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Seal Strength Test of Black and White PE Film used for Liquid Milk

Sep 14,2020

Source: Link Testing Instruments Co., Ltd

The liquid milk is usually packed in bags, boxes and bottles. Bags are very common packaging for liquid milk for its portability and low cost. The bags used for liquid milk include prepak and tetra pak. The latter one is higher in cost than the other one, so prepak is more often used than the tetra pak. Milky white film and black & white film are the two main materials used for prepak. Both of them are co-extruded films. The milky white film can only guarantee a 3-5-day shelf life. The black & white film containing high barrier resin material can meet the requirement of 14-21-day storage at low temperature for the liquid milk. Therefore, most of the prepak used for liquid milk are made of black and white PE films. 

The edges of liquid milk bag must have sufficient heat seal strength, otherwise, the milk may flow outside when the sealing area breaks. Generally, the heat seal strength of the PE bags for liquid milk depends on the raw material and sealing parameters. This article discusses the heat seal strength test method of black and white PE bags for liquid milk. The focus of this article are sealing parameters.

The test specimens are one brand pure milk bag made of black and white PE film. The testing instrument is Link Testing’s LTS-05 Auto Tensile Tester. In the tests, 3 sides of each bag which are processed by heat sealing are tested.

Figure 1. Black and White PE Bags for Liquid Milk

The tests shall be performed according to the following procedures.

(1) Cut 10 pieces of specimens from the sealing edges of the sample material (3 different bags of the same product). The width of the specimen shall be 15mm and the length of spread specimen shall be 100mm.

(2) Set specimen width, thickness, test speed and other test parameters.

(3) Spread the specimen and clamp it in the sample grips. Make sure the longitudinal axis coincides with the line of centers of sample grips.

(4) Click Test button to start the test. The system will calculate and record the test results automatically.


According to the test results, the seal strength can be improved by adjusting the composition of raw materials or changing the sealing parameters if necessary.

For more details about LTS-05 Auto Tensile Tester, please visit

About Link Testing Instruments Co., Ltd:

Link Testing Instruments Co., Ltd is one leading supplier of packaging testing instruments, which is headquartered in Jinan, China

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