Packaging Material Testing – Stretch Film

Packaging material testing involves evaluating the physical, mechanical, optical, and other properties of packaging materials. Some materials, like paper, can be tested even before they are transformed into finished packaging products—such as a sheet of raw paper. In contrast, materials like plastic must first be molded into specific packaging shapes, such as bottles or cans, before their performance can be assessed.

Conducting necessary tests on packaging materials, containers, and packaging itself can optimize design, enhance quality, and amplify product impact—making it critically important for boosting both corporate and societal economic benefits. Today, we’ll introduce a testing procedure for a specific packaging material: stretch film.

Stretch film securely wraps goods into a single unit, reducing damage, collapse, and unpacking during storage and transportation. One ton of palletized goods, when wrapped with stretch film, can withstand a maximum tilting angle of 45。

• Dimensions

Dimensional deviations—such as length, width, and thickness variations—should all remain within the specified tolerance limits.

Xiaomin on Testing: You can check the specifications of the honeycomb cardboard you produce and verify whether it meets the technical requirements of the sample part.

• 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 whether the surface of the stretch film is smooth, free from defects such as broken ends, air bubbles, punctures, tears, or rigid lumps, by examining the sample's physical appearance.

• Tensile strength

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

Xiaomin on Testing: Stretch film is typically pre-stretched before coming into contact with the goods, ensuring a consistent tensile force is applied as the film wraps around the item. This evenly distributes the tension across the entire width of the film. However, the longer the film, the greater the likelihood of weak spots—resulting in reduced overall strength.

• Elongation at break

Elongation at break is expressed as a percentage (%) and typically refers to the ratio of the sample’s displacement at the moment of fracture to its original length.

Xiaomin on Testing: This test is a parameter indicator that reflects the toughness of a material! It’s a crucial metric in tensile performance, typically referring to the elongation rate throughout the entire process.

• Mass per unit area

Unit area mass refers to the ratio of a sample's mass—measured with fixed dimensions—to its surface area, expressed in g/cm². m ² Indicates.

After conditioning the sample under the specified temperature and humidity conditions, weigh the sample with a known surface area and calculate its mass per unit area.

• Viscosity

Adhesion refers to the stickiness between two layers of stretch or non-stretch film.

When stretch wrapping with cling film, the film's self-adhesive properties ensure that it doesn’t loosen after being cut, maintaining a secure hold on the goods over time. During packaging, individual layers of the film adhere tightly to one another yet remain cleanly separated from the wrapped items, preventing any contamination of the product. Additionally, the cling film retains its adhesive quality across a suitable temperature range—from cold to warm environments.

• Permanent deformation

Permanent deformation is the ratio of the increased length of a stretch film after being stretched by a certain percentage to its original length, expressed as a percentage. 。

When the film is stretched, the load increases rapidly. Once the yield point is exceeded, the film undergoes permanent deformation.

• Elastic recovery

The ratio, expressed as a percentage, of the recovered size to the original size after the stretch film is stretched to a certain extent, held in that state for a specific period, and then released.

Memory function is a characteristic of polymer materials. Heat-shrink film restores its original shape by "remembering" and shrinking when heated, while stretch wrap film, on the other hand, can be pre-stretched at room temperature and then return to its original form—allowing it to securely wrap around packaged items. Moreover, the elasticity of stretch wrap film increases as the stretching ratio grows.

• Tension maintenance

Tensile retention refers to the ratio—expressed as a percentage—of the holding force, measured after the wrapping film is stretched to a specific percentage and maintained in that stretched state for a certain period of time, relative to the initial tensile force F.

It relates to the snugness or comfort of the packaging material.

• F Force value

F The force value is the maximum tensile force exerted by the stretch film at a predetermined elongation rate.

The risk value for tear resistance must be measured horizontally—perpendicular to the machine's direction of operation—because tearing in this direction would cause the packaging to loosen. Even if a longitudinal tear occurs, the package should still remain securely intact. Therefore, the greater the toughness of the stretch film, the better its quality.

• Puncture-resistant

Puncture resistance refers to the ability of a stretch film to withstand being pierced by a needle, measured by the force required to cause the film to rupture during the piercing process, as well as the displacement observed when penetration occurs.

Xiaomin Talks Testing: If the stretch film has poor puncture resistance, it can easily be pierced by sharp objects. Such damage can cause products or goods to become loose and separated, leaving them unprotected.

• Light transmittance, haze

Transmittance and haze are determined according to the provisions of GB/T 2410—2008. Transmittance is defined as the ratio of transmitted light to incident light, typically expressed as the percentage of light that passes through the sample. Haze, on the other hand, refers to the percentage of transmitted light that deviates from the original direction due to forward scattering as it passes through the sample.

Xiaomin on Testing: Both transmittance and haze are critical indicators for measuring the transparency of plastics. Generally speaking, transmittance and haze have an inverse relationship—materials with high transmittance tend to have low haze, and vice versa. However, this correlation doesn’t always hold true; in some cases, the two can even move in opposite directions. Thus, transmittance and haze are both opposing yet interconnected optical metrics. By considering these two factors, manufacturers can select packaging materials that strike the right balance in terms of desired transparency levels.

• Water vapor transmission rate

Xiaomin on Testing: Can stretch film effectively protect products from moisture or humidity—such as preventing dampness or moisture absorption—in maritime shipping environments or areas with high humidity, thanks to its airtight seal after wrapping?

Xiaomin on Testing: Can stretch film effectively protect products from moisture or humidity—such as preventing dampness or moisture absorption—in海运 environments or areas with high humidity, thanks to its airtight seal after wrapping?