1. Package Breakage in Filling Process
With the application of Form-Fill-Seal machine in food, pharmaceutical, cosmetics and other industries, the time gap and space gap between package making and product filling have been completely bridged. In the new type filling line, flexible packages are made simultaneously with product filling. Yet, when filling, the heat sealing area of the package bottom is not completely cooled and would withstand less impact.
Heat seal strength refers to the bond strength of two films that have undergone heat sealing process and completely cooled down. However, in production line, there is no sufficient time for the materials to cool down. Therefore, heat seal strength is not suitable to evaluation of the heat seal property of the packaging materials in process of product filling. Hot tack force which refers to the peel strength when the sealing area of the packaging material has not cooled down completely should be applied when selecting packaging material for the production line.
1.2 Test and Application of Hot tack force
As is shown in Fig.1, there is certain regularity between hot tack force and heat seal temperature and dwell time. There is a seal temperature (peak shown in Figure below) at which the hot tack force is optimum. When the seal temperature is higher than the peak value, hot tack force would be decreased. Additionally at the same seal temperature , hot tack force could be increased by prolonging the dwell time properly. For example, the hot tack force of the specimen at same seal temperature with dwell time of 0.3s is obviously lower than that of 0.5s.
Fig. 1. The Relationship between Hot tack force, Heat Seal Temperature and Dwell Time
In practical application, the impact on the bottom of the flexible package during filling process should be calculated according to the target content. Then, adjust the heat seal temperature, heat seal pressure and dwell time of the hot tack tester to draw the hot tack force curves and choose the best group of heat seal parameters based on the calculated data and actual condition of the production line.
2. Package Breakage in Storage Process
Except vacuum packages, the inner pressures of flexible packages would increase owing to the stack of finished products in process of storage and transportation, which may cause package breakage. Since breakage occurs mostly in the heat seal area, it could be avoided by testing the heat seal strength of the sealing edge and then replacing the materials or adjusting heat seal parameters.
According to its test method, the heat seal strength could be classified into two groups: stretching heat seal strength and the expanding heat seal strength. Usually, the heat seal strength tested with tensile tester is the stretching heat seal strength which represents the capability of heat seal area of packaging material against a single direction and uniform separation force. It could be used to evaluate the open easiness of flexible package, yet not suitable to solve the problem of package breakage which is caused by inner pressure with uncertain direction as well the material at the sealing area may be deformed due to the partial pressure endured. Therefore the package breakage problem should be solved through the test of expanding heat seal strength.
2.2 The Test and Application of Expanding Heat Seal Strength
Leak and heat seal strength detector is required for the expanding heat seal strength test. Pressure should be increased inside the flexible package. The part with the weakest strength (include but not limited to the sealing area) could be found out. Since expanding heat seal strength can not represent the average heat seal strength, there is no relationship between expanding heat seal strength and stretching heat seal strength. Expanding heat seal strength is related to the size, shape and material of the package. If the material is so soft that it could be extended to several times of its original length and obvious deformation would occur with the testing pressure increases. Then, restraining plate should be applied to restrain the expansion caused by pressure increase and material deformation.
With leak and seal strength detector, the maximum burst strength of the flexible package could be determined. Additionally the duration for the breakage of test package could be obtained by setting the pressure applied on the package. Based on those test data, the operator could decide the stack structure of the package and adjust the heat seal parameters as well the structure problem of the package could be analyzed according to the breakage position in the test package.
With adjustment of the heat seal parameters through the heat seal property test, the breakage ratio of the flexible package in process of filling, storage and transportation could be reduced. However, attention should be paid to the following points as well.
First, the pollution to the sealing area by the content should be especially noticed. The heat seal strength or the hot tack force would be remarkably reduced by the contamination, and thus cause package breakage. Especially for the powder products, simulation tests are required to perform.
Second, based on the design requirement, there should be a margin for the hot tack force and the expanding heat seal strength obtained from the selected heat seal parameters for various materials and equipments in the production line. The homogeneousness of both the heat seal components and packaging materials especially the film used in flexible package is not so good that the accumulated errors would cause unevenness of heat seal effect in the heat sealing area.
Third, through testing the hot tack force and expanding heat seal strength of the material, 2 sets of heat seal parameters for specific product and specific production line could be obtained. The operator should associate the heat seal parameters with the heat seal curves obtained in test to get the optimum solution.
It has been widely accepted as an effective means to shorten packaging cycling time by optimizing the heat seal process. However, the heat seal parameter selection could directly influence the breakage ratio of the package, the filling efficiency and the losses in storage and transportation. Therefore, heat seal parameters could be decided through specific test. Meanwhile, parameter adjustment should be based on the actual content and packaging materials applied in production.