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Discussion on the heat sealing strength of packaging composite film

    As a flexible packaging material, whether it is used for bag making (three-side sealing bag, self-supporting bag, zipper bag, eight-side sealing bag), or as a roll film, the heat-sealing strength value must meet the standard requirements, so that it can be sealed, compressed, Anti-mechanical shock, boiling, steaming and other aspects provide safety protection for the package contents. Heat-sealing strength is a very important physical index of composite film, a key control point in the bag making process, and a major issue that has caused disputes between packaging material producers and users. Composite strength is the focus of frequent discussion and attention in the flexible packaging industry. Maybe everyone has summarized a set of effective control schemes. Now based on production practice experience, we will discuss the factors that affect the heat seal strength to stabilize packaging during the use of composite film. Provide some reference opinions on product quality, reduction and elimination of various potential risks caused by abnormal heat seal strength.

A. Definition and standard of heat seal strength

     Heat-sealing strength is also called heat-sealing strength, bonding strength, sealing strength and sealing strength, etc. It is the inner layer material of the composite film that bonds the molten materials together by heating and pressing, and after cooling to normal temperature, the bonding There is a certain degree of firmness, and the firmness of this fit is the heat seal strength. According to the specified sampling and testing methods, the specific value of the sample can be obtained, that is, the heat seal strength value.

The heat-sealing strength standard can refer to the following two national standards: GB/T10004-2008 “Plastic composite film for packaging, bag dry composite, extrusion composite” and GB/T21302-2007 “General rules for composite film and bag for packaging”. In GB/T21302-2007 “General Rules for Composite Films and Bags for Packaging”, the requirements for the grade of heat seal strength are as follows:

Product Category

Extruded composite bag

Dry compound bag

Ordinary composite bag



Boiled grade composite bag



Semi-high temperature cooking grade composite bag



Cooking grade composite bag



Table 1 Heat sealing strength (N/15mm) requirements

    The heat-sealing strength (N/15mm) is required in GB/T21302-2007 “General Rules for Composite Films and Bags for Packaging”. The heat-sealing strength grades are as follows: Table 2


Level 1

Level 2

 Level 3

Level 4

 Level 5

Heat sealing strength(N/15mm)






Table 2 Classification of heat sealing strength

    The heat-sealing strength test method refers to QB/T2358-1998 “Plastic film packaging bag heat-sealing strength test method”, which will not be explained here. Generally speaking, the supplier and demander will determine the specific heat-seal strength standard according to the national standard, according to different material structure, thickness, packaging bag shape, use conditions, etc. This standard is usually close to or higher than the national standard, which is the process of composite film and bag Basis for strict control.

B.Correlation performance index of composite film and bag-making products and heat sealing strength

     The packaging materials after heat sealing include bag-making products. After filling the contents, they have the index requirements of compression resistance, drop resistance, tear resistance, impact resistance, boiling resistance, penetration resistance, and edge sealing resistance. The sealing strength has a certain relevance.

Although these items are sometimes related to the flexibility, stretch resistance and impact resistance of the composite film itself, the heat seal strength often plays a key role in ensuring the above items. Therefore, the heat seal strength is used as a packaging film and bag production. Enough attention should be paid to the current basic management and control projects.

     Inadequate heat sealing strength (including brittle fracture caused by excessive heat sealing) of bag-making products is not uncommon, such as compression value, drop resistance, penetration resistance, etc., which is sufficient to prove the complexity of the related items of bag-making heat-seal strength .

For example, the most common packaging bag has poor compression resistance and fails to meet the compression value standard. Most of the reasons are that the low sealing strength of the heat-sealed edge causes cracking or direct brittleness along the heat-sealed edge. This situation is a fatal quality problem. , Will cause the packaging bags to be scrapped in batches.

C. The logical relationship of the “three elements” affecting the strength of heat sealing

    During the heat sealing process of the composite film, other conditions remain unchanged. Temperature, pressure and time are the three elements that affect the heat sealing strength. The logical relationship between them is: in order to achieve a stable heat sealing strength above the standard, the three variables can be simultaneously Adjust, you can also adjust one or two of them; after the three reach a certain balance, in the case of stable heat sealing strength, with the appropriate increase in temperature and pressure, the sealing time can be shortened (that is, the sealing speed can be increased). It is also possible to appropriately increase the temperature and speed while the pressure remains unchanged. When the temperature reaches a certain value, the heat-sealing layer can be melted, and the melted material has the property of bonding. Because organic high molecular polymers have no fixed melting point, when heated to the material melting area, the heat-sealing film will be in a molten state. The melting temperature and decomposition temperature of the heat sealing film are the lower and upper limits of the heat sealing temperature. The heat sealing temperature generally cannot exceed this range. Of course, this temperature condition is also related to the heat sealing pressure and time. During the bag making process This range is not absolute.

    The function of pressure is to produce effective interpenetration, diffusion, and cross-linking of polymer chains between the film interface that is already in the molten state, so that the distance between resin polymers is close enough to generate intermolecular forces. If the pressure is too low, the heat seal will not be firm, and if the pressure is too high, the effective chain of the molten part will be squeezed out, and the bonding thickness will be insufficient. The heat-sealed part will be similar to a cut state, resulting in weakened strength. The heat-sealing time refers to the time that the film stays under the hot knife. To extend the heat-sealing time, one can increase the heat transfer so that the heat-sealing layer can fully reach the set temperature of the hot knife. After the material is fully melted, it is easier to seal. In addition, as the time of external force increases, it also helps the polymer chains to produce viscous flow, cross-link each other, and meet the sealing requirements. In summary, the “three elements” that affect the heat-sealing strength of the composite film are interrelated and indispensable. After the sealing conditions are reached, they can work alone or in a comprehensive manner. Under normal circumstances, improper coordination of these three elements will cause insufficient heat sealing and low sealing strength. Sometimes there will be excessive heat sealing, which will cause brittle fracture of the heat sealing edge and result in insufficient strength. qualified. When the composite film is heat-sealed, these three parameters must be determined through machine tests or past experience. They are the focus of heat-sealing process parameter management.

D, other factors

  1. Material thickness and uniformity

    Due to the different thickness of the same material, the heat sealing strength will be significantly different under the same heat sealing conditions. Under normal circumstances, the thicker the composite film, especially the thicker the heat-sealing layer, the better the heat-sealing conditions and the higher the heat-sealing strength. If the film thickness is not uniform, such as the common lotus leaf edge phenomenon, when the film is moved, the sealing edge position is not straight, such a sealing edge is prone to creases, which will reduce the heat sealing strength.

  1. The different composite structure of the composite membrane has a great influence on the heat sealing strength.             The same heat sealing material, two-layer structure, three-layer structure and four-layer structure can be adjusted to obtain very different heat sealing conditions. Seal strength value. For example, a composite film with aluminum foil layer, due to the good thermal conductivity of aluminum foil, a good heat sealing effect can be achieved even if the heat sealing time is not long. If the composite film has a flexible material such as nylon, the heat sealing strength can be effectively improved . Commonly used PE film and CPP film heat-sealing materials, due to their different properties and other composite materials, the heat-sealing strength value will be very different. The same PE film or CPP film, due to the difference in raw material properties and processing technology The difference also has a great influence on the heat seal strength. For example, metallocene PE has better heat seal adaptability and heat seal strength value than ordinary PE.
  1. The influence of hot knives, hot cloths, and silicone plates.

    Commonly used hot knives are made of copper, iron, aluminum, etc. The shape of the hot knives and the size of the effective heat sealing area are determined according to the shape of the bag making machine and packaging machine and the size of the packaging products. ; The ironing cloth is also called Teflon insulation cloth or polytetrafluoroethylene cloth. There are differences in thickness and surface texture, such as ironing cloth flatness, durability, ironing knife edge wear, bruises, edge regularity, etc., silicone The board hardness, flatness, heat resistance and durability all have an impact on the heat seal strength of the composite film.

  1. Composite film composite processing method and composite film quality

     Composite film processing methods include dry compounding, wet compounding, extrusion compounding and co-extrusion compounding. Solvent-free compounding is usually classified as dry compounding, and co-extrusion compounding is classified as extrusion compounding. In this way, even under the same type of heat-sealing material, the same thickness, and the same type of heat-sealing conditions, the heat-sealing strength is quite different. There are also the impact of the quality of the composite film, such as high solvent residue, insufficient curing, curling, uneven thickness, and contamination of the heat-sealed area.

  1. Heat seal shape and area

    Regular strip straight line heat sealing and irregular arc curve heat sealing show different heat sealing shapes. Due to the large difference between the cut-in point and the force area during tearing, the heat sealing strength value will be different; The difference in the sealing area is the difference in the width of the heat seal, and also the difference in the amount of polymer cross-linking in the bonding area. The main function of the heat seal width is to extend the stress time, stabilize the heat seal strength value, and improve the balance of the heat seal strength. Play a role.

  1. Composite strength of composite membrane,

    Especially the composite strength of the heat-sealing layer. Generally, the composite film composite strength and the heat-sealing strength have a positive relationship, that is, the higher the composite strength within a certain range, the higher the heat-sealing strength, but in practice, this often occurs. A regular situation, that is, the composite strength is high, but the heat seal strength is low, and the composite strength is low, which does not affect the heat seal strength much. Because composite strength and heat seal strength have different microscopic mechanisms of bonding, there is a certain correlation between them. Composite strength has an important influence on heat seal strength, especially when the composite film is delamination during testing, the heat seal strength will suddenly decrease. However, in terms of the properties of various materials, the composite strength is not the decisive factor of the heat seal strength.

  1. Cooling method and effect of heat sealing area

After the composite film is heat-sealed, it generally needs to be quickly cooled. At present, water cooling is the main method. The function is to ensure the flatness of the heat-sealed area and enhance the aesthetics. The heat-sealing effect can also be determined by the condition of the cooling pattern, and the heat-sealing area is increased at the same time The sealing firmness. The main factors affecting the strength of the heat seal here are the temperature of the cooling knife, the distance between the heat sealing knife and the cooling knife, and the clarity of the lines in the sealing area after cooling. These can all affect the strength of the heat seal.

  1. Surface condition of heat sealing material

The surface condition of the heat-sealing material directly affects the heat-sealing strength value, such as moisture in the sealing area due to environmental humidity, grease pollution on the sealing surface, overdue composite film, and surface performance aging.

  1. Material

The anti-powder pollution performance is improved by formula. Some heat-sealing materials have good anti-powder pollution, but many heat-sealing materials will reduce the heat-sealing strength due to the influence of slippery powder, environmental dust, paper tube dust and auxiliary powder. 

  1. Printing ink status in the heat-sealed area of the composite film

Mainly refers to the thickness and uniformity of the ink. The thicker the thickness, especially the ordinary ink and metal ink that are not resistant to high temperatures, the high temperature affects the fastness of the ink, which in turn affects the peeling strength. If the material is layered after heat sealing, it will cause heat sealing The intensity is weakened.

  1. Corona breakdown

Commonly used heat-sealing materials, PE film and CPP film, have to be corona treated on the non-hot cover to ensure the composite strength. During the corona process, due to various abnormalities, the hot cover will have corona, which will be greatly affected. Reduce the strength of the heat seal, even the normal heat sealing conditions can not be sealed at all.

  1. Sampling and test impact

At present, many special-shaped bag products have irregularities in the heat-sealed edges. For example, the widths are different, and the edges are curved. When sampling, the edges of the heat-sealed area should be as straight as possible. When testing, the stress is the entire sealing line, not A part or one point is to ensure that the entire heat sealing edge is stressed, otherwise the heat sealing strength may be low.

E.the intensity difference under different conditions

    Generally speaking, the heat-sealing strength value is tested at room temperature and normal state just after the bag making or packaging is completed. Compare this value with the national standard, enterprise standard, or customer acceptance standard, and then judge whether it is qualified. This data is used as the data for the first inspection of bag making, and it is also the reference basis for heat-sealing machinery to determine the processing parameters. During the production process, take samples and test according to the sampling plan to verify the stability of the heat seal strength during the processing process. If there is no abnormality, the production will continue until the same batch of products is completed. For the bag-making process, the heat-sealing strength data may be stable, and the customer needs to be sealed when using it. It is difficult to ensure the consistency of the sealing strength of the seal and the heat-sealing strength during bag making, because the processing conditions change too much. In order to be close to the customer’s use conditions, many bag-making manufacturers will imitate the customer’s environment to test the bag-making products, such as testing under hot or cold filling conditions, or placing them for a period of time after filling, and then testing at room temperature. The data under these conditions are difficult to match with the data when the bag making is off the machine, and most of them will have intensity attenuation.

    There are usually two ways of attenuation of heat sealing strength. One is the attenuation caused by time conditions. As time goes by, the normally placed bag products will attenuate due to aging. This process is very slow during the shelf life of the bag and can be almost ignored. ; The second is the attenuation caused by changes in use conditions. Cold packaging, hot packaging and sterilization conditions have a direct impact. This attenuation process is sometimes very rapid. For example, after high-temperature retorting and sterilization, the heat sealing strength of some bags may even exceed 30%. The attenuation of heat seal strength is an objective existence. If the attenuation cannot meet the requirements of use, the bag-making products need to be overcome in the use of materials and process selection.

F. How to effectively stabilize the heat seal strength


     The packaging material has a certain degree of firmness after heat sealing. Its essence is the re-establishment of molecular bonds and the effect of intermolecular attraction. From a microscopic analysis, the more molecular bonds in the heat sealing layer, or the greater the intermolecular attraction, the higher the heat sealing strength. . The heat sealing of the composite film is the result of “heat” and “force”. The thickness and composite structure have an important influence on the heat sealing effect, and the influence of heat sealing parameters is formed under certain conditions. For example, the thicker the composite film, the longer it takes for the heat-sealing layer to reach the sealing temperature, but it can be adjusted by increasing the temperature and pressure. To effectively stabilize the heat seal strength of the composite film, it is necessary to cut from the above-mentioned “three elements” and twelve influencing factors. Here are some details to pay attention to:

  1. Ensure the processing parameters, especially the stability of the “three elements” and the machine inspection requirements for the bag making process. The most important thing is to ensure that the heat seal strength is up to standard and stable. The processing “three elements” are guaranteed by the machine and equipment. The calibration of the machine temperature control system, the calibration of the sealing knife level and the calibration of the machine speed are the primary conditions. The frequency of daily inspection is once per shift. In the case of continuous production, the frequency of maintenance should not exceed one month. Make sure The stability of the machine state. In addition to setting processing parameters based on experiments and experience, standard processing instructions must be formulated, and the temperature range of the cooling knife must be determined.
  2. According to the specifications of the bag-making products and the types of contents, reasonably design the structure and thickness of the composite film, the size of the bag, the shape and specifications of the sealing edge, whether it is liquid or solid, how much to pack, and whether the packaging conditions are at room temperature or high temperature. The heat seal strength of the bag is different, and the structure and thickness of the composite film should be designed according to the specific purpose of use.
  3. The role of composite film preheating When the ambient temperature is low or the composite film is thick, preheating is very important, which is easy to overlook by many people. In practice, there are two ways to achieve preheating: one is to put the composite film in the 45~50℃ maturation chamber for 8~12h before making the bag and fully preheat it, and the other is to install a preheating knife on the bag making machine. , The function of this hot knife is to preheat the heat-sealing part to prepare for the subsequent heat-sealing directly. Under special conditions, preheating measures are very helpful to improve the heat seal strength.
  1. The management aids of bag-making auxiliary products mainly refer to heating tubes, hot knives, silicone plates and Teflon cloth. General bag-making manufacturers are not strict enough in the management of auxiliary products, and often do not perform until the heat sealing strength is unqualified. Replace it, or the damage is too serious to be used. A reasonable approach is to manage the use time and processing volume of auxiliary products according to the product type and daily experience. After a certain use time or the specified processing volume, they must be replaced to take precautions.
  2. The quality of the composite film After printing, laminating, curing, slitting and other processes, the composite film may have quality problems in each process. Some quality problems have a fatal impact on the heat sealing of the bag, such as the curing of the glue. Sufficient, contaminated heat-sealing layer material, unqualified composite peel strength, etc. Therefore, ensuring the quality of the composite film before bag making is an important condition for stable heat sealing strength.

G, conclusion

     The heat-sealing strength of packaging composite film is a topic that cannot be avoided in the bag making process. The improvement of heat-sealing material properties, the improvement of processing equipment and processes, and the accumulation of operator experience provide a guarantee for the stability of the bag-making heat-sealing strength. As long as the bag-making operator pays attention to the management and control of daily details, is diligent in thinking, practice more, and summarize, when dealing with abnormal heat seal strength problems, they can perform comprehensive analysis and accurate judgments. Such operators can quickly become Excellent engineer with expertise.


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