Essential for zipper bag making, you need one too!

The most important function of packaging bags is to protect the quality of all goods. This means that throughout the entire process of storage, transportation, and sales, the goods remain undamaged, unlost, leak-proof, and unchanged, regardless of the different stages and environments. This requires that the sealing process during bag making must have a certain strength and sealing performance, able to withstand the pressure of a certain weight of contents, ensuring that the goods do not crack or leak during the circulation process, achieving the purpose of protecting the goods. Bag making is the final production process before delivery to the user, making quality control even more important.

I. Material Selection

1. Surface Material

Mostly non-heat-sealable materials. Materials with good heat resistance and low heat deformation should be considered, such as BOPA, BOPET, BOPP, and paper.

2. Inner Layer Material

This is the heat-sealable material. It should be heat-sealable at a temperature 30℃ lower than the surface material. If the temperature difference between the surface material and the heat-sealing layer material is too small, it will make it difficult to control the bag-making temperature. Because the temperature is low, the sealing strength is poor and does not meet the requirements. To improve the sealing strength, the temperature is increased, but because the temperature difference between the inner and outer layers is too small, the surface material is easily scorched.

For the same heat-sealing material, the heat-sealing strength increases with the thickness of the heat-sealing material. Therefore, for back-sealed bags and stand-up pouches, the inner layer material should be at least 30μm thick. If the inner layer material is too thin, it will affect the sealing performance of the finished bag.

3. Zipper

A zipper sealing strip with a lower heat-sealing temperature should be selected. Generally, it is best to use a temperature 5-10℃ lower than the heat-sealable temperature of the inner layer material. For example, the heat-sealing temperature of the PE L-101 zipper sealing strip from Zhongshan Zhongcheng Packaging Materials Co., Ltd. is about 10℃ lower than its PE LL-201, providing options for bag making with different PE inner layer materials to achieve the optimal state of heat sealing with the inner layer material.

II. Preparation before Bag Making

1. Ensure that the composite film composite layer is completely cured; otherwise, it is easy to cause deformation of the heat-sealed edges after bag making, affecting the appearance of the bag;

2. Select the appropriate heat-sealing mouth according to the requirements, clean it, ensure that the heat-sealing knife surface is flat, and check whether the heat insulation cloth is intact and flat;

3. Check the status of each component of the bag-making machine, pre-set the sealing temperature according to experience (refer to the production conditions of similar materials in the past), preheat, and start formal production only after reaching the pre-set temperature for 20 minutes. It should be noted that reaching the pre-set temperature does not mean that the heat-sealing mouth has reached the pre-set temperature; there is a process of heat conduction to reach equilibrium;

4. Adjustment of bag-making process conditions: Temperature, pressure, and time (speed) are the three indispensable elements of heat sealing and the most important process parameters. Temperature, pressure, and time (speed) need to be adjusted in coordination to achieve the optimal state.

III. Heat Sealing Temperature

Heat sealing temperature has the most direct impact on heat sealing strength. It is the main basis for selecting the optimal viscous flow temperature state. The heat sealing temperature should be set according to the material, thickness, and processing speed of the composite material. In actual production, due to the influence of many factors such as heat sealing pressure, bag making speed, and the thickness of the composite substrate, the heat sealing temperature is often higher than the melting temperature of the heat sealing material.

If the heat sealing temperature is lower than the melting temperature of the heat sealing material, even if the pressure is increased and the heat sealing time is extended, the heat sealing layer cannot be truly sealed; if the heat sealing temperature is too high, it is easy to damage the heat sealing material at the sealing mouth, greatly reducing the sealing strength.

IV. Sealing Pressure

Generally, pressure is the first parameter to be adjusted and determined. Generally, equipment does not have a pressure indication. It is advisable to use about 3mm at the pressure end of the heat sealing knife. Pressure balance (left and right or front and back) is very important. Too little compression will cause insufficient pressure, making it difficult for the heat sealing surface to truly melt together, resulting in locally weak heat sealing, and sometimes bubbles trapped in the weld cannot be cleaned.

Excessive compression will cause excessive pressure, causing the heat sealing and traction to be out of sync. Moreover, when the heat sealing temperature is high, the material at the sealing mouth is in a semi-molten state. If the pressure is too high, it is easy to squeeze out some of the heat sealing material, making the edge of the sealing mouth in a semi-cut state, resulting in the material at the sealing mouth becoming thinner and more brittle, and the heat sealing strength being reduced.

V. Sealing Time (Speed)

Sealing time not only affects the heat sealing strength but also affects the flatness of the composite packaging bag. When the heat sealing temperature and pressure are fixed, the longer the heat sealing time, the more fully the heat sealing layer is melted, the stronger the bonding, and the higher the sealing strength. However, if the heat sealing time is too long, the film is easy to shrink, which can cause wrinkles at the sealing mouth, affecting the flatness of the appearance.

VI. Cooling

The cooling process is to use a lower temperature to shape the weld that has just been melted and heat-sealed. The cooling temperature should be the lowest temperature at which the cooling knife does not accumulate condensation. The cooling water should circulate smoothly. If there is no condition to use cooling water, the water flow should be increased as much as possible to ensure the cooling effect. If the heat-sealed edge is not cooled enough, resulting in insufficient shaping and failure to eliminate internal stress, it will affect the heat sealing strength and appearance.

VII. Number of Heat Sealings

The more heat sealing times, the higher the heat sealing strength. Generally, at least two heat sealing times are required. The number of longitudinal heat sealing times depends on the ratio of the effective length of the longitudinal heat sealing knife to the bag length; the number of transverse heat sealing times is determined by the number of sets of the transverse heat sealing device of the bag making machine.

VIII. Process Control

The quality of finished products should be continuously checked during production. In addition to conducting dimensional and visual inspections, heat sealing strength and sealing performance checks should also be conducted. Problems should be addressed promptly, and detailed records should be kept. Taking the production of three-side sealed zipper bags as an example, the following items should be checked:

1. Heat sealing strength;

2. Size, flatness, appearance quality of the bag, alignment of the front and back of the bag, and other requirements of the customer, such as the position of the tear opening; the placement direction of the zipper, especially at the zipper connection section;

3. Internal and external opening force of zippers: The internal opening force of PE zippers generally requires ≥50N/50mm, and the internal opening force of PP zippers generally requires ≥60N/50mm. Zhongshan Zhongcheng Packaging Materials Co., Ltd.'s PE LL-201 can reach ≥70 N/50mm, and PE Q501 can reach ≥80 N/50mm. The external opening force has different requirements according to customer usage habits;

4. Sealing performance of the bag: Its detection can be carried out by spraying a dye-added permeation liquid onto the inside of the seal to observe whether it permeates, or by filling the bag with water and hanging it upside down for 24 hours to observe whether it leaks;

5. Flatness of the zipper on the bag.

Heat-sealing bag making is a heat transfer process, and its process is relatively simple, but there are many factors that affect its quality. In addition to the above points, the peel strength of the composite film, the heat resistance of the printing ink, the exudation of the heat-sealing layer additive, the width of the heat-sealing edge, and the thickness uniformity of the composite film are all related to the final quality of the composite film bag, and should be considered comprehensively during production.