What tests are conducted on composite films? (Part 1)

Today, we're going to talk about a common type of packaging material testing. Plastic composite film for packaging As a multi-layered membrane structure, it boasts inherent advantages such as high tensile strength, excellent tear resistance, uniform longitudinal and transverse deformation, outstanding wear resistance, and superior water-impermeability. However, when it comes to complex structural packaging materials, the associated testing processes become remarkably intricate and extensive.

Today, we’ll first explain the relevant tests for the physical properties of composite films: appearance, dimensional deviation, print quality, peel strength, heat-seal strength, tensile strength, pendulum impact resistance, 90-degree tear strength, coefficient of friction, puncture resistance, light transmittance, and haze.

Appearance

Conduct an visual inspection of the sample under natural light (by eye), including checks for flatness, dimensional stability, surface defects, transparency, and other relevant criteria.

Xiaomin on Testing: This test aims to assess the physical appearance of the sample, identifying any obvious scratches or punctures on the surface of the plastic composite film used for packaging, as well as the presence of foreign objects, unusual odors, adhesion issues, uneven coating, contamination around the sealing area, interlayer separation or significant damage, bubbles, wrinkles, or dirt—essentially uncovering any visible defects in the material.

Dimensional deviation

The dimensional deviations of plastic composite films used for packaging—including length, width, thickness, and heat-seal width—must all remain within the specified tolerance ranges.

Xiaomin on Testing: This is an important technical indicator for determining whether plastic composite films used in packaging affect the overall aesthetics and functionality of the packaging material. In recent years, it has also become a test that customers place great emphasis on.

Print quality

  Based on the relative positioning of image and non-image areas on the printing plate, common printing methods can be categorized into four main types: letterpress printing, gravure printing, offset printing, and screen printing: (1) Letterpress Printing (2) Gravure Printing (3) Offset Printing (4) Screen Printing. Relevant tests are conducted according to the standards GB/T 7705, GB/T 7706, and GB/T 7707.

Xiaomin on Inspection: By testing plastic composite film packaging, we can determine whether the printed images and text are clear, complete, and free from distortions or defects; whether the ink color is uniform and accurate; whether there are any noticeable watermarks; how smoothly the print layers transition; and whether the halftone dots remain sharp without obvious imperfections. This inspection of printing quality directly reveals the packaging’s aesthetic appeal and overall visual impact. (More details about specific printing quality metrics will be shared later.)

Peeling force

Peel strength is calculated by analyzing the changes in force and displacement during the test process, determining the maximum force required to peel the two layers of the specimen per unit width.

Xiaomin on Testing: Peel strength measures the bonding integrity between layers in composite films used for food packaging, serving as a key indicator to assess whether the composite film material is prone to delamination. This test is crucial for evaluating the physical and mechanical properties of plastic composite films intended for packaging, as well as the product image of the contents they protect. Ultimately, this is one of the factors most likely to lead to quality issues.

Thermal Bond Strength

Thermal sealing strength is a critical performance indicator for plastic composite films used in packaging materials, as it determines the quality of heat sealing between films or between films and other substrates—ultimately influencing the effectiveness of the sealing process.

Xiaomin on Testing: Heat-seal strength directly affects both the appearance and sealing integrity of packaging. To ensure that products can withstand external forces during packaging, transportation, storage, and consumption—preventing cracks, leaks, and ultimately safeguarding the goods—it’s crucial that the sealed areas of composite film packaging materials possess sufficient strength and reliable sealing performance. Therefore, heat-seal strength is a key testing indicator for composite film bags.

Tensile strength

   Tensile strength refers to the stress at which a material undergoes its maximum uniform plastic deformation.

Xiaomin on Testing: Tensile performance is a crucial physical-mechanical property indicator for composite films. By applying tensile loads along the transverse and longitudinal axes of the sample, the material is deformed—ultimately leading to failure—allowing us to assess the tensile stress of the composite film packaging material. Notably, tensile strength largely determines the overall quality and usability of the packaging.