With more and more enterprises using high-speed automatic packaging machines, quality problems such as bag breakage, cracking, delamination, weak heat sealing, and sealing contamination that often occur in the high-speed automatic packaging process of flexible packaging film have gradually become key process issues that enterprises need to control.
When producing roll film for high-speed automatic packaging machines, flexible packaging enterprises should pay attention to the following points:
Strict material selection
1. Material requirements for each layer of rolled film
Due to the different equipment structure of the high-speed automatic packaging machine compared to other bag making machines, its pressure only relies on the force of two rollers or hot pressing strips squeezing each other to achieve heat sealing, and there is no cooling device. The printing layer film directly contacts the heat sealing device without the protection of insulation cloth. Therefore, the selection of materials for each layer of the high-speed printing drum is particularly important.
2. The other properties of the material must comply with:
1) Balance of film thickness
The thickness, average thickness, and average thickness tolerance of plastic film ultimately depend on the thickness balance of the entire film. In the production process, the thickness uniformity of the film should be controlled well, otherwise the produced product is not a good product. A good product should have a balanced thickness in both the longitudinal and transverse directions. Because different types of films have different effects, their average thickness and average thickness tolerance are also different. The thickness difference between the left and right sides of high-speed automatic packaging film is generally not more than 15um.
2) Optical properties of thin films
Refers to the haze, transparency, and light transmittance of a thin film.
Therefore, there are special requirements and controls for the selection and amount of masterbatch additives in film rolling, as well as good transparency. At the same time, the opening and smoothness of the film should also be considered. The opening amount should be based on the principle of facilitating the winding and unwinding of the film and preventing adhesion between the films. If the amount is added too much, it will affect the increase in haze of the film. The transparency should generally reach 92% or more.
3) Friction coefficient
The coefficient of friction is divided into static friction and dynamic friction systems. For automatic packaging roll products, in addition to testing the friction coefficient under normal conditions, the friction coefficient between the film and the stainless steel plate should also be tested. As the heat sealing layer of the automatic packaging film is in direct contact with the automatic packaging molding machine, its dynamic friction coefficient should be less than 0.4u.
4) Add dosage
Generally, it should be controlled within 300-500PPm. If it is too small, it will affect the functionality of the film such as opening, and if it is too large, it will damage the composite strength. And it is necessary to prevent a large amount of migration or penetration of additives during use. When the dosage is between 500-800ppm, it should be used with caution. If the dosage exceeds 800ppm, it is generally not used.
5) Synchronous and asynchronous shrinkage of composite film
Non synchronous shrinkage is reflected in the changes of material curling and warping. Non synchronous shrinkage has two forms of expression: “inward curling” or “outward curling” of the bag opening. This state shows that there is still asynchronous shrinkage inside the composite film in addition to synchronous shrinkage (with different sizes and directions of thermal stress or shrinkage rate). Therefore, when purchasing thin films, it is necessary to conduct thermal (wet heat) shrinkage longitudinal and transverse tests on various composite materials under the same conditions, and the difference between the two should not be too much, preferably about 0.5%.
Reasons for Damage and Control Techniques
1. The effect of heat sealing temperature on heat sealing strength is the most direct
The melting temperature of various materials directly determines the minimum heat sealing temperature of composite bags.
During the production process, due to various factors such as heat sealing pressure, bag making speed, and the thickness of the composite substrate, the actual heat sealing temperature used is often higher than the melting temperature of the heat sealing material. High speed automatic packaging machine, with lower heat sealing pressure, requires higher heat sealing temperature; The faster the machine speed, the thicker the surface material of the composite film, and the higher the required heat sealing temperature.
2. Thermal adhesion curve of bonding strength
In automatic packaging, the filled contents will have a strong impact on the bottom of the bag. If the bottom of the bag cannot withstand the impact force, it will crack.
The general heat sealing strength refers to the bonding strength after two thin films are bonded together by heat sealing and completely cooled. However, on the automatic packaging production line, the two-layer packaging material did not receive sufficient cooling time, so the heat sealing strength of the packaging material is not suitable for evaluating the heat sealing performance of the material here. Instead, thermal adhesion, which refers to the peeling force of the material’s heat sealed part before cooling, should be used as the basis for selecting the heat sealing material, so as to meet the requirements of the material’s heat sealing strength during filling.
There is an optimal temperature point for achieving the best thermal adhesion of thin film materials, and when the heat sealing temperature exceeds this temperature point, the thermal adhesion will show a decreasing trend. On the automatic packaging production line, the production of flexible packaging bags is almost synchronized with the filling of the contents. Therefore, when filling the contents, the heat sealed part at the bottom of the bag is not completely cooled, and the impact force it can withstand is greatly reduced.
When filling the contents, for the impact force at the bottom of the flexible packaging bag, a thermal adhesion tester can be used to draw the thermal adhesion curve by adjusting the heat sealing temperature, heat sealing pressure, and heat sealing time, and select the optimal combination of heat sealing parameters for the production line.
When packaging heavy packaged or powdered items such as salt, laundry detergent, etc., after filling these items and before heat sealing, the air inside the bag should be discharged to reduce the stress on the packaging bag wall, allowing the solid material to be directly stressed to reduce bag damage. In the post-processing process, special attention should be paid to whether the puncture resistance, pressure resistance, drop rupture resistance, temperature resistance, temperature medium resistance, and food safety and hygiene performance meet the requirements.
Reasons and control points for stratification
A major problem with automatic packaging machines for film wrapping and bagging is that the surface, printed film, and middle aluminum foil layer are prone to delamination at the heat sealed area. Usually, after this phenomenon occurs, the manufacturer will complain to the soft packaging company about the insufficient composite strength of the packaging materials they provide. The soft packaging company will also complain to the ink or adhesive manufacturer about the poor adhesion, as well as the film manufacturer about the low corona treatment value, floating additives, and severe moisture absorption of the materials, which affect the adhesion of the ink and adhesive and cause delamination.
Here, we need to consider another important factor: the heat sealing roller.
The temperature of the heat sealing roller of the automatic packaging machine sometimes reaches 210 ℃ or above, and the heat sealing knife pattern of the roller sealing can be divided into two types: square pyramid shape and square frustum shape.
We can see in the magnifying glass that some of the layered and non layered samples have intact roller mesh walls and clear hole bottoms, while others have incomplete roller mesh walls and unclear hole bottoms. Some holes have irregular black lines (cracks) at the bottom, which are actually traces of the aluminum foil layer being fractured. And some of the mesh holes have an “uneven” bottom, indicating that the ink layer at the bottom of the bag has undergone a “melting” phenomenon.
For example, BOPA film and AL are both materials with certain ductility, but they rupture at the moment of processing into bags, indicating that the elongation of the packaging material applied by the heat sealing knife has exceeded the acceptable level of the material, resulting in rupture. From the heat seal imprint, it can be seen that the color of the aluminum foil layer in the middle of the “crack” is noticeably lighter than the side, indicating that delamination has occurred.
In the production of aluminum foil roll film packaging, some people believe that deepening the heat sealing pattern looks better. In fact, the main purpose of using a patterned heat sealing knife for heat sealing is to ensure the sealing performance of the heat seal, and aesthetics are secondary. Whether it is a flexible packaging production enterprise or a raw material production enterprise, they will not easily change the production formula during the production process, unless they adjust the production process or make important changes to the raw materials.
If the aluminum foil layer is crushed and the packaging loses its sealing, what’s the use of having a good appearance? From a technical perspective, the pattern of the heat sealing knife must not be pyramid shaped, but should be frustum shaped.
The bottom of the pyramid shaped pattern has sharp corners, which can easily scratch the film and cause it to lose its heat sealing purpose. At the same time, the temperature resistance of the ink used must exceed the temperature of the heat sealing blade to avoid the problem of ink melting after heat sealing. The general heat sealing temperature should be controlled between 170~210 ℃. If the temperature is too high, the aluminum foil is prone to wrinkling, cracking, and surface discoloration.
Precautions for winding solvent-free composite slitting drum
When rolling solvent-free composite film, the winding must be neat, otherwise tunneling is prone to occur at the loose edges of the winding. When the taper of the winding tension is set too small, the outer layer will generate a large squeezing force on the inner layer. If the friction force between the inner and outer layers of the composite film is small after winding (if the film is too smooth, the friction force will be small), winding extrusion phenomenon will occur. When a larger winding tension taper is set, the winding can be neat again.
Therefore, the winding uniformity of solvent-free composite films is related to the tension parameter setting and the friction force between the composite film layers. The friction coefficient of PE film used for solvent-free composite films is generally less than 0.1 to control the friction coefficient of the final composite film.
The plastic plastic composite film processed by solvent-free composite processing will have some appearance defects such as adhesive spots on the surface. When tested on a single packaging bag, it is a qualified product. However, after packaging the dark colored adhesive content, these appearance defects will appear as white spots.
Conclusion
The most common problems during high-speed automatic packaging are bag breakage and delamination. Although the breakage rate generally does not exceed 0.2% according to international standards, the losses caused by contamination of other items due to bag breakage are very serious. Therefore, by testing the heat sealing performance of materials and adjusting the heat sealing parameters in the production process, the probability of soft packaging bag damage during filling or storage, post-processing, and transportation can be reduced. However, special attention should be paid to the following issues:
1) Special attention should be paid to whether the filling material will contaminate the seal during the filling process. Contaminants can significantly reduce the thermal adhesion or sealing strength of the material, leading to the rupture of the flexible packaging bag due to its inability to withstand pressure. Special attention should be paid to powder filling materials, which require corresponding simulation tests.
2) The material thermal adhesion and expansion heat sealing strength obtained through the selected production line heat sealing parameters should leave some margin on the basis of design requirements (specific analysis should be carried out according to the equipment and material situation), because whether it is heat sealing components or soft packaging film materials, the uniformity is not very good, and accumulated errors will lead to uneven heat sealing effect at the packaging heat sealing point.
3) By testing the thermal adhesion and expansion heat sealing strength of materials, a set of heat sealing parameters suitable for specific products and production lines can be obtained. At this time, comprehensive consideration and optimal selection should be made based on the material heat sealing curve obtained from testing.
4) The rupture and delamination of plastic flexible packaging bags are a comprehensive reflection of materials, production processes, production parameters, and production operations. Only after detailed analysis can the true causes of rupture and delamination be identified. Standards should be established when purchasing raw and auxiliary materials and developing production processes. By keeping good original records and continuously improving during production, the damage rate of plastic automatic flexible packaging bags can be controlled to the optimal level within a certain range.
Post time: Dec-02-2024