How to make high-quality packaging bags? Essential steps to overcome these "obstacles"!

The most important function of a packaging bag is to protect the quality of the packaged goods, meaning that the goods should not be damaged, lost, leaked, or spoiled during the entire circulation process of storage, transportation, and sales, through different links and environments. This requires that the seal during the bag-making process must have certain strength and sealing performance, be able to withstand the pressure of a certain weight of contents, and ensure that the goods do not crack or leak during circulation, thereby achieving the purpose of protecting the goods.

Below, let's follow Shunxingyuan Packaging, a professional flexible packaging manufacturer, to see how high-quality packaging bags ensure their quality.

I. Material Selection

1. Surface Layer Material

Mostly non-heat-sealable materials. It is advisable to choose materials with good heat resistance and low thermal deformation, such as BOPA, BOPET 、 BOPP , paper, etc.

2. Inner Layer Material

This refers to heat-sealable materials, ideally with a heat deformation temperature 30℃ lower than the surface layer material for heat sealing. If the temperature difference in thermal deformation between the surface layer material and the heat-sealable layer material is too small, it will cause difficulties in controlling the bag-making temperature. This is because if the temperature is too low, the seal strength will be poor and not meet requirements. To improve seal strength, the temperature must be increased. However, if the thermal deformation temperature difference between the inner and outer layers is too small, the surface layer material can easily be scorched.

For the same heat-sealable material, its heat-seal strength will increase with the increase in thickness of the heat-sealable material. Therefore, for back-seal bags and stand-up pouches, the inner layer material should be at least 30μ m in thickness. 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-seal temperature should be chosen, typically 5~10℃ lower than the heat-sealable temperature of the inner layer material. For example, Shunxingyuan Packaging's PE L-101 zipper sealing strip heat-seal temperature is lower than its PE LL-201 by 10 ℃, for different PE inner layer materials when making bags, to achieve the best heat-sealing state with the inner layer material.

II. Preparation Before Bag Making

1. Ensure that the composite film's composite layer is fully cured; otherwise, it can easily cause deformation of the heat-sealed edge after bag making, affecting the bag's appearance.

2. Select the appropriate heat-seal jaw according to requirements and clean it thoroughly, ensuring the heat-seal blade surface is flat, and checking if the insulation cloth is intact and flat.

3. Check the status of each part of the bag-making machine. Based on experience (referring to past production conditions for similar materials), preset the sealing pressure temperature, preheat, and after reaching the preset temperature for 20 minutes, formal production can begin. It should be noted that reaching the preset temperature does not mean the heat-seal jaw has reached the preset temperature; a period of heat conduction to reach equilibrium is required.

4. Adjusting the 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 conjunction with each other to achieve the optimal state.

III. Heat Seal Temperature

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

If the heat-seal temperature is lower than the melting temperature of the heat-sealable material, even increasing pressure and extending heat-seal time cannot truly seal the heat-seal layer. If the heat-seal temperature is too high, it can easily damage the heat-sealable material at the seal, significantly reducing the seal strength.

IV. Sealing Pressure

Generally, pressure is the first parameter to be adjusted and determined. Most equipment does not display pressure values; a heat-seal blade compression of about 3mm at the end is appropriate. Pressure balance (left-right or front-back) is very important. Too little compression will result in insufficient pressure, making it difficult for the heat-sealed surfaces to truly fuse together, leading to localized weak seals. Sometimes, this can also cause bubbles trapped in the weld not to be fully expelled.

Excessive compression, on the other hand, will result in too much pressure, causing the heat seal to be out of sync with the traction. Furthermore, when the heat-seal temperature is high, the material at the seal is already in a semi-molten state. If the pressure is too high, it can easily squeeze out some of the heat-sealable material, causing the sealing edge to be in a semi-cut state, making the material at the seal thinner and more brittle, thus reducing the heat-seal strength.

V. Sealing Time (Speed)

Sealing time not only affects heat-seal strength but also impacts the flatness of the composite packaging bag's appearance. When heat-seal temperature and pressure are fixed, a longer heat-seal time allows for more thorough fusion of the heat-seal layer, leading to a stronger bond and higher seal strength. However, if the heat-seal time is too long, the film tends to shrink, easily causing wrinkles at the seal and affecting appearance flatness.

VI. Cooling

The cooling process involves shaping the newly melted and heat-sealed seam using a lower temperature. The cooling temperature should be the lowest temperature at which the cooling blade does not accumulate condensed water. Cooling water circulation must be smooth. If cooling water is not available, the water flow rate should be increased as much as possible to ensure the cooling effect. If the sealed edge is not sufficiently cooled after heat sealing, leading to insufficient shaping, internal stress cannot be eliminated, which will affect heat-seal strength and appearance.

VII. Number of Heat Seals

The more heat seals, the higher the heat-seal strength. Generally, at least two or more heat seals are required. The number of longitudinal heat seals depends on the ratio of the effective length of the longitudinal heat-seal blade to the bag length; the number of transverse heat seals is determined by the number of sets of transverse heat-seal devices on the bag-making machine.

VIII. Process Control

During production, the quality of finished products should be continuously checked. In addition to checking the dimensions and appearance visually, the sealing strength and sealing performance should also be checked. Problems should be addressed promptly, and detailed records should be kept. Taking three-side sealed zipper bags as an example, the following items should be checked:

1. Heat sealing strength;

2. The size, flatness, and appearance quality of the bag, the alignment of the front and back of the bag, and other requirements specified by the customer, such as the location of the tear notch; the direction of the zipper placement, especially at the zipper connection section;

3. The inner and outer opening force of the zipper, PE The inner opening force of the zipper generally requires ≥ 50N/50mm， PP The inner opening force of the zipper generally requires ≥ 60N/50mm ，Shunxingyuan Packaging's PE LL-201 can reach ≥ 70 N/50mm ， PE Q501 can reach ≥ 80 N/50mm ，The outer opening force has different requirements according to customer usage habits;

4. The sealing performance of the bag, which can be tested by spraying a dye-containing penetrating liquid into the seal to observe whether it penetrates, or by filling the bag with water and hanging it upside down 24 for hours to observe whether it leaks;

5. The 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 leaching 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 comprehensively considered during production.

Long press the QR code to follow, Customize your exclusive packaging

www.dgsunpak.cn  Packaging bags