How to Make Zipper Bags?
How to Make Zipper Bags
The most important function of a packaging bag is to protect the quality of the product. This means ensuring that the product, during storage, transportation, and sales throughout the entire flow process, remains intact without damage, loss, leakage, or spoilage, even through different stages and environments. This requires that the sealing of the bag during production must have sufficient strength and sealing performance, capable of withstanding the pressure of the contents' weight, ensuring that the product does not crack, leak, or break during the flow process, thus achieving its protective purpose. Bag making is the last production process before delivery to the user, making quality control even more important. If you need to purchase high-quality, low-cost zipper bags, please contact LIJIE factory in China.
Material Selection
1. Outer Layer Material
Zipper bags are mainly made from non-heat-sealable materials. It is advisable to choose materials with good heat resistance and minimal thermal deformation, such as BOPA, BOPET, BOPP, paper, etc.
2. Inner Layer Material
This refers to heat-sealable materials, which should have a heat deformation temperature 30°C lower than that of the outer layer materials for better heat sealing. If the temperature difference between the outer and heat-seal materials is too small, it can cause difficulty in controlling the sealing temperature. A low temperature would lead to weak sealing, failing to meet requirements. To improve sealing strength, the temperature would be raised, which can easily burn the outer layer material if the temperature difference between the inner and outer layers is too small.
For the same heat-seal material, the sealing strength will increase with the thickness of the heat-seal layer. Therefore, for back-seal bags and stand-up pouches, the inner layer material should be at least 30μm thick. If the inner layer is too thin, it will affect the sealing performance of the finished bag.
3. Zipper
Choose zipper sealing strips with lower heat sealing temperatures. Generally, it is best to choose a zipper strip that has a heat-sealing temperature 5-10°C lower than the inner layer material. For example, the PE L-101 zipper sealing strip from Zhongcheng Packaging Materials Co., Ltd. in Zhongshan has a heat-sealing temperature that is about 10°C lower than the PE LL-201 strip. This allows for optimal heat sealing with various PE inner layer materials.
Preparation Before Bag Making
1. Ensure the composite film layers are fully cured; otherwise, it may cause deformation in the heat-seal edges after bag making, affecting the appearance of the bags.
2. Select the appropriate heat seal areas and clean them thoroughly. Ensure that the heat seal knives are flat and check if the heat-insulating cloth is intact and smooth.
3. Inspect the condition of the bag-making machine components. Based on experience (referencing previous similar materials), set the sealing temperature, preheating time, and wait for 20 minutes to reach the preset temperature before starting formal production. It is important to note that reaching the preset temperature does not mean the heat-sealing area is at the correct temperature—there is a thermal balance process.
4. Adjusting the process conditions—temperature, pressure, and time (speed)—is essential for heat sealing. These three factors are critical process parameters and need to be adjusted in coordination to achieve the optimal state.
Heat Sealing Temperature
The heat sealing temperature has the most direct effect on sealing strength, and it is the main basis for choosing the optimal sealing flow temperature. The heat-sealing temperature should be set according to the material of the composite, its thickness, and the processing speed. In actual production, factors such as sealing pressure, bag-making speed, and composite material thickness usually require the heat sealing temperature to be higher than the melting temperature of the heat-seal material.
If the sealing temperature is lower than the melting temperature of the heat-seal material, increasing pressure and extending the sealing time will not effectively seal the layer. If the sealing temperature is too high, it can damage the sealing material, greatly reducing the sealing strength.
Sealing Pressure
Generally, pressure is the first parameter to adjust. Most equipment does not have a pressure indicator, but it is typically around 3mm at the heat-seal knife pressing end. It is crucial to maintain pressure balance (left and right or front and back). Insufficient compression will result in inadequate pressure, causing the sealing surface to fail to properly fuse, leading to weak seals or air bubbles trapped in the weld.
Excessive compression, on the other hand, can cause the sealing edge to become unsynchronized with the pulling process. If the sealing temperature is high, the material at the sealing area is semi-melted, and too much pressure may force some of the sealing material out, causing the sealing edges to thin and become brittle, reducing the sealing strength.
Sealing Time (Speed)
The sealing time affects both the sealing strength and the smoothness of the composite bag’s appearance. With the sealing temperature and pressure fixed, longer sealing times allow for better fusion and stronger bonding, which increases sealing strength. However, if the sealing time is too long, the film may shrink, causing wrinkles in the sealing area, which negatively affects the appearance.
Cooling
The cooling process involves cooling the freshly melted seal to set the weld. The cooling temperature should be at the lowest temperature where no condensation water accumulates on the cooling knife. The cooling water should circulate smoothly, and if no cooling water is available, the water flow should be increased to ensure proper cooling. If the heat-sealed edge is not sufficiently cooled, the internal stress will not be eliminated, which can affect sealing strength and the appearance of the bag.
Heat Sealing Times
The more times the heat-seal is applied, the higher the sealing strength. Generally, it is required to apply heat-seal at least twice. The number of longitudinal heat-seals depends on the effective length of the longitudinal sealing knife and the length of the bag. The number of transverse heat-seals is determined by the number of transverse sealing units in the bag-making machine.
Process Control
During production, continuous inspection of the finished product quality is necessary. In addition to checking size, appearance, and visual quality, it is important to inspect sealing strength and sealing performance. If any issues are detected, adjustments should be made, and detailed records should be kept. For example, in the production of three-side-seal zipper bags, the following items should be checked:
1. Sealing strength
2. Bag size, flatness, appearance quality, front-back alignment, and other customer requirements such as tear-notch position, etc. The direction of the zipper, especially at the zipper connection.
3. Zipper internal and external opening force: PE zippers typically require an internal opening force of ≥50N/50mm, while PP zippers typically require ≥60N/50mm. Zhongcheng Packaging Materials Co., Ltd.'s PE LL-201 achieves ≥70N/50mm, and PE Q501 can reach ≥80N/50mm. The external opening force may vary based on customer usage habits.
4. Bag sealing performance: This can be tested by spraying a penetrant with dye into the sealed area to check for leakage, or by filling the bag with water, inverting it, and observing for leaks after 24 hours.
5. The flatness of the zipper area.
Heat-seal bag making is a heat transfer process, and the technology is relatively straightforward. However, many factors affect its quality, including the peel strength of the composite film, heat resistance of printing inks, additives in the heat-seal layer, the width of the sealing edge, and the uniformity of the composite film's thickness. These all impact the final quality of the composite film bag, and need to be considered during production.
