should be flat and free of creases or folds.
  • Tape the specimen to the plane with the machine direction of the specimen. Smooth the specimen to eliminate wrinkles if necessary, taking care not t alter the specimen surface through figure oils, etc.
  • Take the edges of the square film specimen to the back of the sled, using adhesive tape and pulling the specimen tight to eliminate wrinkles without stretching it.
  • Attach the specimen-covered sled through its eye screw to the nylon filament.
  • Start the driving mechanism. The test will be performed automatically. The test results will be given by the instrument after tests.

Link Testing Instruments Co., Ltd is one high-tech enterprise that specializes in research & development and manufacturing of packaging testing instruments.

For more details about our packaging testing instruments and testing services, please visit

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