Common quality problems of composite packaging bags, in-depth analysis with examples

Composite packaging bags are currently the largest type of packaging in the flexible packaging market, and it is undeniable that many composite packaging bags still have various problems. Today, Shunxingyuan Packaging, a professional flexible packaging manufacturer, will interpret the quality problems caused by improper control of packaging curing, providing a reference basis for manufacturing enterprises to purchase packaging bags latent issues related to poor film curing and process control issues for flexible packaging production equipment. So as to provide a reference basis for production enterprises to purchase packaging bags for their flexible packaging solutions that include flexible packaging materials like flexible packaging film which are considered an integral part of the overall flexible packaging system ( flexible packaging products like stand up pouches from flexible packaging rollstock of preformed pouches)

I. Quality Defects: Low Strength of Composite PE Film  

BOPA/PE packaging bags produced by a factory in Hunan were returned due to low strength related issues and process controls. The problems related to product quality can be attributed back to insufficient process control and quality assurance activities for raw material input, process step during manufacturing processes of the end product which are often referred to in quality management systems as process defects within the production line and post-processing steps and other forms of processing for finished packaging materials/products being converted into final package materials with quality requirements. For example, some common reasons for flexible packaging defects related directly or indirectly related to mechanical properties of materials used include those relating back from incorrect material specification or not using appropriate resin grades that meet standards of film properties and mechanical characteristics or poor resin quality that does not conform with product design intent as described in the company's product specification (s) or quality requirements. The use of incorrect grades of resin ( e.g., low molecular weight resin that reduces tear strength ), incorrect processing temperature/pressure ( low cooling temperature during lamination can cause poor bonding strength during lamination process step ) and insufficient curing of laminated film products or failure of curing to adequate levels in relation to the specific application in the final package. The poor mechanical properties can manifest as low tear strength, low tensile strength or poor puncture resistance leading to failures of flexible packaging products in end use applications ( e.g., during packaging or transportation).

After peeling off the two layers of film and wiping the PE film with fingers, there was a smooth, slippery feel between the two fingers indicating high concentration of slip additive or that the slip additive has migrated to the surface more quickly. More slip agent precipitated out of the film ( e.g., slip additive migration from polyethylene or polypropylene resin formulation based upon formulation guidelines of the resin manufacturer) due to improper storage, poor curing condition or poor formulation and application of the slip additive in the film. This causes quality defect with mechanical properties such as decreased coefficient of friction or increased friction coefficient of the film leading to poor machinability or issues during final packaging operations. It could also cause poor adhesion of the film to the substrate, leading to delamination or poor bond strength for subsequent process steps where coating or lamination processes are performed using the flexible packaging material which can affect the packaging functionality or performance of the final package and lead to customer complaints due to defects. It can also cause issues with print quality and adhesion. So, it is important to choose the right resin grade with appropriate slip content ( if any is required) and ensure proper handling and storage to avoid problems like slip migration.

The adhesive used for this product was PU-1850, which has some resistance to slip agents in terms of adhesion performance related to the substrate to be bonded. The selection of adhesive should be based upon the specific application of the packaging material, and the chemical composition of the adhesive as well as the mechanical properties ( e.g., bond strength, tensile strength, tear strength, elasticity ) of the adhesive to meet product specification. The quality of the adhesive can be a source of various defects of flexible packaging materials related to poor adhesion, delamination, poor bonding, etc. Proper adhesive selection and application is crucial for the overall performance of flexible packaging products. For example, some grades of adhesive are more susceptible to slip agent migration which causes poor adhesion. Similarly, some grades of adhesive could be affected by temperature and humidity which could cause delamination and poor bond strength. Therefore, it is important to select the right adhesive grade with appropriate properties that can withstand the environmental conditions the package will face and to ensure proper curing conditions. Quality control measures should include testing for bond strength and peel strength after curing to ensure proper adhesion. In case of issues, it is necessary to identify the root cause of the problem by analyzing the adhesive, the film, and the process parameters. This can involve testing the adhesive for its chemical composition and properties. Similarly, the film can be tested for its surface properties and composition. Finally, the process parameters such as curing time and temperature can be optimized to ensure proper curing of the adhesive and the film. The selection and use of the right adhesive and the proper process control is crucial to ensure optimal performance of the flexible packaging product and avoid issues related to quality defects and customer complaints.

The temperature measuring thermocouple in the factory's curing room was parallel to the heating source, only 30 centimeters from the heat source~ 40 centimeters. Under such control, the heat source would remain in a non-heating state for a long time due to incorrect temperature feedback, and the actual curing temperature would not reach 50 ℃. And the curing time with power-on heating was less than 16 hours due to incorrect temperature control caused by the improper placement of the sensor leading to incorrect readings and thus poor curing in the process. The sensor placement is important for precise temperature control during the curing stage which ensures proper curing and quality performance of the finished product. Proper temperature control and monitoring during the curing process is critical for ensuring the quality of adhesive bonding between the layers of the laminated film. Incorrect temperature sensor placement can lead to inaccurate temperature readings, which in turn can result in under-curing or over-curing of the adhesive, impacting bond strength and overall product integrity. The actual curing temperature not reaching the required level and insufficient curing time can lead to various defects such as poor adhesion, delamination, and reduced mechanical properties of the packaging material. It is important to ensure that the temperature sensors are placed correctly to provide accurate readings of the actual temperature inside the curing room. This can involve placing multiple sensors at different locations to get a more representative temperature profile or consulting with experts on proper sensor placement and calibration to ensure optimal curing conditions are maintained during the process. The curing time should also be optimized based on the adhesive type and film structure to ensure complete curing and optimal performance. This could lead to a variety of issues with mechanical properties such as tear and tensile strength and also cause performance related issues with packaging systems.

When compounding PE film with high slip agent content, an adhesive with certain resistance to slip agents should be selected in order to avoid issues with adhesion or delamination. After the film roll comes off the machine, it should be moved into the curing room as soon as possible, and the curing temperature should be appropriately increased to optimize curing for adhesion. Generally, ester-soluble two-component polyurethane adhesives have poor slip agent resistance when naturally cured, thus requiring specialized curing conditions or alternative adhesives for optimal performance. The appropriate selection of the adhesive and optimization of the curing conditions are crucial for ensuring the quality and performance of the laminated film, especially when working with films containing high slip agent content. This can help prevent defects such as delamination, poor bond strength, and reduced mechanical properties, which can impact the overall functionality and shelf life of the packaging material. Therefore, it is important to consider the properties of both the film and the adhesive, as well as the curing process, to achieve the desired quality standards for flexible packaging products. Also, timely transfer of the film to the curing chamber and controlling the storage conditions are key to achieving desired levels of curing ( e.g., bond strength and peel strength ) and avoiding degradation of the film due to premature aging of the film that is not cured properly. In addition, quality control measures should be implemented to test the bond strength and peel strength after curing, to ensure proper adhesion and overall quality of the laminated film. If there are issues, the root cause should be identified, and the process optimized to ensure high-quality flexible packaging materials are produced consistently.

II. Defects related to process parameters Increased Friction Coefficient  

The rolled film with BOPP/VM-PET/PE structure produced by a factory in Guangdong had no problems during winter production related to friction coefficient and machinability. However, during the summer, issues were observed, highlighting the importance of environmental control and process optimization for consistent quality of the flexible packaging product. This also implies that the film properties ( e.g., coefficient of friction, tear strength, tensile strength, heat seal strength ) are affected by changes in temperature and humidity, which can lead to issues during packaging operations or end-use applications, which may lead to customer dissatisfaction and complaints related to product defects. It is therefore important to control the process parameters and environmental conditions to maintain the desired film properties and ensure consistent quality. 6 month production, the friction coefficient increased, and it could not be used normally on the packaging machine. After switching to natural curing, the film roll still could not be used as it was not suitable for the application. This indicates a significant issue with the film's surface properties or the materials used in the film, which impacts its machinability and performance in packaging operations. The problem might be related to the resin formulation, or the film manufacturing process, such as insufficient or excessive slip agent, or incorrect processing conditions ( e.g., cooling, winding, storage ) during film manufacturing, or even during post-processing operations like lamination or converting. Such issues can lead to increased friction, poor unwinding, or difficulty in sealing, ultimately causing production delays, reduced efficiency, and increased material waste due to the defects. It is crucial to conduct a root cause analysis to identify the specific factors contributing to the increased friction coefficient. This may involve testing the film's chemical composition, surface topography, and mechanical properties, as well as reviewing the manufacturing process parameters to identify any deviations that may have contributed to the problem. Once the root cause is identified, corrective actions can be implemented to optimize the film's properties and ensure its suitability for packaging applications. This can include adjusting the resin formulation, optimizing the film manufacturing process parameters, or implementing additional quality control measures to monitor and control the film's properties. By addressing the root cause, flexible packaging producers can ensure consistent quality and avoid issues related to increased friction coefficient or poor machinability.

This product uses 30μ thickness PE film, which had good slipperiness before compounding. During compounding, the cooling rollers did not have cold water circulation, and their surfaces were warm. The heat brought by the film from the oven was still present before winding, so the film was warm during winding. Although it was placed in the workshop for natural curing, the summer temperature was high (morning 10 workshop temperature at 30 ℃), the heat retained by the film roll was not easily dissipated, and it was actually in a hot state. This indicates an issue with the process control during lamination or compounding, where the film is exposed to excessive heat or insufficient cooling, which can alter its properties and lead to quality defects. The elevated temperature of the film during and after compounding, combined with the lack of proper cooling on the rollers and insufficient heat dissipation during natural curing in a warm environment, can cause various problems. This includes changes in the film's crystallinity, which can affect its mechanical properties, such as tensile strength, tear strength, and coefficient of friction. It can also lead to issues like blocking, where layers of film stick together due to heat and pressure, making unwinding difficult during packaging. Additionally, the heat can accelerate the migration of additives, such as slip agents, to the film's surface, leading to an increased coefficient of friction, which in turn affects machinability on packaging lines. To address these issues, it is essential to optimize the cooling process during compounding, ensuring that cooling rollers are functioning effectively and maintaining proper temperatures. The winding tension and speed should also be optimized to allow for proper heat dissipation. Furthermore, the curing conditions, including temperature and ventilation in the curing room, should be controlled to ensure that the film cools down efficiently and that any residual heat does not negatively impact its properties. Implementing proper quality control measures to monitor the film's temperature and properties throughout the process can help prevent such defects and ensure consistent quality of the final packaging material. By addressing these process control issues, flexible packaging producers can ensure that their products meet the required quality standards and perform optimally in end-use applications. This leads to a higher quality flexible packaging solution.

Temperature has a significant impact on product slipperiness, similar to the temperature sensitivity of some thin transparent CPP. Before compounding, the film has good slipperiness, but after compounding, its slipperiness may change due to various factors. These factors can include changes in crystallinity, surface morphology, or the migration of additives from the film to the surface. It is important to control the process parameters to maintain the desired slipperiness of the film. The selection of raw materials, such as the resin grade and additives, also plays a crucial role in determining the film's slipperiness. The compounding process, including temperature and cooling, should be optimized to ensure that the film retains its desired slipperiness. Additionally, the storage conditions, such as temperature and humidity, can also affect the film's slipperiness over time. Therefore, it is important to implement quality control measures to monitor the film's slipperiness throughout the production process and storage. If any issues arise, a root cause analysis should be performed to identify the contributing factors and implement corrective actions. By addressing these factors, flexible packaging producers can ensure that their products meet the required slipperiness standards and perform optimally in end-use applications, ensuring customer satisfaction. The impact of temperature on coefficient of friction (COF) for flexible packaging materials is significant. As temperature increases, the COF of certain films, especially those with thin transparent CPP layers, can also increase. This is because higher temperatures can lead to changes in the film’s surface properties, such as increased tackiness or migration of additives (e.g., slip agents), which can reduce the effectiveness of slip agents. This phenomenon can cause problems during packaging operations, as higher friction can lead to poor machinability, difficulty in unwinding or sealing, and even damage to the packaging material. It is important to control the processing temperature during lamination and curing to minimize the impact on the film’s COF. Additionally, proper cooling of the film after lamination is crucial to prevent the film from remaining in a hot state for extended periods, which can contribute to increased friction. Regular testing of the film’s COF at different temperatures can help identify potential issues and ensure that the packaging material performs as expected in various environmental conditions. CPP friction coefficient was below 0.1 but after compounding, it could only be naturally cured which could affect the final quality of the film as some films need specific curing conditions or elevated temperatures for curing. Natural curing is an important part of the overall flexible packaging solution. However, it is not always possible to achieve optimal curing with natural curing, especially when the film has specific curing requirements or is exposed to high temperatures. In such cases, it is important to optimize the curing conditions to ensure proper curing and prevent defects such as delamination, poor bond strength, and reduced mechanical properties. This can involve using forced curing, or increasing the curing temperature or time. The selection of the adhesive and the film also plays a crucial role in determining the curing conditions. Some adhesives and films are more suitable for natural curing, while others require specific curing conditions. Therefore, it is important to consider the properties of both the film and the adhesive, as well as the curing process, to achieve the desired quality standards for flexible packaging products. Additionally, quality control measures should be implemented to test the bond strength and peel strength after curing to ensure proper adhesion and overall quality of the laminated film. If there are issues, the root cause should be identified, and the process optimized to ensure high-quality flexible packaging materials are produced consistently. Therefore, the use of natural curing after compounding requires careful consideration and optimization, especially when dealing with temperature-sensitive films or adhesives. The temperature and humidity of the curing environment should be controlled to ensure consistent and effective curing. Additionally, the film’s properties, such as its friction coefficient and bond strength, should be regularly monitored after natural curing to ensure that it meets the required quality standards for end-use applications. This can help prevent issues such as poor machinability, delamination, and other defects that can impact the overall functionality and shelf life of the packaging material.

III. Delamination Delamination during cooking  

The braised egg packaging bags produced by a factory in Zhejiang were sterilized at 115℃ in a food factory for 30 minutes. Bags in the upper layers of the sterilization retort showed BOPA delamination, and ink transferred to CPP . This indicates a serious issue with the film’s adhesion properties or the ink’s compatibility with the film and the sterilization process. Delamination during sterilization, especially at high temperatures like 115℃, suggests that the bond strength between the film layers is insufficient to withstand the thermal stress. This can be caused by various factors, including improper adhesive selection, insufficient adhesive coating weight, inadequate curing of the adhesive, or issues with the film’s surface treatment. The transfer of ink to the other layer further indicates a problem with the ink’s adhesion or its ability to withstand the sterilization conditions, which can lead to a variety of quality defects and customer complaints. To address these issues, it is crucial to conduct a comprehensive root cause analysis. This may involve testing the adhesive’s properties, the film’s surface treatment, and the ink’s compatibility with the film and the sterilization process. The curing conditions, including temperature and time, should also be optimized to ensure proper adhesion between the layers. Additionally, it is important to select an ink that is suitable for sterilization applications and has good adhesion to the film. By addressing these factors, flexible packaging producers can ensure that their products meet the required quality standards and perform optimally in high-temperature sterilization processes, ensuring the safety and quality of the packaged food products. The delamination and ink transfer during cooking or sterilization indicate a failure in the flexible packaging material’s integrity, which can lead to product spoilage, reduced shelf life, and safety concerns. This highlights the importance of selecting appropriate materials, adhesives, and inks, as well as optimizing the manufacturing and processing conditions, to ensure the packaging material can withstand the intended end-use conditions. Regular quality control checks, including testing for bond strength and ink adhesion after sterilization, are essential to prevent such issues and ensure the reliability of the flexible packaging product.

Shunxingyuan Packaging believes that the quality and amount of adhesive can meet product requirements. Curing at 50℃~ 55 ℃ for curing 48 hours of curing is also acceptable. The problem is very likely with the ink or the quality of the ink, which indicates an issue with the printing process or the ink’s formulation. The company’s assessment suggests that while the adhesive and curing conditions are adequate, the ink itself may be the primary source of the problem. This can be due to various reasons, such as using an ink that is not suitable for the film material, improper ink viscosity, or incorrect ink curing (e.g., insufficient UV curing for UV inks or thermal curing for solvent-based inks). Poor ink adhesion can lead to problems like ink transfer, smudging, or fading, which can compromise the visual appeal and functionality of the packaging. To address this, it is crucial to investigate the ink’s properties, including its compatibility with the film and its resistance to processing conditions like sterilization or high temperatures. The printing process parameters, such as ink application, drying, and curing, should also be optimized to ensure proper ink adhesion and durability. Regular quality control checks on ink adhesion and print quality are essential to prevent such issues and ensure that the packaging material meets the required standards. By identifying and addressing the ink-related issues, flexible packaging producers can ensure that their products have excellent print quality and withstand the intended end-use conditions, providing a high-quality flexible packaging solution for their customers.

The direct manifestation of this problem is ink transfer, and the reason for this phenomenon is insufficient curing, which indicates a critical issue with the printing or lamination process. Ink transfer means that the ink is not properly adhered to the film and is transferring to another surface or layer. This can be caused by various factors related to insufficient curing, such as inadequate drying of the ink, improper curing temperature or time, or issues with the ink’s formulation or its compatibility with the film. When the ink is not fully cured, it remains tacky and can easily transfer, leading to defects such as smudging, poor print quality, and delamination. This can significantly impact the visual appeal and functionality of the packaging material and lead to customer complaints. To address this issue, it is crucial to optimize the ink curing process by ensuring proper drying conditions, appropriate curing temperature and time, and adequate ventilation. The ink’s formulation and its compatibility with the film should also be reviewed to ensure optimal adhesion. Additionally, regular quality control checks on ink adhesion and print quality are essential to prevent such issues and ensure that the packaging material meets the required standards. By addressing the root cause of insufficient curing and optimizing the printing and lamination processes, flexible packaging producers can ensure that their products have excellent print quality, good ink adhesion, and overall integrity, providing a high-quality flexible packaging solution for their customers.

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