Recyclable Stand up Pouch is a common type of packaging bag in the current flexible packaging market. In addition to regular rectangular bags, there are also irregular shaped bags. Bag making is the most important part of vertical bag production, and the formation of bags requires bag making to achieve. This article focuses on analyzing how to ensure the quality of upright bags during the bag making process, especially how to ensure the quality of heat sealing, avoid leakage and rupture, resist pressure, drop, heat, cold and acid alkali corrosion, design reasonable capacity, and eliminate appearance defects. Upright bags, also known as self-supporting bags, are a common packaging bag shape in the current flexible packaging market. In addition to regular rectangular bags, there are also irregular shaped bags. There are those with welded plastic straws on the top, those with plastic zippers on the top, and those that can be directly heat sealed. A common feature of upright bags is that there is an inward folding bottom strip at the bottom of the bag. Before filling, this bottom strip is embedded between the upper and lower layers of material. After filling, the bottom will be stretched open by the contents. After the bottom of the bag is extended, it can be placed vertically to stand up, which has a very good display effect. The production process of upright bags, especially those with straws, is slightly different from other flexible packaging materials, requiring printing, lamination, slitting, bag making, and connecting processes. Bag making is the most important part of vertical bag production, and the formation of bags requires bag making to achieve.
This article will discuss how to ensure the quality of upright bags during the bag making process, especially how to ensure the quality of heat sealing, avoid leakage and breakage, which is also the focus of ensuring the functional requirements of upright bags. 1、 At present, the sealing method for plastic flexible packaging bags is mainly heat sealing. For those with special processes or requirements, ultrasonic sealing or pulse sealing will be used as an auxiliary method. Heat sealing is a process in which the material is heated, pressurized, and sealed for a certain period of time. The high-temperature metal sealing blade contacts the composite film, and the heat transferred melts the heat sealing layer. The polymer molecules in the molten state pass through the pressure of the heat sealing blade and maintain a certain contact time. The upper and lower layers of heat sealing thin films can be fused together, and after cooling and shaping, a certain heat sealing strength can be maintained. Regarding poor heat sealing, the main manifestations include excessive heat sealing, insufficient heat sealing, incorrect heat sealing position, and poor heat sealing appearance. For upright bags, there are two layers of film and four layers of film in the heat sealing area. There are heat seals for punching holes in the bottom material, as well as arc-shaped or trapezoidal heat seals for the bottom of the bag. The heat sealing area and shape of upright bags are slightly more complex, and the requirements for heat sealing are also very high. For example, the heat sealing of irregular upright bags not only needs to ensure that the shape of the irregular heat sealing mold matches, but also needs to be able to match the pattern design of the bag, which is more complex than ordinary three sided and middle sealed bags.
- Excessive heat sealing is mainly caused by high heat sealing temperature and slow machine speed, or high temperature and high pressure. However, when the machine speed is slow and the pressure is high, as long as the temperature is appropriate, it generally does not cause excessive heat sealing. Excessive heat sealing can cause abnormalities in the appearance and characteristics of packaging bags. For example, in terms of appearance, the surface of the film appears hot paste, wrinkles, and hot edge breaks.
Excessive heat sealing can cause serious damage to the heat sealing layer material, resulting in loss of toughness and decreased sealing strength, commonly known as “brittle fracture”. This situation will also reduce the compressive performance of the bag. To solve the problem of excessive heat sealing, the following aspects should be considered: (1) Based on the starting temperature of the bag’s heat sealing layer and the thickness of the film, the temperature of the sealing knife should be preliminarily selected. For example, if the heat sealing layer is a PE film or CPP film of a certain thickness, the heat sealing temperature at each heat sealing position should be selected based on the total thickness of the composite film and the thickness of the heat sealing layer. If the initial temperature is too high, it may cause excessive heat sealing. It should be noted that for the same PE film, or due to different molding processes, formulas, or the proportion of recycled materials added, even for materials of the same thickness, there may be significant deviations in the suitable heat sealing temperature, even up to 20 ℃ or more. (2) After selecting the heat sealing temperature, it is necessary to test it on the bag making machine and choose the appropriate heat sealing pressure and machine speed. If the pressure is too high and the machine speed is too slow, it can also cause the problem of excessive heat sealing, and in severe cases, it can damage the surface of the composite film. (3) Too many heat seals are applied to the same area, including individual heat seals (such as point seals and arc seals) and overlapping heat seals (such as edge seals). There are many heat sealing positions for upright bags, and the heat sealing positions are complex. Only one heat sealing is performed at the same position, which often cannot meet the requirements of heat sealing strength or sealing performance. In order to ensure sealing requirements, the temperature is increased and the heat sealing pressure is increased, resulting in embrittlement of the heat sealing layer and a decrease in compressive performance. To ensure that the material of the heat sealed part is not severely damaged, the number of heat seals for the same part should not exceed three times. For multiple heat seals at the same position, the temperature and pressure of the heat sealing mold should be appropriately reduced to ensure that the bag has good comprehensive performance. (4) The insulation cloth on the heat sealing knife and the silicone plate underneath are abnormal. If the insulation cloth is damaged or the underlying silicone board is too hard due to prolonged use, the impact of the sealing knife on the heat sealing material will increase, causing excessive heat sealing and reducing its strength. So the insulation cloth on the heat sealing knife should be replaced immediately, and the hardness of the silicone plate should be suitable for the heat sealing material. (5) Insufficient and untimely cooling. After the heat sealing is completed, timely and sufficient cooling is required. Due to excessive waste heat, it not only affects the strength of the heat sealing, but also causes the phenomenon of virtual sealing along the inner edge of the heat sealing edge. Virtual sealing is when heat is transferred to the heat sealing layer next to the sealing blade, melting the heat sealing layer and causing sealing without sufficient pressure. Virtual sealing can affect the appearance of the seal and the capacity of the upright bag. To avoid virtual sealing, it is necessary to shorten the distance between the heat sealing blade and the cooling blade, and ensure that the temperature of the cooling water is maintained at 18-25 ℃, so as not to produce a large number of water droplets on the cooling mold to meet the cooling requirements.
- Insufficient heat sealing is mainly due to weak sealing and failure to meet the sealing strength standard. The main reasons are as follows: (1) low heat sealing temperature and insufficient melting of the heat sealing layer. (2) Insufficient sealing pressure or uneven pressure on both sides is usually caused by wear of machine rod bearings or improper adjustment of spring expansion and contraction force on the mold. (3) The machine speed is too fast and the heat sealing time is insufficient. (4) The sealing blade is worn or uneven. (5) There are foreign objects between the heat sealing layers, such as precipitates of thin films, fragments of thin films, and fragments of punched holes. To solve the problem of insufficient heat sealing, in addition to selecting the appropriate processing temperature, pressure, and heat sealing time based on the characteristics of the heat sealing material, it is also necessary to inspect the key parts of the machine that are related to the heat sealing performance, ensuring that the surface of the heat sealing knife is flat, the insulation cloth is intact, and the machine components are normal. At the same time, attention should be paid to the hygiene of the heat sealing layer to avoid foreign objects appearing in the heat sealing parts.
- Heat sealing position error mainly refers to the abnormal width of the heat sealing edge and the deviation of the heat sealing blade position. According to the specifications of the bag and the size requirements of the heat sealing edge, the heat sealing position is basically fixed. If there is an abnormality in the tracking electric eye or if the film on the bag making machine is not smooth, especially if the bottom seal is overlapped and misaligned, it will cause the heat sealing position to be incorrect. There are several reasons that affect the heat sealing position: (1) the friction coefficient of the composite film is too high or too low, that is, the composite film is too rough or slippery, causing the composite film to not flow smoothly on the bag making machine. The friction coefficient of composite films is not only affected by the type of adhesive particles used in the film itself, but also by the additives, printing ink, composite glue, and film thickness used in single-layer films. It is necessary to determine the formula of the film and the use of auxiliary materials through actual bag making adaptability. (2) The bag making machine has abnormal eye tracking. When the electric eye cannot find or continuously and stably locate the cursor, and the tracking is abnormal, the sealing position of the bag will change, that is, it cannot be sealed to the set position. The main reasons for abnormal tracking of the electric eye are not only related to problems with the electric eye itself, such as electric eye faults, incorrect adjustment methods, and improper selection of the working state of the electric eye, but also to the color contrast depth and spacing of the cursor on the composite film. For example, the cursor design is unreasonable, the available scanning area is too small or the color is too light, and the contrast between the cursor and the surrounding color is not obvious. In addition, there have been changes in the spacing between the cursor. Due to the stretching and shrinking effects of printing and composite processes, the originally designed pattern spacing has changed, especially irregular changes that do not match the actual input size, which can cause abnormal tracking of the electric eye. (3) The pressure at both ends of the mold is unbalanced. Uneven pressure on both sides of the mold can cause the composite film to not flow smoothly on the bag making machine, resulting in abnormal heat sealing position and bag size. To avoid this problem, it is necessary to have a method to test the mold pressure and ensure pressure balance. (4) The position of the heat sealing mold is incorrect. The main reason is that the mold was not adjusted to the correct position during the bag making and machine adjustment process, and was not adjusted in place immediately after discovering abnormalities. (5) For the misalignment of the bottom material, it mainly refers to the separate unwinding of the bottom material of the upright bag on the bag making machine. The bottom material and body material have been separated on the slitting machine. The card strips, clamps, and upper limit baffles of the guide rollers on the bag making machine are insufficient to ensure smooth film flow. If there is a direct sealing area between the upper and lower layers of the body material, the bottom material will be hidden inside. When filling the contents, the bottom of the bag will not open and cannot stand upright, resulting in a three sided sealing shape. If the bottom and body materials are not separated when making bags, just like folding the edges of an accordion bag, there is no problem of misalignment of the bottom material. However, to use bottom folding material, it also depends on whether the unwinding section and film passage of the bag making machine are constructed properly. (6) Inconsistent thickness of composite film and lotus leaf edges. (7) The tightness of the composite film before bag making is not suitable. If the winding is too tight or too loose, it will affect the smooth flow of the film. The initial tension and taper of the slitting machine for winding should be determined according to the bag making practice. If there is a pressure roller on top of the film during slitting and winding, the pressure at both ends of the pressure roller should be as balanced as possible.
- The poor appearance of heat sealing mainly includes: wrinkling of the sealing edge, foaming of the sealing edge, curling of the sealing edge, unclear mold lines of the heat sealing knife, and dirt on the sealing edge. (1) Wrinkles in edge sealing are very common in penguin bag making, and there are many influencing factors, mainly including: uneven insulation cloth on the sealing knife or silicone board; The thickness of the composite film is uneven, with significant deviations; The tension of the upper and lower layers of film is inconsistent during heat sealing; The stacking of the upper and lower layers of film is not aligned; The heat sealing temperature is too high. (2) Edge sealing bubbles are mainly caused by the quality of the composite film itself. For example, when heat sealing is performed on a material structure containing nylon film, the nylon film will absorb moisture and produce heat sealing bubbles. Unbalanced heat sealing pressure and uneven heat sealing molds can also cause bubbles, as well as high residual solvents in the composite film. (3) The reason for edge curling: Improper tension matching control of the composite substrate leads to residual stress even after curing and shaping; Insufficient curing of composite adhesive, or insufficient addition of curing agent and curing time; The thermal shrinkage of the composite film is too high. Sometimes, the raised edge of the sealed bag can cause unevenness and wrinkling in the areas without heat sealing. (4) When the mold pattern of the heat sealing knife is unclear, the main consideration is insufficient heat sealing pressure or temperature; Poor contact or insufficient cooling time of the cooling mold; The edge of the heat sealing knife is uneven and worn; After a long period of use, the patterns on the insulation cloth have been worn flat. The unclear pattern not only affects the appearance, but also indicates insufficient heat sealing. (5) The edge sealing is dirty. In addition to the contamination of the composite film itself before bag making, it is also possible that the heat sealing mold has been stuck with carbonized plastic. After the mold is pressed on the composite film, the dirt is transferred to the edge sealing area. The dirt on the heat sealing mold should be removed immediately. The main factors affecting the poor appearance of the edge banding mentioned above are heat sealing parameters, heat sealing molds, and problems with the composite film itself. By identifying the causes, targeted solutions can be taken. 5. The poor heat sealing caused by previous process reasons and material abnormalities is not only affected by production processes such as printing, lamination, slitting, and winding, but also by the use of materials themselves. (1) The low composite strength causes delamination of the composite film during sealing, thereby affecting the heat sealing strength. At present, most upright bag composite films contain AL foil or aluminum plated materials. If packaging liquid items, in order to ensure composite strength, the adhesive between each composite layer must have sufficient dry coating amount, with each layer not less than 3.5g/㎡. If packaging solid powder materials, if the density is not too high, a dry coating amount of 3.5g/㎡ is sufficient. (2) During the process of compounding and curing, the adhesive penetrates into the interior of the film, affecting the heat sealing strength. (3) The adhesive in the composite film is not fully matured, resulting in edge warping after heat sealing and lower heat sealing strength. (4) The hot sealing surface of the heat sealing layer has corona, causing ineffective sealing. (5) Due to excessive release of lubricating powder or additives in the heat sealing layer, the strength after heat sealing is insufficient. (6) The stiffness and toughness of the heat sealing layer are not suitable, mainly related to the characteristics of the resin, and it is necessary to choose the appropriate resin model and formula requirements. (7) Insufficient or inconsistent thickness of composite film may result in insufficient heat sealing strength; Inconsistent thickness can not only cause the composite film to not pass smoothly on the bag making machine, but also lead to leakage of point seals and abnormal heat sealing positions. (8) The influence of heat sealing processing technology, such as excessive addition of recycled materials or improper selection of blown film process. If too much recycled material is used during film blowing, and the recycled material undergoes multiple thermal degradations, its heat sealing temperature will be greatly increased. Mixing it into the main material will change the heat sealing performance of the heat sealing layer. For commonly used blow molded heat sealing film substrates, their heat sealing performance is not only affected by the resin type, but also by processing parameters such as extrusion temperature, inflation ratio and traction ratio, condensate。
